2- Methyl-2-butene from 2-bromo-2-methylbutane

To synthesize 2-methyl-2-butene from 2-bromo-2-methylbutane, you would use Sn2/E2 conditions (a high concentration of HO in an aprotic polar solvent) because a tertiary alkyl halide gives only the elimination product under those conditions. If Sn1/E1 conditions were used (a low concentration of HO in water), both elimination and substitution products would be obtained. 

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. 

Write a three-dimensional representation for the transition state structure leading to formation of 2-methyl-2-butene from reaction of 2-bromo-2-methylbutane with sodium ethoxide. 

Some electrophilic addition reactions give products that are clearly not the result of the addition of an electrophile to the sp carbon bonded to the greater number of hydrogens and the addition of a nucleophile to the other sp carbon. For example, the addition of HBr to 3-methyl-1-butene forms 2-bromo-3-methylbutane (minor product) and 2-bromo-2-methylbutane (major product). 2-Bromo-3-methylbutane is the product you would expect from the addition of H to the sp carbon bonded to the greater number of hydrogens and Br to the other sp carbon. 2-Bromo-2-methylbutane is an unexpected product, even though it is the major product of the reaction. 

One way to test for the intermediacy of carbocations in reaction mechanisms is to look for rearrangements, for example, from a 2° carbocation to a 3° carbocation. In the addition of bromine to 3,3-dimethyl-l-butene (7) in methanol, however, the only products observed were l,2-dibromo-3,3-di-methylbutane (8), 45%, and 2-bromo-l-methoxy-3,3-dimethylbutane (9), 44%. There was no evidence for products such as 10, which might have been expected if a free 2° carbocation were formed and then rmderwent a methyl shift to yield a 3° carbocation. Therefore, the intermediate in the addition of bromine to alkyl-substituted alkenes appears not to behave like a carbocation. 

In Chapter 7 we discussed how haloalkanes (or alkyl sulfonates) in the presence of strong base can nndergo elimination of the elements of HX with simultaneons formation of a carbon-carbon donble bond. With many substrates, removal of a hydrogen can take place from more than one carbon atom in a molecule, giving rise to constitutional (donble-bond) isomers. In snch cases, can we control which hydrogen is removed—that is, the regio-selectivity of the reaction (Section 9-9) The answer is yes, to a limited extent. A simple example is the elimination of hydrogen bromide from 2-bromo-2-methylbutane. Reaction with sodinm ethoxide in hot ethanol fnmishes mainly 2-methyl-2-butene, but also some 2-methyl-1 -butene. 

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