See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: In electrophilic aromatic substitution (EAS), the rate is governed by how electron-rich the aromatic ring is. Electron-donating groups (EDGs) activate the ring and increase the rate, while electron-withdrawing groups (EWGs) deactivate the ring and decrease the rate. Step 1 - Analyze each compound's effect on the aromatic ring: (a) Tetralin: The fused cyclohexane ring has no heteroatom. The ring is a simple alkyl-fused benzene. Alkyl groups are weakly activating via hyperconjugation/induction. (b) 1,2,3,4-Tetrahydroquinoline: The nitrogen (NH) is directly adjacent to the benzene ring at position 1. The nitrogen lone pair can donate into the aromatic ring through resonance (similar to aniline), making the nitrogen a powerful ortho/para director and strong activating group. (c) 1,2,3,4-Tetrahydroisoquinoline: The nitrogen (NH) is at position 2, one carbon removed from the benzene ring. The nitrogen lone pair cannot conjugate directly with the aromatic ring because there is a CH2 group between N and the ring. It acts more like a weak inductive donor. (d) 3,4-Dihydroisoquinolin-1(2H)-one: Contains a carbonyl group (C=O) conjugated with the aromatic ring, making it an electron-withdrawing group. This deactivates the ring toward EAS. Step 2 - Rank the activating ability: - Option (b): N lone pair directly conjugated with the aromatic ring (vinylogous amine / enamine-like resonance), strongest activation. - Option (c): N is not directly conjugated with the ring (saturated CH2 intervenes), weaker activation than (b). - Option (a): Only weak alkyl activation. - Option (d): Carbonyl EWG deactivates the ring, slowest rate. Step 3 - Why (b) is faster than (c): In tetrahydroquinoline, the N is bonded directly to the aromatic carbon (C8a), allowing direct resonance donation of the nitrogen lone pair into the ring, strongly activating it — analogous to aniline. In tetrahydroisoquinoline, the N is at position 2 with a CH2 spacer between it and the ring, so direct resonance into the ring is not possible. Step 4 - Why other options fail: (a) Only alkyl activation, much weaker than nitrogen lone pair donation. (c) Nitrogen not directly conjugated with aromatic ring. (d) Carbonyl group withdraws electrons from the ring, strongly deactivating. Therefore, the correct answer is B.