Organic compounds containing the group –CO.X, where X is a halogen atom (see formula). Acyl chlorides, for instance, have the general formula RCOCl. The group RCO– is the acyl group. In systematic chemical nomenclature acyl-halide names end in the suffix -oyl; for example, ethanoyl chloride, CH3COCl. Acyl halides react readily with water, alcohols, phenols, and amines and are used in acylation reactions. They are made by replacing the –OH group in a carboxylic acid by a halogen using a halogenating agent such as PCl5.