2- Bromo-2,3-dimethylbutane

The product, 2,3-dimethyl-2-butene, is formed by elimination of HBr from 2-bromo-2,3-dimethylbutane. The product has the more substituted double bond. 

When 3,3-dimethyl-1-butene is treated with HBr alone, the major product is 2-bromo-2,3-dimethylbutane. When the same alkene is treated with HBr and peroxide, the sole product is 1-bromo-3,3-dimethylbutane. Explain these results by referring to the mechanisms. 

Draw a reaction coordinate diagram for the E2 reaction of 2-bromo-2,3-dimethylbutane with sodium tert-butoxide. 

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. 

In Section 10.1, we saw that the difference in the sizes of the methoxide ion and the tert-butoxide ion causes marked differences in the rates of Sj 2 reactions. We noted that steric factors are much less important in determining the ease of abstraction of a proton. However, such effects do exist, and they are in the direction that we would expect. That is, more highly hindered bases tend to abstract the least sterically hindered protons. This fact is established for the reaction of 2-bromo-2,3-dimethylbutane with various alkoxide ions. The major product with methoxide ion is the more highly substituted alkene—the Zaitsev product. However, with rerr-butoxide, the major product is the least substituted alkene. 

The unsaturated compounds obtained in the reaction of 2-bromo-2,3-dimethylbutane with the alkoxide of 3-ethyl-3-pentanol are 92% 2,3-dimethyl-l-butene and 8% 2,3-dimethyl-2-butene. Compare these data with the data for reaction of rcrr-butoxide with the same compound (Section 10.5). 

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