Conformational isomers tert-butylcyclohexane

Problem 11.19 Which isomer would you expect to undergo E2 elimination faster, f/uus-l-bromo-4-fr7t-butyJcyclohexane or ds-l-bromo-4-tert-butylcyclohexane Draw each mole-, cule in its more stable chair conformation, and explain your answer. 

The trans and cis isomers of l-bromo-4-tert-butylcyclohexane can be taken as models to estimate the chemical shift of the proton of the CHBr group when it is axial and equatorial, respectively, in the two chair conformations of bromocyclohexane. An axial proton is more shielded (5 3.81 ppm for fran -l-bromo-4-ferf-butylcyclohexane) than an equatorial one (S 4.62 ppm for cis-l-bromo-4-tert-buty Icy clohexane). 

When cfs-l-bromo-4-fert-butylcyclohexane reacts with sodium ethoxide in ethanol, it reacts rapidly to yield 4-tert-butylcyclohexene. Under similar conditions, trans-l-bromo-4-tert-butylcyclohexane reacts very slowly. Using conformational structures, explain the difference in reactivity of these cis-trans isomers. 

TraKS-l,4-di-ferf-butylcyclohexane exists in a normal chair conformation. Cis-l,4-di-tert-butylcyclohexane, however, adopts a twist-boat conformation. Draw both isomers and explain why the cis isomer is more stable in a twist-boat conformation. 

Consider the cis and trans isomers of 1,3-di-tert-butylcyclohexane (build molecular models). What unusual feature accounts for the fact that one of these isomers apparently exists in a twist boat conformation rather than a chair conformation ... 

Thus, cis-l,2-di-t rt-butylcyclohexane exists as two interconverting chair conformations for preference, with one axial and one equatorial substituent The trans-isomer could exist as a chair with two equatorial substituents, but the close proximity of the substituents disfavours this by several kcalmol" compared with the diaxial chair conformation and the most favoured twist conformation, which are of similar energy according to calculations. There is some historical irony here, for the improbability of a chair with an axial tert-h xiy group was the starting assumption for significant early trans-1,3-Di-tm-butylcyclohexane, the object of... 

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