Methylcyclohexane equatorial

A monosubstituted cyclohexane, e.g. methylcyclohexane, exists theoretically in two isomeric forms with a chair-form ring, and the methyl substituent either axial or equatorial. Since these rapidly interconverl through a CH3... 

Ring inversion in methylcyclohexane differs from that of cyclohexane m that the two chair conformations are not equivalent In one chair the methyl group is axial m the other It IS equatorial At room temperature approximately 95% of the molecules of methylcyclohexane are m the chair conformation that has an equatorial methyl group whereas only 5% of the molecules have an axial methyl group... 

When two conformations of a molecule are m equilibrium with each other the one with the lower free energy predominates Why is equatorial methylcyclohexane more sta ble than axial methylcyclohexane ... 

We can relate the conformational preference for an equatorial methyl group m methylcyclohexane to the conformation of a noncyclic hydrocarbon we discussed ear her butane The red bonds m the following structural formulas trace paths through four carbons beginning at an equatorial methyl group The zigzag arrangement described by each path mimics the anti conformation of butane... 

Recall from Section 3 10 that the equatorial conformation of methylcyclohexane is 7 kJ/mol (1 7 kcal/mol) lower in energy than the confer mation with an axial methyl group... 

If a disubstituted cyclohexane has two different substituents then the most stable conformation is the chair that has the larger substituent m an equatorial orientation This IS most apparent when one of the substituents is a bulky group such as tert butyl Thus the most stable conformation of cis 1 tert butyl 2 methylcyclohexane has an equatorial tert butyl group and an axial methyl group... 

Substitution on a cyclohexane ring does not greatly affect the rate of conformational inversion but does change the equilibrium distribution between alternative chair forms. All substituents that are axial in one chair conformation become equatorial on ring inversion, and vice versa. For methylcyclohexane, AG for the equilibrium... 

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 case (about 1.9 kcal/mol) and is close to the — A(j value of the methyl group (1.8 kcal/mol). This implies that there are no important interactions present that are not also present in methylcyclohexane. This is reasonable since in each case the axial methyl group interacts only with the 3,5-diaxial hydrogens, just as in methylcyclohexane. 

Other substituted cyclohexanes are similar- to methylcyclohexane. Two chair confonnations exist in rapid equilibrium, and the one in which the substituent is equatorial is more stable. The relative fflnounts of the two confor-rnations depend on the effective size of the substituent. The size of a substituent, in the context of cyclohexane confor-rnations, is related to the degree of branching at the atom connected to the ring. A single... 

Even though the methyl group occupies an equatorial site, the conformation shown is not the most stable one for methylcyclohexane. Explain. 

Methylcyclohexane exists as a mixture of equatorial and axial chair conformers. 

Obtain the energies of the equatorial and axial chair conformers of methylcyclohexane. Which conformer is more stable What would be the composition of a methylcyclohexane sample at 298 K Use equation (1). 

CHa-ring interactions might also be expected to control the conformational preferences of dimethylcylohexanes. It might even be anticipated that the energy differences between a diequatorial conformer and a diaxial conformer will be twice the equatorial-axial energy difference in methylcyclohexane. 

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 ... 

Because chair cyclohexane has two kinds of positions, axial and equatorial, we might expect to find two isomeric forms of a monosubstituted cyclohexane. In fact, we don t. There is only one methylcyclohexane, one bromocydohexane, one cycJohexanol (hydroxycyclohexane), and so on, because cyclohexane rings are confbnnationally mobile at room temperature. Different chair conformations readily interconvert, exchanging axial and equatorial positions. This interconversion, usually called a ring-flip, is shown in Figure 4.11. 

Even though cyclohexane rings rapidly flip between chair conformations at room temperature, the two conformations of a monosubstituted cyclohexane aren t equally stable. In methylcyclohexane, for instance, the equatorial conformation is more stable than the axial conformation by 7.6 kj/mol (1.8 kcal/mol). The same is true of other monosubstituted cyclohexanes a substituent is almost always more stable in an equatorial position than in an axial position. 

Figure 4.13 Interconversion of axial and equatorial methylcyclohexane, as represented in several formats. The equatorial conformation is more stable than the axial conformation by 7.6 kJ/mol.

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 (a) and the others equatorial (e). 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... 

Figure 4.20 (a) The conformations of methylcyclohexane with the methyl group axial (1) and and equatorial (2). (b) 1,3-Diaxial interactions between... 

The conformation of methylcyclohexane with an equatorial methyl group is... 

In the equilibrium mixture, the conformation of methylcyclohexane with an equatorial methyl group is the predominant one (-95%). 

It is interesting that the difference between axial and equatorial methyl chemical shifts observed in methylcyclohexanes (100,101) is considerably diminished if one 3-methylene group is replaced by sulfur (54) in 2-methyl-1,3-dithiane (55) (two 3-methylene groups replaced by S) this difference is even inverted (165,166) (cf. Table 11). This is another example of specific effects in thianes and dithianes (167,168). 

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