Acyl Halides react rapidly with water to form carboxylic acids and HCl gas. This reaction is used in tear gas. The release of benzoyl chloride, tear gas, into the air results in a reaction with the moisture in the air to produce benzoic acid and the HCl irritant. Acid Anhydrides Acid anhydrides can be prepared from carboxylic acids by loss of water through heating. Mixed acid anhydrides can be prepared by the reaction of an acyl halide with a carboxylate salt.
Anhydrides undergo nucleophilic acyl substitution reactions with a variety of nucleophiles.
Reaction between acetic anhydride and salicylic acid, which contains an alcohol group, is used to manufacture aspirin, acetylsalicylic acid. This reaction is similar to the aldol condensation reaction of aldehydes and ketones. A strong base extracts the -hydrogen, yielding a resonance stabilized ester enolate ion. The ester enolate ion then can act as a strong nucleophile towards a carbonyl carbon. Attack of the ester enolate ion on the carbonyl carbon of another ester results in the formation of a -keto ester.