High Quality Content by WIKIPEDIA articles! In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group -CO-Cl. Their formula is usually written RCOCl, where R is a side chain. They are usually considered to be reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride, CH3COCl. Acyl chlorides are the most important subset of acyl halides, e.g. acetyl bromide.