Question

Benzaldehyde on treatment with concentrated NaOH undergoes Cannizzaro reaction to form:

• A. Benzyl alcohol and sodium benzoate
• B. Benzoic acid and ethanol
• C. Benzyl chloride and sodium benzoate
• D. Benzene and sodium formate

Hint:

To quickly identify if an aldehyde undergoes the Cannizzaro reaction, check for the presence of $\alpha$-hydrogen atoms.
Aldehydes like formaldehyde (HCHO) and benzaldehyde ($\text{C}_6\text{H}_5\text{CHO}$) lack $\alpha$-hydrogen and will undergo Cannizzaro reaction, whereas acetaldehyde ($\text{CH}_3\text{CHO}$) contains $\alpha$-hydrogens and undergoes aldol condensation.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question belongs to the topic "Aldehydes, Ketones and Carboxylic Acids," specifically focusing on the Cannizzaro reaction.
We need to determine the reaction products when benzaldehyde ($\text{C}_6\text{H}_5\text{CHO}$) is treated with a concentrated solution of sodium hydroxide (NaOH).

Step 2: Key Formula or Approach:
The Cannizzaro reaction is a self-redox (disproportionation) reaction of aldehydes that lack an $\alpha$-hydrogen.
In the presence of concentrated alkali, one molecule of the aldehyde undergoes oxidation to form a salt of carboxylic acid, while the second molecule is reduced to form a primary alcohol.

Step 3: Detailed Explanation:

• Benzaldehyde ($\text{C}_6\text{H}_5\text{CHO}$) does not contain any $\alpha$-hydrogen atoms because the carbon adjacent to the carbonyl carbon (the $\alpha$-carbon) is part of the aromatic ring and has no hydrogen attached to it.

• Because of the lack of $\alpha$-hydrogens, it cannot undergo base-catalyzed aldol condensation.

• When treated with a concentrated alkali like NaOH, a nucleophilic attack by a hydroxide ion ($\text{OH}^-$) occurs at the carbonyl carbon of one benzaldehyde molecule.

• This forms a tetrahedral intermediate which then transfers a hydride ion ($\text{H}^-$) to the carbonyl carbon of another benzaldehyde molecule.

• This hydride transfer is the rate-determining step.

• Consequently, the first benzaldehyde molecule is oxidized to benzoic acid, which quickly reacts with the sodium hydroxide to yield its salt, sodium benzoate ($\text{C}_6\text{H}_5\text{COONa}$).

• Meanwhile, the second molecule of benzaldehyde, which accepted the hydride ion, is reduced to benzyl alcohol ($\text{C}_6\text{H}_5\text{CH}_2\text{OH}$).



Step 4: Final Answer:
The products formed in this reaction are benzyl alcohol and sodium benzoate, which corresponds to option (A).