Rearrangements in dehydration reactions

Because dehydration of secondary and tertiary alcohols occurs via carbocation intermediates, rearrangement reactions are common. For example, in the dehydration of 3,3-dimethyl-2-butanol, only 3% of the dehydration product maintains the original carbon skeleton. The remaining 97% is a mixture of two isomeric alkenes with a rearranged carbon skeleton. 

The rearrangement reaction is favored because tertiary carbocations are more stable than secondary carbocations. We recall that the same rearrangement occurs in the addition reaction of HCl with 3,3-di-methyl-l-butene, which gives not only the expected product, 2-chloro-3,3-dimethylbutane, but2-chlo-ro-2,3-methylbutane as well (Section 6.4). 

Alkyl groups other than the methyl group can also migrate from one carbon atom to an adjacent carbon atom if the resulting carbocation is more stable. Therefore, mixtures of alkenes result, and the dehydration of alcohols is somewhat limited as a synthetic method to form specific alkenes. 

We recall that the dehydrohalogenation of primary and secondary alkyl halides occurs via an E2 mechanism, and that rearranged products are not obtained. Thus, dehydrohalogenation of an alkyl halide using a strong base is a better synthetic method to form alkenes than the dehydration of an alcohol. 

In Section 6.4, we saw that 1,2 hydtide shifts occur in the carbocations formed in addition reactions. We have also seen that 1,2-hydtide shifts can also occur in carbocations that are generated in dehydration reactions if a more stable carbocation results. Such 1,2-hydtide shifts occur even in the dehydration of primary alcohols. For example, the dehydration of 1-decanol gives 1-decene as a minor product, which may result from an E2 mechanism. However, the major product is largely a mixture of cis- and rw r-2-decenes. This product could result from loss of a proton by an El mechanism from a secondary carbocation formed by a hydride shift of a primary carbocation. 

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