Chair conformation of methylcyclohexane

Make a molecular model of each chair conformation of methylcyclohexane and compare their energies... 

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). 

The two chair conformations of methylcyclohexane interconvert at room temperature, so the one that is lower in energy predominates. Careful measurements have shown that the chair with the methyl group in an equatorial position is the most stable conformation. It is about 7.6 kJ/mol (1.8 kcal/mol) lower in energy than the conformation with the methyl group in an axial position. Both of these chair conformations are lower in energy than any boat conformation. We can show how the 7.6 kJ energy difference between the axial and equatorial positions arises by examining molecular models and Newman projections of the two conformations. First, make a model of methylcyclohexane and use it to follow this discussion. 

In Section 2.10, we saw that the gauche interaction between the methyl groups of butane caused the gauche conformer to be 0.9 kcal/mol (3.8 kJ/mol) less stable than the anti conformer. Because there are two such gauche interactions in the chair conformer of methylcyclohexane when the methyl group is in an axial position, this chair conformer is 1.8 kcal/mol (7.5 kJ/mol) less stable than the chair conformer with the methyl group in the equatorial position. 

Two chair conformations of methylcyclohexane.The two axial-axial interactions (steric strain) make conformation (b) less stable than conformation (a) by approximately 7.28 kJ/mol (1.74 kcal/mol). 

Two chair conformations of methylcyclohexane.The steric strain introduced by two diaxial interactions makes the axial methyl conformation less stable by approximately 7.28 kj (1.74 kcal)/mol. 

Identify carbon atoms in the chair conformation of methylcyclohexane that have intramolecular interactions corresponding to those found in the gauche and anti conformations of butane. Which of the chair conformations has the greatest number of gauche interactions How many more If we assume, as in the case for butane, that the anti interaction is 3.8 kJ mol more favorable than gawc/ie, then what is the relative stability of the two chair conformations of methylcyclohexane Hint Identify the relative number of gauche interactions in the two conformations. 

Solution 15 Interconversion of the two chair conformations of methylcyclohexane changes the methyl group fi om an axial to a less crowded equatorial orientation, or the methyl that is equatorial to the more crowded axial position. 

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 ... 

Draw the two chair conformations of cis-l-chloro-2-methylcyclohexane. Which is more stable, and by how much ... 

FIGURE 1.7 The two chair conformations of cyclohexane a = axial hydrogen atom and e = equatorial hydrogen atom. The middle and bottom panels show methylcyclohexane in the chair form with the methyl group equatorial (middle) and axial (bottom). 

A-8. Draw clear depictions of two nonequivalent chair conformations of cis- l-isopropyl-4-methylcyclohexane, and indicate which is more stable. 

Conformations of methylcyclohexane. Plat ring perspective, 0 CHAIR CONFORMATIONS, AND Q NEWMAN PROJECTION. 

The methyl is closer to the top of the page than the H on C-3, and the H is closer to the top of the page than the ethyl on C-l, so the ethyl is trans to the methyl. Thus, this is one chair conformation of trans-1 -ethyl-3-methyIcyclohexane. In the conformation shown, the methyl is equatorial and the ethyl is axial. The ring-flipped conformation, with the methyl axial and the ethyl equatorial, is slightly more stable (by only 0.1 kcal/mol, from Table 6.2). cfr-l-Ethyl-3-methylcyclohexane, the stereoisomer of the compound shown, is more stable because it has a conformation in which both the ethyl and the methyl groups are equatorial. 

A substituent on a cyclohexane ring (in the chair conformation) can occupy either an axial or an equatorial position. In many cases, the reactivity of the substituent depends on whether its position is axial or equatorial. The two possible chair conformations for methylcyclohexane are shown in Figure 3-23. These conformations are in equilibrium because they interconvert at room temperature. The boat (actually the twist boat) serves... 

Chair-chair interconversion of methylcyclohexane. The methyl group is axial in one conformation, and equatorial in the other. 

For testing the ability of the force fields to reproduce the energy difference between an axial and equatorial substituent, methylcyclohexane and aminocyclohex-ane have been chosen as examples. The experimental value for the energy difference between the two chair conformers in methylcyclohexane is 1.75 kcal/mol [45]. All force fields correctly calculate the equatorial conformer to be the most stable one as displayed in Fig. 8. Again, the energy difference is strongly overestimated by CVFF and UFF1.1. 

---