The functional group of acid anhydrides is -CO-O-CO-. These may be regarded as derived from two molecules of acids with the elimination of a molecule of water.
Preparation.
1. By heating acetyl chloride with anhydrous sodium acetate.
2. By the dehydration of anhydrous acetic acid in presence of P2O5.
Properties.
It is a colourless liquid with an irritating smell. It is less reactive than acetyl chloride (CH3COCI).
1. Acetylation (replacement of hydrogen atom by acetyl group).
(i) \(Hydrolysis \,\,\,\,\underset{ACetic\,anhydride}{(CH_3CO)_2O+HOH}\)
\(\longrightarrow \underset{Acetic\,acid\,(2\,molecules)}{CH_3COOH+CH_3COOH}\)
(ii) \((CH_3CO)_2O +HOC_2H_5 \longrightarrow \underset {Ethyl\,acetate}{CH_3COOC_2H_5}+CH_3COOH\)
(iii) \((CH_3CO)_2O+NH_3 \longrightarrow \underset {Acetamide}{CH_3CONH_2}+CH_3COOH\)
(iv) \((CH_3CO)_2O +H_2NR \longrightarrow CH_3CONHR +CH_3COOH\)
2. Friedel-Craft reaction.
\(\underset{Benzene}{C_6H_6}+(CH_3CO)_2O \xrightarrow {anhy.AlCl_3} \underset {Acetophenone}{C_6H_5.CO.CH_3}+CH_3COOH\)
(3) Reaction with PCl5.
\((CH_3CO)_2O +PCl_5 \longrightarrow \underset{Acetyl\,ehloride}{2CH_3COCl }+POCl_3\)
Uses: Acetic anhydride is used
(i) as an acetylating agent in preference to acetyl chloride because it reacts less violently.
(ii) for the detection of -OH and -NH2 groups,
(iii) in the manufacture of dyes and acetate rayon.
(iv) in the manufacture of aspirin (acetylsalicylic acid).
