4. Alkanes and Cycloalkanes

Draw the two chair conformations of each compound, and label the substituents as axial and equatorial. In each case, determine which conformation is more stable.

c. cis-1-ethyl-3-methylcyclohexane

d. trans-1-ethyl-3-methylcyclohexane

When a pyranose is in the chair conformation in which the CH2OH group and the C-1 OH group are both in axial positions, the two groups can react to form an acetal. This is called the anhydro form of the sugar (it has 'lost water'). The anhydro form of d-idose is shown here. Explain why about 80% of d-idose exists in the anhydro form in an aqueous solution at 100 °C, but only about 0.1% of d-glucose exists in the anhydro form under the same conditions.

Draw the two chair conformers for each of the following and indicate which conformer is more stable:

d. cis-1,2-diethylcyclohexane

Draw the two chair conformations of each compound, and label the substituents as axial and equatorial. In each case, determine which conformation is more stable.

e. cis-1-ethyl-4-methylcyclohexane

f. trans-1-ethyl-4-methylcyclohexane

Draw the most stable chair conformation for the following trisubstituted cyclohexane.

Draw the most stable conformation of

b. trans-1-tert-butyl-2-methylcyclohexane.

Draw the more stable chair conformer of cis-1-ethyl-2-methylcyclohexane.

Predict which side of the equilibrium is favored for each of the chair–chair interconversions (ring flips) shown.

(a)

Draw the most stable conformation of

c. trans-1-tert-butyl-3-(1,1-dimethylpropyl)cyclohexane.

Draw the two chair conformers for each of the following and indicate which conformer is more stable:

c. trans-1-ethyl-2-isopropylcyclohexane

Draw the two chair conformers for each of the following and indicate which conformer is more stable:

e. cis-1-ethyl-3-isopropylcyclohexane

Draw two chair conformations of the molecules below. Indicate which is most stable.

(a)

For each pair of conformations shown, choose which is most stable. If both are equally stable, then write 'no difference.' [If both conformations have the same number of axial substituents, choose the one with the smallest axial substituents.]

(e)

a. Draw both chair conformations of cis-1,4-dimethylcyclohexane, and determine which conformer is more stable.

b. Repeat for the trans isomer.

c. Predict which isomer (cis or trans) is more stable.