See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Heat of hydrogenation (ΔH_hyd) measures the energy released when an alkene is hydrogenated to an alkane. More substituted alkenes are more stable (lower energy) due to hyperconjugation and inductive effects, so they release LESS heat upon hydrogenation. Less substituted alkenes are less stable (higher energy) and release MORE heat. Step 1: Identify the substitution level of each alkene. - Structure 1: Monosubstituted alkene (one alkyl substituent on the double bond carbons, e.g., but-1-ene type). Least stable → highest heat of hydrogenation. - Structure 4: Disubstituted alkene (two alkyl groups, both on the same carbon, i.e., 2-methylpropene / isobutylene). Moderately stable. - Structure 3: Disubstituted alkene (one methyl on each carbon, trans-2-butene type). Similar level to structure 4 but trans isomers are slightly more stable than gem-disubstituted in some comparisons; however, internal disubstituted trans alkenes are generally slightly more stable than 1,1-disubstituted. - Structure 2: Trisubstituted alkene (2-methylpent-2-ene: three alkyl substituents). Most stable → lowest heat of hydrogenation. Step 2: Rank by decreasing heat of hydrogenation (least stable releases most heat first). - Structure 1 (monosubstituted) > Structure 4 (1,1-disubstituted) > Structure 3 (1,2-disubstituted trans) > Structure 2 (trisubstituted) Step 3: This gives the order 1 > 4 > 3 > 2, which corresponds to option (d). Why other options fail: - (a) and (b) put structure 2 (most stable, trisubstituted) first, which would mean it releases the MOST heat — incorrect, it releases the least. - (c) puts 1 > 3 > 4 > 2, placing the 1,2-disubstituted (3) above the 1,1-disubstituted (4); while close, 2-methylpropene (gem-disubstituted) is generally less stable than internal trans disubstituted alkenes in terms of heat of hydrogenation values, but the standard ordering places monosubstituted > gem-disubstituted > trans-disubstituted > trisubstituted for these specific structures, making option (d) correct. Therefore, the correct answer is D.