Conformations of Disubstituted Cyclohexanes

The steric interference between substituents in axial positions is particularly severe when there are large groups on two carbon atoms that bear a 1,3-diaxial relationship (cis on Cl and C3, or Cl and C5), as in the two chair conformations of c/s-l,3-dimethylcyclohexane shown here. The less stable conformation has both methyl groups in axial positions. The more stable conformation has both methyl groups in equatorial positions. Note the strongly unfavorable 1,3-diaxial interaction between the two methyl groups in the diaxial conformation. The molecule can relieve this 1,3-diaxial interference by flipping to the diequatorial conformation. Use your models to compare the diaxial and diequatorial forms of c/s-l,3-dimethylcyclohexane. 

Either of the chair conformations of /rans-l,3-dimethylcycIohexane has one methyl group in an axial position and one in an equatorial position. These conformations have equal energies, and they are present in equal amounts. 

Now we can compare the relative stabilities of the cis and trans isomers of 1,3-dimethylcyclohexane. The most stable conformation of the cis isomer has both methyl groups in equatorial positions. Either conformation of the trans isomer places one methyl group in an axial position. The trans isomer is therefore higher in energy than the cis isomer by about 7.6 kJ/mol (1.8 kcal/mol), the energy difference between axial and equatorial methyl groups. Remember that the cis and trans isomers cannot interconvert, and there is no equilibrium between these isomers. 

If you number the carbons in a cyclohexane, the odd-numbered carbons are similar, as are the even-numbered carbons. If the odd-numbered carbons all have their up bond axial and their down bond equatorial, the even-numbered carbons will all have their down bond axial and their up bond equatorial. For example, cis-1,3 (both up, both odd) will be both axial or both equatorial c/s-1,2 (both up, one odd, one even) will be one axial, one equatorial This tip allows you to predict the answers before you draw them. 

Use your results from Problem 3-25 to complete the following table. Each entry shows the positions of two groups arranged as shown. For example, two groups that are trans on adjacent carbons (/rans-1,2) must be both equatorial (e,e) or both axial (a,a). 

Monosubstituted cyclohexanes are more stable with their substituent in an equatorial position, but the situation in disubstituted cyclohexanes 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. 

Let s look at 1,2-dimethy]cyclohexane as an example. There are two isomers, c/s-l,2-dimethylcyclohexane and trcinsA, 2-dimethylcyclohexane. which 

Interactive to learn to draw and assess the stability of substituted cyclohexanes. 

The same kind of conformational analysis just carried out for cis- and fraus-l,2-dimethylcydohexane can be done for any substituted cyclohexane, such as as-l-tert-butyl-4-chlorocydohexane (see Worked Example 4.3). As you might imagine, though, the situation becomes more complex as the number of 

Thomson fOV Click Organic Interactive to learn to recognize the most stable conformations of cyclohexanes following ring-flips. 

Why do you suppose an axial cyano (-CN) substituent causes practically no 1,3-diaxial steric strain (0.4 kj/mol) Use molecular models to help with your answer. 

FIGURE 4.15 Conformations ofc/s-i,2-dimethylcyclohexane. The two chair conformations are equal in energy because each has one axial methyl group and one equatorial methyl group. 

CHAPTER 4 ORGANIC COMPOUNDS CYCLOALKANES AND THEIR STEREOCHEMISTRY 

FIGURE 4.16 Conformations oftrans-i,2-dimethylcyclohexane. The conformation with both methyl groups equatorial (top) is favored by 11.4 kJ/mol (2.7 kcal/mol) overthe conformation with both methyl groups axial (bottom). 

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Problem-Solving Strategy Drawing Chair Conformations 116 3-14 Conformations of Monosubstituted Cyclohexanes 117 3-15 Conformations of Disubstituted Cyclohexanes 120... 

DRAWING BOTH CHAIR CONFORMATIONS OF DISUBSTITUTED CYCLOHEXANES... 

SkillBuilder 4.12 Drawing Both Chair Conformations of Disubstituted Cyclohexanes... 

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