Pinacol pinacolone rearrangement ppt.

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 H 2 SO 4 pinacol give 70% yield of pinacolone . Pinacol (2,3-dimethyl-2,3-butanediol) is a vicinal diol, which on treatment with H 2 SO 4 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 H 2 SO 4 , HCl , HBr , etc. are used. Elimination of water without rearrangement can be achieved under drastic condition (Al 2 O 3 , 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. T he 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 N 2 , - 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 HNO 2 (produced in-situ by mixing NaNO 2 with HCl at low T), the amino group is converted into a diazonium group (N 2 + ). 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 AgNO 3 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.

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