See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Br2 reacts in the dark at room temperature via ionic (electrophilic) mechanisms, not radical mechanisms. Key reactions that proceed without light or heat are: (1) electrophilic addition to alkenes and alkynes (Br2 adds across the pi bond), and (2) electrophilic aromatic substitution on highly activated aromatic rings (those with strongly activating groups like -OH) which can react with Br2 without a Lewis acid catalyst. Step 1 - Analyze each compound: (a) Benzene: Aromatic ring with no activating substituent. Electrophilic aromatic substitution with Br2 requires a Lewis acid catalyst (FeBr3). Does NOT react with Br2 in the dark at room temperature without catalyst. (b) Cyclohexene: Contains a C=C double bond. Br2 undergoes electrophilic addition across the double bond in the dark at room temperature (bromine water/CCl4 test for unsaturation). REACTS. (c) Cyclohexane: Saturated hydrocarbon. Reacts with Br2 only via free radical halogenation, which requires UV light or heat. Does NOT react in the dark. (d) Propanoic Acid (C2H5CO2H): Carboxylic acid. The alpha-halogenation (Hell-Volhard-Zelinsky) requires red phosphorus and Br2, not just Br2 alone at room temperature in dark. Does NOT react under these conditions. (e) Phenol (C6H5OH): The hydroxyl group is a very strong activating, ortho/para-directing group. It activates the ring so strongly that phenol reacts with Br2 water immediately in the dark at room temperature without any catalyst, giving 2,4,6-tribromophenol. REACTS. (f) Nitrobenzene (C6H5NO2): The nitro group is a strong deactivating group. Even electrophilic aromatic substitution requires harsh conditions (fuming H2SO4, high temperature). Does NOT react with Br2 in the dark at room temperature. (g) Hexyne (C6H10): Contains a C≡C triple bond. While alkynes do react with Br2 by electrophilic addition, they are generally less reactive than alkenes toward electrophilic addition because the sp-hybridized carbons hold the pi electrons more tightly. However, hexyne (a terminal or internal alkyne) can still add Br2, but the question's answer does not include (g). The answer provided excludes hexyne, suggesting that under the specific conditions stated (room temperature, dark), the reactivity of the alkyne is considered insufficient or the question specifically focuses on the most reactive cases. Alternatively, some sources note that internal alkynes react more slowly. Given the provided answer is b and e only, hexyne is excluded. (h) 2,2-dichloropropane (C3H6Cl2): Saturated haloalkane. No pi bonds, no activated aromatic ring. Would require radical conditions (light) for further halogenation. Does NOT react with Br2 in the dark at room temperature. Step 2 - Summary: Only cyclohexene (b) reacts via electrophilic addition and phenol (e) reacts via electrophilic aromatic substitution without requiring light, heat, or a catalyst. Therefore, the correct answer is b, e.