What is Pinacol Pinacolone Rearrangement？

Description

Pinacol Pinacolone rearrangement is an essential process in organic chemistry for converting 1,2 diols into carbonyl compounds containing a carbon-oxygen double bond. This is done via a 1,2-migration, which takes place under acyl conditions.

The IUPAC name of Pinacolone is 3,3-dimethyl-2-butanone. Pinacolone is a very important ketone. It has a peppermint or camphor-like odor and is a colorless liquid.

Pinacol is a compound that has two hydroxyl groups, each attached to a vicinal carbon atom. It is a solid organic compound that is white with the following structure. When pinacol is treated with a strong sulfuric acid (H2SO4), a new ketone is formed, and a water molecule is lost from the diol. Upon inspection, it can be seen that the carbon skeleton has undergone rearrangement:

The most important thing to note is that one of the highlighted CH3 groups on C2 has moved to C3. This reaction has become known as the pinacol rearrangement, and it is a relatively common rearrangement of 1,2-diols, first observed as far back as 1860. It was also the first molecular rearrangement identified as such by early chemists.

Mechanism

The Pinacol Pinacolone rearrangement mechanism proceeds via four steps. Each of these steps is explained below.

Step 1: Since the reaction is carried out in an acidic medium, the acid protons the hydroxide group of the pinacol.

Step 2: Water is removed from the compound, leaving behind a carbocation. This carbocation is tertiary and, therefore, stable.

Step 3: The methyl group shifts to the positively charged carbon in a rearrangement of the compound.

Step 4: The oxygen atom, which is doubly bonded to the carbon, is deprotonated, giving rise to the required Pinacolone.

This reaction mechanism can be illustrated as follows:

Thus, the required Pinacolone product is generated. It is important to note that this rearrangement is regioselective. The rearrangement of the more stable carbocation yields the primary product.

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