CBSE Class 12-science Answered

The reactions in which a nucleophile adds to a given reactant are called as nucleophilic addition reaction. The carbonyl carbons (aldehydes and ketones) contain carbonyl group C = O, which provides site for addition reaction through the following mechanism.

Nucleophilic addition reactions involve the initial attack of a nucleophile on the slightly positive carbon centre of the carbonyl group. The carbon is electron-poor. Hence, it is an electrophile. It is a great target for attack by an electron-rich nucleophilic group. When a nucleophile attacks an aldehyde or ketone carbon there is no leaving group – the incoming nucleophile simply ‘pushes’ the electrons in the pi bond up to the oxygen. There will be an attack by a nucleophile on a carbocation. After the carbonyl is attacked by the nucleophile, the negatively charged oxygen has the capacity to act as a nucleophile.  However, most commonly the oxygen acts instead as a base, abstracting a proton from a nearby acid group in the solvent or enzyme active site.)