See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

**Step 1: Set up the thermochemical equations** $$\text{Cyclohexene} + H_2 \rightarrow \text{Cyclohexane} \quad \Delta H_1 = -28.6 \text{ kcal/mol}$$ $$\text{Benzene} + 3H_2 \rightarrow \text{Cyclohexane} \quad \Delta H_2 = -49.8 \text{ kcal/mol}$$ **Step 2: Find the equation for benzene hydrogenation** Subtract equation 1 from equation 2 to get benzene hydrogenation: $$\text{Benzene} + 3H_2 - (\text{Cyclohexene} + H_2) \rightarrow \text{Cyclohexane} - \text{Cyclohexane}$$ $$\text{Benzene} - \text{Cyclohexene} + 2H_2 \rightarrow 0$$ Rearranging: $$\text{Benzene} + 2H_2 \rightarrow \text{Cyclohexene} \quad \Delta H = -49.8 - (-28.6) = -21.2 \text{ kcal/mol}$$ **Step 3: Calculate resonance energy** The resonance energy is the difference between the theoretical (localized) heat of hydrogenation and the observed heat: $$\text{Resonance Energy} = \Delta H_{\text{theoretical}} - \Delta H_{\text{observed}}$$ If benzene were a hypothetical triene with 3 localized double bonds, its heat of hydrogenation would be: $$\Delta H_{\text{theo}} = 3 \times (-28.6) = -85.8 \text{ kcal/mol}$$ $$\text{Resonance Energy} = -85.8 - (-49.8) = -36 \text{ kcal/mol}$$ **Answer: (D) –36 K cals mol⁻¹** The negative sign indicates that benzene is stabilized by resonance (lower energy than expected).