Compounds bearing two hydroxyl groups are known as diols. When the hydroxyl groups are located on adjacent carbon atoms, the diols are called vicinal diols or glycols. Under acidic conditions, vicinal diols undergo a specific reaction called pinacol rearrangement.
The reaction begins with transferring a proton from the acid catalyst to one of the hydroxyl groups, producing an oxonium ion.
In the second step, the oxonium ion loses H2O, forming a tertiary carbocation intermediate.
In the following step, a methyl group migrates from one carbon to the adjacent carbon, producing a resonance-stabilized cation intermediate where carbon and oxygen have complete octets of valence electrons.
Finally, the subsequent proton transfer from the resonance-stabilized cation intermediate to the solvent water completes the reaction to give pinacolone.
Since the reaction causes an overall change in the carbon backbone, it is termed a rearrangement.
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