Pinacol- Pinacolone RearrangementPinacol

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Pinacol- Pinacolone Rearrangement-

The conversion of 1.2-glycols (pinacols) to ketone or aldehyde by means of acids is known  as Pinacol- Pinacolone Rearrangement.

source: Chemistry@

Mechanism –

This rearrangement is general  acid catalysed reaction for alpha glycols.

source: Chemistry@

In this mechanism migration of alkyl group & elimination of  OH by proton catalyst take place simultaneously .Both the           & rearranged carbonium ions are tertiary.The rearranged cation is  resonance stabilised oxonium ion.Loss of proton from the rearranged ion produces pinacolone.

Features of Pinacol- Pinacolone Rearrangement-

1-Stability of Carbonium ion –

When two -OH groups are different then that -OH group will be removed which produces more stable carbonium ion.

2- Migratory aptitude of the group –

a) The migratory group in the pinacols may be alkyl, aryl or even hydrogen atom . If  hydrogen atom is available as migratory group then it migrates.

b) When each of the carbon atoms of glycol has an aryl and an alkyl group ,then more electron rich nucleophillic aryl group  preferentially migrates.

c) When one carbon atom of glycol possesses two aryl groups while other two alkyl groups, then alkyl group migrates to form more stable  carbonium ion.