The acyl group is the R–C(=O)– fragment obtained from a carboxylic acid by removing the hydroxyl (–OH) group. It matters because it is the unit that is transferred from one nucleophile to another in nucleophilic acyl substitution, the characteristic reaction of acid derivatives. For example, transferring an acyl group from an acyl chloride to an alcohol produces an ester.

酰基是从羧酸去掉羟基(–OH)而得到的 R–C(=O)– 片段。它之所以重要,是因为它是在亲核酰基取代(酸衍生物的特征反应)中从一个亲核试剂转移到另一个亲核试剂的单元。例如,把一个酰基从酰氯转移到醇上即生成酯。