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

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 + PbO₂ + 2 H₂SO₄ → 2 PbSO₄ + 2 H₂O

• 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 + 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 chemistry:

NH₄⁺ + NO₂⁻ → N₂ + 2H₂O

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

2 FeCl₃ + Fe → 3 FeCl₂

References