4. Alkanes and Cycloalkanes

Estimate the equilibrium composition of the chair conformers of the following cyclohexanes at room temp:

cis-1,3-diethylcyclohexane

Estimate the equilibrium composition of the chair conformers of trans-1-methyl-3-phenylcyclohexane at room temperature.

Using the data obtained in Problem 85, calculate the amount of steric strain in each of the chair conformers of 1,1,3-trimethylcyclohexane. Which conformer predominates at equilibrium?

Using the data obtained in Problem 81, calculate the percentage of molecules of trans-1,2-dimethylcyclohexane that will have both methyl groups in equatorial positions.

The A value of a substituent on a cyclohexane ring is essentially the ∆G° for a substituent going from the equatorial to the axial position in a chair–chair interconversion. Because most substituents prefer to be in the equatorial position, A values are, by definition, positive numbers. Use the table of A values to calculate ∆G° and K_eq for the chair–chair interconversions shown.

(b)

The A value of a substituent on a cyclohexane ring is essentially the ∆G° for a substituent going from the equatorial to the axial position in a chair–chair interconversion. Because most substituents prefer to be in the equatorial position, A values are, by definition, positive numbers. Use the table of A values to calculate ∆G° and K_eq for the chair–chair interconversions shown.

(c)

The A value of a substituent on a cyclohexane ring is essentially the ∆G° for a substituent going from the equatorial to the axial position in a chair–chair interconversion. Because most substituents prefer to be in the equatorial position, A values are, by definition, positive numbers. Use the table of A values to calculate ∆G° and K_eq for the chair–chair interconversions shown.

(d)

The ∆G° for conversion of “axial” fluorocyclohexane to “equatorial” fluorocyclohexane at 25 °C is -0.25kcal/mol. Calculate the percentage of fluorocyclohexane molecules that have the fluoro substituent in an equatorial position at equilibrium.

Why is the percentage of molecules with the substituent in an equatorial position greater for isopropylcyclohexane than for fluorocyclohexane?

The A value of a substituent on a cyclohexane ring is essentially the ∆G° for a substituent going from the equatorial to the axial position in a chair–chair interconversion. Because most substituents prefer to be in the equatorial position, A values are, by definition, positive numbers. Use the table of A values to calculate ∆G° and K_eq for the chair–chair interconversions shown.

(a)

Using the data in Table 3.9, calculate the percentage of molecules of cyclohexanol that have the OH group in an equatorial position at 25 °C.

Rank the following compounds in order of their total heat of combustion. These compounds are constitutional isomers, each with a molecular formula of C₆H₁₂.

Which constitutional isomer, A or B, would you expect to have the highest heat of combustion (∆H_combustion)?

Which of the following cycloalkanes would you expect to produce the least heat upon combustion when measured per CH₂? Explain your answer.