By What is the pinacol–pinacolone rearrangement method?
Pinacol and pinacolone rearrangement method was first propagated by Wilhelm Rudolph Fittig. Fittig wrongly presumed the molecular formula of acetone as Later it was corrected by Aleksandr Butlerov.
What is Pinacol and Pinacolone?
Pinacol has alcohol functional group and pinacolone has ketone functional group. Pinacol is a compound containing two hydroxyl groups. Each of them is attached to a carbon atom. 2,3-Dimethylbutane-2,3-diol is the IUPAC name of pinacol.
Pinacolone is an organic compound which contains ketone functional group. It does have any color and the smell is like that of peppermint or camphor. 3,3-Dimethyl-2-butanone is the IUPAC name of pinacolone.
Pinacol rearrangement is regioselective where the only product is obtained from the rearrangement of the more stable carbocation.
Mechanism of Pinacol Rearrangement
This reaction happens with many different fully substituted 1,2-diols. It forms carbanion ion intermediate which then undergoes a rearrangement.
What is Pinacol Pinacolone Rearrangement Reaction?
When an alcohol, which has two adjacent alcohol groups (pinacol) is converted to a ketone (pinacolone) by the action of an acid (catalyst), it is known as Pinacol Pinacolone rearrangement reaction.
Step 1. Protonation of a hydroxyl group
Here, proton or hydrogen ion which comes out form acid gets attached to hydroxide group of pinacol.
Step2. Loss of water molecule
Here, water molecules get separated from the compound and gives carbocation.
Step 3. Methyl migration
Here, methyl group which gets migrated from one carbon to another carbon atom, gets attached to positively charged carbon atom of carbocation.
Step4. Deprotonation
Here, proton which was attached to pinacol in step 1, now gets detached. Because the proton was formed by acid catalyst during the reaction. The final product that is formed is pinacolone.
Applications of Pinacolone (Product of Pinacol Rearrangement)
Pinacolone is a colorless liquid having the chemical formula It is the product that is formed of pinacol pinacolone rearrangement reaction.
Some of the uses of pinacolone are as follows:
- It is used in the production of fungicides, herbicides, pesticides and pharmaceuticals.
- It is useful in the analysis of vibunazole.
- It is useful in the synthesis of pinacidil and naminidil.
- It is used in the production of fungicides, herbicides and pesticides.
- It is also useful in the production of triadimefon, paclobutrazol, uniconazole and metribuzin.
The pinacol rearrangement was the first and foremost rearrangement which was discovered by the chemists. Pinacol is obtained by the magnesium reduction of acetone by means of a ketyl intermediate. As the diol is symmetrical, there is protonation and loss of water with equal probability at either hydroxyl group. The result which is 3degree-carbcation is steady and it returns to pinacol by reaction which takes place in isotopically labeled water. A 1,2-methyl shift produces a steadier carbocation where the charge is delocalized by heteroatom resonance. This new cation is the product of repeated rearrangements catalyzed by transfer of proton which is the conjugate acid of the ketone pinacolone. The steps involved in this rearrangement can be reversed, as shown by the acid catalyzed dehydration of pinacolone and pinacol to 2,3-dimethyl-1,3-butadiene under various rough conditions.
There are many factors which must be considered while analyzing the course of a pinacol rearrangement which are given below.
- Which hydroxyl group is lost as water and which intermediate carbocation is steadier?
- What is the shifting tendency (migratory aptitude) present in different substituent groups?
- What does steric hindrance and other strain have effects on the rearrangement?
- Does epoxides formed as intermediates in the pinacol rearrangements?
- Does product stability have an influence on the outcome of competing rearrangements?
- Do reaction conditions such as type of acid, concentration, solvent and temperature affect the course of rearrangement?
In the first reaction given below, there is an example of kinetic versus thermodynamic product control. Under conditions of mild acid treatment, the diol rearranges itself to an aldehyde by means of a 1,2-hydrogen shift to diphenyl 3degree -carbonation which was previously formed. Under conditions of vigorous acid treatment, the diol or the aldehyde produces a steadier phenyl ketone.
The second example talks about a similar reacting system giving more information from stereochemical and isotopic labeling feature. There is loss of water from the 3degree carbinol site and then it is followed by a reversible 1,2-hydride shift, which results in the conjugate acid of the ketone product. When there are short reactions, racemization of recovered diol happens at the same pace as rearrangement. The phenyl shift that occurs to the already formed 3degree-carbocation produces the aldehyde conjugate acid, and the aldehyde isomerize to the same rearranged ketone under the conditions of this pinacol rearrangement. An isotopic carbon label (colored green) present in either the diol or aldehyde is moved rapidly (colored brown) in the course of these reactions, pointing to an epoxide intermediate.
For any doubts or queries, refer: alcohols, phenols and ethers from class 12 chemistry – find all the questions & answers