Question

Which of the following structures represents the lowest-energy form of (1S, 2S, 4R)-trimethyl-cyclohexane?

Answer 1

The lowest-energy structure is the chair conformation with all three methyl substituents in equatorial positions. (Assuming the options are labeled, this corresponds to the option showing all equatorial methyl groups.)

Answer 2

For substituted cyclohexanes the lowest‐energy chair conformation is the one in which the bulky substituents occupy the equatorial positions to minimize 1,3‐diaxial interactions. In the case of (1S, 2S, 4R)–trimethylcyclohexane the optimal conformation is one in which the methyl groups at C–1, C–2, and C–4 are all equatorial, even though the stereochemical descriptors force a specific spatial orientation. Thus, among the given options the lowest‐energy structure is the chair conformer having all three methyl substituents in the equatorial positions.

1. In cyclohexane, equatorial substituents are lower in energy.

2. For (1S,2S,4R)–trimethylcyclohexane, the conformation placing all three methyl groups in equatorial positions minimizes steric strain.

3. Hence, the conformer with all methyls equatorial is the most stable.