See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The hydrogenation of benzene proceeds stepwise. The activation energy for each step reflects the difficulty of adding H2 across a double bond in each intermediate, which is governed by the stability of the starting material (resonance/aromatic stabilization) and the nature of the double bond. Step 1 - Benzene to cyclohexadiene (E1): Benzene is aromatic and has extra stability due to delocalization (resonance energy ~36 kcal/mol). Breaking into this aromatic system to add the first H2 requires overcoming the aromatic stabilization energy ON TOP of the normal alkene hydrogenation barrier. Therefore E1 is very high. Step 2 - Cyclohexadiene to cyclohexene (E2): Cyclohexadiene has two isolated or conjugated double bonds but no aromatic stabilization. Adding H2 to a diene is somewhat easier than to benzene, but the conjugated diene still has some extra stability (conjugation energy). So E2 is intermediate. Step 3 - Cyclohexene to cyclohexane (E3): Cyclohexene has a simple, non-conjugated, non-aromatic double bond. This is the easiest step with the lowest activation energy. E3 is the smallest. Ordering: The first step has the highest activation energy because it disrupts aromaticity. The second step is easier than the first but harder than the third. The third step is the easiest. Thus: E1 > E2 > E3. Why other options fail: - (a) E2 > E1 > E3: This would mean hydrogenating a diene is harder than hydrogenating benzene, which is incorrect since benzene has much greater stabilization. - (b) E3 > E1 > E2: This would mean the simplest alkene hydrogenation has the highest barrier, which contradicts known chemistry. - (c) E3 > E2 > E1: Same issue as (b), incorrectly ranks the simplest step as hardest. Therefore, the correct answer is D.