Dehydrohalogenation of 2-bromo-2-methylbutane

FIGURE 5 12 The El mechanism for the dehydrohalogenation of 2 bromo 2 methylbutane in ethanol... 

There is a strong similarity between the mechanism shown m Eigure 5 12 and the one shown for alcohol dehydration m Eigure 5 6 The mam difference between the dehy dration of 2 methyl 2 butanol and the dehydrohalogenation of 2 bromo 2 methylbutane IS the source of the carbocation When the alcohol is the substrate it is the correspond mg alkyloxonmm ion that dissociates to form the carbocation The alkyl halide ionizes directly to the carbocation... 

The El Mechanism for Dehydrohalogenation of 2-Bromo-2-methylbutane in Ethanol THE OVERALL REACTION ... 

Dehydrohalogenation of many alkyl halides, however, yields more than one product. For example, dehydrohalogenation of 2-bromo-2-methylbutane can yield two products 2-methyl 2-butene and 2-methyl-1-butene, as shown here by pathways (a) and (b), respectively ... 

Discuss the differences observed in the IR and NMR spectra of 2-bromo-2-methylbutane and of 2-methyl-l-butene and 2-methyl-2-butene that are consistent with dehydrohalogenation occurring in this experiment. 

Dehydrohalogenation may give a mixture of products if the halogen is unsymmetrically located on the carbon skeleton. Eor example, 2-bromo-2-methylbutane (6), the substrate you will use in this experiment, yields both 2-methyl-2-butene (7) and 2-methyl-l-butene (8) on reaction with strong base (Eq. 10.5). Because such elimination reactions are normally irreversible under these experimental conditions, the alkenes 7 and 8 do not undergo equilibration subsequent to their production. Consequently, the ratio of 7 and 8 obtained is defined by the relative rates of their formation. These rates, in turn, are determined by the relative free energies of the two transition states, 9 and 10, respectively, rather than by the relative free energies of the alkenes 7 and 8 themselves. 

List the alkenes that would be formed when each of the following alkyl halides is subjected to dehydrohalogenation with potassium ethoxide in ethanol and use Zaitsev s rule to predict the major product of each reaction (a) 2 bromo-3-methylbutane and (b)2 -bromo-2,3-dimethylbutane. 

Stereoelectronic effects are also important in the dehydrohalogenation of acyclic alkyl halides by an E2 pathway. Again, the most favorable arrangement for the hydrogen and the halide being lost is anti coplanar. In the formation of 2-methyl-2-butene from 2-bromo-2-methylbutane shown on page 208, the elimination of HBr occurs readily from the conformation on the left but not from the one on the right. 

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