Pinacol Pinacolone Rearrangement

In this article, we have explained Pinacol-Pinacolone Rearrangement including a detailed step by step mechanism, structure, and stereochemistry involved in it.

What is Pinacol-Pinacolone Rearrangement?

When Vicinal–diols (1,2-diols) are treated with acids, they can be rearranged to give carbonyl compounds (aldehydes or ketones). This is known as Pinacol – Pinacolone rearrangement.

Mechanism

The mechanism of Pinacol – Pinacolone rearrangement involves a simple 1, 2 shift.

Step-1: The first step is the protonation of one -OH group and the formation of oxonium ion.

Step-2: In this step, the oxonium ion loses a water molecule and forms a stable carbocation.

Step-3: In this step, the migration of an alkyl group from the adjacent carbon to the carbocation center occurs. The driving force for the migration of the alkyl group comes from the resonance stability of the carbocation.

Step-4: In this step, pinacolone is formed from the conjugate acid of pinacolone by losing a proton.

Important Facts of Pinacol – Pinacolone Rearrangement

1. In the Pinacol – Pinacolone rearrangement, the migrating group may be alkyl, aryl or hydrogen.

2. The formation of intermediacy of carbocation can be proved by the following isotope experiment. When the pinacol is treated with acid in H₂O¹⁸ and the reaction is stopped before completion, the recovered pinacol is found to contain the ¹⁸O isotope. This suggests that pinacol rearrangement involves the reversible formation of a carbocation.

3. When a pinacol with two different groups on Carbinol carbon atom is allowed to react with acid, the two groups may migrate at different rates. The differences are related to the Migratory Aptitudes of these groups. Since rearrangement involves movement of the migrating groups with its bonding electrons to an electron-deficient center, the group which is more electron-rich migrates. A general order of relative migratory aptitudes of some alkyl and aryl groups in pinacol rearrangement is found to be

p-MeOC₆H4- > p-MeC₆H4- > p-PhC₆H4- > m-MeOC₆H4- > C₆H4- > p-ClC₆H4- > o-MeOC₆H4- > H < Me₃C- > Me₂CH- > MeCH₂– > Me-

4. In the case of unsymmetrically substituted pinacol, the carbocation is formed on that carbon atom which is best able to stabilize it. Some examples are given below:

5. Pinacol rearrangement is stereospecific and occurs in an anti manner. An alkyl or an aryl group which is situated trans- to the leaving -OH₂⁺ group may migrate or ring expansion may occur for cyclic products.