A<unk>  is an atom or<unk>  of atoms that detaches from a chemical substance. The remaining molecule or fragment remaining is known as the residual or main part. The term<unk>  is dependent on the context of the statement.
The ability for a functional<unk>  to leave is called lability.<unk> s affect the intrinsic reactivity, not the nucleophilic discrimination factors.
The lower the pKa of the conjugate acid, the better the<unk> . This is because the lower the pKa of the conjugate acid of the<unk> , the more it stays in its anion (l.g.'s may be neutral),<unk> , form. This anion stability means that the<unk>  is less likely to react with the cation formed by the<unk> 's absence (this is the case with bad<unk> s). Without stabilization, a<unk>  will become a nucleophile due to its negative charge, and if this happens, the reaction goes in circles.  This is why a strong base is a poor<unk> . SN1 reactions prefer halides, pseudohalides, and non-coordinating ions as<unk> s. Halide salts are particularly useful<unk> s because they can be abstracted by silver ions, to form insoluble silver halides.
In room temperature water, the sequence of lability is:
Less lability
amine NH2−
methoxy CH3O−
hydroxyl HO−
carboxylate CH3COO−
F−
water
Cl−
Br−
I−
azide N3−
thiocyanate SCN−
nitro NO2
Greater Lability
NO3 is also weaker than F−.
A direct application of the difference in<unk>  stability is in the carboxylic acid derivatives.  The worse the<unk> , the more stable the chemical, which makes these form a hierarchy that can be separated in the lab.  Amines, which are the worst<unk> , are the most stable compound, and attacking any carboxylic acid derivative with an amine produces an amide.  Esters are the second most stable followed by anhydrides and finally halides.
During the SN2 nucleophilic attack, a partial negative charge forms on the<unk> . During SN1 reactions, the<unk>  anionizes and leaves.
In general, amine, methoxy and hydroxyl<unk> s never act as<unk> s in a substitution reaction.
In the non-mechanistic transformations,<unk>  is the actual substituent<unk>  present in the substrate and product.
The structure of the<unk>  affects the rates of both Sn1 and Sn2 reactions. In general, the more stable the<unk>  is as a free species--that is after it has left--the faster it will leave. This stability also reflects the basicity of the species: the more stable it is, the weaker base it is.
Leaving group
Protecting group
Prevention template
Theoretical plate
Clinical trials terminology
In silico
Algorithm
Validation therapy