Question

When a student treated butane dioic acid with thionyl chloride, she was surprised to find that the product she obtained was an anhydride rather than an acyl chloride. Propose a mechanism to explain why she obtained an anhydride.

Short answer

Mechanism followed as:

Step-by-step solution

Reaction of butane dioic acid with thionyl chloride

When butane dioic acid reacts with the thionyl chloride lead to the formation of cyclic anhydride the oxygen of the acid react with the oxygen of the carbonyl of the thionyl chloride to make the better leaving group and cyclic anhydride formed.

Mechanism

Mechanism followed as:

1. Abstraction of the proton of the acid

2. The oxygen on the acid attack on S of thionyl chloride

3. Chloride ion reduced

4. Cyclisation occurred

5. -SOCl is a leaving group so leave

6. Cyclic anhydride formed