The reaction that involves simultaneous oxidation and reduction of atoms of same element from one oxidation state (OS) to two different oxidation states is known as disproportionation reaction.
The minimum requirement for this reaction to occur is: the element undergoing disproportionation should exhibit minimum three different oxidation states.
Dissociation of hydrogen peroxide is a disproportionation reaction. The oxygen atom in H2O2 is in -1 oxidation state. It is both oxidized to O2 (ox.st = 0) and reduced to H2O (ox.st = -2).
In above reaction, the relatively less stable peroxide disproportionate into relatively more stable compounds i.e. water and dioxygen.
Except fluorine, halogens (Cl2 ,Br2 , I2) undergo disproportionation in alkaline medium.
They form halide and hypohalite in cold and dilute alkali.
Here, one chlorine atom is oxidized to +1 and second is reduced to -1 oxidation states.
Whereas, halide and halate are formed in hot and concentrated alkali.
In this case, one chlorine atom loses 5 electrons and get oxidized to +5 state and five chlorine atoms are reduced to -1 state by accepting one electron each.
Phosphorus disproportionates to phosphine and hypophosphite in alkaline medium. In this case, one P atom is reduced to -3 oxidation number (in PH3) and three P atoms get oxidized to +1 (in NaH2PO2).
Mercurous chloride undergoes disproportionation under UV light to give mercury and mercuric chloride. The Hg22+ ion is oxidized to Hg2+ and reduced to Hg.
Nitrogen dioxide, NO2 reacts with water to give nitric and nitrous acids (Ostwald process). It is a disproportionation reaction. The Nitrogen in NO2 is in +4 oxidation state. It is reduced to nitrous acid, HNO2 in which the OS of nitrogen is +3 and oxidized to nitric acid, HNO3 in which the OS of nitrogen is +5.