An Acid – catalyzed conversion of polysubstituted ethylene glycols to carbonyl compounds as ketones or aldehydes with elimination of water is known as pinacol or pinacol-pinacolone rearrangement. When 1,2 diols such as 2,3 dimethyl 2,3 butenediol called pinacol on treatment with minarel acid (HCl, H2SO4) they undergo dehydration and rearrangement to form ketone commonly known as Pinacolone.
The reaction involves the general characteristics of carbocation rearrangement in which the driving force is the stabilization of the resulting carbocation. The reaction starts with the protonation of the hydroxyl group followed by elimination of water and formation of carbocation.
The carbocation is then stabilized by Whitmore 1, 2-shift. Finally, elimination of a proton from the stable carbocation gives the carbonyl compound.
(i) Anionotropic rearrangement
(ii) Carbon to Carbon 1,2- migration
(iii) Acids used are –Mineral acids
Mechanism : The mechanism involves the loss of water from protonated diol. Then 1,2-nucleophilic shift of a group takes place. In case of unsymmetrically substituted pinacols elimination of that OH group would occur which generates a positive charge on that C which is best able to support it i.e., where more stable carbonium ion is formed. The mechanism are involved are various step as follows:
StepI: Protonation of the 1,2 diols.
StepII: Formation of Carbanium ion by loss of water.
StepIII: Rearrangement of Carbanium ion.
StepIV: Formation of ketone by loss of Proton.
The carbocation though tertiary, prefers to form for its resonance stability. The mechanism is supported by the fact that any carbocation in which the positive charge is on the carbon adjacent to the one bearing the hydroxyl group -C-C-OH also undergoes similar rearrangement. Thus,
The loss of water and migration of the alkyl group may be very rapid or simultaneous. Probably the migrating group does not become completely free before it is partially bonded to the adjacent positively charged carbon, i.e., a type of intramolecular rearrangement is suggested.
1. Synthesis of Carbonyl compounds from alkenes
2. Ring expansion of cyclic ketones
3. Synthesis of ketones from cyclic diols