Equatorial - Axial (larger view)

Axial - Equatorial (larger view)

(How to manipulate JSmol structures)

 

cis-1,4-(1-Bromo-4-isopropylcyclohexane)

Having practiced a number of dimethylcyclohexane structures in the conformational module, how do I handle a chair cyclohexane having different substituents? One cannot readily handle systems with interacting substituents as was done with methyl groups. Cyclohexanes that are 1,4-substituted are a safe bet. The left hand structure has an equatorial substituent and an axial bromine. In this conformer only the axial bromine needs to be considered. In the A-values table, an axial bromine in bromocyclohexane is 0.6 kcal/mol. In the right hand conformer it is the isopropyl group that is axial while the bromine is equatorial. The A-value for the axial isopropyl group in isopropylcyclohexane is 2.1 kcal/mol. Therefore, the difference in energy [Δ(ΔG^o)] is 1.5 kcal/mol. Using ΔG^o = -1.38 log₁₀K_eq at 27^oC, K_eq = 12.3, which is 92% of the axial bromine and 5% of the axial isopropyl conformer.

Had the stereoisomer of cis-1,4-(1-bromo-4-isopropylcyclohexane), namely trans-1,4-(1-bromo-4-isopropylcyclohexane) been considered, then one of the chair conformations would have had two equatorial substituents [ΔG^o = 0 kcal/mol] while the other conformation would have had two axial groups [ΔG^o = 2.1 + 0.6 = 2.7 kcal/mol]. Clearly, the energy difference in this case would have been 2.7 kcal/mol.