Q.10 The migratory aptitude of (alkyl or aryl) substituents in Baeyer-Villiger oxidation is
Baeyer-Villiger Oxidation Migratory Aptitude: CSIR NET Solved Question
The correct answer to this CSIR NET question is option (A) methyl < primary < secondary < tertiary, representing the standard increasing order of migratory aptitude for alkyl substituents in Baeyer-Villiger oxidation.
Reaction Overview
Baeyer-Villiger oxidation converts ketones to esters or lactones using peracids, with oxygen insertion between the carbonyl carbon and the migrating group. The migratory aptitude determines which substituent migrates, following the order that correlates with carbocation-like stability in the transition state.
Option Analysis
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(A) methyl < primary < secondary < tertiary: Correct for alkyl groups, as more substituted alkyls migrate preferentially due to better positive charge stabilization (tertiary > secondary > primary > methyl).
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(B) tertiary < secondary < primary < methyl: Incorrect; reverses the established order, as tertiary groups migrate fastest among alkyls.
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(C) phenyl < methyl < primary < tertiary: Incorrect; places phenyl below methyl, but phenyl typically exceeds primary alkyls and rivals secondary/tertiary depending on conditions.
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(D) tertiary < primary < methyl < phenyl: Incorrect; contradicts alkyl order and overstates phenyl above all alkyls, though phenyl often migrates over primary/methyl.
Migratory Aptitude Factors
The full order is H > tertiary > secondary ≈ phenyl/aryl > primary > methyl, with aryl migration enhanced by electron-donating substituents. Migration retains stereochemistry and aligns anti-periplanar in the Criegee intermediate.
Introduction
Baeyer-Villiger migratory aptitude determines regioselectivity in ketone-to-ester conversions, following tertiary > secondary > primary > methyl for alkyl groups and competing with phenyl/aryl substituents. This guide covers the order, mechanism, and CSIR NET-style questions for exam success.
Mechanism Essentials
Peracid adds to the carbonyl, forming a Criegee intermediate where the higher aptitude group migrates to oxygen, expelling the leaving group. The process is stereospecific with retention.
Detailed Aptitude Order
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Alkyl groups: Tertiary alkyl > secondary alkyl > primary alkyl > methyl, mirroring carbocation stability.
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Aryl involvement: Phenyl ≈ secondary; electron-rich aryls migrate better than primary but below tertiary.
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Exceptions: H migrates best; electron-withdrawing groups reduce aptitude.
| Group Type | Relative Aptitude | Examples |
|---|---|---|
| Tertiary alkyl | Highest | t-butyl > phenyl in many cases |
| Secondary/phenyl | Medium-high | Cyclohexyl ≈ phenyl |
| Primary | Low | n-propyl |
| Methyl | Lowest | CH₃ |
CSIR NET Application
For unsymmetrical ketones like tert-butyl phenyl ketone, tertiary migrates preferentially. Practice predicts products accurately.
