12.10 : 羰基的亲核加成：一般机制

In the carbonyl group, the electrophilicity of carbon—arising from its bonding with the more electronegative oxygen—makes the carbonyl group susceptible to a nucleophilic attack.

The strength of the attacking nucleophile determines the type of mechanism involved during their addition to the carbonyl group.

Strong nucleophiles directly attack the carbonyl carbon, moving the π electrons to the carbonyl oxygen to give the more basic alkoxide intermediate.

In the subsequent step, the negatively charged oxygen atom is protonated to give the addition product.

Weaker nucleophiles are less likely to add directly to the carbonyl group. The electrophilicity of the carbonyl carbon must be significantly enhanced to invite a nucleophilic attack.

This is done using an acid catalyst to generate a protonated carbonyl species: an oxonium cation. As the positive charge shifts from oxygen to carbon, the oxonium ion is resonance stabilized, and the carbon atom becomes strongly electrophilic.

The weak nucleophile then readily adds to the activated carbonyl group to give the addition product.