See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Heat of hydrogenation (ΔH_hyd) is the energy released when a double bond is hydrogenated. A more stable (more substituted or more conjugated) alkene has lower potential energy, so it releases less heat upon hydrogenation. Conversely, a less stable alkene releases more heat. Therefore, stability order is inverse to heat of hydrogenation order. Step 1 – Assess stability of each alkene: Structure (I): 1,2-dimethylcyclohex-1-ene has a tetrasubstituted endocyclic double bond (two ring carbons each bearing a methyl group and two additional ring carbons as substituents). This is the most highly substituted double bond → most stable → lowest heat of hydrogenation. Structure (II): 1-methylene-2-methylcyclohexane has an exocyclic double bond (=CH2). The exocyclic carbon bears no substituents (=CH2 is terminal), and the ring carbon bears the ring and a methyl group. Exocyclic double bonds are generally less stable than endocyclic disubstituted ones. This double bond is monosubstituted on the ring side → less stable than (I) but must be compared to (III). Structure (III): 3,4-dimethylcyclohex-1-ene has an endocyclic double bond between C1 and C2, with no substituents directly on the double bond carbons (the methyls are on C3 and C4, not on C1 or C2). This is essentially a disubstituted endocyclic double bond (each vinyl carbon has one ring carbon substituent) but with no additional alkyl groups directly attached — making it less substituted than (I) but comparable to or slightly more stable than (II). Step 2 – Compare (II) and (III): Exocyclic methylene (=CH2) double bonds are less stable than endocyclic double bonds of comparable substitution because endocyclic double bonds benefit more from hyperconjugation and ring geometry. A monosubstituted exocyclic double bond (II) is less stable than a disubstituted endocyclic double bond (III). Therefore stability: I > III > II. Step 3 – Heat of hydrogenation (inverse of stability): Stability order: I > III > II Heat of hydrogenation order: II > III > I Step 4 – Verify against options: Option (c) states II > III > I, which matches our analysis. Why other options fail: (a) I > II > III would mean I is least stable — incorrect, I is most stable. (b) III > II > I would mean I is most stable but III releases most heat — incorrect ordering of II and III. (d) III > I > II would mean I is intermediate — incorrect, I is most stable and should have lowest heat of hydrogenation. Therefore, the correct answer is C.