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When a 1,2-diol ‘pinacol’ is treated with acid, a rearrangement takes place. The lone pair on the oxygen atom can stabilize the carbocation. By rearranging, the first-formed carbocation gets the positive charge into a position where the oxygen can stabilize it, and loss of a proton gives a stable ketone.
Z.-L. Song, C.-A. Fan and Y.-Q. Tu, Chem. Rev., 2011, 111, 7523–7556.