The key reactive intermediate in the acid-catalysed pinacol-pinacolone rearrangement is a

Acarbocation
Bcarbanion
Cfree radical
Dcarbene

Answer: A

Protonation of one OH and loss of water generate a carbocation. A 1,2-shift of an alkyl, aryl or hydride group to this cation centre then gives a protonated carbonyl.

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