Pinacol pinacolone rearrangement involves conversion of 1,2 - diols to carbonyl compounds in presence of acid catalyst with change in carbon skeleton. It is an example of whitmore shift.
2. Introduction:
• Conversion of 1,2- diol ( vicinal diols) into
carbonyl compounds in presence of acid
catalyst with change in carbon skeleton.
• Example of 1,2- shift ( Whitmore shift)
5. Migratory Aptitude:
• The most electron-rich alkyl group (more
substituted carbon) migrates first. The
general migration order:
Tertiary alkyl > cyclohexyl > secondary alkyl > benzyl >
phenyl > primary alkyl > methyl >> H
• For substituted aryls:
p-MeO-Ar > p-Me-Ar > p-Cl-Ar > p-Br-Ar > p-MeOAr > p-O2N-Ar
6. Migratory Aptitude:
• The OH group lost should be the one that leaves behind the
more stable carbocation
Order of migratory aptitude :
Ar > H > CH3
• In case of alkyl group : tert. Alkyl > sec. > pri. > CH3
• In case of aryl group:
Eg. p- Cl Phenyl< Phenyl < p- tolyl < p- methoxy phenyl
EDG EWG
> >
7. Migratory Aptitude:
• In case of electron rich aryl ring, the position of EDG will
decide the migratory aptitude.
para > meta > ortho
• In case of electron deficient ring, the position of EWG will
decide migratory aptitude.
meta preferred
• Reaction occurs in anti manner i.e. migrating group should
be anti to leaving group.
13. Example 3. mechanism
OHOH
H3CO OCH3
H3CO OCH3
H+
OH2
+
OH
H3CO OCH3
H3CO OCH3
C
+
OH
H3CO OCH3
H3CO OCH3
O
OCH3
OCH3
H3CO
OCH3
migration of
p - methoxy phenyl
- H+
14. Example 4.
Of the two OH groups that one departs which leaves behind more stable carbocation.
This factor takes precedence over migratory aptitude.
OH
Me
MePh
Ph
OH
H+
O
Me
Ph
Ph
Me Ph
Me
Me
Ph
O
Formed Not formed
