See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The stability of cyclohexane conformers is determined by the proportion of time the substituents can occupy equatorial positions, which minimizes 1,3-diaxial interactions. Step 1 - Identify the substitution pattern. We have 1-ethyl-3-methylcyclohexane, meaning the two substituents are at C1 and C3, which are in a 1,3-relationship (both on the same side of the ring in terms of numbering). Step 2 - Analyze the cis isomer (A). In cis-1-ethyl-3-methylcyclohexane, the ethyl and methyl groups are on the same face of the ring. For a 1,3-disubstituted cyclohexane, when substituents are cis they are in a 1,3-relationship where both can simultaneously occupy equatorial positions in one chair conformation. When both groups are on the same face at C1 and C3, in the most stable chair, both substituents can be placed equatorial (since C1 and C3 are 1,3 positions, cis substituents at these positions are both equatorial in one chair). This gives a highly stable diequatorial conformation. Step 3 - Analyze the trans isomer (B). In trans-1-ethyl-3-methylcyclohexane, the two substituents are on opposite faces of the ring. For 1,3-disubstituted cyclohexane, the trans isomer forces one group equatorial and the other axial in both chair conformations. There is no conformation in which both substituents are simultaneously equatorial. This means the molecule always bears axial strain. Step 4 - Compare. The cis isomer (A) can adopt a diequatorial conformation (both ethyl and methyl equatorial), making it more stable. The trans isomer (B) always has one axial substituent regardless of which chair conformation it adopts. Step 5 - Why other options fail: - Option (b) is wrong because B cannot achieve a diequatorial arrangement. - Option (c) is wrong because there is a clear difference in conformational stability. - Option (d) is wrong because a direct comparison is possible using conformational analysis. Therefore, the correct answer is A.