Benzonitrile Hydrolysis Mechanism

The acid-catalyzed hydrolysis of benzonitrile involves a sequence of reactions where the nitrile group is converted into a carboxylic acid group in the presence of an acid catalyst and water.

Mechanism Steps Explained

1. Protonation: The reaction initiates with the protonation of the nitrogen atom of the nitrile group ($Ph-C \equiv N$) by the acid catalyst ($H^+$). This step enhances the electrophilicity of the carbon atom.

   $Ph-C \equiv N + H^+ \rightleftharpoons [Ph-C \equiv \stackrel{+}{N}H]$

2. Nucleophilic Attack: A water molecule ($H_2O$), acting as a nucleophile, attacks the activated electrophilic carbon atom of the protonated nitrile.

   $[Ph-C \equiv \stackrel{+}{N}H] + H_2O \rightleftharpoons Ph-C(OH_2^+)=NH$

3. Proton Transfer: Subsequent proton transfers occur within the molecule. This rearrangement leads to the formation of a protonated imidic acid intermediate.

   $Ph-C(OH_2^+)=NH \rightleftharpoons Ph-C(OH)=NH_2^+$

4. Tautomerization: The protonated imidic acid undergoes deprotonation and tautomerization to yield the corresponding amide ($Ph-C(=O)NH_2$).

   $Ph-C(OH)=NH \rightleftharpoons Ph-C(=O)NH_2$

5. Amide Hydrolysis: The amide intermediate is further hydrolyzed under the acidic conditions to produce benzoic acid ($Ph-COOH$) and ammonia ($NH_3$).

Identifying the Correct Pathway

The mechanism requires the initial activation of the nitrile group via protonation, followed by the nucleophilic addition of water. The correct representation shows these fundamental steps, leading towards the formation of the hydrolysis products. Option B accurately illustrates the initial stages of this acid-catalyzed reaction pathway.