See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Mercuric acetate (Hg(OAc)2) promotes electrophilic cyclization of polyene systems. This is an oxymercuration-type electrophilic addition to alkenes, but here applied intramolecularly to a diene (cationic olefin cyclization mediated by Hg(II)). Hg(II) acts as an electrophile activating one alkene toward intramolecular attack by the second alkene, analogous to biomimetic polyene cyclizations. Step 1 - Identify the reactive sites: The starting material contains two alkene units. One is a more electron-rich trisubstituted alkene (internal, with methyl substituents), and the other bears the electron-withdrawing CO2CH3 group (less electron-rich). Hg(OAc)2 coordinates to the more nucleophilic alkene, generating a mercurinium-like ion that is attacked by the other alkene intramolecularly. Step 2 - Determine regiochemistry of cyclization: Following Markovnikov-like regioselectivity in electrophilic cyclization, the electrophile (Hg) ends up on the more substituted carbon. The internal olefin with gem-dimethyl substitution is attacked, placing HgOAc on the quaternary (more substituted, tertiary->quaternary) carbon bearing the two methyl groups. The ring closure forms a new C-C bond at the less substituted end of that alkene. Step 3 - Stereochemistry: Electrophilic cyclizations with Hg(II) proceed via anti addition across the activated double bond. The ring junction proton (H) is introduced with defined stereochemistry relative to the HgOAc group - they are anti (trans) to each other. Option (b) explicitly shows an H at the ring junction, consistent with trans-diaxial (anti) addition stereochemistry, which is the hallmark of electrophilic anti addition. Step 4 - Product structure: A bicyclic decalin-type product forms. The remaining double bond (bearing CO2CH3 and methyl) is preserved. HgOAc ends up on the quaternary carbon (bearing two methyl groups). An explicit H appears at the ring junction in anti relationship to HgOAc. This matches option (b) exactly. Why other options fail: - Option (a): No H shown at ring junction, inconsistent with anti addition mechanism that generates a ring-junction H; HgOAc placement on ring junction rather than gem-dimethyl carbon is incorrect regiochemistry. - Option (c): Shows OH instead of HgOAc - this would be the oxymercuration-demercuration product, not the direct mercuration product; the question asks for the Hg(OAc)2 addition product itself. - Option (d): Shows OH and incorrect skeleton with what appears to be a tert-butyl group rather than a ring-fused gem-dimethyl; this is neither the correct regiochemistry nor the correct product type. Therefore, the correct answer is B.