See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The heat of hydrogenation (ΔH_hyd) measures the stability of an alkene — more stable alkenes release less heat upon hydrogenation (lower ΔH_hyd), while less stable alkenes release more heat. The stability order of alkenes follows Zaitsev/substitution rules: more substituted (more alkyl groups on the double bond) = more stable = less heat evolved. Exocyclic double bonds are generally less stable than endocyclic ones with equivalent substitution due to ring strain considerations, and terminal/exocyclic alkenes are less stable than internal ones. Step 1 — Identify the degree of substitution and type of each alkene: (a) 3,6-dimethylcyclohex-1-ene: The double bond has one direct methyl substituent on one sp2 carbon and one methyl on the adjacent sp3 carbon — this is a disubstituted endocyclic alkene (monosubstituted on the double bond carbons if counting only direct substituents on the C=C: one methyl on one vinyl carbon, the ring provides two substituents). Overall: trisubstituted or disubstituted endocyclic. (b) 1,6-dimethylcyclohex-2-ene (or equivalent): The double bond is between C1 and C2 of the ring, with methyl groups on adjacent carbons — trisubstituted endocyclic alkene. (c) 1,2-dimethylcyclohex-1-ene: Both carbons of the endocyclic double bond bear methyl groups — this is a tetrasubstituted (or highly substituted) endocyclic alkene, making it the most stable endocyclic alkene here. (d) 1-methylmethylenecyclohexane (exocyclic double bond): The double bond is exocyclic (=CH2), which is less stable than an endocyclic double bond of similar substitution. This is essentially a 1,1-disubstituted exocyclic alkene, but exocyclic alkenes are destabilized relative to endocyclic ones. Step 2 — Stability ranking (most stable → least stable → least heat to most heat evolved): Most stable (least heat): (c) tetrasubstituted/highly substituted endocyclic > (b) trisubstituted endocyclic > (a) disubstituted/less substituted endocyclic > (d) exocyclic (least stable, most heat evolved) Step 3 — Heat of hydrogenation order (most heat to least heat = decreasing): d > a > c > b This matches option (b): d > a > c > b. Step 4 — Why other options fail: (a) b > c > a > d: Incorrect because d (exocyclic) should evolve the most heat, not b. (c) d > c > a > b: Incorrect ordering of the endocyclic alkenes; c is more substituted than b, so c should evolve less heat than b. (d) c > b > c > d: Contains a repeated letter and is inconsistent. Therefore, the correct answer is B.