Question

Why are acyl chlorides so reactive?

Answer 1

Because they contain (i) an unsaturated carbon centre, and (ii) a potential leaving group in the chloride ion.

Explanation:

The business end of an acyl halide, `RC(=O)X (X=Cl, Br)`, contains a reactive `sp_2` hybridized carbon, and a potential leaving group in the chloride ion. If I ever wanted to perform an esterification reaction, I would always boil up my starting carboxylic acid with thionyl chloride, to give the acid chloride, which would easily react with added alcohol or amine in the presence of a non-nucleophilic base.