Methylcyclohexanes, conformational

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 ... 

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

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... 

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

Which of the structures shown for the axial conformation of methylcyclohexane do you r-think IS more stable A or Why" ... 

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... 

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.

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

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). 

Methyl-3-buten-l-ol, NMR spectrum of, 647 Methylcyclohexane, 1,3-diaxial interactions in, 123 conformations of, 123 mass spectrum of, 4H molecular model of, 123, 293... 

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

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