Why alkynes undergo nucleophilic addition reaction while simple alkenes do not ?

Explanation

It can be explained on the basis of relative stability of the intermediate carbanions. These intermediate carbanions are formed by the nucleophilic attack on alkenes and alkynes.

The carbanion formed from alkynes is `sp^2` hybridised, so carbon atom has greater s-character. Greater s-character can accomodate more negative charge. Whereas alkyl carbanion is `sp^3` hybridised and can accomodate lesser negative charge. So, carbanions formed from alkynes are more stable than the carbanions of alkène. Hence, alkynes undergo nucleophilic addition reaction easily while alkenes do not give these reactions.