Chlorine gas is known to undergo disproportionation in alkaline conditions to give chloride and chlorate ions .

Does chlorine show disproportionation reaction?

For example, chlorine disproportionates to sodium chloride and sodiumchlorite when treated with cold and dilute sodium hydroxide solution. In above disproportionation reaction, one chlorine atom is oxidized to +1 while the second one is reduced to -1 oxidation states.

Why is the reaction of chlorine with water a disproportionation reaction?

The same chlorine gas is being oxidized to hypochlorous acid by giving away one electron. This is shown by an increase in the oxidation state of chlorine atom from 0 to +1. Since chlorine is simultaneously oxidized and reduced, the reaction between chlorine gas and water is a disproportionation reaction.