Neighboring dehydrohalogenation

Another route commonly used in the laboratory to produce alkenes is the dehydrohalogenation of haloalkanes, the removal of a hydrogen atom and a halogen atom from neighboring carbon atoms ... 

Anti periplanar geometry for E2 reactions is particularly important in cyclohexane rings, where chair geometry forces a rigid relationship between the substituents on neighboring carbon atoms (Section 4.9). As pointed out by Derek Barton in a landmark 1950 paper, much of the chemical reactivity of substituted cyclohexanes is controlled by their conformation. Let s look at the E2 dehydrohalogenation of chlorocyclohexanes to see an example of such conformational control. 

This elimination reaction is carried out in hot ethanol containing sodium ethoxide, CH3CH20Na. Dehydrohalogenation occurs by attack of the ethoxide ion on a hydrogen atom of the methyl group. A H atom is removed as a proton and CH3CH2O-H is formed. When the methyl C atom forms a second bond to its neighbor, the Br" ion departs. 

The efficiency of these compounds depends also on their chemical structure. For instance, it is easier to break a carbon-halide bond from a ternary carbon than from a primary carbon, but this difference decreases as temperature increases. Furthermore, dehydrochlorination is possible only when the carbon next to the carbon-chlorine bond carries a hydrogen atom. The rate of the dehydrohalogenation is larger when more than one halogen atom is bound to the same carbon but when two neighboring carbons carry one halogen on each of them, the rate of this reaction is lower [33]. 

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