The meaning of percent

WEBVTT
Kind: captions
Language: en

00:00:00.480 --> 00:00:04.530
We're asked to shade 20%
of the square below.

00:00:04.530 --> 00:00:06.700
Before doing that, let's
just even think about

00:00:06.700 --> 00:00:08.380
what percent means.

00:00:08.380 --> 00:00:09.440
Let me just rewrite it.

00:00:09.440 --> 00:00:15.890
20% is equal to-- I'm just
writing it out as a word-- 20

00:00:15.890 --> 00:00:26.260
percent, which literally
means 20 per cent.

00:00:26.260 --> 00:00:30.380
And if you're familiar with the
word century, you might

00:00:30.380 --> 00:00:33.940
already know that cent
comes from the Latin

00:00:33.940 --> 00:00:36.820
for the word hundred.

00:00:36.820 --> 00:00:39.230
This literally means you
can take cent, and that

00:00:39.230 --> 00:00:40.700
literally means 100.

00:00:40.700 --> 00:00:49.610
So this is the same thing
as 20 per 100.

00:00:49.610 --> 00:00:53.970
If you want to shade 20%, that
means, if you break up the

00:00:53.970 --> 00:00:57.880
square into 100 pieces, we
want to shade 20 of them.

00:00:57.880 --> 00:00:59.130
20 per 100.

00:00:59.130 --> 00:01:01.240
So how many squares have
they drawn here?

00:01:01.240 --> 00:01:05.970
So if we go horizontally right
here, we have one, two, three,

00:01:05.970 --> 00:01:09.400
four, five, six, seven, eight,
nine, ten squares.

00:01:09.400 --> 00:01:13.450
If we go vertically, we have
one, two, three, four, five,

00:01:13.450 --> 00:01:15.630
six, seven, eight, nine, ten.

00:01:15.630 --> 00:01:17.620
So this is a 10 by 10 square.

00:01:17.620 --> 00:01:21.680
So it has 100 squares here.

00:01:21.680 --> 00:01:24.580
Another way to say it is that
this larger square-- I guess

00:01:24.580 --> 00:01:26.270
that's the square that they're
talking about.

00:01:26.270 --> 00:01:31.970
This larger square is a broken
up into 100 smaller squares,

00:01:31.970 --> 00:01:34.220
so it's already broken
up into the 100.

00:01:34.220 --> 00:01:38.990
So if we want to shade 20% of
that, we need to shade 20 of

00:01:38.990 --> 00:01:42.420
every 100 squares that
it is broken into.

00:01:42.420 --> 00:01:45.470
So with this, we'll just
literally shade in 20 squares.

00:01:45.470 --> 00:01:47.330
So let me just do one.

00:01:47.330 --> 00:01:51.210
So if I just do one square, just
like that, I have just

00:01:51.210 --> 00:01:54.050
shaded 1 per 100
of the squares.

00:01:54.050 --> 00:01:56.080
100 out of 100 would
be the whole.

00:01:56.080 --> 00:01:58.640
I've shaded one of them.

00:01:58.640 --> 00:02:02.170
That one square by itself
would be 1%

00:02:02.170 --> 00:02:04.300
of the entire square.

00:02:04.300 --> 00:02:08.009
If I were to shade another one,
if I were to shade that

00:02:08.009 --> 00:02:12.190
and that, then those two
combined, that's 2% of the

00:02:12.190 --> 00:02:12.950
entire square.

00:02:12.950 --> 00:02:15.680
It's literally 2 per 100,
where 100 would

00:02:15.680 --> 00:02:16.640
be the entire square.

00:02:16.640 --> 00:02:23.490
If we wanted to do 20, we do
one, two, three, four-- if we

00:02:23.490 --> 00:02:26.860
shade this entire row, that
will be 10%, right?

00:02:26.860 --> 00:02:34.160
One, two, three, four, five,
six, seven, eight, nine, ten.

00:02:34.160 --> 00:02:36.430
And we want to do 20, so that'll
be one more row.

00:02:36.430 --> 00:02:39.050
So I can shade in this whole
other row right here.

00:02:39.050 --> 00:02:43.460
And then I would have shaded
in 20 of the 100 squares.

00:02:43.460 --> 00:02:45.010
Or another way of thinking about
it, if you take this

00:02:45.010 --> 00:02:48.750
larger square, divide it into
100 equal pieces, I've shaded

00:02:48.750 --> 00:02:57.590
in 20 per 100, or 20%, of the
entire larger square.

00:02:57.590 --> 00:02:59.720
Hopefully, that makes sense.


Solvent Effects on Sn1 and Sn2 Reactions

WEBVTT
Kind: captions
Language: en

00:00:00.530 --> 00:00:02.890
Let's talk a little bit more
about the comparison between

00:00:02.890 --> 00:00:09.660
Sn1 and Sn2 reactions, and maybe
we can also talk about

00:00:09.660 --> 00:00:12.760
what type of conditions make
one or the other more

00:00:12.760 --> 00:00:14.110
favorable, especially
the solvent.

00:00:14.110 --> 00:00:15.550
We've talked about it a little
bit, but we'll go into a

00:00:15.550 --> 00:00:16.760
little bit more detail.

00:00:16.760 --> 00:00:22.200
So just as a bit of a review,
an Sn1 reaction, you might

00:00:22.200 --> 00:00:25.000
have some type of a-- let me
just draw it like this.

00:00:25.000 --> 00:00:27.410
So you'll have some
carbon that's

00:00:27.410 --> 00:00:28.610
attached to three things.

00:00:28.610 --> 00:00:30.720
I'm making these three things
just arbitrary colors.

00:00:30.720 --> 00:00:32.549
They could be other
carbons, CH3.

00:00:32.549 --> 00:00:33.830
It could be hydrogens.

00:00:33.830 --> 00:00:35.480
It could be other things.

00:00:35.480 --> 00:00:40.640
And then it's also attached
to a leaving group.

00:00:40.640 --> 00:00:45.660
Let me do this in a color that
I haven't used yet: in pink.

00:00:45.660 --> 00:00:50.280
And then you also have a
nucleophile floating around.

00:00:50.280 --> 00:00:51.700
That'll be our nucleophile.

00:00:51.700 --> 00:00:53.740
It has one excess electron.

00:00:53.740 --> 00:00:55.270
It has a negative charge.

00:00:55.270 --> 00:00:59.660
And in an Sn1 reaction, the
first step only involves one

00:00:59.660 --> 00:01:01.500
of the reactants, one
of the reagents.

00:01:01.500 --> 00:01:03.720
That's why it's called Sn1.

00:01:03.720 --> 00:01:06.590
So in the very first step, the
nucleophile just waits

00:01:06.590 --> 00:01:09.520
passively while the leaving
group leaves.

00:01:09.520 --> 00:01:12.570
So maybe there's an electron
right there, the leaving group

00:01:12.570 --> 00:01:13.850
already has an electron.

00:01:13.850 --> 00:01:17.130
It swipes one, so it has to be
reasonably electronegative.

00:01:17.130 --> 00:01:20.480
It swipes one from that carbon,
and so once it does

00:01:20.480 --> 00:01:23.440
that, our situation will
look like this.

00:01:23.440 --> 00:01:24.830
You have your substrate.

00:01:24.830 --> 00:01:27.310
It now just lost an electron.

00:01:27.310 --> 00:01:31.030
It lost that electron, so it
now has a positive charge.

00:01:31.030 --> 00:01:34.820
Let me add that little
trace of something.

00:01:34.820 --> 00:01:36.760
Let me erase this right here
so you don't think that

00:01:36.760 --> 00:01:38.290
electron is still there.

00:01:38.290 --> 00:01:41.210
So it now has a positive
charge.

00:01:41.210 --> 00:01:43.540
The leaving group, which was
neutral, took an electron.

00:01:43.540 --> 00:01:46.340
So it out now has a
negative charge.

00:01:46.340 --> 00:01:49.180
The leaving group now took an
electron, so it now has at

00:01:49.180 --> 00:01:51.180
least two valence
electrons there.

00:01:51.180 --> 00:01:53.170
It has a negative charge,
probably a lot more.

00:01:53.170 --> 00:01:56.050
And then in the next step, this
nucleophile-- so let me

00:01:56.050 --> 00:01:59.110
copy and paste this.

00:01:59.110 --> 00:02:00.632
That's not what I
wanted to do.

00:02:00.632 --> 00:02:03.460
Let me copy and paste
it again.

00:02:03.460 --> 00:02:06.620
So copy and paste.

00:02:06.620 --> 00:02:07.090
OK.

00:02:07.090 --> 00:02:09.180
So that thing that was waiting
patiently during the

00:02:09.180 --> 00:02:10.720
rate-determining steps
says, hey, there's

00:02:10.720 --> 00:02:13.100
a carbocation there.

00:02:13.100 --> 00:02:14.070
I'm a nucleophile.

00:02:14.070 --> 00:02:17.760
I'm not a strong nucleophile in
the case of an Sn1, but I

00:02:17.760 --> 00:02:21.320
still like giving away electrons
to nucleuses.

00:02:21.320 --> 00:02:22.770
I love nucleases.

00:02:22.770 --> 00:02:26.190
I see a positive carbocation
there.

00:02:26.190 --> 00:02:28.520
Actually, a carbocation is
positive by definition, so let

00:02:28.520 --> 00:02:31.090
me give my electrons to him.

00:02:34.150 --> 00:02:39.320
And in the very last step, we
have a situation where this

00:02:39.320 --> 00:02:42.360
guy-- let me copy and
paste this again.

00:02:42.360 --> 00:02:46.310
Let me copy and let me paste.

00:02:46.310 --> 00:02:49.230
That guy is bonded to
the nucleophile.

00:02:52.890 --> 00:02:55.300
Since the nucleophile gave the
electron to this carbon, the

00:02:55.300 --> 00:02:56.770
carbon doesn't have
a charge anymore.

00:02:56.770 --> 00:02:57.840
Since the nucleophile lost that

00:02:57.840 --> 00:02:59.800
electron, it's now neutral.

00:02:59.800 --> 00:03:02.940
So you have it bonded to the
nucleophile, and, of course,

00:03:02.940 --> 00:03:06.650
you have the leaving
group still here

00:03:06.650 --> 00:03:10.000
with a negative charge.

00:03:10.000 --> 00:03:12.690
We've hinted at this a little
bit, but in order for this

00:03:12.690 --> 00:03:17.650
step to happen on its own with
only this reagent happening

00:03:17.650 --> 00:03:20.300
spontaneously to some degree,
or for it to just happen.

00:03:20.300 --> 00:03:21.690
And we won't talk
about the rates.

00:03:21.690 --> 00:03:23.520
And it won't necessarily happen
fast, but in order for

00:03:23.520 --> 00:03:26.940
this to happen at all, this
has to be a stable

00:03:26.940 --> 00:03:27.910
carbocation.

00:03:27.910 --> 00:03:30.760
And we've seen before that in
order for this to be a stable

00:03:30.760 --> 00:03:33.540
carbocation, it has to be bonded
to things that are

00:03:33.540 --> 00:03:37.230
willing to lend it some
electrons so it's not super

00:03:37.230 --> 00:03:38.270
hungry for electrons.

00:03:38.270 --> 00:03:43.220
And in order for that to happen,
this carbocation right

00:03:43.220 --> 00:03:45.870
here has to be tertiary.

00:03:45.870 --> 00:03:47.190
Tertiary is ideal.

00:03:47.190 --> 00:03:49.710
Tertiary carbocation means that
these three things over

00:03:49.710 --> 00:03:52.290
here are all carbons
or carbon chains.

00:03:52.290 --> 00:03:55.450
So ideally, it would be tertiary
and secondary will

00:03:55.450 --> 00:03:59.650
also work if at least two of
these guys are carbon.

00:03:59.650 --> 00:04:02.830
That'll also work if the solvent
is favorable, if the

00:04:02.830 --> 00:04:05.290
environment we're dealing with
is a favorable environment.

00:04:05.290 --> 00:04:07.840
If only one of these were
carbon, so if this was a

00:04:07.840 --> 00:04:11.630
primary carbocation or this was
a methyl carbocation where

00:04:11.630 --> 00:04:14.890
nothing here is a carbon,
then you're unlikely

00:04:14.890 --> 00:04:17.060
to have an Sn1 reaction.

00:04:17.060 --> 00:04:21.709
Now, on the other side of the
picture, Sn2 reaction, this is

00:04:21.709 --> 00:04:24.040
the one where you have a super
strong nucleophile, or at

00:04:24.040 --> 00:04:27.020
least a strong nucleophile, and
the whole reaction happens

00:04:27.020 --> 00:04:30.740
in one step involving both
of the reagents.

00:04:30.740 --> 00:04:36.305
So you start here with
your leaving group.

00:04:36.305 --> 00:04:38.580
The way you can think about it,
Sn1, the leaving group is

00:04:38.580 --> 00:04:40.750
the thing that does something
first. In Sn2, it all happens

00:04:40.750 --> 00:04:42.940
at once, but the nucleophile's
kind of the aggressor.

