An acyl halide (also known as an acid halide) is an organic compound containing a -COX functional group, which consists of a carbonyl group singly bonded to a halogen atom such as chlorine ( Cl ). The general formula for an acyl halide could be written as RCOX, where R represents an organic radical group, CO is the carbonyl group, and X represents the halogen atom. See simple diagram at right. The RCO- group in an acyl halide can be thought of as an acyl group and the -X radical as the halide part of the molecule; for example, -Cl is chloride. Acyl chlorides are the most commonly used acyl halides. Acyl halides are commonly made by halogenating a carboxylic acid, hence the name acid halide.

An example of an acyl halide is acetyl chloride CH₃COCl. See simple diagram at right.

Chemical reactions

Acyl halides are artificial, rather reactive compounds often synthesized to be used as intermediates in the synthesis of other organic compounds. For example, an acyl halide can chemically react with:

• water to form a carboxylic acid

• an alcohol to form an ester

• an amine to form an amide

• an aromatic compound, using a catalyst such as AlCl₃, to form an aromatic ketone. See Friedel-Crafts acylation.

In the above reactions, HX (hydrogen halide or hydrohalic acid) is also formed. For example, if the acyl halide is an acyl chloride, HCl (hydrogen chloride or hydrochloric acid) is also formed.

Because acyl halides are rather reactive compounds, they have toxicity and special precautions should be taken handling them. They are lachrymatory chemicals because they can react with water at the surface of the eye producing hydrohalic and organic acids irritating to the eye. Similar problems can result if one inhales acyl halide vapors.

Multiple functional groups

A molecule can have more than one acyl halide functional group. For example, adipoyl dichloride or adipyl dichloride (sometimes simply called adipoyl chloride or adipyl chloride), has two acyl chloride functional groups]]; see the structure at right. It is the dichloride (i. e., double chloride) of the 6-carbon dicarboxylic acid adipic acid. An important use of adipoyl chloride is polymerization with an organic di-amino compound to form a polyamide called nylon or polymerization with certain other organic compounds to form polyesters.

Phosgene (carbonyl dichloride, Cl-CO-Cl) is a very toxic gas which may be thought of as the dichloride of carbonic acid (HO-CO-OH). Both chloride radicals in phosgene can undergo reactions analogous to the preceding reactions of acyl halides. Phosgene is used a reactant in the production of polycarbonate polymers, among other industrial applications.