10.9 : Preparation of Diols and Pinacol Rearrangement

Recall that diols are compounds with two hydroxy groups. The numbers before the parent indicate the positions of the hydroxy groups, and its name ends with the suffix diol.

Diols can be prepared by reducing diketones with hydride, lithium aluminum hydride, or sodium borohydride. Alternatively, 1,1-diols, or geminal diols, form through the hydration of aldehydes and ketones, a reversible process. 

If the hydroxy groups in a diol are present on two adjacent carbon atoms, as seen in the dihydroxylation of alkenes, they are called vicinal diols or glycols.

When a vicinal diol like 2,3-dimethyl-2,3-butanediol, or pinacol, is treated with sulfuric acid, it undergoes a rearrangement that leads to a ketone called pinacolone. This acid-catalyzed conversion of pinacol to pinacolone is called pinacol rearrangement.

The reaction begins with transferring a proton from the acid catalyst to one of the hydroxy groups, producing an oxonium ion. Next, 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 both 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.

In the case of asymmetric diols, the hydroxy group that becomes protonated and leaves is the one that gives rise to the more stable carbocation.