A type of organic compound of the general formula RCOX, where X is a halogen (acyl chloride, acyl bromide, etc.). Acyl halides can be prepared by the reaction of carboxylic acid with a halogenating agent. Commonly, phosphorus halides are used (e.g. PCl5) or a sulfur dihalide oxide (e.g. SOCl2):
RCOOH + PCl5 → RCOCl + POCl3 + HCl
RCOOH + SOCl2 → RCOCl + SO2 + HCl
The acyl halides have irritating vapors and fume in moist air. They are very reactive to the hydrogen atom of compounds containing hydroxyl (–OH) or amine (–NH2) groups.