Guilford Techno Consultants, Inc

Tuesday, May 16, 2023 by Guilford Techno Consultants, Inc. | Mechanism

Acid catalyzed dehydration of alcohols yields alkenes, while acid catalyzed dehydration of vicinal diols (glycols) yields ketones or aldehydes. The mechanism, known as the pinacol rearrangement, involves the protonation of one of the hydroxyl groups, followed by loss of water to produce a carbocation. Rearrangement of the carbocation gives rise to a resonance stabilized cation. Deprotonation of this cation forms the final product. Below is shown the acid catalyzed dehydration of 2,3-dimethyl-2,3-butanediol (pinacol) to 3,3-dimethyl-2-butanone (pinacolone). The mechanism involves the rearrangement of a tertiary carbocation to a resonance stabilized cation through migration of a methyl group. In the case of a symmetrical diol, the same carbocation is generated after loss of water on either hydroxyl bearing carbon. When the diol is not symmetrical, the most stable carbocation is initially formed prior to rearrangement. Rearrangements can involve migration of alkyl groups and hydrides, as well as ring expansion or compression. Practice problems can be found under the OChem I tab. For each reaction, predict the product. For extra practice, propose a mechanism for each reaction.