See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Resonance stabilization energy (delocalization energy) of benzene is measured by comparing the actual heat of hydrogenation of benzene with the hypothetical heat of hydrogenation of 1,3,5-cyclohexatriene (a Kekulé structure with three isolated double bonds). Step 1: Establish the reference point. Cyclohexene (one double bond) has a heat of hydrogenation of approximately -120 kJ/mol. A hypothetical 1,3,5-cyclohexatriene with three isolated (non-interacting) double bonds would be expected to release 3 × 120 = 360 kJ/mol upon complete hydrogenation to cyclohexane. Step 2: Actual measurement. The actual heat of hydrogenation of benzene is approximately -208 kJ/mol (it releases only 208 kJ/mol). Step 3: Calculate the difference. The hypothetical 1,3,5-cyclohexatriene would give off 360 kJ/mol, but benzene actually gives off only 208 kJ/mol. The difference is 360 - 208 = 152 kJ/mol. Step 4: Interpret the result. Because benzene is more stable than the hypothetical 1,3,5-cyclohexatriene by 152 kJ/mol (due to aromatic delocalization), benzene releases 152 kJ/mol LESS heat than the hypothetical cyclohexatriene would. In other words, benzene gives off 152 kJ/mol less heat than the cyclohexatriene. Why other options fail: - (a) Benzene does not absorb heat; hydrogenation is exothermic for both. - (b) Benzene gives off less heat, not more, because it is more stable. - (c) Benzene does not absorb heat relative to cyclohexatriene; it simply releases less heat. Therefore, the correct answer is D.