Acyl halide
An acyl halide is a chemical compound derived from an oxoacid by replacing a hydroxyl group with a halide group.
In organic chemistry, the term typically refers to acyl halides of carboxylic acids, which contain a functional group consisting of a carbonyl group singly bonded to a halogen atom. The general formula for such an acyl halide can be written RCOX, where R may be, for example, an alkyl group, CO is the carbonyl group, and X represents the halide, such as chloride. Acyl chlorides are the most commonly encountered acyl halides, but acetyl iodide is the one produced on the largest scale. Billions of kilograms are generated annually in the production of acetic acid.
Preparation
Aliphatic acyl halides
On an industrial scale, the reaction of acetic anhydride with hydrogen chloride produces a mixture of acetyl chloride and acetic acid:Common syntheses of acyl chlorides also entail the reaction of carboxylic acids with phosgene, thionyl chloride, and phosphorus trichloride Phosphorus pentabromide is used for acyl bromides, which are rarely of value.
Aromatic acyl chlorides
Benzoyl chloride is produced from benzotrichloride using either water or benzoic acid:As with other acyl chlorides, it can be generated from the parent acid and other chlorinating agents phosphorus pentachloride or thionyl chloride.
Representative laboratory routes to aromatic acyl halides are comparable to those for aliphatic acyl halides. For example, chloroformylation, a specific type of Friedel-Crafts acylation which uses formaldehyde as a reagent, or by the direct chlorination of benzaldehyde derivatives.
Acyl bromides and iodides
Acyl bromides and iodides are synthesized accordingly but are less common.Reactions
Acyl halides are rather reactive compounds often synthesized to be used as intermediates in the synthesis of other organic compounds. For example, an acyl halide can react with:- water, to form a carboxylic acid. This hydrolysis is the most heavily exploited reaction for acyl halides as it occurs in the industrial synthesis of acetic acid.
- an alcohol to form an ester
- an amine to form an amide
- an aromatic compound, using a Lewis acid catalyst such as AlCl3, to form an aromatic ketone. See Friedel-Crafts acylation.
- carboxylic acids to form an organic acid anhydrides.
Multiple functional groups
A molecule can have more than one acyl halide functional group. For example, "adipoyl dichloride", usually simply called adipoyl chloride, has two acyl chloride functional groups; see the structure at right. It is the dichloride 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 is a very toxic gas that is the dichloride of carbonic acid. Both chlorine atoms 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.