00:04:45.610 --> 00:04:49.500
It really wants to give that
electron away badly.

00:04:49.500 --> 00:04:52.740
So in the very first step, it
comes in here and gives it to

00:04:52.740 --> 00:04:55.035
that carbon, even though that
carbon didn't necessarily want

00:04:55.035 --> 00:04:56.920
it that badly.

00:04:56.920 --> 00:04:58.970
It would have a partial positive
charge if this thing

00:04:58.970 --> 00:05:01.400
is really electronegative and
might be hogging this electron

00:05:01.400 --> 00:05:04.026
already, but that thing that it
was already hogging, now it

00:05:04.026 --> 00:05:05.350
could say, hey, you already
got an electron.

00:05:05.350 --> 00:05:06.810
I can take it from you.

00:05:06.810 --> 00:05:09.240
And so in literally one
step, we end up with

00:05:09.240 --> 00:05:10.490
something like this.

00:05:13.530 --> 00:05:14.970
Actually, let me just
redraw that.

00:05:14.970 --> 00:05:16.680
So we have our leaving
group has left.

00:05:19.180 --> 00:05:20.950
It's taken the electron
that the

00:05:20.950 --> 00:05:22.600
carbon didn't need anymore.

00:05:22.600 --> 00:05:27.030
So it now has this electron and
that one and maybe more,

00:05:27.030 --> 00:05:28.380
and it has a negative charge.

00:05:28.380 --> 00:05:34.370
And now this carbon is attached
to the blue thing.

00:05:34.370 --> 00:05:38.040
I'll just draw the purple thing,
and maybe in back, it's

00:05:38.040 --> 00:05:39.330
attached to the orange thing.

00:05:39.330 --> 00:05:43.730
Let me draw it so you can see
what I'm talking about, so the

00:05:43.730 --> 00:05:45.890
blue thing and then
the purple thing.

00:05:45.890 --> 00:05:48.360
And then the nucleophile had
come from the other direction,

00:05:48.360 --> 00:05:51.230
so now it's also bonded
to the nucleophile.

00:05:51.230 --> 00:05:54.340
The nucleophile is giving an
electron, so it loses its

00:05:54.340 --> 00:05:54.945
negative charge.

00:05:54.945 --> 00:05:57.400
And essentially, the negative
charge gets transferred to the

00:05:57.400 --> 00:05:59.890
leaving group, which is able to
take an electron, and then

00:05:59.890 --> 00:06:02.000
that carbon in the middle
stays neutral.

00:06:02.000 --> 00:06:05.060
And we saw before that this
can only reasonably happen

00:06:05.060 --> 00:06:06.990
with a strong nucleophile.

00:06:06.990 --> 00:06:09.640
That has to be a strong
nucleophile, and there can't

00:06:09.640 --> 00:06:11.420
be a lot of steric hindrance.

00:06:11.420 --> 00:06:13.730
These things can't
really block this

00:06:13.730 --> 00:06:15.290
nucleophile from attacking.

00:06:15.290 --> 00:06:23.740
So this will work best if this
is a methyl or primary carbon.

00:06:23.740 --> 00:06:26.150
And if the conditions are just
right and if the nucleophile

00:06:26.150 --> 00:06:32.880
is strong enough,
maybe secondary,

00:06:32.880 --> 00:06:34.680
maybe a secondary carbon.

00:06:34.680 --> 00:06:38.250
Because once you have too many
carbons or big carbon chains

00:06:38.250 --> 00:06:40.390
over here, then it blocks
the nucleophile.

00:06:40.390 --> 00:06:41.190
It cannot attack.

00:06:41.190 --> 00:06:43.060
If these are all hydrogens,
there's a lot of space for

00:06:43.060 --> 00:06:44.210
this nucleophile to attack.

00:06:44.210 --> 00:06:47.400
If only one of these are carbon,
still a lot of space.

00:06:47.400 --> 00:06:50.490
Now, that's just kind of a
general comparison, and it

00:06:50.490 --> 00:06:51.520
gives you some clues.

00:06:51.520 --> 00:06:53.600
Weak nucleophile, you're
dealing with Sn1.

00:06:53.600 --> 00:06:55.760
Strong nucleophile, Sn2.

00:06:55.760 --> 00:06:58.550
If this is a tertiary or
secondary carbon, so if it's

00:06:58.550 --> 00:07:00.830
bonded to a lot of other
carbons, Sn1

00:07:00.830 --> 00:07:01.960
should be in your brain.

00:07:01.960 --> 00:07:04.220
If this isn't bonded to a lot
a carbons and you have a

00:07:04.220 --> 00:07:07.740
strong nucleophile, Sn2 should
be in your brain.

00:07:07.740 --> 00:07:09.550
And now the last thing you
should have to think about is

00:07:09.550 --> 00:07:10.970
the solvent itself.

00:07:10.970 --> 00:07:13.700
What are these reagents
actually dissolved in.

00:07:13.700 --> 00:07:15.210
So let's think about it.

00:07:15.210 --> 00:07:20.670
So in either an Sn1 or an Sn2
reaction, we're dealing with

00:07:20.670 --> 00:07:22.300
things that have charge.

00:07:22.300 --> 00:07:25.180
And in general, if you're
dealing with ions or things

00:07:25.180 --> 00:07:30.210
that have charge, they're going
to be more soluble in

00:07:30.210 --> 00:07:32.680
polar solvents because the
charge likes to be around

00:07:32.680 --> 00:07:34.670
other things with charge so that
they can mix and match.

00:07:34.670 --> 00:07:41.050
So in both situations, you want
to have polar solvents.

00:07:44.740 --> 00:07:46.990
And you are familiar with
polar solvents.

00:07:46.990 --> 00:07:49.730
Examples of polar solvents
are, well,

00:07:49.730 --> 00:07:52.170
water is a polar solvent.

00:07:52.170 --> 00:07:55.500
H2O, and it's polar because
oxygen is so electronegative

00:07:55.500 --> 00:07:58.740
it hogs electrons.

00:07:58.740 --> 00:08:02.590
So you have a partial negative
charge on the oxygen side and

00:08:02.590 --> 00:08:07.870
partial positive charges
on the hydrogen side.

00:08:07.870 --> 00:08:10.890
Another example of a polar
solvent that you may have not

00:08:10.890 --> 00:08:18.620
seen before is something
like diethyl ether.

00:08:18.620 --> 00:08:21.800
So diethyl ether would
look like this.

00:08:21.800 --> 00:08:29.350
CH2, CH3, CH2, CH3.

00:08:29.350 --> 00:08:31.050
And I know we havn't
named ethers yet.

00:08:31.050 --> 00:08:31.670
Let me write this.

00:08:31.670 --> 00:08:37.630
This is diethyl-- let me make
sure I get that "y" right.

00:08:40.320 --> 00:08:42.510
This is diethyl ether.

00:08:42.510 --> 00:08:43.890
And this is a bit of a review.

00:08:43.890 --> 00:08:45.410
This is probably the best
way to learn things, as

00:08:45.410 --> 00:08:46.970
we learn as we go.

00:08:46.970 --> 00:08:48.590
An ether is something
that has a form.

00:08:48.590 --> 00:08:51.970
You have some carbon chain or
some-- you could call it a

00:08:51.970 --> 00:08:55.920
radical-- some group attached to
a carbon chain, I guess is

00:08:55.920 --> 00:08:59.120
the way my brain processes it,
to an oxygen which is attached

00:08:59.120 --> 00:09:00.230
to another carbon chain.

00:09:00.230 --> 00:09:01.800
It doesn't have to be
the exact same one.

00:09:01.800 --> 00:09:03.760
And diethyl ether is the same.

00:09:03.760 --> 00:09:05.400
And you're literally just
saying diethyl.

00:09:05.400 --> 00:09:08.220
You have to ethyl groups
attached to an ether.

00:09:08.220 --> 00:09:09.480
Ethyl is two carbons.

00:09:09.480 --> 00:09:12.340
So two carbons, oxygen,
two more carbons.

00:09:12.340 --> 00:09:13.650
Diethyl ether.

00:09:13.650 --> 00:09:19.740
And this would also be a polar
solvent because oxygen is more

00:09:19.740 --> 00:09:22.880
electronegative then carbon, so
you still have this partial

00:09:22.880 --> 00:09:25.890
negative charge-- let me do this
in a different color-- on

00:09:25.890 --> 00:09:32.640
the oxygen end and you still
have a partial positive charge

00:09:32.640 --> 00:09:34.680
on your carbon end.

00:09:34.680 --> 00:09:36.710
So this is also a
polar solvent.

00:09:36.710 --> 00:09:38.410
Both of these are
polar solvents.

00:09:38.410 --> 00:09:41.080
Now, the difference between
these, and this will probably

00:09:41.080 --> 00:09:46.130
be a new term for you, is this
water and most types of

00:09:46.130 --> 00:09:48.980
alcohol, something that has
hydrogen that could be knocked

00:09:48.980 --> 00:09:51.050
off, this is a protic sovent.

00:09:54.040 --> 00:09:55.940
And protic just comes
from proton.

00:09:55.940 --> 00:09:57.970
And that means that if something
is dissolved in

00:09:57.970 --> 00:09:59.860
this, or if you have a solution
of this, then these

00:09:59.860 --> 00:10:02.750
hydrogens might be able to get
knocked off and you might have

00:10:02.750 --> 00:10:04.830
free protons lying around.

00:10:04.830 --> 00:10:07.620
Because let's say this
oxygen swipes this

00:10:07.620 --> 00:10:09.770
electron from this hydrogen.

00:10:09.770 --> 00:10:12.530
If it swipes that electron from
that hydrogen, then you

00:10:12.530 --> 00:10:15.750
have a situation where you
have OH minus, right?

00:10:15.750 --> 00:10:17.090
The oxygen now has a
negative charge.

00:10:17.090 --> 00:10:20.760
And this hydrogen, all it had to
its name was that electron.

00:10:20.760 --> 00:10:23.110
If you take that away, all
it is is a proton.

00:10:23.110 --> 00:10:27.370
So when someone draws H plus,
that literally is a proton.

00:10:27.370 --> 00:10:29.510
That literally is a proton.

00:10:29.510 --> 00:10:33.440
And this can occur in water,
that you have some protons

00:10:33.440 --> 00:10:38.320
floating around, so water
is fundamentally a

00:10:38.320 --> 00:10:40.860
protic polar solvent.

00:10:40.860 --> 00:10:42.950
Diethyl ether, on the other
hand, it doesn't have

00:10:42.950 --> 00:10:46.480
hydrogens where oxygens can
take its hydrogens away.

00:10:46.480 --> 00:10:49.200
The hydrogens here are all
bonded to carbon, which is not

00:10:49.200 --> 00:10:53.300
that electronegative, so this
is an aprotic solvent.

00:10:53.300 --> 00:10:54.550
Let me draw this.

00:11:04.410 --> 00:11:08.510
Now, let's think about whether
an Sn1 or Sn2 is better and--

00:11:08.510 --> 00:11:12.780
well, I already said the
solvents have to be polar.

00:11:12.780 --> 00:11:14.690
That's because we're dealing
with charged

00:11:14.690 --> 00:11:16.700
things in both reactions.

00:11:16.700 --> 00:11:17.790
But which one will be better?

00:11:17.790 --> 00:11:18.760
So let me write this down.

00:11:18.760 --> 00:11:22.360
So in both situations, we want
a polar solvent, so let me

00:11:22.360 --> 00:11:24.790
write that down.

00:11:24.790 --> 00:11:27.730
And I'll give you a hint: one
of them will be good in a

00:11:27.730 --> 00:11:30.640
protic environment and one of
them will not be good in a

00:11:30.640 --> 00:11:31.960
protic environment.

00:11:31.960 --> 00:11:34.690
So in an Sn2, if you had-- let's
just think it through.

00:11:34.690 --> 00:11:36.040
Let's just not memorize this.

00:11:36.040 --> 00:11:42.010
If we had protons floating
around in an Sn2 situation,

00:11:42.010 --> 00:11:43.680
what is likely to happen?

00:11:43.680 --> 00:11:45.920
We have this strong
nucleophile.

00:11:45.920 --> 00:11:49.960
We have this thing that wants
to give away electrons.

00:11:49.960 --> 00:11:53.010
Now, it could give it away to a
carbon that might maybe have

00:11:53.010 --> 00:11:55.450
a partial positive charge and
then the leaving group could

00:11:55.450 --> 00:11:58.780
take it away, but what
wants electrons more?

00:11:58.780 --> 00:12:01.420
A carbon that has a partial
negative charge or a proton,

00:12:01.420 --> 00:12:03.770
something that has a full
positive charge?

00:12:03.770 --> 00:12:05.450
Well, a proton would
want it more.

00:12:05.450 --> 00:12:08.500
So if you had protons floating
around, the nucleophile would

00:12:08.500 --> 00:12:11.730
just give its electrons to the
proton and the Sn2 reaction

00:12:11.730 --> 00:12:13.670
wouldn't actually occur.

00:12:13.670 --> 00:12:15.810
The protons would take
all the electrons.

00:12:15.810 --> 00:12:18.270
So you can't have protons
floating around.

