Conformational Analysis of Disubstituted Cyclohexanes

Monosubstituted cyclohexanes always have the substituent in an equatorial position. In disubstituted cyclohexanes, however, the situation is more complex because the steric effects of both substituents must be taken into account. All steric interactions in both possible chair conformations must be analyzed before deciding which conformation is favored. 

Conformations of cis- and trons- ,2-dimethyi yclohexane. In tfie cis isomer (top), the two chair conformations are equal in energy because each has one axial methyl group and one equatorial methyl group. In the trans isomer (bottom), the conformation with both methyl groups equatorial is favored by 11.4 k /mol (2.7 kcal/mol) over the conformation with both methyl groups axial. 

Practice Problem 4.3 Draw the most stable conformation of cis-I-teri-butyl-4-chIorocycIohexane. 

Strategy Draw the possible conformations, and calculate the strain energy in each. 

Remember that equatorial substituents cause less strain than axial substituents. 

WeTl begin with cis- and fran.y-l,4-dimethylcyclohexane. A conventional method to represent cis and trans stereoisomers in cyclic systems nses wedge-and-dash descriptions as shown. 

Wedge-and-dash drawings fail to show conformation, and it s important to remember that the rings of cis- and tra i-l,2-dimethylcyclohexane exist in a chair conformation. This fact must be taken into consideration when evaluating the relative stabilities of the 

Their heats of combustion (Table 3.2) reveal that tra i-l,4-dimethylcyclohexane is 7 kJ/mol (1.6 kcal/mol) more stable than the cis stereoisomer. It is umealistic to believe that van der Waals strain between cis substituents is responsible, because the methyl groups are too far away from each other. To understand why tranx-l,4-dimethylcyclo-hexane is more stable than c/x-l,4-dimethylcyclohexane, we need to examine each stereoisomer in its most stable conformation. 

Orientation of methyl groups in most stable conformation 

The methyl groups are described as cis because both are up relative to the hydrogen present at each carbon. If both methyl groups were down, they would still be cis to each other. Notice that ring flipping does not alter the cis relationship between the methyl groups. Nor does it alter their up-versus-down quality substituents that are up in one conformation remain up in the ring-flipped form. 

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Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

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