See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Conformational equilibria (ring flips, bond rotations) are generally rapid at room temperature because they involve low energy barriers (typically 10-12 kcal/mol or less). In contrast, configurational changes, such as cis-trans (geometric) isomerism about a C=C double bond, require breaking a pi bond, which has a very high energy barrier (~60 kcal/mol or more) and is NOT rapid at room temperature. Step-by-step reasoning: Option (a): Shows a ring flip equilibrium of a disubstituted cyclohexane (1-chloro-4-methylcyclohexane) between two chair conformations. Ring flips involve only conformational changes with a barrier of approximately 10-12 kcal/mol, which is readily overcome at room temperature. This equilibrium IS rapid. Option (b): Shows interconversion between two geometric isomers (cis and trans, or E and Z) of a trisubstituted alkene containing Cl, Br, and Me groups on the double bond carbons. To interconvert E and Z isomers about a C=C double bond, the pi bond must be broken and reformed. The energy barrier for this process is approximately 60 kcal/mol, which is far too high to be overcome at room temperature. This equilibrium is NOT rapid at room temperature. Option (c): Shows rotation about a C-C single bond in a Newman projection (ethane-like with methyl substituents), interconverting gauche and anti conformers. Rotation about a C-C single bond has a very low barrier (~3 kcal/mol), making this equilibrium very rapid at room temperature. Option (d): Shows interconversion between the chair and boat conformations of cyclohexane. This conformational change has a barrier of about 10-12 kcal/mol and is rapid at room temperature. Why other options fail: Options (a), (c), and (d) all involve conformational changes (ring flips or bond rotations) with low energy barriers that are easily overcome at room temperature. Only option (b) involves a configurational change requiring pi bond breaking, which has an insurmountably high barrier at room temperature. Therefore, the correct answer is A.