Pinacol Pinacolone Rearrangement

Pinacol and pinacolone rearrangement was first described by Wilhelm Rudolph Fittig. Although Fittig wrongly assumed the molecular formula of acetone as (C3H3O)n. Then Aleksandr Butlerov correctly identified the reaction products. 

What is Pinacol and Pinacolone?

As the name suggests, pinacol contains alcohol functional groups while pinacolone contains ketone functional groups. Pinacol is a compound which has two hydroxyl groups each attached to an adjacent carbon atom. The IUPAC name of pinacol is 2,3-Dimethylbutane-2,3-diol. Pinacol structure is given below – 

Pinacolone is an organic compound containing ketone functional groups. It is a colorless liquid and has an odor of peppermint or camphor. The IUPAC name of pinacolone is 3,3-Dimethyl-2-butanone. Pinacolone structure is given below – 

Pinacol Pinacolone Rearrangement Reaction 

Conversion of an alcohol having two adjacent alcohol groups (pinacol) to a ketone (pinacolone) by the action of an acid (catalyst) is known as pinacol pinacolone rearrangement reaction. The rearrangement reaction goes back to the observation by Fittig in the year 1859. The 2, 3-dimethyl-2, 3-butanediol reacts within an acid promoting rearrangement. The exact product constituents were not clear till time. Burlerov uncovered the structure of the product as 3,3-dimethyl-2-butanone and referred to it as the result in case of a subtle carbon skeleton rearrangement. The mechanistic context was elaborated for proper studies.

Pinacol Pinacolone Rearrangement Reaction Mechanism 

The Pinacol rearrangement is a circular method and it is used in converting 1,2 diol to a carbonyl compound. The 1, 2 rearrangement always takes place under acidic conditions. This reaction proceeds through the positively charged intermediate in which the methyl group migrates from one carbon atom to another.

First, the hydroxide is protonated in the presence of a carbonation with the removal of water. The tertiary carbocation is more stable. The next is the  rearrangement where the methyl group gets shifted to the positive carbon when there is a removal of the proton by  water and a carbonyl compound is formed.

Supposedly, the procedure of pinacol rearrangement will commence by weighing a 3g of pinacol as the solute and 30ml of deionized water as a solvent. The Pinacol has to be dissolved in deionized water placed in a 50 ml round-bottom flask. It will then be heated gradually to allow the solute to dissolve. The heated solution has to be cooled using the tap water and brought to room temperature and then the boiling chips added to the solution. After that, 0.5 ml of concentrated sulfuric acid has to be measured cautiously, and gradually added to the contents of the flask. The reacting contents of the flask will be mixed together for ten minutes and cooled in the tap water because the reaction produces immense heat. Set up a small distillation apparatus and it is used in distilling the contents in the flask. The distillate needs to be collected in a 25 ml round-bottom flask and the distillation process has to be halted when the temperature of the vapor reaches 100 degrees celsius. 5 ml of saturated sodium chloride is added to the flask to absorb water from the distillate, and take a transfer pipette to take out the aqueous layer. The remaining organic phase is dried by using anhydrous sodium sulfate, which will then be transferred into a pre weighed sample vial and capped for further analysis. The capped liquid will be weighed and the percent yield has to be calculated. After that, qualitative analysis will confirm the formation of pinacol by using the tests of IR, NMR, and 2,4-DNP.

Pinacol rearrangement is an organic reaction and it is characterized by dehydration of alcohol which forms a ketone. This reaction is possible when the concentrated sulfuric acid, heat, and boiling chips catalyze the pinacol rearrangement among 1,2 diols such as glycols. The name of the reaction is always derived from its reactant. Pinacol is an alcohol and it undergoes rearrangement into ketone. This reaction follows the reaction process of SN1 for it leads to the formation of a carbonyl group from a diol. The 2,4-DNP test is used for confirmation of pinacolone. The test helps to detect the presence of aldehydes and ketones in organic compounds. In this case, the test helps to determine if the product is a ketone i.e. pinacolone. After adding a few drops of the reagent to the product it leads to the formation of an organic precipitate that indicates the presence of the carbonyl group. 

The detection of the carbonyl group in the product tells that the pinacol goes under rearrangement to form pinacolone. The 2,4 DNP reagent has a lone pair of electrons. It becomes nucleophilic and attacks the carbonyl groups because they are electrophilic and leads to the formation of a compound 2,4-DNP dinitrophenylhydrazone which is used as an orange precipitate. The pinacol rearrangement and the formation of pinacolone can also be confirmed by using the NMR test.

Analysis of the NMR spectrum of the product gives two peaks which reflect the two different types of protons characterizing pinacolone. One type of protons is the ones on the methyl groups of pinacolone, whereas another type of protons is the ones on tertiary-butyl of pinacolone. The apparent peak of the NMR spectrum shows the protons attached to tertiary-butyl, and thus, greater than other peaks in the spectrum.

The difference in peaks is because tertiary-butyl has 9 protons while the methyl group has 3 hydrogen atoms. Moreover, the spectrum shows that these peaks are singlet for methyl groups and are found near carbonyl groups without protons. The tertiary butyl group has protons that are attached to carbon that is without protons. The same arrangement is seen in the NMR spectrum of pinacol because it shows unique peaks, number of peaks and split. The apparent difference in the NMR spectrum of pinacol gives a peak at 2.1ppm. The nature of the peak gives the down shielded attachment of oxygen atoms on the pinacol. Pinacol Pinacolone rearrangement reaction mechanism can be explained by following four steps which takes place in the reaction- 

Step 1. Protonation of a hydroxyl group 

In this step proton or hydrogen ion coming from acid (H2SO4 breaks into H+ and HSO4- ions) gets attached to the hydroxyl group of pinacol. Reaction is given below – 

Step 2. Loss of water molecule 

In this step water molecules get detached from the compound and give carbocation. Reaction is given below-

Step 3. Methyl migration 

In this step the methyl group migrated from one carbon to another carbon atom. It gets attached to a positively charged carbon atom of carbocation. Reaction is given below – 

Step 4. Deprotonation 

In this step a proton which got attached to pinacol in 1st step gets detached as this proton was generated by an acid catalyst during the reaction. When a proton gets detached from carbocation, the final product pinacolone is formed. Reaction is given as follows – 

Applications of Pinacolone (Product of Pinacol Rearrangement)

Pinacolone is a colorless liquid which has the chemical formula C6H12O. It is the product of pinacol pinacolone rearrangement reaction. It is very useful in production of fungicides, herbicides, pesticides and pharmaceuticals. Few uses of pinacolone are as follows - 

• Pinacolone is used in the analysis of vibunazole. 

• It is used in the synthesis of pinacidil and naminidil.

• It is used in fungicides, herbicides and pesticides. 

• It is used in the production of triadimefon, paclobutrazol, uniconazole and metribuzin. 

• It is useful in the production of pharmaceuticals. 

If you are looking for more name reactions, then you can go through the article ‘Important chemical reactions for Class 12 Chemistry’ available on Vedantu website. You can register yourself on or download the Vedantu app to get free PDFs of NCERT Solutions, revision notes, study material and mock tests.