Pinacol rearrangement

The Pinacol rearrangement is a chemical reaction in which di-tert- glycols migrate under the action of a protic acid to α - tertiary ketones. The name of the reaction, first described in 1860 comes from the common name of the simplest of these glycol, the pinacol. The reaction was in 1874 by the Russian chemist Aleksandr Mikhailovich Butlerov ( 1828-1896 ) elucidated as rearrangement of the carbon skeleton.

The reaction consists of four steps:

Mechanism of a symmetrical pinacol rearrangement

First, a hydroxyl group is protonated, then one molecule of water is split off, with a tertiary carbonium ion is formed. This is then deposited in the third step of the [1,2 ] shift of a methyl group to the more stable carbenium ion ( carboxonium Ion) to which then leads to the ketone by deprotonation.

Mechanism of an unsymmetrical pinacol rearrangement

With asymmetrical di-tert- glycols under the same acidic conditions as above, the rearrangement is always the more stable carbenium ion. Thus, the methyl group and not the phenyl moiety migrates:

Both intermediate compounds are tertiary carbenium ions. The carbenium ion is left, however, more stable than the right, because in it a resonance stabilization is possible.