00:12:18.270 --> 00:12:20.060
You cannot have protons
floating

00:12:20.060 --> 00:12:23.000
around in an Sn2 reaction.

00:12:23.000 --> 00:12:24.790
Then this carbon will never
get the electron.

00:12:24.790 --> 00:12:26.090
The proton will take it.

00:12:26.090 --> 00:12:33.340
So you need an aprotic
solvent.

00:12:33.340 --> 00:12:36.650
So Sn2 would work well
in diethyl ether.

00:12:40.250 --> 00:12:42.705
If this is a strong nucleophile,
it's a solution

00:12:42.705 --> 00:12:46.310
in diethyl ether, and these were
all hydrogens, then you'd

00:12:46.310 --> 00:12:49.300
be just ready to go for
an Sn2 reaction.

00:12:49.300 --> 00:12:53.660
Now, on the other end of the
spectrum, the key in an Sn1

00:12:53.660 --> 00:12:57.250
reaction is that this
carbocation be stable.

00:12:57.250 --> 00:13:03.320
And for a carbocation to be
stable-- or actually even more

00:13:03.320 --> 00:13:05.690
important, instead of the
carbocation being stable,

00:13:05.690 --> 00:13:09.070
that's dependent on it being a
tertiary or secondary carbon.

00:13:09.070 --> 00:13:12.060
But order for the leaving group
to be stable when it's

00:13:12.060 --> 00:13:14.840
negative, it actually would
be nice if there was

00:13:14.840 --> 00:13:17.960
some protons available.

00:13:17.960 --> 00:13:21.820
Maybe it could give electrons
to the proton.

00:13:21.820 --> 00:13:24.200
Maybe it could bond
with the proton.

00:13:24.200 --> 00:13:27.560
So in this situation, the
protons will actually help

00:13:27.560 --> 00:13:29.270
stabilize the situation.

00:13:29.270 --> 00:13:35.260
So in an Sn1 reaction, the
ideal is actually a polar

00:13:35.260 --> 00:13:38.550
protic solvent.

00:13:38.550 --> 00:13:41.580
So if you see a situation where
you have a secondary or

00:13:41.580 --> 00:13:48.700
tertiary carbon, it is a weak
nucleophile, and you're

00:13:48.700 --> 00:13:51.720
dealing with a polar protic
solvent, the most famous of

00:13:51.720 --> 00:13:55.010
which is water, but an alcohol
would also work, then you

00:13:55.010 --> 00:13:57.053
should be thinking Sn1.

00:13:57.053 --> 00:14:00.434
If you see a strong nucleophile,
there's not much

00:14:00.434 --> 00:14:01.400
steric hindrance.

00:14:01.400 --> 00:14:03.815
There's space that
it can attack.

00:14:03.815 --> 00:14:06.954
This is a methyl or a primary,
maybe a secondary carbon, and

00:14:06.954 --> 00:14:10.650
it's happening in an aprotic
polar solvent like diethyl

00:14:10.650 --> 00:14:12.390
ether, then you know you're
ready for an Sn2.


Sn2 Stereochemistry

WEBVTT
Kind: captions
Language: en

00:00:00.560 --> 00:00:03.810
In this video, not only are we
going to pay attention to how

00:00:03.810 --> 00:00:07.800
these two molecules will react,
but we'll also pay

00:00:07.800 --> 00:00:08.500
attention to the

00:00:08.500 --> 00:00:11.560
stereochemistry of the end product.

00:00:11.560 --> 00:00:13.040
To do that, we have to
pay attention to the

00:00:13.040 --> 00:00:15.460
stereochemistry of the things
that we're beginning with.

00:00:15.460 --> 00:00:19.110
And this is just regular
hydroxide anion, we've seen

00:00:19.110 --> 00:00:20.200
this show before.

00:00:20.200 --> 00:00:22.200
This is going to act
as the nucleophile.

00:00:22.200 --> 00:00:23.940
It's not the strongest
nucleophile.

00:00:23.940 --> 00:00:26.750
it's not iodide, but it's a
pretty good nucleophile.

00:00:26.750 --> 00:00:28.560
So we have a good nucleophile
here.

00:00:35.700 --> 00:00:38.210
And then over here,
we have a carbon.

00:00:38.210 --> 00:00:40.790
It's not a methyl carbon.

00:00:40.790 --> 00:00:42.130
It's not bonded to a
bunch of hydrogens.

00:00:42.130 --> 00:00:44.790
That would be a super
fast Sn2 reaction.

00:00:44.790 --> 00:00:45.970
This guy could attack it.

00:00:45.970 --> 00:00:47.900
And it's not even a primary,
it's a secondary.

00:00:47.900 --> 00:00:50.860
But we said if we have a pretty
good nucleophile, this

00:00:50.860 --> 00:00:53.310
guy's going to want to give his
electrons enough to that,

00:00:53.310 --> 00:00:54.230
that this will happen.

00:00:54.230 --> 00:00:56.150
It won't be the fastest
reaction, but we can get an

00:00:56.150 --> 00:01:00.230
Sn2 reaction, especially if the
solvent is the right type

00:01:00.230 --> 00:01:00.800
of solvent.

00:01:00.800 --> 00:01:03.650
And we'll talk more about this
in future videos on what type

00:01:03.650 --> 00:01:08.200
of solvents are good for Sn2
reactions versus Sn1.

00:01:08.200 --> 00:01:10.410
So we're going to have
an Sn1 reaction.

00:01:10.410 --> 00:01:15.570
We're going to have this
situation where one of the

00:01:15.570 --> 00:01:19.210
electrons from the nucleophile
will be given to this

00:01:19.210 --> 00:01:20.180
secondary carbon.

00:01:20.180 --> 00:01:21.820
It's bonded to two
other carbons.

00:01:21.820 --> 00:01:24.770
So it'll be given to the
secondary carbon, and then

00:01:24.770 --> 00:01:28.840
that will allow this bromine
to swipe the carbon that it

00:01:28.840 --> 00:01:30.370
was a bonded with.

00:01:30.370 --> 00:01:32.410
And we know bromine is pretty
electronegative, so it's going

00:01:32.410 --> 00:01:32.760
to do that.

00:01:32.760 --> 00:01:34.960
It's going to swipe
that electron.

00:01:34.960 --> 00:01:41.370
And then at the end, you're
going to end up with a

00:01:41.370 --> 00:01:45.130
bromine, or I should say bromide
now that it's an

00:01:45.130 --> 00:01:46.890
anion, a bromide anion.

00:01:46.890 --> 00:01:48.300
It swiped that electron.

00:01:48.300 --> 00:01:53.520
Before bromine had one, two,
three, four, five, six, seven

00:01:53.520 --> 00:01:55.510
valence electrons, and now it's
going to swipe this one

00:01:55.510 --> 00:01:59.080
over here, so now it'll have
eight: one, two, three, four,

00:01:59.080 --> 00:02:01.160
five, six, seven, eight.

00:02:01.160 --> 00:02:04.280
Put a negative charge, it
just gained an electron.

00:02:04.280 --> 00:02:07.960
And then the other stuff
will look like this.

00:02:07.960 --> 00:02:12.050
Let me draw that carbon in the
center, that's just like this.

00:02:12.050 --> 00:02:14.310
And notice, bromine is leaving
from the right, and the

00:02:14.310 --> 00:02:18.120
hydroxide is attacking, or it's
giving its electron from

00:02:18.120 --> 00:02:19.550
the left, or it's bonding
from the left.

00:02:19.550 --> 00:02:20.080
So what's it going to look?

00:02:20.080 --> 00:02:24.380
Well, We have this ethyl group
coming straight out

00:02:24.380 --> 00:02:36.140
forward, CH2, CH3.

00:02:36.140 --> 00:02:38.690
We have the hydrogen that
is still going to

00:02:38.690 --> 00:02:40.210
be behind the carbon.

00:02:43.590 --> 00:02:48.010
So I could draw it like this.

00:02:48.010 --> 00:02:49.460
The bromine is no
longer there.

00:02:49.460 --> 00:02:50.910
You still have this
methyl group above

00:02:50.910 --> 00:02:54.060
it, so that is CH3.

00:02:54.060 --> 00:02:58.930
And now this hydroxide has
attacked right in the same

00:02:58.930 --> 00:03:01.300
direction that the bromine is
leaving in, but from the

00:03:01.300 --> 00:03:01.950
opposite side.

00:03:01.950 --> 00:03:04.640
So that's leaving to the right,
this guy's entering

00:03:04.640 --> 00:03:06.840
from the left, so then
the hydroxide will be

00:03:06.840 --> 00:03:12.990
bonded like this: O-H.

00:03:12.990 --> 00:03:19.510
Now we have just described
a classic Sn2 reaction.

00:03:19.510 --> 00:03:22.650
And I always like to emphasize
the Sn2, because my brain

00:03:22.650 --> 00:03:26.350
always confuses it with Sn1,
so I like to remind myself,

00:03:26.350 --> 00:03:30.680
this is Sn2 because it is
substitution with a

00:03:30.680 --> 00:03:31.880
nucleophile.

00:03:31.880 --> 00:03:34.790
That's the hydroxide
in this case.

00:03:34.790 --> 00:03:37.740
And that number two says the
rate-determining step, and

00:03:37.740 --> 00:03:41.080
there is only one step in this
reaction, involves both of the

00:03:41.080 --> 00:03:43.990
reactants, and this definitely
is involving both of the

00:03:43.990 --> 00:03:47.020
reactants, so this
is a classic Sn2.

00:03:47.020 --> 00:03:49.960
The point of this video is to
see what happened to the

00:03:49.960 --> 00:03:52.930
stereochemistry of
this molecule.

00:03:52.930 --> 00:03:55.205
Obviously, it's a different
molecule on this side, but it

00:03:55.205 --> 00:03:57.780
had switched from left to right,
or I guess we should

00:03:57.780 --> 00:04:02.510
say that it switched from R to
S, or S to R, or did it stay

00:04:02.510 --> 00:04:03.300
in the same direction?

00:04:03.300 --> 00:04:06.210
So let's name this one using our
naming mechanisms. I guess

00:04:06.210 --> 00:04:09.240
you could call it the
R-S naming scheme.

00:04:09.240 --> 00:04:11.050
And then we'll name this one,
and then we'll see if we

00:04:11.050 --> 00:04:13.770
switched our handedness.

00:04:13.770 --> 00:04:17.120
So first of all, let's just
name the base molecule.

00:04:17.120 --> 00:04:18.170
This is good review.

00:04:18.170 --> 00:04:19.550
So what's our longest
carbon chain?

00:04:19.550 --> 00:04:23.670
We have one, two, three,
four carbons.

00:04:23.670 --> 00:04:25.240
And actually, I probably
shouldn't have named it that

00:04:25.240 --> 00:04:28.160
way, because this bromo group
is closer to that end.

00:04:28.160 --> 00:04:30.460
So I should number it
starting from here.

00:04:30.460 --> 00:04:35.450
We have one, two, three,
four carbons.

00:04:35.450 --> 00:04:38.150
So methyl is one carbon.

00:04:38.150 --> 00:04:39.370
Ethyl is two carbons.

00:04:39.370 --> 00:04:43.730
Propyl is three carbons, or
prop- is three, so it's but-,

00:04:43.730 --> 00:04:46.780
But- is four carbons, so
it's going to but-.

00:04:46.780 --> 00:04:48.945
It's all single bond, so it's
going to be a butane.

00:04:52.220 --> 00:04:55.355
And on the number two carbon, we
have a bromo group, so it's

00:04:55.355 --> 00:04:56.605
2-bromobutane.

00:05:01.660 --> 00:05:04.620
And this is a chiral carbon,
so we have to describe it's

00:05:04.620 --> 00:05:05.090
handedness.

00:05:05.090 --> 00:05:07.420
And we know it's a chiral
carbon, because everything

00:05:07.420 --> 00:05:09.620
it's bonded to is different.

00:05:09.620 --> 00:05:11.000
It's bonded to two carbons,
but this is

00:05:11.000 --> 00:05:12.260
a whole ethyl group.

00:05:12.260 --> 00:05:13.100
There's two carbons here.

00:05:13.100 --> 00:05:14.600
There's only one over there,
and obviously, it's a

00:05:14.600 --> 00:05:15.700
hydrogen, and that's
a bromine.

00:05:15.700 --> 00:05:17.280
So what's it's handedness?

00:05:17.280 --> 00:05:21.130
And to do that, you want to
put the group that has the

00:05:21.130 --> 00:05:23.630
lowest atomic number
in the back.

00:05:23.630 --> 00:05:26.260
And we know hydrogen has the
lowest atomic number, so it's

00:05:26.260 --> 00:05:27.290
already in the back.

00:05:27.290 --> 00:05:28.110
So that's convenient.

00:05:28.110 --> 00:05:30.250
We don't have to do any
visualization of how you

00:05:30.250 --> 00:05:32.130
rotate it, so you put
it in the back.

00:05:32.130 --> 00:05:33.480
So that's already in the back.

