 Nucleophilic addition

A nucleophilic addition forms a bond by attack of a nucleophile on an electrophilic multiple bond, reducing its bond order. A common case is attack on the carbon of a carbonyl group, with its pi electrons displaced toward oxygen.

Protonation of the resulting oxygen-centered intermediate can give an alcohol. Unlike nucleophilic substitution, the basic addition step does not expel a leaving group from the attacked carbon. Carbonyl polarization and orbital geometry explain both why carbon is attacked and why the incoming bond follows a preferred approach direction.