Question 4.3: Drawing the Most Stable Conformation of a Substituted Cycloh...
Drawing the Most Stable Conformation
of a Substituted Cyclohexane
Draw the most stable conformation of cis-1-tert-butyl-4 chlorocyclohexane. By how much is it favored?
Strategy
Draw the possible conformations, and calculate the strain energy in each. Remember that equatorial substituents cause less strain than axial substituents.
First draw the two chair conformations of the molecule:
In the left-hand conformation, the tert-butyl group is equatorial and the chlorine is axial. In the right-hand conformation, the tert-butyl group is axial and the chlorine is equatorial. These conformations aren’t of equal energy because an axial tert-butyl substituent and an axial chloro substituent produce different amounts of steric strain. Table 4.1 shows that the 1,3-diaxial interaction between a hydrogen and a tert-butyl group costs 11.4 kJ/mol (2.7 kcal/mol), whereas the interaction between a hydrogen and a chlorine costs only 1.0 kJ/mol (0.25 kcal/mol). An axial tert-butyl group therefore produces (2 × 11.4 kJ/mol) – (2 × 1.0 kJ/mol) = 20.8 kJ/mol (4.9 kcal/mol) more steric strain than does an axial chlorine, and the compound preferentially adopts the conformation with the chlorine axial and the tert-butyl equatorial.
| TABLE 4.1 | |||
| Steric Strain in Monosubstituted Cyclohexanes | |||
| 1,3-Diaxial strain |  |
||
| Y | (kJ/mol) | (kcal/mol) | |
| F | 0.5 | 0.12 | |
| Cl, Br | 1 | 0.25 | |
| OH | 2.1 | 0.5 | |
| CH_{3} | 3.8 | 0.9 | |
| CH_{2}CH_{3} | 4.0 | 0.95 | |
| CH(CH_{3})_2 | 4.6 | 1.1 | |
| C(CH_{3})_{3} | 11.4 | 2.7 | |
| C_{6}H_{5} | 6.3 | 1.5 | |
| CO_{2}H | 2.9 | 0.7 | |
| CN | 0.4 | 0.1 | |