00:05:37.900 --> 00:05:40.570
So what we have in the
front is this guy,

00:05:40.570 --> 00:05:42.130
this guy, this guy.

00:05:42.130 --> 00:05:44.550
And what you do is you start
with the guy with the highest

00:05:44.550 --> 00:05:48.260
atomic number, and that would
be bromine right there.

00:05:48.260 --> 00:05:52.630
And then you go in a circle in
the direction of the next

00:05:52.630 --> 00:05:53.930
highest atomic number.

00:05:53.930 --> 00:05:55.790
Carbon and carbon is a tie.

00:05:55.790 --> 00:05:57.305
The tie breaker is that
this carbon is

00:05:57.305 --> 00:05:58.390
bonded to another carbon.

00:05:58.390 --> 00:06:00.530
Well, this carbon is only
bonded to hydrogens.

00:06:00.530 --> 00:06:02.330
So you're going to go in this
direction right here.

00:06:02.330 --> 00:06:05.150
You're going to go clockwise.

00:06:05.150 --> 00:06:07.790
Let me do that in
another color.

00:06:07.790 --> 00:06:09.190
You are going to go clockwise.

00:06:12.280 --> 00:06:16.060
And the way I like to think
about is, on top, we're going

00:06:16.060 --> 00:06:17.500
to the right.

00:06:17.500 --> 00:06:20.480
We're going clockwise,
so this is R.

00:06:20.480 --> 00:06:23.460
S is to the left for sinister,
R for the right.

00:06:23.460 --> 00:06:26.250
That's the way I think of it.

00:06:26.250 --> 00:06:27.505
So this is R-2-bromobutane.

00:06:30.100 --> 00:06:32.070
So it's handedness is R.

00:06:32.070 --> 00:06:33.690
Now, what's this thing
over here?

00:06:36.430 --> 00:06:39.000
Well, first of all, once again
we have four carbons.

00:06:39.000 --> 00:06:46.330
We have one, two, three,
four carbons, so

00:06:46.330 --> 00:06:48.100
but- will be our prefix.

00:06:48.100 --> 00:06:52.210
But since the group that we
are attached to is an OH

00:06:52.210 --> 00:06:55.240
group, it's a hydroxide group,
this is actually an alcohol.

00:06:55.240 --> 00:06:57.810
So this a little bit of practice
of naming alcohols.

00:06:57.810 --> 00:06:59.955
So we still have but-
as our prefix.

00:07:03.160 --> 00:07:04.840
But we wouldn't call
it butane.

00:07:04.840 --> 00:07:07.050
Since it has an OH there,
we'll call it butanol.

00:07:10.470 --> 00:07:17.150
And maybe I'll write that in
blue to show the -anol comes

00:07:17.150 --> 00:07:18.970
from the OH.

00:07:18.970 --> 00:07:24.450
Butanol, or maybe we could say
the -ol comes from-- maybe

00:07:24.450 --> 00:07:25.170
that's even better.

00:07:25.170 --> 00:07:26.420
Let me just write butanol.

00:07:28.660 --> 00:07:30.640
That's a different color.

00:07:30.640 --> 00:07:36.020
But- -an- and then the -ol comes
from the fact that we

00:07:36.020 --> 00:07:37.050
have the OH there.

00:07:37.050 --> 00:07:38.050
We're dealing with an alcohol.

00:07:38.050 --> 00:07:41.250
And it's attached to the number
two carbon, so we'll

00:07:41.250 --> 00:07:42.500
call it 2-butanol.

00:07:45.040 --> 00:07:48.350
So we went from having
2-bromobutane to 2-butanol,

00:07:48.350 --> 00:07:50.220
but what is the handedness
of this?

00:07:50.220 --> 00:07:52.750
So same exercise, you put
the lowest atomic

00:07:52.750 --> 00:07:54.200
number in the back.

00:07:54.200 --> 00:07:54.980
That's the hydrogen.

00:07:54.980 --> 00:07:56.200
It's already in the back.

00:07:56.200 --> 00:07:59.200
And then you start with the atom
or the group that has the

00:07:59.200 --> 00:08:01.520
highest atomic number of the
things that are bonded to this

00:08:01.520 --> 00:08:02.570
chiral carbon.

00:08:02.570 --> 00:08:03.300
It's still chiral.

00:08:03.300 --> 00:08:04.990
All of these things
are different.

00:08:04.990 --> 00:08:07.980
You start with whatever has
the highest atomic number.

00:08:07.980 --> 00:08:11.410
This oxygen has the highest
atomic number.

00:08:11.410 --> 00:08:14.865
And you go in the direction of
the next highest. This carbon

00:08:14.865 --> 00:08:17.560
and that carbon are tied, but
this one is also attached to

00:08:17.560 --> 00:08:18.260
other carbons.

00:08:18.260 --> 00:08:21.230
Well, this is attached only to
hydrogens, so we go in this

00:08:21.230 --> 00:08:26.040
direction, or counterclockwise,
or in the

00:08:26.040 --> 00:08:28.120
sinister direction, or the
leftward direction.

00:08:28.120 --> 00:08:30.650
So this is S, S for sinister.

00:08:30.650 --> 00:08:32.480
S-2-butanol.

00:08:32.480 --> 00:08:35.640
So the whole point of this video
is to show you that when

00:08:35.640 --> 00:08:44.270
you have an Sn2 reaction on a
chiral carbon, that it changes

00:08:44.270 --> 00:08:46.220
the carbon's stereochemistry.

00:08:46.220 --> 00:08:48.550
It changes its handedness.

00:08:48.550 --> 00:08:51.830
It went from being right-handed
to left-handed.

00:08:55.390 --> 00:08:56.750
It changes its chirality.

00:08:56.750 --> 00:09:00.000
You can even imagine a situation
where a bromine

00:09:00.000 --> 00:09:02.070
comes and attacks, and
this bromine leaves.

00:09:02.070 --> 00:09:04.830
And then you would actually have
the same 2-bromobutane,

00:09:04.830 --> 00:09:07.180
but it would actually switch
its handedness.

00:09:07.180 --> 00:09:10.290
This is super important, because
this matters if you

00:09:10.290 --> 00:09:13.390
care whether or not you
want to flip the

00:09:13.390 --> 00:09:14.680
stereochemistry or not.

00:09:14.680 --> 00:09:18.440
What we're going to see in the
next video, or maybe the video

00:09:18.440 --> 00:09:23.520
after that, is that in Sn1
reactions, not only do you not

00:09:23.520 --> 00:09:26.390
flip your stereochemistry, that
you kind of lose-- if all

00:09:26.390 --> 00:09:30.670
of your molecules were
one-handedness, your end

00:09:30.670 --> 00:09:33.900
product is going to be a mix of
the two handedness when you

00:09:33.900 --> 00:09:35.210
do an Sn1 reaction.

00:09:35.210 --> 00:09:38.030
But Sn2, you flip
the handedness.

00:09:38.030 --> 00:09:38.570
You saw here.

00:09:38.570 --> 00:09:40.910
We started with R-2-bromobutane.

00:09:40.910 --> 00:09:42.470
The Sn2 reaction happened.

00:09:42.470 --> 00:09:43.280
We ended up with S.

00:09:43.280 --> 00:09:44.420
It makes complete sense.

00:09:44.420 --> 00:09:46.790
You view this as coming from the
back, so you view this as

00:09:46.790 --> 00:09:49.090
kind of an umbrella.

00:09:49.090 --> 00:09:50.540
This was kind of an
upturned umbrella.

00:09:50.540 --> 00:09:51.540
This was the handle.

00:09:51.540 --> 00:09:53.480
And when this comes in, he
becomes the handle, and the

00:09:53.480 --> 00:09:55.770
umbrella flips in
that direction.

00:09:55.770 --> 00:09:57.050
So that's one way to
think about it.

00:09:57.050 --> 00:10:00.500
So visually, it also should make
a little bit of sense.


Sn2 Reactions

WEBVTT
Kind: captions
Language: en

00:00:00.520 --> 00:00:01.790
Let's see if we can
learn a thing or

00:00:01.790 --> 00:00:08.189
two about Sn2 reactions.

00:00:08.189 --> 00:00:10.790
And you're going to see a lot of
these in organic chemistry.

00:00:10.790 --> 00:00:12.880
This sounds like a
very fancy term.

00:00:12.880 --> 00:00:16.100
But it really just stands
for-- the S stands for

00:00:16.100 --> 00:00:17.350
substitution.

00:00:21.360 --> 00:00:24.600
The n stands for nucleophilic.

00:00:31.590 --> 00:00:34.810
And then the 2, and this is
probably maybe the least

00:00:34.810 --> 00:00:38.040
obvious part of Sn2, the 2 comes
from the fact that the

00:00:38.040 --> 00:00:41.570
rate-determining step
in this reaction

00:00:41.570 --> 00:00:43.230
involves both reactants.

00:00:43.230 --> 00:00:44.595
And I'm going to show
you what we mean.

00:00:44.595 --> 00:00:47.970
And it'll actually be probably
more clear when we compare it

00:00:47.970 --> 00:00:50.800
to Sn1 reactions.

00:00:50.800 --> 00:00:53.740
So let me show you
an Sn2 reaction.

00:00:53.740 --> 00:00:56.840
Let's say we have
bromomethane.

00:00:56.840 --> 00:00:58.270
And let me draw it in
three dimensions.

00:00:58.270 --> 00:01:00.190
You have a hydrogen
sticking out.

00:01:00.190 --> 00:01:03.980
Maybe the bromine is
right over there.

00:01:03.980 --> 00:01:08.900
And then you have another
hydrogen in the back.

00:01:08.900 --> 00:01:14.080
And then you have a hydrogen
that comes up just like that.

00:01:14.080 --> 00:01:16.570
And here, the three dimensions
won't matter that much.

00:01:16.570 --> 00:01:21.160
This is not a chiral carbon.

00:01:21.160 --> 00:01:22.800
There's no handedness here.

00:01:22.800 --> 00:01:25.560
But in the future, we'll think
about chiral carbons and what

00:01:25.560 --> 00:01:29.640
might happen to them as they
undergo Sn2 reactions.

00:01:29.640 --> 00:01:32.440
So I have this bromomethane
here.

00:01:32.440 --> 00:01:37.920
And let's say we also have
some hydroxide in

00:01:37.920 --> 00:01:39.810
solution with it.

00:01:39.810 --> 00:01:42.640
So it's a hydroxide,
and this oxygen has

00:01:42.640 --> 00:01:44.590
seven valence electrons.

00:01:44.590 --> 00:01:48.070
You could imagine that the way
that you get a hydroxide is if

00:01:48.070 --> 00:01:52.620
you start off with water,
that looks like this.

00:01:57.200 --> 00:02:03.160
And maybe there's another water
someplace else that

00:02:03.160 --> 00:02:04.630
looks like that.

00:02:04.630 --> 00:02:08.860
And maybe in one step, this
water right here takes this

00:02:08.860 --> 00:02:09.509
guy's hydrogen.

00:02:09.509 --> 00:02:11.390
Remember, there's a partially
positive charge on the

00:02:11.390 --> 00:02:14.220
hydrogen, partially negative
charge on the oxygen.

00:02:14.220 --> 00:02:20.070
So maybe he gives one of his
electrons to the hydrogen,

00:02:20.070 --> 00:02:21.850
which will end up with
a positive charge.

00:02:21.850 --> 00:02:24.190
It'll be a hydronium cation.

00:02:24.190 --> 00:02:29.740
And then this electron takes
back that extra electron from

00:02:29.740 --> 00:02:30.560
the hydrogen.

00:02:30.560 --> 00:02:33.180
And so now, he's going to have
seven valence electrons.

00:02:33.180 --> 00:02:34.220
He's going to have this one.

00:02:34.220 --> 00:02:35.970
That was at this end of the
bond of the one that he's

00:02:35.970 --> 00:02:36.870
taking back.

00:02:36.870 --> 00:02:40.560
And then that's two, three,
four, five, six, and then this

00:02:40.560 --> 00:02:42.680
electron right there, seven.

00:02:42.680 --> 00:02:48.440
So he's going to have one, two,
three, four, five, six,

00:02:48.440 --> 00:02:50.740
seven valence electrons.

00:02:50.740 --> 00:02:52.250
And he's going to have
a negative charge.

00:02:52.250 --> 00:02:53.610
It's neutral when it's water.

00:02:53.610 --> 00:02:56.060
When he takes an extra electron,
it's going to have a

00:02:56.060 --> 00:02:56.670
negative charge.

00:02:56.670 --> 00:02:59.600
And you could imagine this is in
a solution with water, that

00:02:59.600 --> 00:03:00.570
water is our solvent.

00:03:00.570 --> 00:03:04.390
So let me put a negative
charge right here.

00:03:04.390 --> 00:03:07.010
I can write minus 1 if I want.

00:03:07.010 --> 00:03:10.850
And in this reaction, just so
we know what's what, this

00:03:10.850 --> 00:03:15.610
hydroxide molecule, or this
hydroxide anion, is our

00:03:15.610 --> 00:03:17.240
nucleophile.

00:03:17.240 --> 00:03:19.900
It is the nucleophile
right there.

