Q17-68E - Organic Chemistry (Mcmurry)

Question

2,3-Dimethyl-2,3-butanediol has the common name pinacol. On heating with aqueous acid, pinacol rearranges to pinacolone, 3,3-dimethyl-2-butanone. Suggest a mechanism for this reaction.

Step-by-Step Solution

Answer

The mechanism of 2,3-dimethyl-2,3-butanediol (pinacol) rearranges to 3,3-dimethyl-2-butanone (pinacolone) as;

Step1: Pinacol-pinacolone reaction is the conversion of the 1,2-diol compound to the corresponding carbonyl compound.

Pinacol-pinacolone reaction is the conversion of the 1,2-diol compound to the corresponding carbonyl compound. The following steps are completed by the reaction;              

Protonation of one of the alcohol groups.

Removal of a water molecule (leaving group) to generate a carbocation.

The above-formed carbocation will be rearranged to give a more stable carbocation by migrating a group (in the given case, the methyl group will migrate from the adjacent carbon to form a stable carbocation) which further rearranges to give the corresponding carbonyl compound.

Step2: Mechanism of 2,3-dimethyl-2,3-butanediol (pinacol) rearranges to 3,3-dimethyl-2-butanone (pinacolone)

Consider the mechanism of 2,3-dimethyl-2,3-butanediol (pinacol) rearranges to 3,3-dimethyl-2-butanone (pinacolone) as;