Pinacol pinacolone rearrangement ppt. MOLECULAR REARRANGEMENT

1. PINACOL-PINACOLONE REARRANGEMENT
Presentation By: Humna Mehmood
BS CHEMISTRY (2017-2021)
GPCSF

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).

5. MECHANISM
The Pinacol Pinacolone
rearrangement mechanism
proceeds via four steps.
 Step 1: Protonation
 Step 2: Dehydration
 Step 3: Rearrangement
 Step 4: Dehydrogenation

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.

9. EXAMPLE:

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.

13. SYNTHETIC APPLICATIONS
Synthesis of Spiro-compounds:
 This rearrangement provides a synthetic route for the synthesis of Spiro-compounds.

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.