Tert-Butylcyclohexane conformations

A tert butyl group is so large that tert butylcyclohexane exists almost entirely in the conformation m which the tert butyl group is equatorial The amount of axial tert butylcyclohexane present is too small to measure... 

Compare energies for equatorial and axial chair conformers for methylcyclohexane, R = Me, and tert-butylcyclohexane, R = CMe3. Which is more stable in each molecule Use equation (1) to calculate the ratio of major to minor conformers for each system at 298 K. Which molecule shows a larger preference Why (Hint Compare nonbonded interactions and/or geometrical distortions in the higher-energy conformers that are absent in the lower-energy conformers.)... 

Which is more stable, the equatorial or axial chair conformer of i-propylcyclohexane, R=CHMe2 Calculate the ratio of major to minor conformers at 298 K. Is it more like that found for tert-butylcyclohexane or for methylcyclohexane Why ... 

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 conformation of tert-butylcyclohexane with tert-butyl group equatorial is more than 21 kJ mol-1 more stable than the axial form. 

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). 

Allinger and Freiberg s experiments discussed above, shows NMR spectral features which confirm the dominance of non-chair conformations. cis-l,4-Di-tert-butylcyclohexane is concluded from experiment and calculation to exist as a mixture of conformations of which twist ones are more stable, but there is a significant amount of a chair conformation with an axial substituent . 

Wiberg, K. B., Hammer, J. D., Castejon, H., et al. (1999). Conformational Studies in the Cyclohexane Series. 1. Experimental and Computational Investigation of Methyl, Ethyl, Isopropyl, and tert-Butylcyclohexanes. The Journal of Organic Chemistry, 64(6), 2085-2095. 

In cyclohexane derivatives with larger alkyl substituents, the strain caused by 1,3-diaxial interactions is even more pronounced. The conformation of tert-butylcyclohexane with the rm-butyl group equatorial is estimated to be approximately 21 kJ moP more stable than the axial form (Fig. 4.20). This large energy difference between the two conformations means that, at room temperature, 99.99% of the molecules of tert-butylcyclohexane have the r rt-butyl group in the equatorial position. (The molecule is not conformationally locked, however it still flips from one chair conformation to the other.)... 

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. 

Draw Haworth projections for cis-1,3-di-tert-butylcyclo-hexane and trans-1,3-di-tert-butylcyclohexane. One of these compounds exists in a chair conformation, while the other exists primarily in a twist boat conformation. Offer an explanation. 

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 ... 

Open the energy-minimized 3D Molecular Models on the book s website for trani -l-tert-butyl-S-methylcyclohexane and trani-l,3-di-tert-butylcyclohexane. What conformations of cyclohexane do the rings in these two compounds resemble most closely How can you account for the difference in ring conformations between them ... 

Using chair conformational structures (Section 4.11), show the nucleophilic substitution reaction that would take place when trans-l-bromo-d-tert-butylcyclohexane reacts with iodide ion. (Show the most stable conformation of the reactant and the product.)... 

The energy differences between the axial and the equatorial forms of many monosubsti-tnted cyclohexanes have been measured several are given in Table 4-3. In many cases (but not all), particnlarly for alkyl substituents, the energy difference between the two forms increases with the size of the snbstituent, a direct consequence of increasing unfavorable 1,3-diaxial interactions. This effect is particularly pronounced in (l,l-dimethylethyl)cyclohexane (tert-butylcyclohexane). The energy difference here is so large (about 5 kcal moF ) that very little (about 0.01%) of the axial conformer is present at equilibrium. 

On the other hand, for trani-l-bromo-4-tert-butylcyclohexane to assume an anti copla-nar transition state, the molecule must assume a conformation in which the large tert-butyl group is axial ... 

Coleman, Kobylecki, and Utley studied the electrochemical reduction of the conformationally fixed ketones 4-tert-butylcyclohexanone and 3,3,5-tri-methylcyclohexanone 82>. Stereochemically, the cleanest reductions took place at a platinum cathode in a mixture of hexamethylphosphoramide and ethanol containing lithium chloride. Under these conditions the equatorial alcohol predominated heavily (95% from 4-fer/-butylcyclohexane and 91% from 3,3,5-trimethylcyclohexanone).In acidic media roughly equal quantities of axial and equatorial alcohol were produced. It was suggested that organo-lead intermediates are involved in the reductions in aqueous media. This is reasonable, based upon the probable mechanism of reduction in acid 83F Reductions in acid at mercury cathodes in fact do result in the formation of... 

At room temperature, 99.99% of the molecules of /ert-butylcyclohexane have the tert-butyl group in the equatorial position due to the large energy difference between the two conformations. 

Use SpartanBuild to construct models of axial and equatorial conformations of methylcyclohexane and (er/-butylcyclohexane. Minimize each structure, and use the energy differences to predict the relative conformational preferences of methyl and tert-butyl groups. 

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