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9.23 The most stable chair conformation of the trans- isomer has both the bromine and the tert-butyl tBu group equatorial. However, elimination cannot Br occur from this conformation because bromine is trans-isomer not axial. Therefore, elimination occurs very slowly for the trans-isomer. For the cis-isomer, on the other hand, the most stable chair conformation is also the conformation Br needed for elimination, with the tert-butyl group equatorial and the bromine axial. The cis-isomer tBu will react much faster than the trans-isomer cis-isomer because the conformation required for elimination is the more stable conformation. 9.24 The cis-isomer has a faster rate of E2 elimination because its more stable chair conformation has the methyl equatorial and chlorine axial. Elimination can occur from this conformer. The more stable conformation of the trans- isomer has both the chlorine and the methyl group equatorial. Elimination cannot occur from this conformation because chlorine is not axial. So the trans-isomer must ring flip to its less stable conformation before it can react. 9.25 The upper route, using the conjugate base of cyclopentanol and methyl iodide, will yield only the substitution product because methyl iodide cannot give an elimination product. In the case of the lower route, with bromocyclopentane (a secondary substrate) and methoxide anion (a strong base), E2 elimination will predominate. Therefore, the upper route will give the higher yield of cyclopentyl methyl ether. 9.26 a) An E2 elimination product can be obtained from a primary alkyl halide using a strong, hindered base. Use potassium tert-butoxide in tert-butanol. b) An SN1 substitution product can be obtained from a tertiary alkyl halide using a weak nucleophile in a polar solvent. Use methanol as both the solvent and the nucleophile. c) An E2 elimination product can be obtained from a tertiary substrate by reaction with a strong base such as sodium methoxide in methanol. The conjugated product is favored. d) An E2 elimination product can be obtained from a secondary substrate by reaction with a strong base such as sodium ethoxide in ethanol. 141