Question

Which of the following carbonyl compounds will undergo a self-oxidation and self-reduction reaction (Cannizzaro Reaction) when treated with a concentrated solution of a strong base (\( \text{NaOH} \))?

• A. Acetaldehyde (\( \text{CH}_3\text{CHO} \))
• B. Benzaldehyde (\( \text{C}_6\text{H}_5\text{CHO} \))
• C. Acetone (\( \text{CH}_3\text{COCH}_3 \))
• D. Propanal (\( \text{CH}_3\text{CH}_2\text{CHO} \))

Hint:

The two most common examples of aldehydes that lack \(\alpha\)-hydrogens in the NCERT curriculum are Formaldehyde (\(\text{HCHO}\)) and Benzaldehyde (\(\text{C}_6\text{H}_5\text{CHO}\)). Keep these memorized for quick name reaction classification questions.

Answer 1

The Correct Option is B

Concept: The Cannizzaro Reaction is a redox disproportionation reaction characteristic of aldehydes that completely lack any \(\alpha\)-hydrogen atoms. When exposed to a concentrated strong base, one molecule of the aldehyde is reduced to a primary alcohol, while another molecule is oxidized to a carboxylic acid salt.

Step 1: Check for the presence of \(\alpha\)-hydrogens in each option.
An \(\alpha\)-hydrogen is any hydrogen atom attached to the carbon atom immediately adjacent to the carbonyl functional group (\(\text{C}=\text{O}\)).

• Acetaldehyde (\(\text{CH}_3\text{CHO}\)): The methyl carbon has 3 active \(\alpha\)-hydrogens. It undergoes Aldol Condensation instead.
• Acetone (\(\text{CH}_3\text{COCH}_3\)): Contains 6 total \(\alpha\)-hydrogens, making it highly reactive under Aldol pathways.
• Propanal (\(\text{CH}_3\text{CH}_2\text{CHO}\)): The central \(\text{CH}_2\) group contains 2 active \(\alpha\)-hydrogens.

Step 2: Identify the compound lacking \(\alpha\)-hydrogens.
In Benzaldehyde (\(\text{C}_6\text{H}_5\text{CHO}\)), the carbonyl group is attached directly to an aromatic ring carbon that has no remaining hydrogen atoms. Because it completely lacks an \(\alpha\)-hydrogen, it undergoes the Cannizzaro reaction.