10.9
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…
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 complete octets of valence electrons.
Finally, the subsequent proton transfer from the resonance-stabilized cation intermediate to the solvent water completes the reaction to give pinacolone.
In the case of asymmetric diols, the hydroxy group that becomes protonated and leaves is the one that gives rise to the more stable carbocation.
View the full transcript and gain access to JoVE Core videos
Q1: What are diols and how are they named?
Diols are organic compounds containing two hydroxy groups. The numbers in their names indicate the positions of these hydroxy groups on the carbon chain, and the name ends with the suffix 'diol'. For example, 2,3-butanediol has hydroxy groups at the second and third carbon positions. Diols are classified by the location of their hydroxy groups, such as vicinal diols where the groups are on adjacent carbons.
Q2: How can diols be prepared from carbonyl compounds?
Diols can be prepared by reducing diketones using hydride reagents such as lithium aluminum hydride or sodium borohydride. These reducing agents convert the carbonyl groups into hydroxy groups. Alternatively, geminal diols (1,1-diols) form through hydration of aldehydes and ketones, though this is a reversible process that typically favors the carbonyl form.
Q3: What is a vicinal diol and how does it form?
Vicinal diols, also called glycols, have two hydroxy groups on adjacent carbon atoms. They form through dihydroxylation of alkenes, where both hydroxy groups add to neighboring carbons. Pinacol (2,3-dimethyl-2,3-butanediol) is a common example of a vicinal diol used in organic synthesis and rearrangement reactions.
Q4: What is the pinacol rearrangement and what does it produce?
The pinacol rearrangement is an acid-catalyzed conversion of vicinal diols into ketones. When pinacol is treated with sulfuric acid, it undergoes rearrangement to form pinacolone, a ketone product. This reaction involves protonation, carbocation formation, methyl group migration, and proton transfer, resulting in an overall change to the carbon backbone structure.
Q5: What are the key steps in the pinacol rearrangement mechanism?
The mechanism begins with protonation of a hydroxy group by the acid catalyst, forming an oxonium ion. This loses water to generate a tertiary carbocation intermediate. A methyl group then migrates to the adjacent carbon, producing a resonance-stabilized cation where both carbon and oxygen have complete octets. Finally, proton transfer to solvent water completes the reaction, yielding pinacolone.
Q6: How does the structure of a diol affect which hydroxy group is lost in pinacol rearrangement?
In asymmetric diols, the hydroxy group that becomes protonated and leaves as water is the one that generates the more stable carbocation intermediate. This selectivity is driven by carbocation stability, ensuring the reaction proceeds through the most favorable pathway. The resulting carbocation stability determines the regioselectivity of the rearrangement.
Q7: Why is the pinacol rearrangement classified as a rearrangement reaction?
The pinacol rearrangement is termed a rearrangement because it causes an overall change in the carbon backbone structure through methyl group migration. Unlike simple elimination or substitution reactions that only remove or replace functional groups, this reaction reorganizes the carbon skeleton itself, fundamentally altering the molecular structure to form a new ketone product.