Question

a. Propose a mechanism for the formation of succinic anhydride in the presence of acetic anhydride.

b. How does acetic anhydride make it easier to form the anhydride?

Short answer

Mechanism followed as:

b. Acetic anhydride makes it easier to form anhydride because in the absence of acetic anhydride the leaving group will be hydroxide ion and here, the reaction takes two successive addition elimination reactions.

Step-by-step solution

Formation of succinic anhydride

Succinic acid on reacting with acetic anhydride leads to formation of succinic anhydride and the two equivalent of carboxylic acids followed by the loss of water when heated.

Mechanism

Mechanism followed as:

1. The abstraction of the proton occurs from succinic acid

2. The negative charge oxygen of acid attack the carbonyl carbon of anhydride

3. Following the base reaction abstract the proton

4. Loss of water molecule occur

5. Succinic anhydride formation occurs

Acetic anhydride make it easier to form anhydride

Acetic anhydride makes it easier to form anhydride because in the absence of acetic anhydride the leaving group will be hydroxide ion and here, the reaction takes two successive addition elimination reactions.

Here, the leaving group is the acetate ion which is less basic than the hydroxide ion and the best leaving group.