Final answer:
Nucleophilic addition at the carbonyl carbon occurs when a nucleophile attacks the carbonyl group, transferring a pair of electrons to the carbonyl carbon. This forms a new bond and can result in the generation of positive and negative charges at the nucleophile and carbonyl oxygen, respectively. There are two types of nucleophilic additions: direct and conjugate. The reaction is reversible, but the reversibility can be limited based on the stability of the leaving group.
Step-by-step explanation:
Nucleophilic addition at the carbonyl carbon occurs when a nucleophile attacks the carbonyl group, transferring a pair of electrons to the carbonyl carbon. This results in the formation of a new bond between the nucleophile and the carbon atom. If the nucleophile is neutral, it acquires a positive charge, which is compensated by deprotonation of the introduced group. If the nucleophile is anionic, no positive charge is generated. In both cases, a negative charge is generated at the carbonyl oxygen, which is usually neutralized through protonation.
There are two types of nucleophilic addition reactions: direct or [1,2]-additions, and conjugate or [1,4]-additions. In direct additions, the nucleophile directly adds to the carbonyl carbon, while in conjugate additions, the nucleophile adds to the β-position of an α,β-unsaturated carbonyl system.
The nucleophilic addition at the carbonyl carbon is a reversible reaction, with the carbonyl group being regenerated in the second step of the reaction. However, the reversibility can be limited if the leaving group is a stable anion. Acyl phosphates, for example, are considered activated analogues of carboxylic acids because their leaving phosphate group is a very stable anion.