See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: OsO4-catalyzed dihydroxylation of alkenes. In the Upjohn modification, catalytic OsO4 is used with a co-oxidant (TMAO, trimethylamine N-oxide) to regenerate OsO4 in situ. The reaction proceeds via a concerted [3+2] cycloaddition of OsO4 across the double bond to form an osmate ester intermediate, which is then hydrolyzed to give the syn-diol (vicinal diol). Step 1 - Identify the substrate: 2,3-dimethyl-2-butene has the structure (CH3)2C=C(CH3)2. The double bond is between C2 and C3. Step 2 - Reaction mechanism: OsO4 adds across the C=C double bond in a syn (cis) fashion, forming a cyclic osmate ester. TMAO then oxidizes Os(VI) back to Os(VIII), releasing the diol product and regenerating OsO4. Hydrolysis of the osmate ester gives the syn-1,2-diol. Step 3 - Product identification: Adding two OH groups across (CH3)2C=C(CH3)2 gives (CH3)2C(OH)-C(OH)(CH3)2, which is 2,3-dimethyl-2,3-butanediol (pinacol). Both carbon atoms that were part of the double bond now each bear a hydroxyl group and two methyl groups. Step 4 - Why other options fail: - Option (a) is an epoxide, which would result from peracid oxidation (e.g., mCPBA), not OsO4 dihydroxylation. - Option (c) acetone and option (d) methyl tert-butyl ketone would result from ozonolysis (oxidative cleavage of the double bond), not dihydroxylation. - Option (b) correctly shows the vicinal diol product of syn-dihydroxylation with both OH groups on adjacent carbons. Step 5 - Stoichiometry check: 0.025 moles alkene, 0.034 moles TMAO (excess co-oxidant), catalytic OsO4 (10^-4 mole). The catalytic cycle is feasible since TMAO is in excess relative to alkene, and OsO4 is regenerated continuously. Therefore, the correct answer is B.