00:03:23.870 --> 00:03:27.430
And the best way to think about
a nucleophile is it

00:03:27.430 --> 00:03:30.970
likes other people's
nucleuses.

00:03:30.970 --> 00:03:33.600
It likes them because nucleuses
are positive.

00:03:33.600 --> 00:03:36.040
It has extra electrons.

00:03:36.040 --> 00:03:38.300
In this case, it has a negative
charge, so it is

00:03:38.300 --> 00:03:40.020
attracted to other nucleuses.

00:03:40.020 --> 00:03:42.530
Nucleuses of atoms are positive,
so it is attracted

00:03:42.530 --> 00:03:43.800
to electrons.

00:03:43.800 --> 00:03:47.950
Phile, I'm not a Greek scholar,
but it means to be

00:03:47.950 --> 00:03:49.230
attracted or to like

00:03:49.230 --> 00:03:51.670
something, so it likes nucleuses.

00:03:51.670 --> 00:03:55.320
And if you want to compare it
to acids or bases, you'd

00:03:55.320 --> 00:03:58.520
classify this actually as a
Lewis base, something that

00:03:58.520 --> 00:03:59.180
gives electrons.

00:03:59.180 --> 00:04:00.910
But we're not going to go
into that right now.

00:04:00.910 --> 00:04:01.990
This is a nucleophile.

00:04:01.990 --> 00:04:05.700
It is attracted to nucleuses,
to positive things.

00:04:05.700 --> 00:04:08.390
It wants to give
away electrons.

00:04:08.390 --> 00:04:11.780
Now, what's going to happen is
that this is going to give an

00:04:11.780 --> 00:04:13.580
electron to this carbon.

00:04:13.580 --> 00:04:15.980
This carbon's going to say,
oh, I got an electron.

00:04:15.980 --> 00:04:18.775
Let me give away an electron to
somebody who probably wants

00:04:18.775 --> 00:04:19.620
it really badly.

00:04:19.620 --> 00:04:21.470
And in this case, it's going
to be the bromine because

00:04:21.470 --> 00:04:25.820
we've seen multiple times
that the bromine is very

00:04:25.820 --> 00:04:26.710
electronegative.

00:04:26.710 --> 00:04:28.770
It already is starting to hog
the electrons, and it would

00:04:28.770 --> 00:04:31.830
like to take an electron
altogether.

00:04:31.830 --> 00:04:33.700
And so let me show
you the reaction.

00:04:33.700 --> 00:04:35.630
It's going to happen
very quickly.

00:04:35.630 --> 00:04:37.172
Or it's actually going to happen
very quickly the way I

00:04:37.172 --> 00:04:39.630
drew it, because it's really
a one-step reaction.

00:04:39.630 --> 00:04:46.780
So you're going to have one of
these electrons right here

00:04:46.780 --> 00:04:49.310
will attack the carbon.

00:04:49.310 --> 00:04:50.310
Or I shouldn't say attack.

00:04:50.310 --> 00:04:51.670
It sounds very aggressive.

00:04:51.670 --> 00:04:55.070
It will be given
to the carbon.

00:04:55.070 --> 00:04:57.430
And then the carbon says, oh,
I'm getting an electron from

00:04:57.430 --> 00:04:57.980
this direction.

00:04:57.980 --> 00:05:00.840
It actually has a slightly
positive charge at this end,

00:05:00.840 --> 00:05:02.960
because the bromine is hogging
some of the electrons, so

00:05:02.960 --> 00:05:04.410
they'll be attracted
to each other.

00:05:04.410 --> 00:05:09.080
They're going to bump into each
other just the right way.

00:05:09.080 --> 00:05:11.820
And then at the exact same time,
this all has to happen

00:05:11.820 --> 00:05:12.800
kind of simultaneously.

00:05:12.800 --> 00:05:15.310
At the exact same time, this
electron, sitting at that end

00:05:15.310 --> 00:05:20.110
of the bond, is going to
go to the bromine.

00:05:20.110 --> 00:05:24.680
And so what is it going to look
like when this reaction

00:05:24.680 --> 00:05:26.120
has actually occurred?

00:05:26.120 --> 00:05:29.630
So it's literally a
one-step reaction.

00:05:29.630 --> 00:05:31.220
So you have your carbon.

00:05:31.220 --> 00:05:35.480
You have the hydrogen
that is pointing up.

00:05:35.480 --> 00:05:36.710
Now, let me color code this.

00:05:36.710 --> 00:05:37.860
So that'll make it
interesting.

00:05:37.860 --> 00:05:39.750
So you have this hydrogen,
that is

00:05:39.750 --> 00:05:41.253
coming out of the screen.

00:05:45.320 --> 00:05:47.960
And it's still just coming
straight out of the screen,

00:05:47.960 --> 00:05:52.070
but I'm going to draw it to the
down right, because this

00:05:52.070 --> 00:05:54.010
hydroxide's attacking
from behind.

00:05:54.010 --> 00:05:57.290
And this bromine, which we call
the leaving group, or

00:05:57.290 --> 00:06:00.420
actually, it's going to be a
bromide anion once it leaves,

00:06:00.420 --> 00:06:02.800
that's going to leave
from the right.

00:06:02.800 --> 00:06:04.330
Attack from the left, leave
from the right,

00:06:04.330 --> 00:06:05.890
from different sides.

00:06:05.890 --> 00:06:08.090
So then this is that hydrogen
that's sticking out.

00:06:08.090 --> 00:06:10.830
The hydrogen in the back--
well, we know it's in the

00:06:10.830 --> 00:06:16.960
back, so I'll just continue to
draw it in the back like that.

00:06:16.960 --> 00:06:19.570
And now this hydroxide
is going to be

00:06:19.570 --> 00:06:22.100
attached to the carbon.

00:06:22.100 --> 00:06:29.140
So this carbon now gets one of
those electrons, and it is

00:06:29.140 --> 00:06:33.530
bonded now to the oxygen.

00:06:33.530 --> 00:06:34.620
Let me draw the oxygen.

00:06:34.620 --> 00:06:36.640
So you have the oxygen.

00:06:36.640 --> 00:06:37.960
Let me see the best way
that I could do this.

00:06:37.960 --> 00:06:41.710
So you have the oxygen here
bonded to a hydrogen.

00:06:41.710 --> 00:06:43.150
And let me draw its
valence electrons.

00:06:43.150 --> 00:06:50.930
So it had one, two, three, four,
five, six, and then it

00:06:50.930 --> 00:06:55.520
had a seventh electron, but
it just gave it away.

00:06:55.520 --> 00:06:57.890
So its seventh electron is going
to bond with this one.

00:06:57.890 --> 00:06:59.480
They were already a
pair over here.

00:06:59.480 --> 00:07:02.800
Let me make it very clear
what the color is.

00:07:02.800 --> 00:07:06.030
This electron right over
there is this electron.

00:07:06.030 --> 00:07:07.800
But it was already paired with
this guy, so when he gives it

00:07:07.800 --> 00:07:12.150
away to the carbon, it forms
this covalent bond right here.

00:07:12.150 --> 00:07:16.500
So now you have this OH group,
this hydroxyl group, off of

00:07:16.500 --> 00:07:17.450
the carbon now.

00:07:17.450 --> 00:07:22.850
And now the bromine, you had a
negatively charged group here.

00:07:22.850 --> 00:07:25.150
Now, what I've drawn so far,
everything is neutral.

00:07:25.150 --> 00:07:26.630
So something has to be
negatively charged.

00:07:26.630 --> 00:07:28.850
It's going to be the bromine
because that's what getting

00:07:28.850 --> 00:07:30.180
the electron.

00:07:30.180 --> 00:07:34.940
So now the bromine, if I wanted
to draw bromine's

00:07:34.940 --> 00:07:37.750
valence shell, before it would
have seven electrons, so one,

00:07:37.750 --> 00:07:41.910
two, three, four, five,
six, seven.

00:07:41.910 --> 00:07:44.040
Now that it's taking that
electron from carbon, it's

00:07:44.040 --> 00:07:49.710
going to have eight: one, two,
three, four, five, six, seven,

00:07:49.710 --> 00:07:51.220
and let me color
code it again.

00:07:51.220 --> 00:07:53.880
So this electron right here,
when it goes to the bromine,

00:07:53.880 --> 00:07:56.850
is going to be that electron
right there, and we're going

00:07:56.850 --> 00:07:59.630
to have a negative charge.

00:07:59.630 --> 00:08:02.510
And so here, in this situation,
just to make sure,

00:08:02.510 --> 00:08:03.950
this is the nucleophile.

00:08:03.950 --> 00:08:09.030
The bromine right here, this
is the leaving group.

00:08:13.400 --> 00:08:16.200
And then the carbon, I guess
the thing that is reacting,

00:08:16.200 --> 00:08:19.850
that's getting substituted,
where the bromine leaves it

00:08:19.850 --> 00:08:24.370
and the hydroxide joins it,
that's called the substrate.

00:08:24.370 --> 00:08:27.610
So I could draw it like this.

00:08:27.610 --> 00:08:28.860
This is the substrate.

00:08:32.059 --> 00:08:33.470
And the whole reason why
I did it, this was

00:08:33.470 --> 00:08:34.720
not a chiral carbon.

00:08:34.720 --> 00:08:37.630
But you could imagine, once we
deal with chiral carbons, when

00:08:37.630 --> 00:08:40.760
they undergo an Sn2 reaction
like this, their chirality

00:08:40.760 --> 00:08:44.039
will actually change.

00:08:44.039 --> 00:08:45.340
So I'll leave you
there right now.

00:08:45.340 --> 00:08:48.490
That'll give you something to
think about, Sn2 reactions.

00:08:48.490 --> 00:08:50.270
And then in the next video,
we'll think a

00:08:50.270 --> 00:08:51.220
little bit about Sn1.

00:08:51.220 --> 00:08:53.810
Oh, and before I leave you, I
left you hanging on the 2.

00:08:53.810 --> 00:08:56.210
I said this is called
substitution.

00:08:56.210 --> 00:09:00.740
You saw that the bromine was
substituted by the hydroxide.

00:09:00.740 --> 00:09:01.710
It's nucleophilic.

00:09:01.710 --> 00:09:05.460
It involved this hydroxide,
which likes nucleuses.

00:09:05.460 --> 00:09:08.610
And then I said the
rate-determining step involves

00:09:08.610 --> 00:09:09.440
both reactants.

00:09:09.440 --> 00:09:12.030
And here, it might not be
completely clear, but the

00:09:12.030 --> 00:09:15.570
rate-determining step
was actually what's

00:09:15.570 --> 00:09:16.690
going on right here.

00:09:16.690 --> 00:09:18.900
And if I were to draw an
intermediate, you could

00:09:18.900 --> 00:09:24.242
actually draw an intermediate
step here.

00:09:24.242 --> 00:09:30.100
Let me copy and paste
this down here.

00:09:32.606 --> 00:09:35.210
So you could actually draw an
intermediate step, so let me

00:09:35.210 --> 00:09:37.210
copy the other one.

00:09:37.210 --> 00:09:39.880
Let me copy this.

00:09:39.880 --> 00:09:43.540
Copy and paste.

00:09:43.540 --> 00:09:46.300
This all happens over one step,
but if we wanted to show

00:09:46.300 --> 00:09:54.290
what happens in between, for a
small amount of time, you have

00:09:54.290 --> 00:09:58.400
this transition state, where the
carbon is going to have a

00:09:58.400 --> 00:10:01.740
partial bond, so it has
this hydrogen in blue.

00:10:01.740 --> 00:10:04.790
Let me draw this hydrogen
in blue sticking out.

00:10:04.790 --> 00:10:09.020
And it has another hydrogen
behind it.

00:10:09.020 --> 00:10:11.960
I'll draw it like it's
right behind it.

00:10:11.960 --> 00:10:15.600
And then for some small fraction
of time, it is bonded

00:10:15.600 --> 00:10:18.415
to both the bromine-- so I'll
draw it with a dotted line.

00:10:18.415 --> 00:10:21.450
It is bonded to both the
bromine, which now will have a

00:10:21.450 --> 00:10:23.450
partial negative charge because
it's starting to get

00:10:23.450 --> 00:10:35.170
an electron, and it's also
bonded to the hydroxide, which

00:10:35.170 --> 00:10:37.360
also has a partial
negative charge.

00:10:37.360 --> 00:10:39.230
You can imagine this negative
charge is now getting split

00:10:39.230 --> 00:10:39.730
between the two.

00:10:39.730 --> 00:10:41.320
This is a transition state.

00:10:41.320 --> 00:10:44.360
And the notation for a
transition state, you put

00:10:44.360 --> 00:10:47.830
brackets around it, or
parentheses, in some cultures,

00:10:47.830 --> 00:10:51.020
they call them, and you write
this little symbol up here.

00:10:51.020 --> 00:10:53.020
This tells us this is
a transition state.

00:10:53.020 --> 00:10:56.100
This is not one of the
semi-stable intermediates.

00:10:56.100 --> 00:10:57.890
This is something that
we're going through.

