pinacols

Pinacols are a class of organic compounds characterized by the presence of two hydroxyl groups attached to adjacent carbon atoms, with each of these carbon atoms also bearing two methyl groups. The general structure is (CH3)2C(OH)C(OH)(CH3)2. The parent compound, pinacol, is 2,3-dimethylbutane-2,3-diol.

Pinacols are typically synthesized through the pinacol coupling reaction, a reductive dimerization of ketones or aldehydes. This reaction is often carried out using magnesium, aluminum, or samarium diiodide as reducing agents. The reaction proceeds via radical intermediates.

The term "pinacol" is also used as a general descriptor for vicinal diols that possess the specific substitution pattern of four methyl groups. While other vicinal diols exist, the term pinacol specifically refers to this highly substituted type.

Pinacols are often crystalline solids at room temperature. They can undergo a characteristic rearrangement reaction known as the pinacol rearrangement. In this acid-catalyzed process, one of the hydroxyl groups is protonated, followed by loss of water to form a carbocation. A methyl group then migrates to the adjacent carbocation, leading to the formation of a more stable carbonyl compound, typically a ketone. This rearrangement is a fundamental transformation in organic synthesis.