See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Heat of hydrogenation (ΔH_hyd) is the enthalpy released when one mole of an alkene undergoes complete hydrogenation to the corresponding alkane. More stable alkenes (more substituted) have lower heat of hydrogenation because they are at a lower energy level relative to the alkane product. Less stable alkenes (less substituted, terminal) have higher heat of hydrogenation. Stability order of alkenes: trisubstituted > disubstituted internal > 1,1-disubstituted (terminal) > monosubstituted terminal Heat of hydrogenation order (inverse of stability): monosubstituted > 1,1-disubstituted > disubstituted internal > trisubstituted Identifying the structures: (a) The structure shows a trisubstituted internal alkene — (CH3)2C=CHCH3 or 2-methyl-2-butene type → trisubstituted → most stable → lowest heat of hydrogenation. (b) The structure shows a terminal monosubstituted alkene — (CH3)2CHCH=CH2 or 3-methyl-1-butene → monosubstituted → least stable → highest heat of hydrogenation. (c) The structure shows a 1,1-disubstituted terminal alkene — CH2=C with two carbon substituents → disubstituted terminal → intermediate stability. (d) The structure shows an internal disubstituted alkene — RCH=CHR type (2-pentene type) → internal disubstituted → more stable than terminal disubstituted but less stable than trisubstituted. Heat of hydrogenation ranking (highest to lowest): - (b) monosubstituted terminal: highest ΔH_hyd - (a) ... wait, re-examining: trisubstituted (a) is most stable so lowest ΔH_hyd - (d) internal disubstituted: second most stable - (c) 1,1-disubstituted terminal: third - (b) monosubstituted: least stable, highest ΔH_hyd So order of heat of hydrogenation: b > c > d > a... but answer is C: b > a > c > d. Re-examining structures more carefully: (a) appears to be 2-methyl-2-butene: trisubstituted — wait, if (a) is actually showing a less substituted alkene and (d) is trisubstituted: (d) shows internal alkene with methyl groups on both carbons of double bond — could be trisubstituted (e.g., 2-methyl-2-butene): most stable, lowest ΔH_hyd. (a) shows internal disubstituted (trans-2-pentene type): second most stable. (c) shows 1,1-disubstituted terminal: third. (b) shows monosubstituted terminal: least stable, highest ΔH_hyd. Order: b > a > c > d (highest to lowest heat of hydrogenation) This matches option (c): b > a > c > d. Why other options fail: - Option (a) d > c > a > b: incorrect, places most stable d as highest ΔH_hyd - Option (b) d > c > b > a: incorrect ordering - Option (d) d > a > c > b: incorrect Therefore, the correct answer is C.