00:10:57.890 --> 00:11:00.680
This is an Sn2 reaction because
this transition state

00:11:00.680 --> 00:11:03.460
is actually what determines
the rate of the reaction.

00:11:03.460 --> 00:11:08.490
This requires the highest energy
state of this reaction.

00:11:08.490 --> 00:11:10.300
This is what's going to-- well,
I said it already--

00:11:10.300 --> 00:11:11.500
determine its rate.

00:11:11.500 --> 00:11:15.000
And in this transition state,
this rate-determining step,

00:11:15.000 --> 00:11:17.270
you have all of the reactants.

00:11:17.270 --> 00:11:21.830
You have your hydroxide right
there, and you also have your

00:11:21.830 --> 00:11:26.070
bromomethane that we
started with, so it

00:11:26.070 --> 00:11:27.860
has two of the reactants.

00:11:27.860 --> 00:11:30.560
That's why it's called
an Sn2 reaction.


Markovnikov's rule and carbocations

WEBVTT
Kind: captions
Language: en

00:00:00.550 --> 00:00:03.300
In the last video, we saw a
potential mechanism where if

00:00:03.300 --> 00:00:06.800
we reacted hydrogen bromide with
this alkene right over

00:00:06.800 --> 00:00:08.910
here, that we could
essentially add.

00:00:08.910 --> 00:00:14.370
We had the addition of this
halide to what started as an

00:00:14.370 --> 00:00:17.800
alkene, and then it ended up as

00:00:17.800 --> 00:00:21.370
2-bromo-pentane, as an alkane.

00:00:21.370 --> 00:00:25.630
But when we did that, we made a
somewhat arbitrary decision,

00:00:25.630 --> 00:00:27.880
or I didn't explain why
we made the decision.

00:00:27.880 --> 00:00:30.010
We said, look, this hydrogen
is going to be partially

00:00:30.010 --> 00:00:32.610
positive, because this guy's so
electronegative, and maybe

00:00:32.610 --> 00:00:34.880
when it's partially positive,
it'll be attracted.

00:00:34.880 --> 00:00:36.970
Maybe it'll just bump in just
the right way into one of

00:00:36.970 --> 00:00:37.660
these carbons.

00:00:37.660 --> 00:00:39.490
It'll maybe swipe
its electron.

00:00:39.490 --> 00:00:42.310
We somewhat arbitrarily in the
last video decided that it

00:00:42.310 --> 00:00:44.300
would swipe this
guy's electron.

00:00:44.300 --> 00:00:46.750
But you could just as easily
imagine a world where it

00:00:46.750 --> 00:00:48.940
swipes an electron
from this guy.

00:00:48.940 --> 00:00:52.100
So let's draw a mechanism for
that and just think about

00:00:52.100 --> 00:00:55.530
which one is more likely
to actually happen.

00:00:55.530 --> 00:00:56.730
So what happens?

00:00:56.730 --> 00:01:01.180
So once again, this guy-- let
me draw all of his valence

00:01:01.180 --> 00:01:03.280
electrons, so this
is the bromine.

00:01:03.280 --> 00:01:08.510
One, two, three, four, five,
six, seven valence electrons.

00:01:08.510 --> 00:01:10.710
You have the hydrogen.

00:01:10.710 --> 00:01:12.250
I'll do it in the same color.

00:01:12.250 --> 00:01:15.500
The hydrogen has its electron
right there.

00:01:15.500 --> 00:01:19.580
This is partially positive and
this is partially negative.

00:01:19.580 --> 00:01:22.640
The hydrogen might want
to swipe one of

00:01:22.640 --> 00:01:23.520
these electrons away.

00:01:23.520 --> 00:01:25.270
Let's do it from this
guy right here.

00:01:28.160 --> 00:01:33.720
So he has this electron right
over here, so the other side

00:01:33.720 --> 00:01:37.000
of that bond, it goes to the
hydrogen when the hydrogen

00:01:37.000 --> 00:01:39.210
goes near it, or maybe
it's attracted to it.

00:01:39.210 --> 00:01:42.070
And when it goes to hydrogen,
then the hydrogen lets go of

00:01:42.070 --> 00:01:44.780
the electron that the bromine
wanted all along, because it's

00:01:44.780 --> 00:01:46.520
so electronegative.

00:01:46.520 --> 00:01:50.760
So then that electron
goes to the bromine.

00:01:50.760 --> 00:01:53.350
So after we do that, what
will it look like?

00:01:53.350 --> 00:01:56.140
What will be the next step
in our reaction?

00:01:56.140 --> 00:01:57.740
And it will look fundamentally
different than

00:01:57.740 --> 00:01:59.360
this right over here.

00:01:59.360 --> 00:02:02.040
So now what happens?

00:02:02.040 --> 00:02:08.190
So we have a carbon bonded to
two hydrogens, and it only has

00:02:08.190 --> 00:02:12.720
a single bond to the other
carbon, which is bonded to the

00:02:12.720 --> 00:02:15.480
original hydrogen right
over there.

00:02:15.480 --> 00:02:17.670
Let me write my hydrogens a
little bit-- actually, let me

00:02:17.670 --> 00:02:20.396
write this whole thing
a little bit neater.

00:02:20.396 --> 00:02:25.080
So you have your carbon bonded
to a hydrogen and another

00:02:25.080 --> 00:02:28.060
hydrogen, and now it only has
a single bond to this carbon

00:02:28.060 --> 00:02:31.270
right here, which is bonded to
a hydrogen and then the rest

00:02:31.270 --> 00:02:32.346
of the chain.

00:02:32.346 --> 00:02:36.820
Let me just draw the rest
of the chain right here.

00:02:36.820 --> 00:02:39.110
And now this electron went
to the hydrogen.

00:02:39.110 --> 00:02:41.670
The other electron that it was
paired with is still with this

00:02:41.670 --> 00:02:45.320
carbon, so now this carbon is
now bonded to that hydrogen

00:02:45.320 --> 00:02:46.240
over there.

00:02:46.240 --> 00:02:48.700
So this blue electron is
now with the hydrogen.

00:02:48.700 --> 00:02:49.790
Let me draw.

00:02:49.790 --> 00:02:54.380
So the blue electron that was
here has now gone over to the

00:02:54.380 --> 00:02:56.910
orange hydrogen.

00:02:56.910 --> 00:02:58.535
Let me draw it a little
bit neater than that.

00:02:58.535 --> 00:03:03.360
It has now gone over to this
hydrogen right over there.

00:03:03.360 --> 00:03:07.800
And then the hydrogen lost its
electron to the bromine.

00:03:07.800 --> 00:03:11.820
So the bromine originally had
seven valence electrons: one,

00:03:11.820 --> 00:03:15.530
two, three, four, five,
six, seven.

00:03:15.530 --> 00:03:19.480
And then it nabbed an extra
electron from the hydrogen, so

00:03:19.480 --> 00:03:21.750
now it will have a
negative charge.

00:03:21.750 --> 00:03:23.370
It is a negative ion.

00:03:23.370 --> 00:03:24.730
It is bromide.

00:03:24.730 --> 00:03:27.340
It's a bromide anion, I guess
you could call it.

00:03:27.340 --> 00:03:30.690
And since this guy lost an
electron, he had four valence

00:03:30.690 --> 00:03:34.980
electrons, lost one to the
hydrogen, he now has a

00:03:34.980 --> 00:03:36.000
positive charge.

00:03:36.000 --> 00:03:37.250
He's a carbocation.

00:03:41.620 --> 00:03:42.670
So notice the difference.

00:03:42.670 --> 00:03:46.090
Before, this guy lost the
electron, and so the hydrogen

00:03:46.090 --> 00:03:47.110
bonded to this carbon.

00:03:47.110 --> 00:03:49.510
In this situation, this guy
lost the electron, so the

00:03:49.510 --> 00:03:51.420
hydrogen bonded to
the other carbon.

00:03:51.420 --> 00:03:53.460
And so you can imagine, from
here, something very similar

00:03:53.460 --> 00:03:55.580
happens as what happened in the
first video, but now it

00:03:55.580 --> 00:03:57.940
happens to this carbon
right over there.

00:03:57.940 --> 00:03:58.720
So let's do that.

00:03:58.720 --> 00:04:00.930
This carbon is positive.

00:04:00.930 --> 00:04:04.960
The bromide ion is obviously
negative, so maybe he'll want

00:04:04.960 --> 00:04:06.690
to swipe his electron away.

00:04:06.690 --> 00:04:12.240
So this electron then goes to
the carbocation and then it

00:04:12.240 --> 00:04:14.560
will form a bond.

00:04:14.560 --> 00:04:16.899
This green will go to the
carbocation and then this

00:04:16.899 --> 00:04:18.839
purple one still stays
with the bromine.

00:04:18.839 --> 00:04:19.930
And so they'll have a bond.

00:04:19.930 --> 00:04:21.720
They're paired up, you
can imagine it.

00:04:21.720 --> 00:04:24.260
So then we're left with,
we have a carbon.

00:04:24.260 --> 00:04:26.770
We have our original
hydrogens.

00:04:26.770 --> 00:04:29.580
We have this carbon, that
hydrogen, the rest of the

00:04:29.580 --> 00:04:36.110
chain: CH2, CH3.

00:04:36.110 --> 00:04:38.140
And then you have this
hydrogen right here

00:04:38.140 --> 00:04:38.960
that it bonded to.

00:04:38.960 --> 00:04:41.520
That was our first step.

00:04:41.520 --> 00:04:43.420
And now the bromine
has bonded to this

00:04:43.420 --> 00:04:44.850
carbon right over here.

00:04:44.850 --> 00:04:49.990
The bromine has bonded over to
that carbon right over there.

00:04:49.990 --> 00:04:51.920
And we're done!

00:04:51.920 --> 00:04:54.440
This is another possible
mechanism.

00:04:54.440 --> 00:04:59.130
This one we ended up with
2-bromopentane, right?

00:04:59.130 --> 00:05:00.600
Because it's on the
number two carbon.

00:05:00.600 --> 00:05:03.080
Here we have 1-bromo-pentane.

00:05:03.080 --> 00:05:04.360
One, two, three, four, five.

00:05:04.360 --> 00:05:05.290
Still five carbons.

00:05:05.290 --> 00:05:07.560
It's just the bromine is
attached to the one carbon

00:05:07.560 --> 00:05:10.080
here, attached to the
two carbon here.

00:05:10.080 --> 00:05:13.040
So we now need to think about
it, because on a first cut,

00:05:13.040 --> 00:05:15.880
these both seemed like
reasonable mechanisms. But if

00:05:15.880 --> 00:05:20.450
you did it experimentally, you
would see that this is the one

00:05:20.450 --> 00:05:22.380
that you'd really observe.

00:05:22.380 --> 00:05:25.830
I actually haven't done this
exact experiment, so I don't

00:05:25.830 --> 00:05:26.580
know the proportions.

00:05:26.580 --> 00:05:29.600
But you're going to observe this
one disproportionately.

00:05:29.600 --> 00:05:32.750
The great majority of the
products that you see are

00:05:32.750 --> 00:05:34.960
going to be this one,
not that one.

00:05:34.960 --> 00:05:37.200
And so the question is, well,
you know, they both seem like

00:05:37.200 --> 00:05:38.750
reasonable things
to do up here.

00:05:38.750 --> 00:05:41.210
Why is this one so much
more likely to

00:05:41.210 --> 00:05:43.360
happen than that one?

00:05:43.360 --> 00:05:47.040
It all comes from something
called Markovnikov's rule.

00:05:57.760 --> 00:05:59.840
And there's a couple of ways
to think about it.

00:05:59.840 --> 00:06:02.950
When Markovnikov thought it
up, or he observed it, it

00:06:02.950 --> 00:06:03.730
seemed to work.

00:06:03.730 --> 00:06:05.620
They weren't 100% sure
about why it worked.

00:06:05.620 --> 00:06:07.880
We can think a little bit
about why it worked.

00:06:07.880 --> 00:06:09.500
So Markovnikov's rule,
a couple of ways you

00:06:09.500 --> 00:06:10.340
can think about it.

00:06:10.340 --> 00:06:13.170
You can think of it as the thing
that already has more

00:06:13.170 --> 00:06:16.290
hydrogens is more likely to get
more hydrogens, so that's

00:06:16.290 --> 00:06:16.950
what happened here.

00:06:16.950 --> 00:06:20.160
This thing had more hydrogens
on it than the right carbon

00:06:20.160 --> 00:06:20.540
right here.

00:06:20.540 --> 00:06:22.860
This right carbon had a
hydrogen, but it had some

00:06:22.860 --> 00:06:26.010
other alkyl group
attached to it.

00:06:26.010 --> 00:06:28.600
And so the thing that had more
hydrogens ended up with the

00:06:28.600 --> 00:06:31.350
hydrogen, and then the thing
that had more groups, this

00:06:31.350 --> 00:06:33.740
character right here had
more groups, right?

00:06:33.740 --> 00:06:34.880
He had one group over here.

00:06:34.880 --> 00:06:36.480
This carbon over here
had no groups.

00:06:36.480 --> 00:06:38.630
He ended up with the bromine.

