Acid anhydrides are the other acylating agents in AQA's specification. An acid anhydride can be thought of as two carboxylic acid molecules joined together with the loss of a molecule of water. Its general formula is (RCO)2O, and the functional group is RCO-O-COR.They are named after the parent acid: (CH3CO)2O, made from two ethanoic acid units, is ethanoic anhydride, and (CH3CH2CO)2O is propanoic anhydride.Acid anhydrides react with the same nucleophiles as acyl chlorides (water, alcohols, ammonia and primary amines) by nucleophilic addition-elimination, and give the same main organic product. There are two key differences:1. They react less vigorously, so the mixture usually needs warming.
2. The by-product is a carboxylic acid instead of hydrogen chloride. Only half of the anhydride becomes part of the product; the other RCO unit leaves as RCOOH.Worked example: ethanoic anhydride and methanol.(CH3CO)2O + CH3OH → CH3COOCH3 + CH3COOH The products are methyl ethanoate and ethanoic acid. With water, ethanoic anhydride simply gives two molecules of ethanoic acid:(CH3CO)2O + H2O → 2CH3COOH Their slower, more controllable reactions and lack of HCl make anhydrides the preferred choice for some industrial acylations.
2. The by-product is a carboxylic acid instead of hydrogen chloride. Only half of the anhydride becomes part of the product; the other RCO unit leaves as RCOOH.Worked example: ethanoic anhydride and methanol.