Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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 an organic radical group. 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.