Methylcyclohexane conformational isomers

Cis alkenes are less stable than their trans isomers because of steric strain between the two larger substituents on the same side of the double bond. This is the same kind of steric interference that we saw previously in the axial conformation of methylcyclohexane (Section 4.7). 

Figure 4.22. A representation of the two chair conformational isomers of methylcyclohexane (a) methyl group axial (b) methyl group equatorial.

Problem 43. Using Newman-type projections, show the gauche methyl-methylene interactions for the conformational isomer of methylcyclohexane with the methyl group axial. 

The danger of relying too strongly on two-dimensional representations of molecules is shown nicely by methylcyclohexane. Two-dimensional structures give no hint of the richness of the complicated, three-dimensional structure of this molecule they even hide the presence of two conformational isomers of methylcyclohexane (Fig. 5.26). 

FIGURE 5.26 The uninformative two-dimensional structure hides the existence of two conformational isomers of methylcyclohexane. In one isomer, the methyl group is equatorial, and in the other it is axial. 

Table 1.4 is useful for many calculations, but its data caimot be used to estimate the differences between the standard enthalpies of formation of conformational isomers. For example, we would obtain the same enthalpy of formation for the equatorial and axial conformers of methylcyclohexane (3 and 4, respectively) if we were to use mean bond enthalpies. However, it has been observed experimentally that these conformers have different standard enthalpies of formation due to the steric repulsions in the axial conformer, which raise its energy relative to that of the equatorial conformer. 

On each carbon, one bond is directed up or down and the other more or less in the plane of the ring. The up or down bonds are called axial and the others equatorial. The axial bonds point alternately up and down. If a molecule were frozen into a chair form, there would be isomerism in monosubstituted cyclohexanes. For example, there would be an equatorial methylcyclohexane and an axial isomer. However, it has never been possible to isolate isomers of this type at room temperature.219 This proves the transient existence of the boat or twist form, since in order for the two types of methylcyclohexane to be non-separable, there must be rapid interconversion of one chair form to another (in which all axial bonds become equatorial and vice versa) and this is possible only through a boat or twist conformation. Conversion of one chair form to another requires an activation energy of about 10 kcal/mol (42 kJ/mol)220 and is very rapid at room temperature.221 However, by... 

Draw the structural formula of aVl-[(R)-sec-butyl]-2-methylcyclohexane in its lowest energy chair conformation. How many isomers of this compound are there ... 

The cis and trans isomers of 2-bromo-l-methylcyclohexane react at different rates and give different E2 elimination products. Use SpartanBuild to build the two chair conformations for both isomers, and minimize the structures of each. Identify the reactive conformation of each molecule and the expected reaction product from each. Is the reactive conformation the one with lower strain energy Which isomer, cis or trans, will undergo elimination more readily ... 

Often interaction energy values of conformations are reported as potential energies E by assuming that the difference in free energy AG° between isomers is equal to E and that AE may be equated with AH° and AS° 0, which is probably true for methylcyclohexane. 

Conformational relationships for the three cis-trans pairs of dimethyl-cyclohexanes were examined by Beckett et al. (34). Here the situation becomes more involved than for methylcyclohexane because some of the isomers are dl pairs, each of which can exist as a pair of chair conformations. However, the conformational analysis of this system merits attention because it will provide a nearly complete basis for the conformational analysis of the deoalins, methyldecalins, and dimethyl-decalins in the following sections. 

Problem 9.14 (a) Draw the possible chair conformational structures for the following pairs of dimethylcy-clohexanes (i) cis- and trans-1,2- (ii) cis- and trans-1,3- (iii) cis- and trans-1,4-. (b) Compare the stabilities of the more stable conformers for each pair of geometric isomers, (c) Determine which of the isomers of di-methylcyclohexane are chiral. 

Now let s look at the geometric isomers of l-ierr-butyl-3-methylcyclohexane. Both substituents of the cis isomer are in equatorial positions in one conformer and in axial positions in the other conformer. The conformer with both substituents in equatorial positions is more stable. 

The more stable diastereomer in each case is the one having both methyl groups equatorial. The free-energy difference favoring the diequatorial isomer is about the same for each equilibrium (about 1.9 kcal/mol), and is close to that for the conformational equilibrium between equatorial and axial methylcyclohexane (1.8 kcal/mol). This near agreement is reasonable, since the equilibria are, in all cases, established between an isomer having no axial substituents and an isomer with one axial methyl substituent. 

The A value of the benzyl group has been determined from an examination of the low temperature n.m.r. spectrum of cis-l-benzyl-4-methylcyclohexane. At — 97.6°C two doublets were observed for the benzyl methylene protons, with the more abundant isomer (57.4 + 2.7 %) corresponding to the conformation with an equatorial benzyl group, being more stable by 0.11 kcal mol . This led to an A value for the benzyl group of 1.81 kcal mol at — 97.6°C however, the room-temperature A value cannot be derived from this. The A value obtained is similar to that for other —CHjX groups, including ethyl. It was concluded from spectral analysis that cis-l-benzyl-4-methylcyclohexane is distorted with respect to benzylcyclohexane and that the conformation around the benzyl-cyclohexane bond is markedly temperature dependent. 

Conformational equilibrium between the axial (left) and equatorial (right) Isomers of methylcyclohexane. The steric Interactions between the axial methyl and the 1,3-dlaxlal hydrogens are evident In the left ball-and-stick structure. 

Draw the chair conformations of cw-l-chloro-2-methylcyclohexane and its trans isomer. For E2 elimination reactions to occur, there must be a H and X trans diaxial to each other. 

Let s consider methylcyclohexane in a chair conformation with an equatorial methyl group. When the ring flips, the equatorial methyl group moves into an axial position (Figure 4.14). These two structures are different conformations, not structural isomers. A methyl group in an axial position is 8.1 kj mole less stable than a methyl group in an equatorial position. At equilibrium, about 95% of the mixture has an equatorial methyl group. 

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