Conformations of Cyclohexanes with Two or More Substituents

Conformation of isopropylcyclohexane with the isopropyl group axial 

Conformation of -butylcyclohexane with the r-butyl group axial 

Which of these compounds will have more of the conformation with the substituent on the cyclohexane ring axial present at equilibrium  

Bromine is larger than chlorine, yet the two atoms have identical axial destabilization energies. Explain. 

The presence of two or more substituents on a ring—any size ring—introduces the possibility of stereoisomers. The existence of stereoisomers is independent of conformations and should be analyzed first because different stereoisomers will have different conformations. It is easiest to examine the stereoisomers of cyclic compounds by considering the rings to be flat, even though they may actually exist in chair or other conformations. Once all the stereoisomers have been identified, the conformations of each can be scrutinized. 

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Cyclohexanes with Two or More Substituents Conformations of Cyclohexanes (page 214)... 

Reasonable predictions for the shift values for trans-decalin can be based on the simple substituent effects outlined in Chapter 1 and the stereochemical considerations mentioned above. Thus, the bridgehead carbons would be predicted to absorb at 45.3 (27.3, + 24.0 for three additional P substituents, and -6.0 for a 3 carbon with four p substituents, 43.7 obs.). Similarly, the prediction for C-2 would be 35.3 (27.3 + 8.0), 34.3 observed, and for C-3 would be 27.3 (26.9 obs.) since there are no additional a or P substituents relative to cyclohexane and the y carbons are anti, not gauche. There is a small upfield shift, typically 0.5 for an anti, y carbon, but in most cases this effect is too small to be of significant use in st eochemical predictions. The situation with cis-decalin is more complicated because of the superposition of two equal energy conformations. Nonetheless, the shifts observed are understandable once this factor is combined with stereochemical considerations to be detailed in Chapter 3. 

Because of the difference in stability of the two chair conformers, a sample of methylcyclohexane (or any other monosubstituted cyclohexane) will, at any point in time, contain more chair conformers with the substituent in an equatorial position than with the substituent in an axial position. The relative amounts of the two chair conformers depend on the substituent (Table 3.9). 

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