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.^[1]

Frost diagrams

In electrochemistry, the tendency of two redox species to disproportionate, or comproportionate, can be determined by examining their Frost diagram. It is a graphical plot of nE° = −ΔG°/F as a function of the oxidation number for the different redox species of a given element.

The Gibbs free energy ΔG° is related to the reduction potential E° by the formula: ΔG° = −nFE° or nE° = −ΔG°/F, where n is the number of transferred electrons, and F is the Faraday constant (F = 96,485 J/(V·mol) = 96,485 Coulomb).^[1]

If the value of nE° = −ΔG°/F for a species is lower than the line joining two adjacent, or more generally, neighboring species, having thus a lower and a higher oxidation number, then this species is more stable than its neighbors, and the two surrounding species will undergo comproportionation to minimize the Gibbs free energy of the system. Example: a mixture of Mn(III) and Mn(VI) will comproportionate towards Mn(IV) as illustrated in the Frost diagram for manganese. Non-adjacent neighboring species of Mn obeying the same general rule will also react together as, e.g., Mn²⁺ and MnO−4 to form MnO₂. So, the more distant Mn(II) and Mn(VII) can also react together to form Mn(IV). The reacting redox species do not have to be necessarily adjacent on a Frost diagram.

The comproportionation reaction cannot easily occur in

• 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

• In selenium chemistry:

15 Se + SeCl₄ + 4 AlCl₃ → 2 Se₈[AlCl₄]₂

• In the Claus process, two gaseous compounds of sulfur comproportionate in the presence of a catalyst to give elemental sulfur:

2 H₂S + SO₂ → 3 S + 2 H₂O

• In halogen chemistry:

IO₃⁻ + 5 I⁻ + 6 H ⁺ → 3 I₂ + 3 H₂O

• In anammox (anaerobic ammonium oxidation) biochemistry:

NH₄⁺ + NO₂⁻ → N₂ + 2 H₂O

• Iron(III) chloride reacts with iron powder to form iron(II) chloride:^[2]

2 FeCl₃ + Fe → 3 FeCl₂

References