See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the structure: The molecule is a 1,2-dichlorocyclohexane drawn in chair conformation. Carbon a (left) bears a Cl that points downward (axial, going into the ring plane from above - shown by the downward arrow, meaning it is in the axial-down position). Carbon b (right) bears a Cl shown by an arrow pointing upper-right (equatorial or axial-up position). Step 2 - Assign substituents at carbon a: Carbon a is a stereocenter. Its four substituents are: Cl, H (the other hydrogen), and two ring carbons (one going to carbon b, one going the other way around the ring). In the chair, the Cl at carbon a is axial and pointing downward. Priority order: Cl (highest, #1) > the ring carbon connected to carbon b carrying Cl (#2, since that branch has Cl substituent) > the other ring carbon (#3) > H (#4, lowest). With Cl axial-down at carbon a, arranging 1→2→3 with H (#4) pointing axially upward (away from viewer in this orientation), the rotation 1→2→3 is clockwise. Since H points away, clockwise = R. So carbon a = R. Step 3 - Assign substituents at carbon b: Carbon b is a stereocenter. Its Cl is shown pointing upper-right (axial-up position in the chair). Substituents: Cl (#1, highest priority), ring carbon toward carbon a with Cl (#2), other ring carbon (#3), H (#4). H at carbon b is axial-down (pointing downward, away from viewer). With H pointing away, the sequence 1→2→3 is clockwise = R. So carbon b = R. Step 4 - This gives a = R, b = R, which corresponds to answer choice (a). Step 5 - Why other options fail: Option (b) a=R, b=S would require b's rotation to be counterclockwise with H pointing away, which it is not. Options (c) and (d) assign S to carbon a, which contradicts the clockwise rotation determined above. Therefore, the correct answer is A.