cetic anhydride can be synthesized by heating acetic acid with a strong dehydration agent through nucleophilic acyl substitution. Why is this method not practical for preparation of acetic benzoic anhydride (a mixed anhydride) ?

Mixed products and difficulties in separation.

Explanation:

Acetic anhydride is prepared by reacting acetyl chloride with glacial acetic acid. It is a self sustaining reaction as both the common groups are acetic acid while leaving hydrogen chloride out of the system.

However, a mixed anhydride is very difficult to prepare because there won't be one product. It will contain three different kinds of anhydride. There will be acetic anhydride, benzoic anhydride and acetic-benzoic anhydride. For preparation of any anhydride, a suitable solvent with at least one common group must be present which is more convenient.