3. Mechanism
♦ Mechanism of the pinacol rearrangement of a
symmetrical glycol
CH3 CH3 CH3 CH3 _ OH CH3 CH
H2SO4 2 3
H3C CH3 H3C CH3 H3C +
OH OH OH OH2 OH CH3
+
1. Protonation of 2. Loss of water
pinacol a hydroxyl group
3. Methyl migration
4. Deprotonation
H3C CH3 H3C CH3 H3C CH3
+ CH3
CH3 _ + CH3 ..
O H H O+ H O
CH3 CH3 CH3
pinacolone Resonance-stablized carbocation
4. Facts
♦ 1, 2 shift takes place in carbonium ion
which is already a tertiary carbonium ion
♦ It is probably due to stabilization of
rearranged ion that can be affected by
delocalization involving the electron pair on
the oxygen atom, and ready loss of proton
from the oxygen to produce a stable end
product
5. The stable cation formed superior for
stabilization
Ph2C CH2 H+ Ph C CH2
+
2+ Ph2CH CH
OH OH OH
OH
Ph2CH CH +Ph2CH CH
+OH H
O
8. Facts
♦ It seems that migration of alkyl group is
extremely fast on the loss of leaving group,
taking place simultaneously
♦ That migrating group is from the opposite
side of the leaving group
– Because rearrangement is much more easy in
compounds having trans configuration than that
having cis configuration
9. Pinacol rearrangement of an asymmetrical
glycol
Me Ph Me
Ph O Me
H2SO4
HO OH HO + Ph
Me Ph Me Ph Me Ph
A B C
Me Ph
+ OH
Me Ph
D
Et Ph Et Ph O Et
H2SO4
HO OH HO + _ + Ph
Ph H
Et Ph Et Et Ph
E F
10. Which group migrates?
H+ PhHC CHPh
PhHC CHPh
+
OH OH OH
+
Ph2CH CH Ph2CH CH + Ph2CH CH
+OH H
OH O
Phenyl group move first because of
greater chances of stabilization due to
delocalization
12. ♦ The steric chemistry of Pinacol
Rearrangements
It is found that o-MeOPh migrate
more than a 1000 time less readily
that p-substituent and even much
less than phenyl. This is due to
interference in the transition state
with non-migrating group
13. Solved problem
Explain the following experimental facts ：
R R R
R
R
C R +
H2SO4 O O
OH OH
(A) (B) (C)
When R=CH3, （ B ） and （ C ） are formed ；
When R=Ph, only （ C ） is formed.
14. When R=Me R R
+ C R
C R
+O H 2 OH OH
(1) (2)
R R
+ +
-H C R
R C R
+H+ :O H O
C R (3) (B)
OH OH
R R
(A)
+
C R C R
+ OH2
OH
(4) OH (5)
R + R
-H
+ R R
:O H
(6) O (C)
