Comproportionation
# Comproportionation

Comproportionation or synproportionation is a chemical reaction where two reactants containing the same element but with different oxidation numbers, form a compound having an intermediate oxidation number. It is the opposite of disproportionation.

## Frost diagrams

The tendency of two species to disproportionate or comproportionate can be determined by examining the Frost diagram of the oxidation states; if a species' value of ΔG/F is lower than the line joining the two oxidation numbers on either side of it, then it is more stable and if in a solution, these two species will undergo comproportionation.

A Frost diagram is another way of displaying the reduction potentials for the various oxidation states of a given element, X. It shows nE against the oxidation number N: here, E is the reduction potential for the X(N)/X(0) couple, and n is the number of electrons transferred in the conversion of X(N) to X(0)

## Examples

* In lead batteries, the spontaneous reaction is:

       Pb + $PbO_2$ + 2 $H_2SO_4$ → 2 $PbSO_4$ + 2 $H_2O$
* The laboratory preparation of manganese dioxide involves comproportionation of Mn(II) and Mn(VII) reagents:

    2 KMnO $_4$ + 3 MnSO $_4$ + 2 H $_2O→$ 5 MnO $_2$ + K $_2SO$ $_4$ + 2 H $_2SO$ $_4$
* 15 Se + $SeCl_4$ + 4 $AlCl_3$ → 2 $Se_8[AlCl_4]_2$
* In the Claus process, two gaseous compounds of sulfur comproportionate in the presence of a catalyst to give elemental sulfur:

       2 $H_2S$ + $SO_2$ → 3 S + 2 $H_2O$
* In halogen chemistry:

       $IO_3^−$ + 5 $I^−$ + 6 H $^+$ → 3 $I_2$ + 3 $H_2O$
* In anammox chemistry:

       $NH_4^+$ + $NO_2^−$ → $N_2$ + $2H_2O$
* Iron(III) chloride reacts with iron powder to form iron(II) chloride:

    2 $FeCl_3$ + Fe → 3 $FeCl_2$
