See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: In electrophilic aromatic substitution (EAS), substituents on a benzene ring are classified by two criteria: (i) whether they activate or deactivate the ring toward EAS, and (ii) whether they direct the incoming electrophile to ortho/para positions or to the meta position. Electron-donating groups (EDGs) activate the ring and are o/p-directors. Electron-withdrawing groups (EWGs) deactivate the ring and are meta-directors. Halogens are a special exception (deactivating but o/p-directing). Groups with lone pairs directly on the atom attached to the ring (N, O, S) can donate by resonance making them o/p-directors. Step 2 - Analysis of (a): N-phenyl-2-piperidinone. The nitrogen of the lactam is attached to the phenyl ring. Although the nitrogen lone pair is partially delocalized into the carbonyl of the lactam (making it less electron-rich than a free amine), there is still residual electron donation from nitrogen to the phenyl ring via resonance. The nitrogen lone pair donation to the ring (through N-to-ring conjugation) makes the group net activating and o/p-directing with respect to EAS on the phenyl ring. Hence (a) matches (p) activating and (r) o/p-director. Step 3 - Analysis of (b): Cyclohexylbenzene. An alkyl group (cyclohexyl) is an electron-donating group by hyperconjugation and induction. Alkyl groups activate the benzene ring and direct o/p. Hence (b) matches (p) activating and (r) o/p-director. Step 4 - Analysis of (c): Phenyl vinyl ether, Ph-O-CH=CH2. The oxygen atom bears lone pairs and is directly attached to the benzene ring. Oxygen is a strong o/p-director and activator through resonance donation of lone pairs into the ring. The vinyl portion does not negate the oxygen donation to the ring. Hence (c) matches (p) activating and (r) o/p-director. Step 5 - Analysis of (d): Ethylthiobenzene, Ph-S-Et. Sulfur, like oxygen, has lone pairs directly on the atom bonded to the ring. Sulfur donates lone pairs by resonance to the ring (though less effectively than oxygen due to poorer orbital overlap with the larger 3p orbital). Nonetheless, -SR groups are classified as activating and o/p-directing in EAS. Hence (d) matches (p) activating and (r) o/p-director. Step 6 - Why not (q) or (s) for any of these: None of the groups here are net EWGs that deactivate the ring in the classical sense for EAS regiochemistry purposes (e.g., -NO2, -CN, -COOH would be deactivating meta-directors). The lactam nitrogen, alkyl, vinyl ether oxygen, and thioether sulfur all provide net activation and o/p direction to the phenyl ring. Therefore, the correct answer is a-p,r; b-p,r; c-p,r; d-p,r.