Pinacolone pinacol rearrangement, mechanism

Pinacol-pinacolone rearrangement 712 Ping-pong mechanism 465,475 rate equation for 466... 

The conversion of pinacol (Expt 5.35) to t-butyl methyl ketone (pinacolone, Expt 5.98) under acid conditions exemplifies a general reaction of 1,2-diols (the pinacol-pinacolone rearrangement). The mechanism, formulated below, involves loss of water from the protonated 1,2-diol accompanied by a 1,2-nucleophilic shift of a methyl group. 

The pinacol-pinacolone rearrangement of 1,2-diols may be interpreted by two possible mechanisms. The concerted mechanism (Scheme 6.15, route a) involves the 49 pentacoordinate intermediate, whereas in the stepwise mechanism (Scheme 6.15, route Z>) the intermediate [i-lix droxy carbenium ion... 

Mechanisms such as the above do not involve or//io-quinonoid or catechol-type intermediates. An alternative suggestion (96) involves ortho-dihy-droxylation (peroxidation) and a pinacol-pinacolone rearrangement ... 

DimethyI-2 3-butanediol has the common name piiuicol. On heating with aqueous acid, pinacol rearranges to pinacolone, 3,3-dimethyl-2-butanone. Suggest a mechanism for this reaction. 

The pinacol rearrangement is a dehydration of an alcohol that results in an unexpected product. When hot sulfuric acid is added to an alcohol, the expected product of dehydration is an alkene. However, if the alcohol is a vicinal diol, the product will be a ketone or aldehyde. The reaction follows the mechanism shown, below. The first hydroxyl group is protonated and removed by the acid to form a carboca-tion in an expected dehydration step. Now, a methyl group may move to fonn an even more stable carbocation. This new carbocation exhibits resonance as shown. Resonance Structure 2 is favored because all tire atoms have an octet of electrons. The water deprotonates Resonance Structure 2, forming pinacolone and regenerating the acid catalyst. 

Problem 28.9 The following reactions have all been found to yield a mixture of pinacol and pinacolone, and in the same proportions treatment of 3-amino-2,3-dimethyl-2-butanol with nitrous acid treatment of 3-chloro-2,3-dimethyl-2-butanol with aqueous silver ion and acid-catalyzed hydrolysis of the epoxide of 2,3-dimethyl-2-butene. What does this finding indicate about the mechanism of the pinacol rearrangement ... 

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. 

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