See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The heat of hydrogenation (ΔH_hyd) is the enthalpy released when one mole of an alkene is hydrogenated to the corresponding alkane. A more negative heat of hydrogenation means more energy is released, which corresponds to a less stable (higher energy) alkene. Conversely, more substituted (more stable) alkenes have less negative heats of hydrogenation. Step 1 – Identify the degree of substitution of each alkene's double bond: (a) Vinylcyclohexane: the double bond is CH2=CH– (terminal, monosubstituted vinyl group attached to cyclohexane). The double bond carbons bear one alkyl substituent total → monosubstituted alkene. (b) Methylenecyclohexane: exocyclic =CH2 on cyclohexane. The sp2 carbon of the ring bears two ring carbons but the exocyclic =CH2 carbon bears none; overall this is a 1,1-disubstituted (geminal) alkene. (c) The structure shown is an exocyclic double bond from cyclohexane to C(CH3)= with an additional methyl, making it a trisubstituted alkene (the exocyclic carbon bears one methyl, and the ring carbon bears two ring carbons). This is more substituted and therefore more stable → less negative heat of hydrogenation. (d) 1-Methylcyclohex-2-ene: endocyclic trisubstituted double bond (the two ring carbons of the double bond each bear one additional carbon substituent, and one also bears the methyl group) → trisubstituted, relatively stable. Step 2 – Rank stability (more stable = less negative ΔH_hyd): Most stable (least negative ΔH_hyd): (c) trisubstituted exocyclic and (d) trisubstituted endocyclic are most stable. Moderately stable: (b) disubstituted exocyclic (methylenecyclohexane). Least stable (most negative ΔH_hyd): (a) monosubstituted terminal alkene (vinylcyclohexane). Step 3 – Key reasoning for (a): Vinylcyclohexane has a monosubstituted terminal vinyl double bond (CH2=CH–), which is the least substituted among the four options. Less substitution = less hyperconjugative/inductive stabilization = higher energy alkene = most energy released upon hydrogenation = most negative heat of hydrogenation. Step 4 – Why other options fail: (b) Methylenecyclohexane is disubstituted (more stable than (a)), so less negative ΔH_hyd than (a). (c) The trisubstituted exocyclic alkene is even more stable, so even less negative ΔH_hyd. (d) The trisubstituted endocyclic alkene is similarly or more stable than (b), so less negative than (a). Therefore, the correct answer is A.