See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Background concept: Hexachlorocyclohexane (HCH) has 8 possible stereoisomers. The key isomers are alpha, beta, gamma, delta, epsilon, zeta, eta, and theta-HCH. In cyclohexane chair/flat representations, the stereochemistry at each carbon (axial vs equatorial, or up vs down) determines whether the molecule is chiral (optically active) or achiral (meso/symmetric). Step 2 - Identifying optical activity (Question A): A molecule is optically active if it has no plane of symmetry and is not superimposable on its mirror image. Among the given structures: - P: All Cl groups are on the same face (all-cis). This structure has multiple planes of symmetry (C6v-like symmetry), so it is achiral (optically inactive). This corresponds to alpha or a symmetric isomer. - R: The structure shown has a specific arrangement that upon analysis possesses an internal plane of symmetry, making it a meso compound - optically inactive. - S: This structure also has planes of symmetry due to its alternating pattern - optically inactive. - T: The arrangement in T lacks any plane of symmetry or center of inversion. It is a chiral molecule with no internal mirror plane, making it optically active. This corresponds to the delta or another chiral HCH isomer. Therefore, T is optically active. Answer A = (d) T. Step 3 - Identifying most stable isomer (Question B): The most stable isomer of hexachlorocyclohexane is the one where the maximum number of Cl substituents are in equatorial positions in the chair conformation. In a cyclohexane ring, equatorial positions are preferred over axial to minimize 1,3-diaxial strain. - The beta-HCH isomer has all Cl groups in alternating up-down pattern (aaaeee in chair form), giving all equatorial positions - this is the most stable isomer. - Structure R corresponds to the isomer where all six Cl atoms can adopt equatorial positions in the chair conformation (the all-equatorial arrangement corresponds to alternating e,e,e,e,e,e). Among the options, R represents the isomer with the most equatorial Cl substituents. - S, T, U have more axial Cl groups and are therefore less stable. Structure R (beta-HCH equivalent) with all or maximum Cl groups equatorial is the most stable. Answer B = (a) R. Step 4 - Why other options fail: - For A: P is all-cis with planes of symmetry (achiral). R and S have internal symmetry planes making them meso/achiral. Only T is chiral. - For B: S, T, U all have more axial Cl substituents than R, leading to greater steric strain and lower stability. Therefore, the correct answer is A-d; B-a.