See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The heat of combustion reflects the stability of a molecule — more stable molecules release less energy upon combustion (lower heat of combustion). Aromatic stabilization (resonance energy) significantly lowers the enthalpy of combustion relative to non-aromatic or antiaromatic analogs. Step 1: Identify the molecular formula and degree of unsaturation for each option to ensure comparison is meaningful. All four structures appear to be C9H10 or similar constitutional isomers, meaning they have the same molecular formula and thus can be directly compared by stability. Step 2: Assess aromaticity: - Option (a): A 7-membered ring with multiple double bonds (tropylium-like but neutral) with a vinyl group — not aromatic in the neutral form; non-aromatic or possibly antiaromatic depending on electron count. - Option (b): A 6-membered ring with two (not three) double bonds plus a vinyl group — this is a non-aromatic cyclohexadiene derivative (not fully conjugated around the ring). - Option (c): A benzene ring (fully aromatic, 6 pi electrons, Hückel 4n+2 with n=1) with a vinyl substituent — this is styrene. Benzene's aromaticity provides ~36 kcal/mol of resonance stabilization energy. Styrene benefits from both aromatic stabilization and extended conjugation of the vinyl group with the ring. - Option (d): An 8-membered ring with alternating double bonds (cyclooctatetraene, COT) with a methyl group — COT is non-planar and non-aromatic (antiaromatic if forced planar, 4n pi electrons with n=2), so it has no aromatic stabilization. Step 3: Determine which is most stable: Option (c), styrene (vinylbenzene), contains a fully aromatic benzene ring conjugated with the vinyl group. This gives it the greatest thermodynamic stability among the four isomers. Greater stability means less energy is released upon combustion, corresponding to the lowest heat of combustion. Step 4: Why other options fail: - (a) 7-membered ring system with vinyl: non-aromatic, higher energy, higher heat of combustion. - (b) Cyclohexadiene-vinyl: non-aromatic (only two double bonds in ring, not three), less stable than benzene analog. - (d) COT with methyl: 8-membered ring is non-aromatic (tub-shaped), no special stabilization, higher heat of combustion than styrene. Therefore, the correct answer is C.