An acyl chloride, also called an acid chloride, is a highly reactive carboxylic acid derivative of the form R–C(=O)–Cl. It matters because chloride is an excellent leaving group, which makes the acyl chloride the most reactive of the common acid derivatives and a convenient reagent for transferring its acyl group to other nucleophiles. It is therefore used to make esters and amides, which are far less reactive themselves.

酰氯,又称酸式氯,是一类反应性很强的羧酸衍生物,其形式为 R–C(=O)–Cl。它之所以重要,是因为氯负离子是极佳的离去基团,这使酰氯成为常见酸衍生物中反应性最强的一种,也是把其酰基转移到其他亲核试剂上的便利试剂。因此,它被用来制备本身活泼得多的酯和酰胺。