See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: The rate of electrophilic aromatic substitution (EAS), including nitration, depends on the electron density of the aromatic ring. Electron-donating groups (EDG) activate the ring (increase rate), while electron-withdrawing groups (EWG) deactivate the ring (decrease rate). Additionally, kinetic isotope effects (KIE) can influence rate when C-H bond breaking is involved in the rate-determining step. Step 2 - Classify each compound: (a) Benzene: reference compound, no substituent. (b) Toluene (Me group): methyl is an EDG via hyperconjugation and induction, activates the ring. Rate > benzene. (c) Benzene-d6 (deuterobenzene): Deuterium has a primary kinetic isotope effect (kH/kD > 1) only if C-H/C-D bond breaking is in the rate-determining step. In EAS, the rate-determining step is the addition of the electrophile (formation of the arenium/sigma complex), NOT the deprotonation step. Therefore, the KIE for nitration is essentially 1, meaning benzene-d6 reacts at the same rate as benzene. Rate of (c) = Rate of (a). (d) Nitrobenzene (NO2 group): NO2 is a strong EWG by both induction and resonance, strongly deactivates the ring. Rate << benzene. This is the slowest. (e) Chlorobenzene (Cl group): Cl is weakly deactivating by induction (EWG) but weakly activating by resonance (lone pair donation). Net effect: Cl is a weak deactivator overall but less deactivating than NO2. Rate is slightly less than benzene but greater than nitrobenzene. Step 3 - Order of increasing rate: Slowest to fastest: nitrobenzene (d) < chlorobenzene (e) < benzene (a) = benzene-d6 (c) < toluene (b) This gives: d < e < a = c < b Step 4 - Match to options: Option (b) states: d < e < a = c < b — this matches exactly. Step 5 - Why other options fail: (a) b < c < a < d < e — incorrect order entirely; places toluene slowest and chlorobenzene fastest. (c) d < a = c < e < b — incorrect; places chlorobenzene faster than benzene, which is wrong since Cl is net deactivating. (d) a < c < b < e < d — incorrect; places nitrobenzene fastest and separates a from c incorrectly. Therefore, the correct answer is B.