See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The nitration of toluene proceeds via an electrophilic aromatic substitution (EAS) mechanism. The rate-determining step in EAS is the attack of the electrophile (NO2+) on the aromatic ring to form the arenium ion (sigma complex / Wheland intermediate). The subsequent step is loss of a proton (or deuteron) from the ipso carbon to restore aromaticity. Since proton loss (or deuteron loss) occurs AFTER the rate-determining step, the identity of the substituent at the position being attacked (H vs D) does NOT affect the overall rate of nitration. Step-by-step reasoning: 1. In EAS, the slow (rate-determining) step is the formation of the sigma complex by electrophilic attack on the ring. 2. The fast step is deprotonation (loss of H+ or D+) to regenerate the aromatic system. 3. Because C-H bond breaking occurs in the fast step (not the rate-determining step), there is no primary kinetic isotope effect on the overall rate of nitration. 4. Therefore, whether the ortho positions bear H or D (or a mixture), the rate of nitration is the same for all three compounds (a), (b), and (c). 5. Option (a) a > b > c would imply a kinetic isotope effect governs the rate, which is incorrect since C-H/C-D cleavage is not rate-determining. 6. Option (b) a > c > b is similarly incorrect for the same reason. 7. Option (d) c > a > b makes no chemical sense. 8. Option (c) a = b = c correctly reflects that substitution of H by D at the ortho positions does not change the rate of the rate-determining electrophilic attack step. Therefore, the correct answer is C.