See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Hydroboration-oxidation is a syn-addition reaction. Using B2D6 (deuteroborane) instead of B2H6 means that deuterium (D) is delivered to the alkene along with boron in a concerted, syn (cis) fashion. The boron adds to the less substituted carbon and deuterium adds to the more substituted carbon (anti-Markovnikov), both from the same face. Step 2 - Substrate: 1-methylcyclopentene has the double bond between C1 (bearing the methyl group) and C2. C1 is more substituted, C2 is less substituted. Step 3 - Regioselectivity: In hydroboration, boron goes to the less hindered (less substituted) carbon. Therefore, boron attaches to C2 and deuterium attaches to C1 (the carbon bearing the methyl group). This is anti-Markovnikov addition of D. Step 4 - Stereochemistry of addition: The addition is syn, meaning B and D add to the same face of the double bond. Since the ring can be attacked from either face equally, but the key point is that B and D end up on the same face (cis to each other on the ring). Step 5 - Oxidation step: Treatment with alkaline H2O2 replaces the C-B bond with C-OH with retention of configuration at the carbon bearing boron (C2). So OH ends up where B was, on the same face as D. Step 6 - Relative stereochemistry of product: D is on C1 and OH is on C2, both on the same face (syn addition). This means D (at C1) and OH (at C2) are cis to each other on the cyclopentane ring. Step 7 - Structure analysis: In option (a), the product shows Me and D both on wedge (same face) at C1, and OH on wedge at C2, with H on the opposite face at C2 - indicating D and OH are on the same face (cis), which is consistent with syn hydroboration-oxidation. The methyl group and D are on C1, OH is on C2, all consistent with: D added to C1 (more substituted), OH replaced B at C2 (less substituted), both on same face. Step 8 - Why other options fail: - Option (b): Shows D at C1 and H at C2 with OH at bottom - OH and D are on opposite faces (trans), inconsistent with syn addition. - Option (c): Shows OH at C1 and D at C2 - this would mean boron went to the more substituted carbon (C1), violating anti-Markovnikov regioselectivity. - Option (d): Shows OH at C1 and D at C2 on opposite faces - both wrong regiochemistry and wrong stereochemistry. Therefore, the correct answer is A.