Carboxylic acid derivatives and nucleophilic acyl substitution

Carboxylic acid derivatives — acid chlorides, anhydrides, esters, and amides — all share a common structural core: a carbonyl carbon bonded to an oxygen (the carbonyl oxygen) and to a second substituent that can, in principle, leave as a stable anion or neutral molecule (a chlorine in acid chlorides, a carboxylate in anhydrides, an alkoxy group in esters, an amine in amides). This shared structure gives them a shared, distinctive reaction pathway with nucleophiles: nucleophilic acyl substitution.

The mechanism begins exactly like nucleophilic addition to a simple ketone: a nucleophile is drawn to the electron-poor carbonyl carbon and attacks it, using the C=O pi electrons to form a new carbon-oxygen bond (building on the same polarisation logic as ordinary carbonyl chemistry), producing a tetrahedral intermediate. For a ketone, this tetrahedral intermediate, once protonated, is the final product. For a carboxylic acid derivative, the tetrahedral intermediate instead has a leaving group still attached, and rather than persisting, it collapses: the carbon-oxygen pi bond re-forms, expelling the leaving group as the carbonyl carbon returns to its flat, sp2-hybridised form. The overall transformation therefore replaces the original leaving group with the incoming nucleophile while regenerating the carbonyl group — an addition followed immediately by an elimination, together amounting to a net substitution at the carbonyl carbon, distinct from the simple addition seen with ketones and aldehydes that lack any leaving group to expel.

How readily a given carboxylic acid derivative undergoes this substitution depends almost entirely on how good a leaving group it carries — a better leaving group (one that is more stable once it has departed, typically because its negative charge or lone pair is better stabilised) is expelled more easily, and this maps directly onto a clear reactivity order. Acid chlorides, with chloride as an excellent, weakly basic leaving group, are the most reactive, reacting readily even with weak nucleophiles. Anhydrides are somewhat less reactive, with a carboxylate leaving group. Esters, with an alkoxide leaving group, are markedly less reactive still. Amides, with a nitrogen-based leaving group that is a comparatively poor leaving group (amide nitrogen holds its lone pair tightly, both because nitrogen is less electronegative than oxygen and because that lone pair is delocalised into the carbonyl, strengthening the C-N bond), are the least reactive of the common derivatives, which is exactly why amide (peptide) bonds are biologically stable enough to hold proteins together under everyday conditions, requiring dedicated enzymatic catalysis to hydrolyse rapidly.