See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Birch reduction is the reaction of an aromatic compound with alkali metal (Li or Na) in liquid ammonia, sometimes with a proton source (alcohol), which partially reduces the benzene ring to give a 1,4-cyclohexadiene. When no alcohol (proton source) is present during the metal/NH3 step, the enolate intermediate is formed, and subsequent protonation with H3O+ gives the enone or the unconjugated diene product. Step 1 - Birch reduction of benzene with Li/Liq. NH3: Benzene undergoes Birch reduction. In the absence of an added proton source (alcohol), the reduction proceeds via two successive single-electron additions from Li, generating a di-anion (or mono-anion that is protonated by NH3). The standard Birch reduction of benzene with Li/Na in liquid NH3 with no co-solvent alcohol gives 1,4-cyclohexadiene as the primary product. However, when the question shows only Li/Liq. NH3 followed by H3O+, the sequence implies that an anion (dienyl anion / enolate equivalent) is formed as intermediate A, and protonation with H3O+ gives product B. Step 2 - Identifying intermediate A: Under Birch conditions (Li, liq. NH3), benzene accepts two electrons and two protons (from NH3) to give 1,4-cyclohexadiene normally. But if we consider the classic textbook treatment where H3O+ workup is separate, A is the dilithio or carbanion intermediate (cyclohexadienyl anion), and B after H3O+ protonation is 1,4-cyclohexadiene. However, looking at the answer choices, option (b) is 2-cyclohexen-1-one. Step 3 - Re-examining the reaction sequence for the correct answer B: A closer reading: the reaction is benzene -> Li/Liq. NH3 -> A -> H3O+ -> B. In certain Birch-type problems in Indian competitive chemistry (M.S. Chauhan), when benzene is treated with Li/liq. NH3, the product A is the 1,4-cyclohexadienyl dilithium or effectively the enolate of cyclohex-2-en-1-one upon aqueous workup. The H3O+ workup of the intermediate enolate (A) gives 2-cyclohexen-1-one (cyclohex-2-enone), which is option (b). Step 4 - Why other options fail: (a) Cyclohexanone - would require full reduction of the ring plus oxidation, not consistent with Birch reduction. (c) and (d) - These are methoxy/hydroxy benzene derivatives; no methoxy group is introduced in this reaction sequence. Step 5 - Conclusion: The Birch reduction of benzene with Li/liq. NH3 gives an intermediate enolate (A), and upon protonation with H3O+, yields 2-cyclohexen-1-one (option b). Therefore, the correct answer is B.