Question

How to convert benzaldehyde to benzyl alcohol in presence of aq. $\mathrm{NaOH}$?

Answer 1

Cannizzaro Reaction:-

• Cannizzaro reaction is a chemical reaction to produce a carboxylic acid and a primary alcohol that involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde.

Mechanism of Cannizzaro Reaction:-

Step 1: Formation of anion

• A nucleophile like a hydroxide ion is used to attack the given aldehyde's carbonyl group, thus causing a disproportionation reaction and forming an anion carrying 2 negative charges:

Step 2: Formation of Carboxylic and Alkoxide anions

• This dianion formed acts as a hydride reducing agent and releases a hydride anion, as it is unstable in nature.
• This hydride anion now proceeds to attack another aldehyde molecule.
• After attacking, the doubly charged anion(dianion) gets converted into a Carboxylate anion and the aldehyde gets converted into an Alkoxide anion

Step 3: Formation of Carboxylic acid and Alcohol

• In this step, the water molecule offers a proton to the Alkoxide anion which produces final alcohol product.
• Since the alkoxide is more basic than water, the reaction proceeds.
• The carboxylate ion is less basic than water and hence cannot obtain proton from water.
• So acid workup is given which produces the final carboxylic acid product.

Conversion of Benzaldehyde to Benzyl alcohol:-

Step 1: Formation of Benzyl alcohol

When Benzaldehyde reacts with aqueous Sodium hydroxide, it undergoes Cannizzaro reaction and forms Benzyl alcohol and Sodium benzoate as its product:

Therefore, Benzaldehyde is converted to Benzyl alcohol in presence of aq. $\mathrm{NaOH}$.