pinacol

Pinacol is the common name for 2,3-dimethyl-2,3-butanediol, a vicinal diol in which two tertiary alcohol groups occupy adjacent carbon atoms. The molecule has the formula C6H14O2 and is highly polar. In its most common form it is a meso compound, having two stereocenters of opposite configuration and no net optical rotation.

Pinacol can be prepared by the reductive coupling of acetaldehyde, a reaction known as pinacol coupling. Under reducing conditions the carbonyl groups of two aldehyde molecules are joined to give the vicinal diol pinacol, which can then be isolated and purified. The reaction can also be used to assemble other 1,2-diols bearing alkyl substituents.

One of the best-known transformations of pinacol is the pinacol rearrangement. In strongly acidic conditions, protonation of one hydroxyl group followed by loss of water generates a carbocation that undergoes a 1,2-alkyl shift. The rearrangement yields pinacolone, or 3,3-dimethyl-2-butanone, after deprotonation and tautomerization. The reaction is a classic named reaction illustrating rearrangements of vicinal diols and is widely used to convert diols to ketones.

Beyond its role as a substrate in rearrangements, pinacol serves as a synthetic building block and reference point in studies of vicinal diols, diol protection strategies, and reaction mechanisms.