Glycols ketones, pinacol rearrangement

The pinacol rearrangement is believed to involve two important steps (i) loss of water from the protonated glycol to form a carbonium ion and (2) rearrangement of the carbonium ion by a 1,2-shift to yield the protonated ketone. 

Pinacol Rearrangement.12 Certainly one of the best-known examples of a carbonium ion rearrangement is the pinacol transformation in which polysubstituted ethylene glycols are converted into substituted ketones by the action of acidic reagents such as mineral acid, acetyl chloride, or acetic acid and iodine. In accordance with Whitmore s theory of carbonium ion rearrangements,11 the mechanism of the reaction can be outlined as follows, with pinacol itself as an example ... 

Collins (1955), using two different carbon-14 labels, was able to demonstrate the contributions of four reaction paths in the pinacol rearrangements of 1,1,2-triphenylethylene glycol (125) to benzhydryl phenyl ketone (126) and triphenylacetaldehyde (127) and, in addition, to calculate the contribution of each path under several different conditions from his radiochemical experiments. In a separate series of experiments, Collins and co-workers (1959) applied the same techniques to the rearrangement of l,l,2-triphenylethylene-2-d (125a). The ratios kjk and kjkj were then evaluated from which... 

Note two features of fhis reaction (1) it involves dehydration of a glycol to form a ketone, and (2) it involves migration of a methyl group from one carbon to an adjacent carbon. Acid-catalyzed conversion of pinacol to pinacolone is an example of a type of reaction called fhe pinacol rearrangement. We accoimt for the conversion of pinacol fo pinacolone by a four-step mechanism. 

Pinacol Rearrangement (Section 10.7) Dehydration of a glycol involves formation of a carbocation intermediate, rearrangement, and loss of H+ to give an aldehyde or a ketone. 

The pinacol rearrangement is an effective method for rearranging completely alkylated 1,2-glycols. The reaction has been extended to l,r-dihydroxyalkanes to yield spiro ketones. 

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