Q.15 The major product in the following reaction is
Major Product: Option (B)
The reaction involves acid-catalyzed dehydration of 2-methylcyclohexane-1,2-diol using p-TsOH in benzene with heat, proceeding via pinacol rearrangement to form 1-acetyl-1-methylcyclopentane.
Reaction Mechanism
p-TsOH protonates one tertiary OH group of the vicinal diol, facilitating water departure and generating a tertiary carbocation at the substituted carbon. The adjacent ring bond then migrates preferentially over the methyl group due to anti-periplanar alignment and strain relief, forming a more stable five-membered ring carbocation. Deprotonation yields the cyclopentyl ketone.
Option Analysis
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(A) 1-methylcyclohexanol: Secondary alcohol; cannot form from rearrangement as it lacks carbonyl and ring contraction.
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(B) 1-acetyl-1-methylcyclopentane: Matches migration to five-membered ring ketone; favored by tertiary carbocation stability post-ring contraction.
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(C) 2-acetyl-2-methylcyclohexanone: Dicarbonyl structure impossible from single dehydration; excess oxygen atoms.
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(D) 2-hydroxycyclohexyl methyl ketone: Retains OH group; violates complete water loss in rearrangement.
The major product of 2-methylcyclohexane-1,2-diol dehydration with p-TsOH benzene heat is a classic pinacol rearrangement example crucial for CSIR NET Life Sciences and organic chemistry exams. This reaction transforms the vicinal diol into 1-acetyl-1-methylcyclopentane (option B) through carbocation formation and ring contraction.
Pinacol Rearrangement Key Phrase Explanation
In pinacol rearrangement of 2-methylcyclohexane-1,2-diol, protonation targets the tertiary OH, leading to water loss and a tertiary carbocation. The C1-C6 bond migrates over the methyl group, driven by better orbital overlap and five-membered ring stability, yielding the observed ketone.
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Tertiary carbocation forms first for maximum stability.
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Ring contraction preferred in cyclic systems like this diol.
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p-TsOH acts as mild acid catalyst, benzene azeotropes water.
Why Not Other Options?
Simple dehydration to alkene unlikely due to 1,2-diol favoring rearrangement over E1. Option A is unrearranged alcohol, C has invalid structure, D retains OH.
This 2-methylcyclohexane-1,2-diol dehydration p-TsOH benzene heat question tests migratory aptitude and carbocation stability for competitive exams.
