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Concept
Acyl halide
Summary
In organic chemistry, an acyl halide (also known as an acid halide) is a chemical compound derived from an oxoacid by replacing a hydroxyl group () with a halide group (, where X is a halogen).
If the acid is a carboxylic acid (), the compound contains a functional group, which consists 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 (transiently) on the largest scale. Billions of kilograms are generated annually in the production of acetic acid.
On an industrial scale, the reaction of acetic anhydride with hydrogen chloride produces a mixture of acetyl chloride and acetic acid:
(CH3CO)2O + HCl -> CH3COCl + CH3CO2H
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.
Benzoyl chloride is produced from benzotrichloride using either water or benzoic acid:
C6H5CCl3 + H2O -> C6H5COCl + 2 HCl
C6H5CCl3 + C6H5CO2H -> 2 C6H5COCl + HCl
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.
Of commercial interest, acyl chlorides react with HF to give acyl fluorides. Aromatic (as well as aliphatic) acyl fluorides are conveniently prepared directly from carboxylic acids, using stable, inexpensive commodity chemicals: , NBS and in a bench-top protocol. Cyanuric fluoride converts carboxylic acids to acyl fluorides.
Official source
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