nucleophilic acyl substitution

This is the single mechanism that ties the whole carboxylic-acid family together. Acid chlorides become esters, esters become amides, amides hydrolyze to acids — and every one of those interconversions runs by the same two-step dance: a nucleophile adds to the carbonyl, then a leaving group departs. Learn this one mechanism and you have learned acyl chemistry.

Here is the dance step by step. The carbonyl carbon is electron-poor (the electronegative oxygen pulls electron density away), so a nucleophile attacks it. The C=O pi bond breaks and its electrons flip up onto the oxygen, turning the flat carbon into a tetrahedral, four-bonded carbon — the tetrahedral intermediate — now bearing a negative oxygen. That negative oxygen is the spring: it re-forms the C=O double bond and, in doing so, pushes out the leaving group (chloride, carboxylate, alkoxide, etc.). Net result: the nucleophile has replaced the leaving group, and the carbonyl is restored. Addition first, then elimination.

The contrast with aldehydes and ketones is the whole point. They undergo nucleophilic addition and simply stop at the tetrahedral intermediate, because the groups on their carbonyl (H or alkyl) are not leaving groups — there is nothing good to expel. Carboxylic-acid derivatives have a leaving group attached to the carbonyl carbon, so the tetrahedral intermediate collapses forward, substituting rather than just adding. The whole reactivity order of the family — acyl chloride down to amide — is simply a measure of how easily each derivative forms that intermediate and how readily its leaving group departs.