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Chapter 10: Problem 54

Compounds containing hyklroxyl groups on two adjacent carhon atoms are called vicinal diols, or alternatively, glycols. Such compounds can be synthesized by a variety of methods, including oxidation of alkenes by \(\mathrm{OsO}_{4}\) (Section 6..a4). The products of arid-catalyzed dehydration of glycols are quite different from those of acid-ratalyzed dehydration of alcohols. For example, treating 2,3-dimethyl-2,3= butanediol (commonly called pinacol) with concentrated sulfuric acid gives 3,3= dimethyl 2 .butanone (commonly called pinacolone): CC(=O)C(C)(C)P 2,3-Dimethyl-2,3-butanediol 3,3-Dimethy-2-butanone (Pinacol) (Pimacolone)

Short Answer

Expert verified
Question: Explain the step-by-step process of pinacol rearrangement in the conversion of 2,3-dimethyl-2,3-butanediol to 3,3-dimethyl-2-butanone using concentrated sulfuric acid as the catalyst. Answer: The pinacol rearrangement involves the following steps: 1. Protonation of the hydroxyl group by sulfuric acid, enabling it to become a good leaving group. 2. Leaving group departure, forming a carbocation intermediate. 3. 1,2-methyl group migration, leading to a rearrangement of the structure and a new carbocation. 4. Deprotonation of the carbocation, forming the final product, 3,3-dimethyl-2-butanone. The sulfuric acid is regenerated and can be used again for another rearrangement reaction.

Step by step solution

01

Protonation of the hydroxyl group

The first step in pinacol rearrangement is the protonation of one of the hydroxyl groups in 2,3-dimethyl-2,3-butanediol by the sulfuric acid catalyst. This protonation converts the hydroxyl group into a good leaving group, which is a crucial step for the subsequent rearrangement to occur.
02

Formation of carbocation and leaving group departure

In the next step, the protonated hydroxyl group will depart as a water molecule, and a carbocation(developing a positive charge on the adjacent carbon atom) will be formed. This step is crucial in this reaction as it involves the formation of a carbon-centered intermediate, which is a highly reactive species.
03

1,2-methyl group migration

At this stage, migration of the methyl group (from the carbon atom bearing the positive charge) occurs, followed by the transfer of a hydride ion from the adjacent carbon atom to the positively charged carbon atom. This migration results in a rearrangement of the original structure and leads to the formation of a new carbocation. This process is called a 1,2-methyl shift.
04

Deprotonation and formation of ketone

Finally, the carbocation formed in the previous step will be deprotonated by the sulfuric acid catalyst, resulting in the formation of 3,3-dimethyl-2-butanone (pinacolone). The sulfuric acid is regenerated in this step and can be used again for another pinacol rearrangement reaction. In summary, the pinacol rearrangement of 2,3-dimethyl-2,3-butanediol involves an acid-catalyzed process where the hydroxyl group is protonated, a carbocation is formed with a leaving group departure, 1,2-methyl group migration occurs, and carbocation deprotonation results in the formation of 3,3-dimethyl-2-butanone.

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