SolutionsDehydration of Vicinal Diols
1. From alkene to ketone
Which carbonyl compound is obtained from 1-methylcyclohexene by epoxidation, acid opening of the epoxide and dehydration of the diol in acid?
trans-1-Methylcyclohexane-1,2-diol is obtained. In acid the tertiary OH is protonated and lost, giving the most stable carbocation. Loss of a proton from the neighboring carbon that bears the other OH gives an enol, in equilibrium with 2-methylcyclohexanone.
2. The pinacol rearrangement
2,3-Dimethyl-2,3-butanediol (pinacol) gives 3,3-dimethyl-2-butanone (pinacolone) in acid, and not an enol of the same chain. Explain.
After loss of water a tertiary carbocation forms next to a carbon bearing an OH. Instead of losing a proton, one of the methyl groups of that carbon migrates to the cationic center: a cation stabilized by the oxygen lone pair (a protonated carbonyl) forms, which loses H+ to give pinacolone.
When the neighboring carbon has no hydrogens, the dehydration of a vicinal diol is accompanied by migration of an alkyl or aryl group (pinacol rearrangement).