The Pinacol-Pinacolone rearrangement is an organic reaction that involves the conversion of a 1,2-diol pinacol to a ketone pinacolone through the migration of an alkyl or aryl group. This reaction is an example of a 1,2-rearrangement and follows a concerted mechanism. Here's a step-by-step description of the mechanism:1. Protonation: The reaction starts with the protonation of one of the hydroxyl groups of the 1,2-diol pinacol by a strong acid, such as sulfuric acid H2SO4 or hydrochloric acid HCl . This step generates an oxonium ion, which is a highly reactive intermediate.2. Rearrangement: The oxonium ion undergoes a 1,2-alkyl or aryl migration, in which one of the alkyl or aryl groups adjacent to the positively charged oxygen atom migrates to the oxygen, simultaneously breaking the C-O bond and forming a new C-C bond. This migration results in the formation of a carbocation intermediate.3. Deprotonation: The carbocation intermediate is then deprotonated by a base, such as water or the conjugate base of the acid used in the first step. This deprotonation step generates the final product, a ketone pinacolone .The Pinacol-Pinacolone rearrangement is a useful reaction in organic synthesis for the preparation of ketones from 1,2-diols. The reaction is stereospecific, and the stereochemistry of the migrating group is retained during the rearrangement.