Rearrangements in Alcohol Dehydration

Like other El reactions, alcohol dehydration follows an order of reactivity that reflects carbocation stability 3° alcohols react faster than 2° alcohols, and 1° alcohols are the least reactive. Rearrangements of the carbocation intermediates are common in alcohol dehydrations. In most cases, Zaitsev s rule applies The major product is usually the one with the most substituted double bond. 

Our belief that carbocations are intermediates in the addition of hydrogen halides to alkenes is strengthened by the fact that rearrangements of the kind seen in alcohol dehydrations (Section 5.13) sometimes occur. For example, the reaction of hydrogen chloride with 3-methyl-1-butene is expected to produce 2-chloro-3-methylbutane. Instead, a mixture of 2-chloro-3-methylbutane and 2-chloro-2-methylbutane results. 

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

The principal complications in the dehydration of simple alcohols arise through the possibilities of rearrangement and alternative directions for elimination. 

In certain cases, bis(dialkylamino)difluoro-/l4-sulfanes have advantages over (dialkyl-amino)trifluoro-24 -sulfanes, for example, in the lluorination of alcohols which are susceptible to rearrangement and/or dehydration.71 The fluorination of but-2-en-l-ol with DAST in isooctane gives the rearrangement product 4 as the major product, whereas (diethylamino)di-methylaminosulfur difluoride gives the unrearranged product 5 as the major product. 

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. 

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