Why are aromatic aldehydes less reactive than aliphatic aldehydes in nucleophilic addition reactions?

Aromatic aldehydes are less reactive than aliphatic aldehydes in nucleophilic addition reactions because the carbonyl group (–CHO) is stabilized by resonance with the aromatic ring. This reduces the positive charge on the carbonyl carbon, making it less attractive for nucleophiles.

In aromatic aldehydes, such as benzaldehyde (C₆H₅CHO), the π-electrons of the benzene ring interact with the carbonyl group through resonance. This electron delocalization decreases the electrophilic nature of the carbonyl carbon and lowers its tendency to undergo nucleophilic attack.

In contrast, aliphatic aldehydes do not have this type of resonance stabilization, so their carbonyl carbon remains more reactive toward nucleophiles. Additionally, the bulky aromatic ring can create some steric hindrance, making nucleophilic addition comparatively difficult.

This difference helps explain variations in the chemical behaviour of aldehydes in organic chemistry reactions.