Question

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

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

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.

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.

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.

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.

Key concepts

Vicinal Diols

Vicinal diols, also known as glycols, are compounds that have two hydroxyl ( OH) groups attached to adjacent carbon atoms. These compounds are significant in organic chemistry due to their versatility. The proximity of the hydroxyl groups often allows vicinal diols to undergo interesting and useful chemical reactions.

A common method of synthesizing vicinal diols is through the oxidation of alkenes using osmium tetroxide ( OsO_4). This reaction adds hydroxyl groups across the doubly bonded carbons, effectively forming diols. Other methods exist too, allowing for a range of chemical transformations in the synthesis process.

• They are building blocks for complex molecule synthesis.
• They serve as intermediates in various organic reactions.
• They can undergo rearrangement reactions, such as pinacol rearrangement, to form different functional groups.

Vicinal diols are particularly important in processes such as the pinacol rearrangement, where their structure facilitates rearrangement to form ketones or aldehydes.

Carbocation Formation

The formation of a carbocation is a crucial step in many organic reactions, including the pinacol rearrangement. In the context of this reaction, a proton from sulfuric acid first protonates a hydroxyl group of 2,3-dimethyl-2,3-butanediol. This converts the hydroxyl group into water, a much better leaving group, which then departs.

The departure of the leaving group (water) leaves behind a positively charged carbon atom, known as a carbocation. This carbocation is a highly reactive and unstable species that sits at the center of many organic transformations.

• Carbocations are intermediates in the reaction.
• The stability of a carbocation often dictates the pathway and outcome of the reaction process.
• They are electrophilic in nature, meaning they easily attract or combine with nucleophiles.

In the pinacol rearrangement, the carbocation is responsible for setting off the chain of events that leads to the structural change of the molecule, culminating in product formation.

Methyl Shift

A 1,2-methyl shift is a key feature in the pinacol rearrangement. It involves the migration of a methyl group from one carbon atom to an adjacent carbon atom. This shift is usually driven by the need to stabilize a carbocation, as more substituted carbocations are generally more stable.

In our case, after the formation of the carbocation, a methyl group from the neighboring carbon migrates to the positively charged carbon. This migration adjusts the structure to form a more stable carbocation.

• This step is necessary for rearranging the carbon skeleton.
• The shift results in a more stable carbocation because it moves towards a more substituted and thus more stable position.
• The migration facilitates the later steps that result in the formation of pinacolone.

The methyl shift is a fascinating example of how molecules dynamically rearrange themselves to achieve more stable and energetically favorable states.

Dehydration of Glycols

The dehydration of glycols is a process whereby vicinal diols lose water molecules to form more complex structures, often facilitated by acid catalysis. In the pinacol rearrangement, dehydration is initiated by the protonation of a hydroxyl group resulting in the departure of it as a water molecule.

This step is vital as it sets the stage for carbocation formation and the subsequent rearrangement steps. Unlike simple dehydration of alcohols, the dehydration of glycols can lead to various rearranged products due to the presence of two hydroxyl groups.

• Dehydration removes water molecules from the original compound, altering its structure significantly.
• This rearranged form is often a ketone or aldehyde, depending on the molecule structure and conditions.
• Acids, like sulfuric acid, commonly catalyze these dehydration reactions.

Through dehydration, the vicinal diol's dual hydroxyl groups facilitate the creation of new chemical species, making this process highly valuable in complex organic syntheses.