= 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.
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