Exercise 8: - Conformational analysis and nomenclature of cyclohexanes

1. The axial conformation of fluorocyclohexane is 1.05 kJ/mol less stable than the equatorial conformation. What is the energetic cost of a 1,3-diaxial hydrogen-fluorine interaction?

2. trans-1,3-Di-tert-butylcyclohexane is one of the few molecules that exists largely in a skew-boat conformation. Why?

The chair conformation would require an axial tert-butyl with at least 20 kJ/mol of steric strain. The skew-boat conformation reduces this strain:

3. The following table lists the steric strain due to one hydrogen - substituent 1,3-diaxial interaction in some monosubstituted cyclohexanes:

Substituent	Strain (kJ/mol)	Substituent	Strain (kJ/mol)
Cl	1.09	CH3	3.56
Br	1.15	CH3CH2	3.66
OH	1.96	(CH3)2CH	4,61
CH3O	1.57	(CH3)3C	~10

a. Draw the most stable conformation of trans-1-chloro-3-methylcyclohexane:

b. Justify your decision using the principles of conformational analysis.

Both possible conformations are free of angle strain and of torsional strain. However the steric strain due to two H – Cl 1,3-diaxial interactions is 2.18 kJ/mol whereas that due to two H – CH₃ 1,3-diaxial interactions is 7.12 kJ/mol

c. One of the two chair structures of cis-1-chloro-3-methylcyclohexane is more stable than the other by 15.5 kJ/mol. Draw the structure of the more stable structure.

d. Calculate the energy cost of a 1,3-diaxial interaction between a chlorine and a methyl group.

15,5 kJ/mol = 3,56 + 1,09 + (Cl – CH₃)

Thus the energy cost of a1,3 diaxial interaction between a chlorine and a methyl group is 10,96 kJ/mol.

e. Draw the most stable conformation of cis-1-tert-butyl-4-isopropylcyclohexane.

f. Calculate the strain in this molecule.

4. Name the following compounds :

a.

cis-3-isopropylcyclohexanol

b.

cis-1-ethyl-4-methylcyclohexane

c.

trans-1,3-cyclohexanediol

d.

2-cyclohexenol