See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: C-H bond dissociation energy (bond energy) depends on the hybridization of the carbon bearing the hydrogen. Higher s-character in the C-H bond leads to higher bond energy (shorter, stronger bond). The order of C-H bond energies by hybridization is: sp C-H (~≥500 kJ/mol) > sp2 C-H (~≈460 kJ/mol) > sp3 C-H (~≈410 kJ/mol). Among sp3 C-H bonds, bond energy order is: primary > secondary > tertiary (primary C-H bonds are slightly stronger than secondary, which are slightly stronger than tertiary). Step 1 - Identify the hybridization of each carbon bearing the labeled proton: - Hd: attached to the vinylic carbon of the exocyclic double bond (=CH-). This carbon is sp2 hybridized. sp2 C-H bond energy ≈ 460 kJ/mol. - Ha: attached to the methyl group (CH3) on the ring carbon via a wedge bond. This is a primary sp3 C-H. Bond energy ≈ 420 kJ/mol (primary). - Hb: attached to a ring carbon that is sp3 hybridized (secondary C-H, flanked by two ring carbons). Bond energy ≈ 410 kJ/mol (secondary). - Hc: attached to the top ring carbon that is sp3 hybridized (secondary C-H, adjacent to the vinylic carbon). Bond energy ≈ 410 kJ/mol (secondary), but slightly less than Hb or similar; effectively in the same secondary sp3 category. Step 2 - Compare bond energies: - Hd (sp2 vinylic C-H) has the highest bond energy among the four. - Ha (primary sp3 C-H, methyl group) has higher bond energy than secondary sp3 C-H. - Hb (secondary sp3 C-H) is next. - Hc (secondary sp3 C-H adjacent to sp2 carbon; slight allylic weakening is not relevant here since we are comparing bond energies not acidity, but the allylic position actually has lower BDE due to radical stabilization — however in terms of bond energy tables for C-H, Hc being on a carbon alpha to the double bond makes it allylic, giving it a lower C-H BDE than a regular secondary C-H like Hb). Step 3 - Refined reasoning: - Hd: sp2 C-H, highest BDE (~460 kJ/mol). - Ha: primary sp3 C-H (~420 kJ/mol), higher than secondary. - Hb: secondary sp3 C-H, not allylic (~410 kJ/mol). - Hc: secondary sp3 C-H that is allylic (alpha to the C=C), so BDE is lower (~360-380 kJ/mol) due to allylic radical stabilization upon homolysis. Thus decreasing order of bond energies: Hd > Ha > Hb > Hc. Why others fail: Any arrangement placing Hc above Hb or Ha above Hd would ignore the hybridization and allylic stabilization effects on C-H bond dissociation energies. Therefore, the correct answer is Hd>Ha>Hb>Hc.