Disproportionation
Disproportionation is a specific type of redox reaction in which an element from a reaction undergoes both oxidation and reduction to form two different products.
For example, the UV photolysis of mercury(I) chloride Hg2Cl2 → Hg + HgCl2 is a disproportionation. Mercury(I) is a diatomic dication Hg2+
2. In this reaction, the chemical bond in the molecular ion is broken, and one mercury atom is reduced to mercury(0), and the other is oxidized to mercury(II).
A similar type of reaction, but in which no element changes oxidation number, is the acid-base disproportionation reaction observed when an amphiprotic species reacts with itself. Two common examples for conjugated bases of polyprotic acids such as bicarbonate and dihydrogenophosphate are respectively:
The oxidation numbers remain constant in these acid-base reactions: O = −2, H = +1, C = +4, P = +5. This is also called autoionization.
Another variant on disproportionation is radical disproportionation, in which two radicals form an alkane and alkene.
The reverse of disproportionation, when a compound in an intermediate oxidation state is formed from compounds in lower and higher oxidation states, is called comproportionation, also known as symproportionation.
The first disproportionation reaction to be studied in detail was:
This was examined using tartrates by Johan Gadolin in 1788. In the Swedish version of his paper he called it 'söndring'.
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