Please Note: The terms "acid halide" and "acyl halide" are synonymous and are both used in this text. In biochemistry, the term "acyl" is used more frequently. Acid Halides An acyl halide (also known as an acid halide ) is a chemical compound derived from a carboxylic acid by replacing a hydroxyl group with a halogen: 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. Acid Halide Synthesis Carboxylic acids react with thionyl chloride (SOCl 2 ) or oxalyl chloride (C 2 O 2 Cl 2 ) to form acid chlorides. Typically the reactions occur in the presence of a proton scavanger like pyridine to minimize unwanted side reactions. During the reaction the hydroxyl group of the carboxylic acid is converted to a chlorosulfite intermediate making it a better leaving group. The chloride anion produced during the reaction acts a nucleophile. Analogous to the reactions of primary and secondary alcohols with PBr 3 to produce the corresponding alkyl bromide, acid bromides can be formed from the reaction of phosphorous tribromide with carboxylic acids. Ester Synthesis from Acyl Chlorides Acid chlorides react with alcohols for form esters are shown in the reaction below. The benefit of using acyl chlorides instead of carboxylic acid is that the reaction becomes irreversible. The synthesis of ethyl benzoate from benzoyl chloride and ethanol is shown as an example. Example: Ester Synthesis from Acid Chlorides Acid Halide Hydrolysis Acid halides are quite reactive and therefore very unstable. They react with water (even moisture) in a hydrolysis reaction as shown below: The hydrolysis of butonyl chloride is shown below as an example. Example: Acyl Chloride Hydrolysis Because of this reaction, it is very important to work with glassware that has been dried, and also solvents that are free from moisture. Volatile acyl halides are lachrymatory 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.