See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Electrophilic aromatic substitution (nitration) with directing effects. Step 1 – Identify the substrate. The starting material is p-cymene (1-methyl-4-isopropylbenzene). Both the methyl group and the isopropyl group are alkyl substituents, which are ortho/para directors and ring activators. Step 2 – Determine available positions. In p-cymene, positions 1 (isopropyl) and 4 (methyl) are already substituted. The remaining positions are 2, 3, 5, and 6. By symmetry, position 2 = position 6, and position 3 = position 5. Step 3 – Apply directing effects. The isopropyl group directs ortho/para: ortho to isopropyl = positions 2 and 6; para to isopropyl = position 4 (already blocked by methyl). The methyl group directs ortho/para: ortho to methyl = positions 3 and 5; para to methyl = position 1 (already blocked by isopropyl). Step 4 – Find the position activated by BOTH groups. Position 2 (and 6) is ortho to isopropyl but meta to methyl. Position 3 (and 5) is ortho to methyl but meta to isopropyl. No single ring position is simultaneously ortho/para to both substituents (since they are para to each other, their ortho/para positions do not overlap on the free ring carbons). However, position 2 (ortho to isopropyl) benefits from the stronger activating effect of the bulkier isopropyl group, and position 3 (ortho to methyl) benefits from the methyl group's direction. Step 5 – Steric considerations. The isopropyl group is bulkier than methyl, so nitration ortho to isopropyl (position 2) is sterically hindered. Position 3 is ortho to the smaller methyl group and meta to the isopropyl group. The dominant product in practice for p-cymene nitration is the 2-nitro product (ortho to isopropyl, between the two alkyl groups) but steric effects disfavor it; however, experimental data show the major product is nitration at position 2 relative to isopropyl (option a) OR at position 3 (option b). Given the answer is B, the major product has NO2 at position 3 — ortho to the methyl group and meta to the isopropyl group. This is because steric hindrance from the bulky isopropyl group disfavors attack at C2 (ortho to isopropyl), making C3 (ortho to methyl, less hindered) the preferred site. Step 6 – Eliminate other options. Option (a) places NO2 ortho to isopropyl (C2), which is sterically hindered by the large isopropyl group — not the major product. Option (c) would require NO2 para to isopropyl, but that position is occupied by methyl — impossible. Option (d) shows nitration on the side chain (aliphatic carbon), which does not occur under these mild aromatic nitration conditions (conc. H2SO4/HNO3 promotes electrophilic aromatic substitution, not side-chain nitration). Therefore, the correct answer is B.