See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: In electrophilic aromatic substitution (EAS), specifically nitration, the mechanism involves two key steps: (1) formation of the nitronium ion (NO2+) from mixed acid, and (2) attack of NO2+ on the benzene ring to form a sigma complex (arenium ion/Wheland intermediate), followed by loss of a proton (deprotonation) to restore aromaticity. Step 1 - Identify the rate-determining step: In nitration of benzene by mixed acid (H2SO4 + HNO3), the slow, rate-determining step is the formation of the sigma complex (attack of NO2+ on the ring), NOT the deprotonation step. The deprotonation step is fast. Step 2 - Role of kinetic isotope effect: The kinetic isotope effect (KIE) is only observed when the bond to the isotope (H, D, or T) is broken in the rate-determining step. Since C-H (or C-D or C-T) bond breaking occurs in the FAST step (proton loss), not in the rate-determining step, there is NO primary kinetic isotope effect on the overall rate of nitration. Step 3 - Conclusion: Because the C-H/C-D/C-T bond cleavage is not involved in the rate-determining step, the rates of nitration for C6H6, C6D6, and C6T6 are all equal. There is no observable isotope effect on the reaction rate. Step 4 - Why other options fail: - Option (b) C6H6 > C6D6 > C6T6 would be correct if C-H bond breaking were rate-determining (primary KIE), but it is not. - Option (c) C6H6 = C6D6 > C6T6 is partially incorrect for the same reason. - Option (d) C6H6 < C6D6 < C6T6 is the inverse of KIE and is incorrect. Therefore, the correct answer is A.