13.0 Resonance Effect

Due to resonance the carbon to leaving group (L) bond acquires a double bond character due to which stabilization occurs. Now more is stabilization, lesser is the reactivity and vice-versa. As the stabilization is the least in the case of acid chloride because of high magnitude of $–I$ effect of Cl atom. Therefore its reactivity is the most.

Nucleophilic acyl substitution should be catalysed by acids because the protonation of the acyl compound would facilitate step (i) for nuceleophilic attack.