Name: pinacol rearrangement of vicinal diols
Uploaded: 2023-04-30T18:28:39.000Z
Duration: 1 min 57 s
Channel: JoVE

Recall that diols are compounds with two hydroxy groups. The numbers before the parent indicate the positions of the hydroxy groups, and its name ends with the suffix diol.
Diols can be prepared by reducing diketones with hydride, lithium aluminum hydride, or sodium borohydride. Alternatively, 1,1-diols, or geminal diols, form through the hydration of aldehydes and ketones, a reversible process. 
If the hydroxy groups in a diol are present on two adjacent carbon atoms, as seen in the dihydroxylation of alkenes, they are called vicinal diols or glycols.
When a vicinal diol like 2,3-dimethyl-2,3-butanediol, or pinacol, is treated with sulfuric acid, it undergoes a rearrangement that leads to a ketone called pinacolone. This acid-catalyzed conversion of pinacol to pinacolone is called pinacol rearrangement.
The reaction begins with transferring a proton from the acid catalyst to one of the hydroxy groups, producing an oxonium ion. Next, the oxonium ion loses H2O, forming a tertiary carbocation intermediate.
In the following step, a methyl group migrates from one carbon to the adjacent carbon, producing a resonance-stabilized cation intermediate where both carbon and oxygen have complet

preparation of diols and pinacol rearrangement

Preparation of Diols and Pinacol Rearrangement

Compounds bearing two hydroxyl groups are known as diols. When the hydroxyl groups are located on adjacent carbon atoms, the diols are called vicinal diols or glycols. Under acidic conditions, vicinal diols undergo a specific reaction called pinacol rearrangement.
The reaction begins with transferring a proton from the acid catalyst to one of the hydroxyl groups, producing an oxonium ion.
<img alt="Organic chemistry acid-base reaction mechanism with equations, showing molecular structure changes." src="/files/ftp_upload/11928/Figure_1-11928.svg" style="text-align: center; height: 75px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
In the second step, the oxonium ion loses H2O, forming a tertiary carbocation intermediate.
<img alt="Organic reaction mechanism, acid-catalyzed dehydration, chemical equation, illustrates water removal." src="/files/ftp_upload/11928/Figure_2-11928.svg" style="text-align: center; height: 75px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
In the following step, a methyl group migrates from one carbon to the adjacent carbon, producing a resonance-stabilized cation intermediate where carbon and oxygen have complete octets of valence electrons.
<img alt="Cyclohexene epoxidation mechanism diagram, showing stepwise reaction and electron movement." src="/files/ftp_upload/11928/Figure_3-11928.svg" style="text-align: center; height: 75px;" data-altupdatedat="1761830439000" data-alt="Figure3" />
Finally, the subsequent proton transfer from the resonance-stabilized cation intermediate to the solvent water completes the reaction to give pinacolone.
<img alt="Organic reaction mechanism, chemical equations diagram, showing protonation of an alcohol group." src="/files/ftp_upload/11928/Figure_4-11928.svg" style="text-align: center; height: 75px;" data-altupdatedat="1761830439000" data-alt="Figure4" />
Since the reaction causes an overall change in the carbon backbone, it is termed a rearrangement.

Compounds bearing two hydroxyl groups are known as diols. When the hydroxyl groups are located on adjacent carbon atoms, the diols are called vicinal ...

preparation-of-diols-and-pinacol-rearrangement

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JoVE Core

Organic Chemistry

Unit 1

Alcohols and Phenols

KEY TERMS AND CONCEPTS

pinacol rearrangement of vicinal diols

Vicinal diols are compounds with two hydroxyl groups on adjacent carbon atoms. They are also called glycols. Under acidic conditions, these diols can undergo a pinacol rearrangement.

The reaction starts when the acid catalyst transfers a proton to one hydroxyl group. This forms an oxonium ion. The oxonium ion then loses water and creates a tertiary carbocation intermediate.

Next, a methyl group migrates from one carbon to the neighboring carbon. This step forms a resonance-stabilized cation intermediate. In that intermediate, both carbon and oxygen have complete octets of valence electrons. A final proton transfer from this cation to solvent water gives pinacolone. Because the carbon backbone changes during the reaction, the process is called a rearrangement.

Pinacol Rearrangement of Vicinal Diols

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Vicinal diols are compounds with two hydroxyl groups on adjacent carbon atoms. They are also called glycols. Under acidic conditions, these diols can undergo a pinacol rearrangement. The reaction starts when the acid catalyst transfers a proton to one hydroxyl group. This forms an oxonium ion. The oxonium ion then loses water and creates a tertiary carbocation intermediate. Next, a methyl group migrates from one carbon to the neighboring carbon. This step forms a resonance-stabilized cation...

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key terms and concepts

Learn how vicinal diols react under acidic conditions through proton transfer, water loss, methyl migration, and formation of pinacolone.

Pinacol Rearrangement of Vicinal Diols | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Pinacol Rearrangement of Vicinal Diols

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