Question:
1,2-dimethylbenzene on treatment with chromic oxide in acetic anhydride at 273-283 K followed by hydrolysis produces P. P on heating with concentrated NaOH followed by hydrolysis provides Q. The functional groups present in the product Q are:
1,2-dimethylbenzene on treatment with chromic oxide in acetic anhydride at 273-283 K followed by hydrolysis produces P. P on heating with concentrated NaOH followed by hydrolysis provides Q. The functional groups present in the product Q are:
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An intramolecular Cannizzaro reaction of a dialdehyde (like phthalaldehyde) is an elegant way to simultaneously create an alcohol and a carboxylic acid on the same molecule without needing external oxidizing or reducing agents.
Updated On: Jun 16, 2026
- One carboxylic acid and one alcohol
- Two aldehydes
- One aldehyde and one carboxylic acid
- Two carboxylic acids
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Question:
The question asks us to identify the chemical functional groups present in the final product Q of a two-step reaction starting from 1,2-dimethylbenzene (o-xylene).
Step 2: Key Formula or Approach:
1. Controlled oxidation of methylbenzenes with chromic oxide ($\text{CrO}_3$) in acetic anhydride ($\text{Ac}_2\text{O}$) converts methyl groups into gem-diacetates, which on subsequent acid hydrolysis yield aldehydes.
2. The Cannizzaro reaction is an base-induced disproportionation of aldehydes lacking $\alpha$-hydrogens, yielding an alcohol and a carboxylate salt.
Step 3: Detailed Explanation:
• The starting material is 1,2-dimethylbenzene (o-xylene).
• Treatment of o-xylene with chromic oxide ($\text{CrO}_3$) in acetic anhydride ($\text{Ac}_2\text{O}$) at $273\text{-}283\ \text{K}$ oxidizes both methyl groups into gem-diacetate groups.
• Acid hydrolysis of these gem-diacetate intermediates yields o-phthalaldehyde as product P:
\[ \text{C}_6\text{H}_4\text{(CH}_3)_2 \xrightarrow{\text{CrO}_3, \text{Ac}_2\text{O}} \text{C}_6\text{H}_4[\text{CH(OAc)}_2]_2 \xrightarrow{\text{H}_3\text{O}^+} \text{C}_6\text{H}_4\text{(CHO)}_2 \text{ (o-phthalaldehyde)} \]
• Compound P is a dialdehyde that lacks any $\alpha$-hydrogen atoms relative to either carbonyl group.
• When heated with concentrated sodium hydroxide (NaOH), P undergoes an intramolecular Cannizzaro reaction.
• In this process, one of the aldehyde groups acts as a hydride donor and is oxidized to a sodium carboxylate group ($-\text{COONa}$), while the neighboring aldehyde group acts as a hydride acceptor and is reduced to a primary alcohol group ($-\text{CH}_2\text{OH}$).
• Subsequent acid hydrolysis/protonation converts the carboxylate salt to a free carboxylic acid group ($-\text{COOH}$).
• This produces 2-(hydroxymethyl)benzoic acid, which contains exactly one carboxylic acid group and one primary alcohol group.
Step 4: Final Answer:
Therefore, the functional groups present in the final product Q are one carboxylic acid and one alcohol (Option A).
The question asks us to identify the chemical functional groups present in the final product Q of a two-step reaction starting from 1,2-dimethylbenzene (o-xylene).
Step 2: Key Formula or Approach:
1. Controlled oxidation of methylbenzenes with chromic oxide ($\text{CrO}_3$) in acetic anhydride ($\text{Ac}_2\text{O}$) converts methyl groups into gem-diacetates, which on subsequent acid hydrolysis yield aldehydes.
2. The Cannizzaro reaction is an base-induced disproportionation of aldehydes lacking $\alpha$-hydrogens, yielding an alcohol and a carboxylate salt.
Step 3: Detailed Explanation:
• The starting material is 1,2-dimethylbenzene (o-xylene).
• Treatment of o-xylene with chromic oxide ($\text{CrO}_3$) in acetic anhydride ($\text{Ac}_2\text{O}$) at $273\text{-}283\ \text{K}$ oxidizes both methyl groups into gem-diacetate groups.
• Acid hydrolysis of these gem-diacetate intermediates yields o-phthalaldehyde as product P:
\[ \text{C}_6\text{H}_4\text{(CH}_3)_2 \xrightarrow{\text{CrO}_3, \text{Ac}_2\text{O}} \text{C}_6\text{H}_4[\text{CH(OAc)}_2]_2 \xrightarrow{\text{H}_3\text{O}^+} \text{C}_6\text{H}_4\text{(CHO)}_2 \text{ (o-phthalaldehyde)} \]
• Compound P is a dialdehyde that lacks any $\alpha$-hydrogen atoms relative to either carbonyl group.
• When heated with concentrated sodium hydroxide (NaOH), P undergoes an intramolecular Cannizzaro reaction.
• In this process, one of the aldehyde groups acts as a hydride donor and is oxidized to a sodium carboxylate group ($-\text{COONa}$), while the neighboring aldehyde group acts as a hydride acceptor and is reduced to a primary alcohol group ($-\text{CH}_2\text{OH}$).
• Subsequent acid hydrolysis/protonation converts the carboxylate salt to a free carboxylic acid group ($-\text{COOH}$).
• This produces 2-(hydroxymethyl)benzoic acid, which contains exactly one carboxylic acid group and one primary alcohol group.
Step 4: Final Answer:
Therefore, the functional groups present in the final product Q are one carboxylic acid and one alcohol (Option A).
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