Question:medium
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 (\( \text{NaOH} \))?
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 (\( \text{NaOH} \))?
Show 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.
Updated On: Jun 3, 2026
- Acetaldehyde (\( \text{CH}_3\text{CHO} \))
- Benzaldehyde (\( \text{C}_6\text{H}_5\text{CHO} \))
- Acetone (\( \text{CH}_3\text{COCH}_3 \))
- Propanal (\( \text{CH}_3\text{CH}_2\text{CHO} \))
Show Solution
The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept:
The Cannizzaro reaction is a redox disproportionation reaction where an aldehyde is simultaneously oxidized to a carboxylic acid salt and reduced to a primary alcohol.
This reaction is highly specific and is only shown by aldehydes that lack \(\alpha\)-hydrogen atoms.
If an aldehyde possesses \(\alpha\)-hydrogens, the presence of a strong base like NaOH leads to the deprotonation of the acidic \(\alpha\)-carbon, triggering an Aldol Condensation instead.
Disproportionation implies that two molecules of the same starting material react such that one becomes oxidized and the other becomes reduced.
Step 2: Detailed Explanation:
To determine which compound undergoes the Cannizzaro reaction, we must examine the molecular structure of each option to identify the presence or absence of \(\alpha\)-hydrogens.
An \(\alpha\)-hydrogen is any hydrogen atom attached to the carbon atom immediately adjacent to the carbonyl (\(C=O\)) group.
(A) Acetaldehyde (\(CH_3CHO\)):
The carbon adjacent to the \(CHO\) group is a methyl group (\(CH_3\)). This carbon has three \(\alpha\)-hydrogens. Because of these active hydrogens, acetaldehyde undergoes Aldol Condensation.
(B) Benzaldehyde (\(C_6H_5CHO\)):
In benzaldehyde, the \(CHO\) group is attached directly to the benzene ring. The carbon of the ring that is bonded to the carbonyl group is quaternary (bonded to two ring carbons and the carbonyl carbon), meaning it has zero hydrogen atoms. Since it lacks \(\alpha\)-hydrogens, it undergoes the Cannizzaro reaction.
(C) Acetone (\(CH_3COCH_3\)):
Acetone is a ketone. It has two methyl groups adjacent to the carbonyl, providing a total of six \(\alpha\)-hydrogens. Ketones with \(\alpha\)-hydrogens undergo base-catalyzed reactions like the Aldol, but rarely the Cannizzaro.
(D) Propanal (\(CH_3CH_2CHO\)):
The \(\alpha\)-carbon in propanal is part of a \(CH_2\) group. This means there are two \(\alpha\)-hydrogens available. Like acetaldehyde, propanal will prefer the Aldol pathway.
The mechanism for the Cannizzaro reaction in Benzaldehyde involve:
1. Nucleophilic attack of \(OH^-\) on one aldehyde carbonyl to form a tetrahedral intermediate.
2. Hydride (\(H^-\)) transfer from the anionic intermediate to the carbonyl carbon of a second benzaldehyde molecule.
3. This step produces one molecule of benzoic acid (which immediately reacts with NaOH to form sodium benzoate) and one molecule of benzyl alcohol.
Step 3: Final Answer:
Based on the structural analysis, Benzaldehyde is the only candidate lacking \(\alpha\)-hydrogens.
Therefore, the correct option is (B).
The Cannizzaro reaction is a redox disproportionation reaction where an aldehyde is simultaneously oxidized to a carboxylic acid salt and reduced to a primary alcohol.
This reaction is highly specific and is only shown by aldehydes that lack \(\alpha\)-hydrogen atoms.
If an aldehyde possesses \(\alpha\)-hydrogens, the presence of a strong base like NaOH leads to the deprotonation of the acidic \(\alpha\)-carbon, triggering an Aldol Condensation instead.
Disproportionation implies that two molecules of the same starting material react such that one becomes oxidized and the other becomes reduced.
Step 2: Detailed Explanation:
To determine which compound undergoes the Cannizzaro reaction, we must examine the molecular structure of each option to identify the presence or absence of \(\alpha\)-hydrogens.
An \(\alpha\)-hydrogen is any hydrogen atom attached to the carbon atom immediately adjacent to the carbonyl (\(C=O\)) group.
(A) Acetaldehyde (\(CH_3CHO\)):
The carbon adjacent to the \(CHO\) group is a methyl group (\(CH_3\)). This carbon has three \(\alpha\)-hydrogens. Because of these active hydrogens, acetaldehyde undergoes Aldol Condensation.
(B) Benzaldehyde (\(C_6H_5CHO\)):
In benzaldehyde, the \(CHO\) group is attached directly to the benzene ring. The carbon of the ring that is bonded to the carbonyl group is quaternary (bonded to two ring carbons and the carbonyl carbon), meaning it has zero hydrogen atoms. Since it lacks \(\alpha\)-hydrogens, it undergoes the Cannizzaro reaction.
(C) Acetone (\(CH_3COCH_3\)):
Acetone is a ketone. It has two methyl groups adjacent to the carbonyl, providing a total of six \(\alpha\)-hydrogens. Ketones with \(\alpha\)-hydrogens undergo base-catalyzed reactions like the Aldol, but rarely the Cannizzaro.
(D) Propanal (\(CH_3CH_2CHO\)):
The \(\alpha\)-carbon in propanal is part of a \(CH_2\) group. This means there are two \(\alpha\)-hydrogens available. Like acetaldehyde, propanal will prefer the Aldol pathway.
The mechanism for the Cannizzaro reaction in Benzaldehyde involve:
1. Nucleophilic attack of \(OH^-\) on one aldehyde carbonyl to form a tetrahedral intermediate.
2. Hydride (\(H^-\)) transfer from the anionic intermediate to the carbonyl carbon of a second benzaldehyde molecule.
3. This step produces one molecule of benzoic acid (which immediately reacts with NaOH to form sodium benzoate) and one molecule of benzyl alcohol.
Step 3: Final Answer:
Based on the structural analysis, Benzaldehyde is the only candidate lacking \(\alpha\)-hydrogens.
Therefore, the correct option is (B).
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