Chemistry - GOC Question with Solution | TestHub

Addition across $C_9$-$C_{10}$ destroys the least resonance energy. The product keeps two intact benzene rings (2 × 36 = 72 kcal/mol), so the loss is 92 - 72 = 20 kcal/mol. Addition in a terminal ring costs 92 - 61 = 31 kcal/mol, so the $C_9$-$C_{10}$ bond reacts.