Acyl transfer is the mode of interconversion between carboxylic acids the carboxylic acid derivatives including acyl halides, carboxylic acid anhydrides, esters, and carboxamides (additionally, nitriles may be easily converted to carboxylic acids or amides by hydrolysis). The relative stability of a particular carboxylic acid derivative derives from the ability of the substituent (hydroxyl, halide, alkoxyl, etc.) to contribute to the resonance stabilization of the group. For example, the great length of the C-Cl bond in an acyl chloride (1.79 Å) works against the donation of its nonbonding pair by resonance, which, combined with its inductive electron withdrawing character, leads to the high susceptibility of acid chlorides to nucleophilic attack. The C-N bond of carboxyamides and the C-O bonds of anhydrides, esters, and carboxylic acids are more conducive to donation by resonance. With the lower electronegativity of nitrogen, compared to oxygen, decreasing its inductive electron withdrawing effect, amides are the most stable carboxylic acid derivatives (except for carboxylate anions), followed by esters then thioesters and anhydrides (followed then by acid halides, the least stable).
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