See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Solution: Heat of Hydrogenation Analysis **Key Principle:** Heat of hydrogenation reflects stability. More stable alkenes release *less* heat upon hydrogenation (lower ΔH). ## Pair (A) Analysis: - **Left structure:** 1,2-dimethylcyclohexene (disubstituted alkene, ortho-substitution) - **Right structure:** 1,4-dimethylcyclohexene (disubstituted alkene, para-substitution) The **ortho isomer** (left) has steric hindrance between the two adjacent methyl groups, which destabilizes the double bond. This makes it less stable and more reactive. Upon hydrogenation, the less stable (strained) alkene releases **more heat** due to greater relief of strain energy. Therefore, the **para isomer** (right, less strained) gives **lower heat of hydrogenation**. ## Pair (B) Analysis: Both structures are unsaturated ketones with different saturation degrees. The tetrasubstituted double bond (left) is more stable than the trisubstituted one (right), but the stability difference is overshadowed by other factors and both undergo exothermic reduction. --- **Answer: (A)** — The para-disubstituted alkene releases less heat because it's more stable (fewer steric interactions) compared to the sterically hindered ortho isomer.