Question

The mechanism for acidic hydrolysis of a nitrile resembles the basic hydrolysis, except that the nitrile is first protonated, activating it toward attack by a weak nucleophile (water). Under acidic conditions, the proton transfer (tautomerism) involves protonation on nitrogen followed by deprotonation on oxygen. Propose a mechanism for the acid-catalyzed hydrolysis of benzonitrile to benzamide.

Accepted Answer

Step 1: Protonation of the nitrile group - The lone pair of electrons on the nitrogen atom in the nitrile group of benzonitrile interacts with a proton (H⁺) from the acidic medium, resulting in the formation of a protonated nitrile intermediate. This step increases the electrophilicity of the carbon in the nitrile group. Step 2: Nucleophilic attack by water - A water molecule, acting as a weak nucleophile, attacks the electrophilic carbon atom of the protonated nitrile. This leads to the formation of an intermediate that contains a positively charged nitrogen atom and a hydroxyl group attached to the carbon. Step 3: Proton transfer (tautomerism) - The intermediate undergoes a proton transfer. The positively charged nitrogen atom loses a proton (deprotonation), and the hydroxyl group gains a proton (protonation), resulting in the formation of an imidic acid intermediate. Step 4: Hydrolysis of the imidic acid - The imidic acid intermediate undergoes hydrolysis. A water molecule attacks the carbon atom of the imidic acid, leading to the cleavage of the C=N bond and the formation of a tetrahedral intermediate. Step 5: Formation of benzamide - The tetrahedral intermediate undergoes proton transfers and rearrangements, ultimately resulting in the loss of an ammonium ion (NH₄⁺) and the formation of benzamide as the final product.

Verified video answer for a similar problem:

Key Concepts

Acid-Base Chemistry

Nucleophilic Attack

Tautomerism