00:06:38.630 --> 00:06:40.210
So the thing that has
more hydrogens ends

00:06:40.210 --> 00:06:41.260
up with more hydrogens.

00:06:41.260 --> 00:06:44.990
The thing that has more groups
ends up with more groups.

00:06:44.990 --> 00:06:46.890
So I guess you kind of go more
in the direction that

00:06:46.890 --> 00:06:47.930
you are going in.

00:06:47.930 --> 00:06:53.350
But that still is just a rule,
so why does that make sense?

00:06:53.350 --> 00:06:56.150
It starts to make sense when you
think about that in both

00:06:56.150 --> 00:07:00.190
mechanisms, we had to
have a carbocation.

00:07:00.190 --> 00:07:01.560
We talked about it in
the last video.

00:07:06.060 --> 00:07:08.160
We had a carbocation
right over there.

00:07:08.160 --> 00:07:10.500
This is the left carbon
being a carbocation.

00:07:10.500 --> 00:07:13.570
This is the right carbon
being a carbocation.

00:07:13.570 --> 00:07:18.450
And Markovnikov's rule all comes
from which carbocation

00:07:18.450 --> 00:07:22.840
is more stable, which one has
a lower energy level.

00:07:22.840 --> 00:07:26.590
It turns out that the
carbocation that is a bonded

00:07:26.590 --> 00:07:34.140
to more electron-rich molecules
or atoms is going to

00:07:34.140 --> 00:07:35.610
be more stable.

00:07:35.610 --> 00:07:38.120
You can imagine it has more
things that, look, it's

00:07:38.120 --> 00:07:40.830
positive, but it has more
carbons around it so it can

00:07:40.830 --> 00:07:43.130
share some of those electrons.

00:07:43.130 --> 00:07:45.700
The electron clouds will help
it out a little bit to be a

00:07:45.700 --> 00:07:46.700
little bit more stable.

00:07:46.700 --> 00:07:49.170
This one right here is only
bonded to one other carbon, so

00:07:49.170 --> 00:07:49.920
not as much sharing.

00:07:49.920 --> 00:07:52.150
This is bonded to two.

00:07:52.150 --> 00:07:57.130
So, in general, when you're
only bonded to one other

00:07:57.130 --> 00:08:01.240
carbon, you're called
a primary carbon.

00:08:01.240 --> 00:08:04.340
And if you're carbocation, this
is a primary carbocation

00:08:04.340 --> 00:08:05.250
right here.

00:08:05.250 --> 00:08:07.710
This guy is bonded to two
carbons, so he would be called

00:08:07.710 --> 00:08:09.020
a secondary carbon.

00:08:09.020 --> 00:08:11.825
Since it's a carbocation, it's
a secondary carbocation, so

00:08:11.825 --> 00:08:16.130
this right here is secondary.

00:08:16.130 --> 00:08:19.390
So a secondary carbocation is
more stable than a primary.

00:08:19.390 --> 00:08:22.280
And actually a tertiary, if you
had another carbon group

00:08:22.280 --> 00:08:23.980
here or something else that had
a lot of electrons around

00:08:23.980 --> 00:08:27.270
it, that would be even
more stable.

00:08:27.270 --> 00:08:30.160
So bonded to three things, more
stable than two things.

00:08:30.160 --> 00:08:32.710
And when I say two things, two
things other than hydrogen,

00:08:32.710 --> 00:08:34.470
and then, more stable
than one.

00:08:34.470 --> 00:08:38.010
So Markovnikov's rule all is a
byproduct of the fact that

00:08:38.010 --> 00:08:40.710
this carbocation is
more stable than

00:08:40.710 --> 00:08:42.179
this one over here.

00:08:42.179 --> 00:08:44.500
That's because it's secondary
versus primary.

00:08:44.500 --> 00:08:47.290
Because it's secondary, it can
borrow electrons from some of

00:08:47.290 --> 00:08:48.210
its friends.

00:08:48.210 --> 00:08:49.580
It has more neighbors to borrow

00:08:49.580 --> 00:08:50.940
electrons than this one.

00:08:50.940 --> 00:08:54.010
And since this is more stable,
this is more likely to happen.

00:08:54.010 --> 00:08:58.250
This is a more likely
intermediate to have. This is

00:08:58.250 --> 00:09:00.690
a less likely outcome
to have in general.

00:09:00.690 --> 00:09:03.230
And that's why you're more
likely to get to this left

00:09:03.230 --> 00:09:06.340
product, the 2-bromo-pentane
than the 1-bromo-pentane.


Introduction to reaction mechanisms

WEBVTT
Kind: captions
Language: en

00:00:00.610 --> 00:00:05.260
Let's think about how hydrogen
bromide might react with this

00:00:05.260 --> 00:00:05.920
thing right here.

00:00:05.920 --> 00:00:07.390
Let's think about what
this would be called.

00:00:07.390 --> 00:00:10.210
We have one, two, three,
four, five carbons.

00:00:10.210 --> 00:00:12.800
It has a double bond right here,
if we start numbering at

00:00:12.800 --> 00:00:15.240
this end, because that's where
the double bond is.

00:00:15.240 --> 00:00:16.490
Then this would be pent.

00:00:19.170 --> 00:00:22.510
The double bond starts at the
number one carbon, pent-1, and

00:00:22.510 --> 00:00:24.080
it's obviously an alkene.

00:00:24.080 --> 00:00:26.330
It has a double bond,
so it's pent-1-ene.

00:00:26.330 --> 00:00:31.490
Sometimes this is called
1-pentene, either way.

00:00:31.490 --> 00:00:34.830
So let's think about how these
two characters might react

00:00:34.830 --> 00:00:36.930
with each other in some
type of solvent.

00:00:36.930 --> 00:00:40.090
Usually when a solvent's not
specified, it's usually water

00:00:40.090 --> 00:00:40.880
or alcohol.

00:00:40.880 --> 00:00:43.810
If this was water, then we
would have a solution of

00:00:43.810 --> 00:00:45.700
hydrobromic acid, but
let's not worry

00:00:45.700 --> 00:00:46.420
about that right now.

00:00:46.420 --> 00:00:49.520
Let's just worry about how these
two characters might

00:00:49.520 --> 00:00:50.920
react with each other.

00:00:50.920 --> 00:00:53.440
Now, the first thing we might
look at is this hydrogen

00:00:53.440 --> 00:00:54.500
bromide right there.

00:00:54.500 --> 00:00:56.920
And as you get more experience
there, you'll say, well, you

00:00:56.920 --> 00:01:00.290
know, bromine is much more
electronegative than hydrogen.

00:01:00.290 --> 00:01:03.090
It likes to hog electrons
much more.

00:01:03.090 --> 00:01:06.090
If that's a completely foreign
concept to you, I'll do a

00:01:06.090 --> 00:01:07.010
little bit of a review.

00:01:07.010 --> 00:01:08.610
This is the Periodic Table.

00:01:08.610 --> 00:01:11.900
Electronegativity increases from
the bottom left to the

00:01:11.900 --> 00:01:15.950
top right of the
Periodic Table.

00:01:15.950 --> 00:01:17.200
Electronegativity.

00:01:20.010 --> 00:01:24.180
And really, that's just a fancy
way of saying how much

00:01:24.180 --> 00:01:26.780
does an atom like to
hog electrons.

00:01:26.780 --> 00:01:31.570
So, for example, fluorine loves
to hog electrons and

00:01:31.570 --> 00:01:34.350
calcium doesn't like to
hog electrons so much.

00:01:34.350 --> 00:01:38.100
So if we think about hydrogen
bromide, hydrogen's way up

00:01:38.100 --> 00:01:41.110
here on the left side of
the Periodic Table.

00:01:41.110 --> 00:01:43.680
Bromine's all the way
to the right.

00:01:43.680 --> 00:01:46.460
Bromine is much more
electronegative.

00:01:46.460 --> 00:01:48.710
So in this situation, since
bromine is more

00:01:48.710 --> 00:01:51.890
electronegative, it will hog
the electrons in this bond.

00:01:51.890 --> 00:01:54.640
And since it's hogging the
electrons in this bond, you'll

00:01:54.640 --> 00:02:01.960
have a partial negative charge
on the bromine end, and you'll

00:02:01.960 --> 00:02:06.090
have a partial positive charge
on the hydrogen end.

00:02:06.090 --> 00:02:08.110
And, whenever you do these
reactions, actually, it's

00:02:08.110 --> 00:02:09.789
useful to draw all the
valence electrons.

00:02:09.789 --> 00:02:14.120
So bromine right here has an
atomic number of 35, which

00:02:14.120 --> 00:02:16.980
means it has 35 electrons
in its neutral state.

00:02:16.980 --> 00:02:18.610
And we don't have any
charge here, so it's

00:02:18.610 --> 00:02:20.160
in a neutral state.

00:02:20.160 --> 00:02:22.010
But you can look at its group.

00:02:22.010 --> 00:02:23.540
It's in Group Number 7.

00:02:23.540 --> 00:02:26.070
If you just count from here,
one, two, three, four, five,

00:02:26.070 --> 00:02:28.360
six, seven, or has seven
valence electrons.

00:02:28.360 --> 00:02:29.440
Let me draw that.

00:02:29.440 --> 00:02:32.470
Seven electrons in its
outermost shell.

00:02:32.470 --> 00:02:34.100
Let me draw them right here.

00:02:34.100 --> 00:02:39.330
So one, two, three, four,
five, six, and then the

00:02:39.330 --> 00:02:41.530
seventh is in the bond
with the hydrogen.

00:02:41.530 --> 00:02:43.180
Hydrogen obviously
has one electron.

00:02:45.690 --> 00:02:46.750
It's right there.

00:02:46.750 --> 00:02:49.380
So these two guys are bonding
with each other and they both

00:02:49.380 --> 00:02:51.960
allow each other to pretend that
they're part of a pair.

00:02:51.960 --> 00:02:54.070
And that's what gives it a lower
energy level or makes it

00:02:54.070 --> 00:02:54.770
a little more stable.

00:02:54.770 --> 00:02:57.910
That's why the bond forms
in the first place.

00:02:57.910 --> 00:02:59.780
But anyway, let's think about
what might happen here.

00:02:59.780 --> 00:03:02.030
This guy is really
electronegative, so maybe he

00:03:02.030 --> 00:03:05.590
wants to take this electron
away, this

00:03:05.590 --> 00:03:07.720
green hydrogen electron.

00:03:07.720 --> 00:03:11.130
And maybe that happens.

00:03:11.130 --> 00:03:12.250
Let me just draw it out.

00:03:12.250 --> 00:03:13.910
So maybe that happens.

00:03:13.910 --> 00:03:16.360
So he takes that electron, that
green electron, away.

00:03:16.360 --> 00:03:17.780
It's just getting closer
and closer.

00:03:17.780 --> 00:03:20.810
If there was only some other
place that this hydrogen could

00:03:20.810 --> 00:03:24.170
get an electron from, then this
guy could just go to the

00:03:24.170 --> 00:03:26.940
bromine, which is what bromine
really wants to happen,

00:03:26.940 --> 00:03:28.410
because it's so electronegative.

00:03:28.410 --> 00:03:31.640
So where can the hydrogen get an
electron to replace the one

00:03:31.640 --> 00:03:33.320
that it's about to lose?

00:03:33.320 --> 00:03:35.820
Well, we have a double
bond here.

00:03:35.820 --> 00:03:39.020
And maybe one of these carbons
lose an electron.

00:03:39.020 --> 00:03:41.220
And in future videos, we'll talk
more about which one is

00:03:41.220 --> 00:03:43.830
more likely to give up
the electron at this

00:03:43.830 --> 00:03:45.650
stage of the reaction.

00:03:45.650 --> 00:03:50.360
But just for simplicity, let's
assume that this carbon right

00:03:50.360 --> 00:03:52.880
here gives up an electron.

00:03:52.880 --> 00:03:54.410
Where are its valence
electrons?

00:03:54.410 --> 00:03:56.250
And just as a bit a review,
here's carbon on

00:03:56.250 --> 00:03:57.710
the Periodic Table.

00:03:57.710 --> 00:04:03.200
it has six protons and six
electrons in its stable state.

00:04:03.200 --> 00:04:06.945
Two are sitting in its first
shell and then the other four

00:04:06.945 --> 00:04:07.920
are its valence electrons.

00:04:07.920 --> 00:04:08.870
And you can see that.

00:04:08.870 --> 00:04:10.310
It's in Group 4, right?

00:04:10.310 --> 00:04:13.775
One, two, three, four.

00:04:13.775 --> 00:04:16.310
We can ignore these for now, and
usually you're not going

00:04:16.310 --> 00:04:18.620
to be dealing with-- well, we'll
ignore those for now.

00:04:18.620 --> 00:04:20.670
It has four valence electrons.

00:04:20.670 --> 00:04:22.620
Most of what you're going to
deal with, you just have to

00:04:22.620 --> 00:04:23.890
count the group numbers
like this.

00:04:23.890 --> 00:04:26.290
We won't worry too much about
all of the metals and all of

00:04:26.290 --> 00:04:27.930
those right now.

