12.13: Nucleophilic Addition to the Carbonyl Group: General Mechanism
The carbonyl carbon in an aldehyde or ketone is the site of a nucleophilic attack due to its electron-deficient nature. Depending on the strength of the incoming nucleophile, the reaction occurs via different mechanistic pathways.
A stronger nucleophile can directly attack the electrophilic center, the carbonyl carbon. The HOMO orbital of the nucleophile interacts with the LUMO (π* antibonding) orbital present on the carbonyl carbon. This interaction breaks the π bond and shifts the π bonding electrons onto the carbonyl oxygen, forming a basic alkoxide anion. The basic alkoxide ion intermediate abstracts a proton, forming the addition product as depicted in the following figure.
On the other hand, a weak nucleophile cannot directly attack the electrophilic carbonyl carbon. For the weak nucleophile to react, the electrophilicity of the carbonyl carbon needs to be enhanced substantially. Thus, the aldehyde or ketone is treated with an acid catalyst to improve the electrophilicity of carbonyl carbon. As shown in the figure below, the acid catalyst protonates the carbonyl oxygen forming an oxonium cation. The oxonium cation is resonance stabilized by delocalizing the positive charge onto the adjacent carbonyl carbon atom. This delocalization of the positive charge increases the electrophilicity of the carbonyl carbon, leading to the attack of the weak nucleophile.