Acetic anhydride is based on acetic acid. An anhydride is basically a dimer of the corresponding carboxylic acid. So acetic anhydride is basically two acetic acid molecules condensed together. Its chemical formula is C4H6O3. The structure of acetic anhydride is:
The C(O) group denotes a carbonyl. The reaction to make acetic anhydride is to condense two molecules of acetic acid with the resulting loss of a water molecule:
2 H3CCO2H ---> H3CC(O)OC(O)CH3 + H2O
The OH group of one acetic acid attacks the carbonyl of the other acetic acid. When the intermediate collapses, water is released. The reverse reaction is also possible and acetic anhydride can be hydrolized with water into two acetic acid molecules.