00:04:27.930 --> 00:04:29.340
And that's four valence
electrons and you

00:04:29.340 --> 00:04:29.980
see it right here.

00:04:29.980 --> 00:04:34.470
It has one, two, three, four
valence electrons.

00:04:34.470 --> 00:04:36.700
It has two in its one shell,
so it actually has six, but

00:04:36.700 --> 00:04:38.720
you only draw the
four out there.

00:04:38.720 --> 00:04:41.890
So what we're saying is this
green electron can go to the

00:04:41.890 --> 00:04:45.480
bromine as long as the hydrogen
can take an electron

00:04:45.480 --> 00:04:47.710
maybe from this carbon
right here.

00:04:47.710 --> 00:04:49.120
So let's draw that.

00:04:49.120 --> 00:04:53.070
So this electron right here is
going to go to the hydrogen.

00:04:53.070 --> 00:04:56.100
And when that electron goes to
the hydrogen, simultaneously

00:04:56.100 --> 00:04:57.770
that will allow this electron
to go to bromine.

00:04:57.770 --> 00:05:00.080
And, obviously, they won't be
this far when it happens.

00:05:00.080 --> 00:05:01.900
There would be some type of
collision that would have to

00:05:01.900 --> 00:05:05.180
occur in just the right way with
just the right level of

00:05:05.180 --> 00:05:06.510
energy for this to occur.

00:05:06.510 --> 00:05:08.970
Maybe the hydrogen is getting
really close to this part

00:05:08.970 --> 00:05:11.930
right at the right moment when
this electron is being sucked

00:05:11.930 --> 00:05:14.960
away from the bromine, so this
has a partial positive charge

00:05:14.960 --> 00:05:16.170
attracted to the electron.

00:05:16.170 --> 00:05:17.630
That electron goes over there.

00:05:17.630 --> 00:05:19.350
It won't always happen,
but this is a

00:05:19.350 --> 00:05:21.010
potential reaction mechanism.

00:05:21.010 --> 00:05:22.730
But this whole step
happens at once.

00:05:22.730 --> 00:05:26.180
This electron goes from this
character to the hydrogen at

00:05:26.180 --> 00:05:28.210
the same time as hydrogen
loses its

00:05:28.210 --> 00:05:31.600
electron to the bromine.

00:05:31.600 --> 00:05:35.150
So what's going to happen
right after this step?

00:05:37.780 --> 00:05:39.500
So right after that
happens, what will

00:05:39.500 --> 00:05:41.150
everything look like?

00:05:41.150 --> 00:05:46.900
Well, the bromine will have
gained the electron, so it's

00:05:46.900 --> 00:05:48.806
now a bromide ion.

00:05:48.806 --> 00:05:50.700
So let me draw it like this.

00:05:50.700 --> 00:05:54.140
So it had its original seven
valence electrons, one, two,

00:05:54.140 --> 00:05:58.650
three, four, five, six, seven,
and now it just stole an

00:05:58.650 --> 00:06:00.650
electron from the hydrogen.

00:06:00.650 --> 00:06:02.950
It was able to swipe an electron
off the hydrogen.

00:06:02.950 --> 00:06:05.250
That's the electron it swiped
off the hydrogen.

00:06:05.250 --> 00:06:08.020
And now what will this thing
that was 1-pentene or

00:06:08.020 --> 00:06:09.790
pent-1-ene, what will
this look like now?

00:06:09.790 --> 00:06:11.080
Let me draw it.

00:06:11.080 --> 00:06:17.710
So we have a carbon and a
hydrogen and a hydrogen, and

00:06:17.710 --> 00:06:21.710
then you have a carbon, and then
a hydrogen, and then you

00:06:21.710 --> 00:06:24.550
have the rest of the chain
right over here.

00:06:27.820 --> 00:06:29.360
But this double bond
was broken.

00:06:29.360 --> 00:06:32.410
This carbon lost an electron,
went to the hydrogen.

00:06:32.410 --> 00:06:36.010
So this bond right here now
forms between this carbon and

00:06:36.010 --> 00:06:36.730
that hydrogen.

00:06:36.730 --> 00:06:37.980
So let me draw that bond.

00:06:40.970 --> 00:06:42.490
You have this electron
right there.

00:06:42.490 --> 00:06:44.640
That electron will now
be with the hydrogen.

00:06:44.640 --> 00:06:46.270
So let me draw the electron.

00:06:46.270 --> 00:06:48.340
And now we have that
orange hydrogen.

00:06:48.340 --> 00:06:50.250
I'll try to keep the colors
consistent so that we know

00:06:50.250 --> 00:06:51.490
where things came from.

00:06:51.490 --> 00:06:55.070
And then we have this bond
now to the hydrogen.

00:06:55.070 --> 00:06:58.580
Now, this carbon now only has
three valence electrons.

00:06:58.580 --> 00:07:00.380
One, two, three.

00:07:00.380 --> 00:07:03.240
It has two sitting in its first
shell, so it actually

00:07:03.240 --> 00:07:04.590
has a total of five electrons.

00:07:04.590 --> 00:07:09.350
It has six protons, so it
has a positive charge.

00:07:09.350 --> 00:07:13.900
This carbon right here has
a positive charge.

00:07:13.900 --> 00:07:16.040
And another way to think about
it is it was completely

00:07:16.040 --> 00:07:17.780
neutral and then it
lost an electron.

00:07:17.780 --> 00:07:20.290
So now it will have a positive
charge right over there.

00:07:20.290 --> 00:07:21.960
So this is what we
are left after

00:07:21.960 --> 00:07:24.980
that step of the reaction.

00:07:24.980 --> 00:07:26.560
Oh, and of course,
we can't forget.

00:07:26.560 --> 00:07:28.730
Bromine over here was neutral.

00:07:28.730 --> 00:07:30.950
It had seven valence electrons,
and that's when

00:07:30.950 --> 00:07:31.550
bromine is neutral.

00:07:31.550 --> 00:07:32.950
But now it has eight,
so now this will

00:07:32.950 --> 00:07:34.200
have a negative charge.

00:07:37.030 --> 00:07:38.290
This will have a negative
charge, because

00:07:38.290 --> 00:07:39.800
it gained an electron.

00:07:39.800 --> 00:07:42.450
And in general, your total
charge-- over here our total

00:07:42.450 --> 00:07:43.630
charge is zero, So?

00:07:43.630 --> 00:07:45.310
Our total charge will
still be zero.

00:07:45.310 --> 00:07:46.820
We have a negative and
we have a positive.

00:07:46.820 --> 00:07:50.240
They would cancel out, so our
total charge is still zero.

00:07:50.240 --> 00:07:54.610
So what's likely to happen for
the next step of our reaction?

00:07:54.610 --> 00:07:56.340
Well, we have this positive
thing here.

00:07:56.340 --> 00:08:01.430
Maybe bromine just bumped, just
the right way, to let go

00:08:01.430 --> 00:08:03.020
of this guy and steal
his electron.

00:08:03.020 --> 00:08:05.820
But now you have this guy who's
negative and this guy

00:08:05.820 --> 00:08:06.740
who's positive.

00:08:06.740 --> 00:08:08.400
Maybe they'll be attracted
to each other.

00:08:08.400 --> 00:08:10.240
Maybe they'll just bump
into each other at

00:08:10.240 --> 00:08:11.340
the exact right way.

00:08:11.340 --> 00:08:15.270
And if they bump in the exact
right way, maybe this guy can

00:08:15.270 --> 00:08:20.300
swipe the electron from the
bromide ion, from this

00:08:20.300 --> 00:08:21.750
negative ion right here.

00:08:21.750 --> 00:08:24.710
And you might say, hey, isn't
bromine more electronegative

00:08:24.710 --> 00:08:25.190
than carbon?

00:08:25.190 --> 00:08:27.500
Well, it might be, but this
guy's electron rich.

00:08:27.500 --> 00:08:31.220
It's not just a regular
bromine atom.

00:08:31.220 --> 00:08:33.190
This is a bromine plus
an extra electron.

00:08:33.190 --> 00:08:35.774
So he's already hogged
an electron, so

00:08:35.774 --> 00:08:36.750
he's electron rich.

00:08:36.750 --> 00:08:38.549
So in this situation,
he's negative.

00:08:38.549 --> 00:08:39.700
He's positive.

00:08:39.700 --> 00:08:42.360
He can give this guy
an electron.

00:08:42.360 --> 00:08:46.940
So if they bump in just the
right way, this electron can

00:08:46.940 --> 00:08:52.070
be swiped by this carbon
right over here.

00:08:52.070 --> 00:08:53.780
And this positive carbon, just
so it gives you a little

00:08:53.780 --> 00:08:56.590
terminology, and we'll go over
it in more detail in future

00:08:56.590 --> 00:08:59.270
videos, is actually called
a carbocation.

00:08:59.270 --> 00:09:01.370
It's a positive ion of carbon.

00:09:01.370 --> 00:09:03.160
That's where the word
comes from.

00:09:03.160 --> 00:09:06.680
But anyway, if this electron
gets swiped by this carbon, it

00:09:06.680 --> 00:09:07.720
will then form a bond.

00:09:07.720 --> 00:09:09.780
Because remember, this was the
electron that was originally

00:09:09.780 --> 00:09:12.730
in a bond with this hydrogen.

00:09:12.730 --> 00:09:15.110
It's still going to be, you
could imagine, paired up with

00:09:15.110 --> 00:09:19.000
this other purple or magenta
electron right over there.

00:09:19.000 --> 00:09:22.370
So if that happens, then we're
going to be left with-- so the

00:09:22.370 --> 00:09:26.190
next step is going to be-- so on
this end of the molecule we

00:09:26.190 --> 00:09:29.540
have C, carbon, hydrogen,
hydrogen.

00:09:29.540 --> 00:09:32.360
Then we have this orange
hydrogen that we stole from

00:09:32.360 --> 00:09:34.270
the hydrogen bromide.

00:09:34.270 --> 00:09:39.440
Then we have this carbon
right here.

00:09:39.440 --> 00:09:40.700
It has a hydrogen.

00:09:40.700 --> 00:09:42.020
You have the rest of the
chain, CH2-CH2-CH3.

00:09:47.150 --> 00:09:50.380
And now, since this guy stole an
electron, a bond will form

00:09:50.380 --> 00:09:53.500
with the bromine.

00:09:53.500 --> 00:09:55.670
Let me draw it.

00:09:55.670 --> 00:09:59.620
So he's going to steal this--
let me draw it this way.

00:09:59.620 --> 00:10:00.640
So a bond will form.

00:10:00.640 --> 00:10:03.340
He's stealing this electron, so
now this electron is with

00:10:03.340 --> 00:10:05.250
this carbon, so I can do it
as one end of the bond.

00:10:05.250 --> 00:10:08.970
The other end of the bond will
be that magenta electron right

00:10:08.970 --> 00:10:10.340
there on the bromine.

00:10:10.340 --> 00:10:13.490
And now the bromine lost an
electron, so it's neutral.

00:10:13.490 --> 00:10:18.140
It once again has one, two,
three, four, five, six, seven,

00:10:18.140 --> 00:10:19.410
valence electrons.

00:10:19.410 --> 00:10:21.660
Carbon is now neutral, because
it gained an electron.

00:10:21.660 --> 00:10:24.930
It once again has one, two
three, four electrons.

00:10:24.930 --> 00:10:27.640
So now everyone is
happy again.

00:10:27.640 --> 00:10:30.360
In this video, we were able
to get a mechanism.

00:10:30.360 --> 00:10:32.200
We can talk in the future about
how likely it is to

00:10:32.200 --> 00:10:34.060
happen, how quickly
it might happen.

00:10:34.060 --> 00:10:36.520
We were able to start with
hydrogen bromide and

00:10:36.520 --> 00:10:40.720
1-pentene, or pent-1-ene, and
get to this thing, just by

00:10:40.720 --> 00:10:43.050
pushing around electrons and
just thinking about what is

00:10:43.050 --> 00:10:45.630
likely to happen based on what's
electronegative and

00:10:45.630 --> 00:10:48.135
what might be able to gain
or lose an electron.

00:10:48.135 --> 00:10:50.500
And just a bit of a review, what
is this thing right here?

00:10:50.500 --> 00:10:53.420
This might be-- This is
one, two, three, four,

00:10:53.420 --> 00:10:54.590
five carbons still.

00:10:54.590 --> 00:10:57.620
So it's going to be a pent.

00:10:57.620 --> 00:10:59.910
It's now an alkane,
no double bond.

00:10:59.910 --> 00:11:01.750
So it's pentane.

00:11:01.750 --> 00:11:04.370
And it has one group on it, so
we'll start numbering closer

00:11:04.370 --> 00:11:04.810
to the group.

00:11:04.810 --> 00:11:06.350
So one, two.

00:11:06.350 --> 00:11:07.600
It's 2-bromopentane.

00:11:12.010 --> 00:11:14.250
So we were able to figure out a
reaction mechanism to get us

00:11:14.250 --> 00:11:18.550
from hydrogen bromide and
1-pentene to 2-bromopentane.