Question

Why are acyl chlorides more reactive?

Answer 1

Acyl chlorides are more reactive than carboxylic acids or carboxylic esters because they have a good leaving group attached to the carbonyl carbon.

Explanation:

The ipso carbon in an acyl halide, `RC(=O)X`, is bound to two electronegative groups in the halogen and the carbonyl. This carbon centre is highly electron deficient and likely to react with an added nucleophile. In addition, the halide is also a good leaving group.

Contrast this with `RC(=O)OR'` or `RC(=O)OH`, where the potential leaving group is alkoxide or hydroxide, both potent nucleophiles themselves. It is reasonable that these centres are less reactive to added nucleophiles, as hydroxide and alkoxide are poor leaving groups.