2. PINACOL-PINACOLONE REARRANGEMENT
Pinacol-pinacolone rearrangement is a very important process in organic chemistry
for the conversion of 1,2-diol (vicinal diol) into ketone or aldehyde in the presence of
an acid.
This reaction is a result of the work of the German chemist William Rudolph Fittig
who first described it in the year 1860.
3. PINACOL-PINACOLONE REARRANGEMENT
This Rearrangement get its name from classical example of conversion of pinacol to
pinacolone.
In the presence of H2SO4 pinacol give 70% yield of pinacolone.
Pinacol (2,3-dimethyl-2,3-butanediol) is a vicinal diol, which on treatment with H2SO4
produces 3,3-dimethyl-2-butanone, commonly called as pinacolone (methyl-t-butylketone).
Pinacol is a solid organic compound which is white. Pinacolone is a very important ketone. It
has a peppermint like or camphor like odour and appears to be a colourless liquid.
4. CHARACTERISTICS
Anionotropic rearrangement ‘C’ to electron deficient ‘C’ migration.
Electron donating groups attached to migrating group increase the rate of reaction.
Mineral acids like H2SO4, HCl, HBr, etc. are used.
Elimination of water without rearrangement can be achieved under drastic condition (Al2O3, 450°C).
6. MIGRATORY APTITUDE
The group with more electron donation has greater migratory aptitude. The ease of migration of
different groups is in order: H >> Aryl >> Alkyl
7. MIGRATORY APTITUDE
The migratory aptitude of the alkyl group
with a longer chain is greater.
Pr- > Et- > Me
Ethyl migrates faster than methyl because it
give more hyper-conjugating stable
carbocation than methyl.
The migratory aptitude of iso-propyl is
greater than n-propyl.
8. MIGRATORY APTITUDE
As migratory group
migrates with its electron
pair, the more
nucleophilic group might
be expected to migrate.
10. Transmigration: The migratory group attacks from the antiperiplanar (backside) to the leaving group. The
two isomers of 1,2-dimethylcyclohexane-1,2-diol give different products due to different orientations of
methyl and hydroxyl groups.
Cis-1,2-dimethyl-cyclohexane-1,2-diol to 2,2-
dimethylcyclohexanone:
Trans-1,2-dimethyl-cyclohexane-1,2-diol to 1-
acetyl-1-methylcyclopentane (Ring Contraction)
11. SYNTHETIC APPLICATIONS
Synthesis of carbonyl compounds from alkenes:
Isobutylraldehyde may be prepared on a large scale from iso butylene.
12. SYNTHETIC APPLICATIONS
Cyclic ketones from cyclic diols:
It is employed to prepare cyclic ketones which are otherwise very difficult to
synthesize.
Many sterically hindered ketones can be produced by this rearrangement.
14. SEMI PINACOL-PINACOLONE REARRANGEMENT
In semi pinacol-pinacolone rearrangement, one hydroxyl group must be present in the
substrate along with the other good leaving group such as N2, -oTs, -X, etc.
Semi-pinacol rearrrangement is more regioselective than pinacol rearrangement.
15. TOSYL AS LEAVING GROUP
In this rearrangement, a less-hindered hydroxyl group is converted to tosylate group (-
OTs) which easily leaves forming a carbocation.
This carbocation rearranges to give a ketone and ring expansion occurs.
Tosyl is a bulky group and replaces the hydrogen of the hydroxyl group, which is not
sterically hindered.
16. DEAMINATION OF AMINO ALCOHOL (TIFFENEAV-
DEMJANOV REARRANGEMENT)
When amino alcohol is treated
with HNO2 (produced in-situ by
mixing NaNO2 with HCl at low
T), the amino group is converted
into a diazonium group (N2
+).
Nitrogen is the best leaving group
and leaves to form a carbocation
which is then rearranged to give a
ketone.
17. EXAMPLES
Ring Contraction
2-Aminocyclohecanol is converted into
cyclopentanecarbaldehyde in the presence of
nitrous acid.
Ring Expansion
1-(Aminomethyl)cyclohexanol is converted into
cycloheptanone in the presence of nitrous acid.
18. DEHALOGENATION OF HALOALCOHOL
Haloalcohol or hydoxy halide is treated with AgNO3 in an aqueous medium.
AgX is precipitated and settled down leaving behind a carbocation.
Halides e.g. -Cl and -Br are good leaving groups.
19. SYNTHETIC APPLICATIONS
Semi pinacol-pinacolone rearrangement is used in synthesis of 7 and 8 membered rings.
Cyclohexanone can be converted into cycloheptanone in good yield.
