14.8 : 羧酸衍生物的相对反应性

Like carboxylic acids, the derivatives also undergo nucleophilic acyl substitution reactions. The relative reactivity depends on the basicity of the leaving group and the resonance stabilization of the derivative.

Recall that the weaker the base, the better the leaving group.

In this series, the halide ion is the weakest base and the best leaving group making acid halides the most reactive.

The reactivity decreases with an increase in the basicity of the leaving group.

Since amide ions are strong bases and poor leaving groups, amides are the least reactive.

Another competing factor is the resonance stabilization of the derivatives.

The electron-donating ability of the substituents increases the electron density on the carbonyl carbon, stabilizing the positive charge. This renders the carbonyl carbon less electrophilic, thereby decreasing the reactivity.

Since resonance stabilization increases with the electron-donating ability of the substituents, acid halides are the most reactive and amides the least reactive.