disproportionation and comproportionation
/ dis-pro-POR-shun-AY-shun /
Most redox reactions need two different partners. Disproportionation is the strange case where a single species plays both roles at once: some of it is oxidized while the rest of it is reduced, splitting into a higher and a lower oxidation state. Picture a crowd of identical people where, with no outsider involved, half rise a step and half fall a step. Comproportionation is the exact reverse: a high state and a low state of the same element meet and settle into a single middle state.
The driving force is whether the intermediate state is energetically a hill or a valley. A species disproportionates when its middle oxidation state is less stable than the average of the states it would split into — on a Frost diagram, when it sits as a convex bump above the line joining its neighbours; on a Latimer diagram, when the potential to its right exceeds the potential to its left. Comproportionation happens in the opposite situation, when the middle state is the stable valley (a concave dip on the Frost diagram), so the high and low states are drawn together to form it.
These reactions are everywhere in inorganic chemistry. Copper(I) disproportionates in water (2 Cu+ -> Cu2+ + Cu), which is why simple Cu+ salts are unstable in solution; chlorine disproportionates in cold base to give chloride and hypochlorite (the chemistry of household bleach). A useful honest reminder: whether a state disproportionates is conditional, not absolute — copper(I) is unstable as the aqua ion but perfectly stable in CuCl or stabilized by soft ligands, and chlorine's behaviour flips with temperature and pH. Always state the conditions.
Cl2 + 2 NaOH -> NaCl + NaOCl + H2O: chlorine (0) splits into chloride (-1, reduced) and hypochlorite (+1, oxidized) — disproportionation, and the reaction behind bleach.
One element, two fates: part reduced to chloride, part oxidized to hypochlorite.
Disproportionation needs no external oxidant or reductant — the species supplies both. Whether it happens depends entirely on conditions (pH, temperature, ligands), so a state stable in one medium can disproportionate in another.