Stability of Allylic and Benzylic
Cations



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Chapter 6
An allylic carbon is an
sp3 carbon adjacent to
an sp2 carbon of an
alkene
These carbons can lose
a basic substituent and
become a carbocation in the process they
become sp2 hybridized
Allylic and benzylic
cations have
delocalized electrons
1
Stability of Allylic and Benzylic
Cations
CHR
CHR
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CHR
Chapter 6
CHR
CHR
2
Stability of Allylic and Benzylic
Cations
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Chapter 6
3
Stability of Allylic and Benzylic
Cations

Alkyl substitution at the allylic carbon
stabilizes allylic and benzylic cations
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Chapter 6
4
Stability of Allylic and Benzylic
Radicals

Allylic and Benzylic radicals also are
stabilized by electron delocalization
CHR
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CHR
CHR
Chapter 6
CHR
CHR
5
Chemical Consequences of
Electron Delocalization


Markovnikov’s rule makes no prediction in
this case
The stability of the benzylic cation is such
as to exclude the other possible product
CH2CHCH3
CHCH2CH3
a secondary benzylic cation
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a secondary carbocation
Chapter 6
6
Chemical Consequences of
Electron Delocalization

Rearrangements also are driven by
electron delocalization
The driving force for the
rearrangement is the formation of a
more stable secondary benzylic
cation
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Chapter 6
7
Effect of Delocalization on pKa

Carboxylic acids (RCOOH) are much
more acidic than and alcohols (ROH)
O
CH3COH
CH3CH2OH
pKa = 4.76
pKa = 15.9
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Chapter 6
8
Effect of Delocalization on pKa


Substituents that withdraw
electrons from the oxygen
of the OH group, such as
(C=O), weaken the bond
between O and H, and
The resulting carboxylate
ion is stabilized by
electron delocalization
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Chapter 6
9
Effect of Delocalization on pKa
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Chapter 6
10
Effect of Delocalization on pKa
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Chapter 6
11
Effect of Delocalization on pKa

A protonated aniline is more acidic than a
protonated cyclohexylamine
NH3
NH2
+ H
NH3
NH2
+ H
pKa = 4.6
pKa = 11.2
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Chapter 6
12
Effect of Delocalization on pKa
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Chapter 6
13
Effect of Delocalization on pKa
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Chapter 6
14