Rearrangements in hydrogen halide addition to alkenes

Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes... 

CARBOCATION REARRANGEMENTS IN HYDROGEN HALIDE ADDITION TO ALKENES... 

This relatively ready rearrangement can be a nuisance in the preparative addition of acids, e.g. hydrogen halides (p. 184) to alkenes, or in their acid-catalysed hydration (p. 187) mixed products that are difficult to separate may result or, in unfavourable cases, practically... 

Our belief that carbocations are intermediates in the addition of hydrogen halides to alkenes is strengthened by the fact that rearrangements 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. 

In general, allenes are prepared by the same types of reactions which lead to alkenes elimination of hydrogen halides, halogens and water from adjacent carbon atoms. However, some methods are characteristic of allene synthesis, e.g. the dehalogenation of gem-dihalocyclopropanes, rearrangement of alkynes and the 1,4-addition to vinylalkynes. 

Although a carbocation is not the only possible intermediate in the addition of hydrogen halides to alkenes, the observation of characteristic carbocation rearrangement products strongly supports the role of a carboca-tion-like species in this reaction under these surface-mediated conditions. Addition of HCI to neat 3,3-dimethyl-l-butene (7) produced 60% of the rearranged product (2-chloro-2,3-dimethylbutane, 33) and about 40% of the imrearranged 2-chloro-33-dimethylbutane (34). Fahey and... 

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