Question

a. Propose a mechanism for the reaction of acetic anhydride with water.

b. How does this mechanism differ from the mechanism for the reaction of acetic anhydride with an alcohol?

Short answer

Mechanism followed as:

Step-by-step solution

Reaction of acidic anhydride with water

Acidic anhydride on reaction with water leads to the formation of two equivalents of carboxylic acids reaction occurs by the incoming nucleophile by losing proton behave has a strong base than the by-product which is carboxylate ion.

Mechanism

Mechanism followed as:

1. Oxygen of water having lone pair attack on the carbonyl carbon and addition of the -HOH group and the electron density gain the carbonyl oxygen having a negative charge.

2. In the presence of base proton is abstracted.

3. The electron comes back from negatively charged oxygen and again carbonyl formation occur along with the breaking of the C-O anhydride bond

4. Two equivalent carboxylate ions are formed

Reaction of acidic anhydride with alcohol

Acetic anhydride on reaction with alcohol leads to the formation of ester and a carboxylic acid whereas in water only two equivalent of a carboxylic acid is formed.

Mechanism

Mechanism followed as:

1. Oxygen of hydroxyl group of water having lone pair attack on the carbonyl carbon and addition to the carbonyl carbon and the electron density gain the carbonyl oxygen having a negative charge.

2. In the presence of base proton is abstracted.

3. The electron comes back from negatively charged oxygen and again carbonyl formation occurs along with the breaking of the C-O anhydride bond

4. Formation of ether along with carboxylate ion is formed