Rearrangement Involving Carbocations

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Lecture 7
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Rearrangement Involving Carbocations
Qn Why is rearrangement quite common to
carbocations, and not in case of radicals or
anions?
Formation of TS involves combination of bonding
orbital (C-R)with the vacant orbital on the
carbocation, singly occupied orbital in case of
radical and doubly occupied orbital in case of
carbanion.This overlap generates one bonding and
one antibonding scenario.
In case of cation, both the electrons occupy the
bonding orbitalhence rearrangement is facile
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Electron Deficient Carbon
Carbene

• Reactions of Carbocations
• Combination with a nucleophile
• Elimination of a proton
• Addition to an unsaturated linkage
• rearrangement

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Qn Why does the methyl migrate to a tertiary
cation? Ans To achieve greater stabilization by
delocalization of charge through electron pair of
oxygen atom.
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For differently substituted pinacols

• How to decide which will be major?
• First decide which carbocation is more stable
• Then consider the relative migratory aptitude of
  the groups that will undergo 1,2-shift.

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Migratory aptitude arylgtalkyl usually
Hgtalkyl Aryl has greater migratory aptitude
because it assists in the departure of leaving
group (anchimeric assistance) Via bridged
carbocation
Electron donating groups at the ortho or para
position will enhance the migratory aptitude
while electron withdrawing groups will lower the
aptitude for migration.
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In case of hydride vs alkyl migration Hydride
generally migrates because of the generation of a
more stable carbocation
Other ways of carrying out pinacol-pinacolone
type rearrangements
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Stereochemistry of migration The migrating group
retains configuration. That means migrating group
is never totally detached
This is also proved by cross over experiments
Home Assignment
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LECTURE 8
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Problems on Pinacol-pinacolone rearrangement
Qn Write down the Fisher projection formula of
the major product
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Migration to electron deficient Nitrogen
Hofman, Curtius, Lossen, Schmidt
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Problem
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Beckman Rearrangement
Anti group always migratesno question of
migratory aptitude!
Mechanism
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Migration to electron Deficient Oxygen Bayer
Villiger Oxidation
Rearrangement Involving Carbanions Favorskii
Rearrangent
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Earlier mechanism
However
Application to acyclic systems
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