See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Benzyne (elimination-addition) mechanism with 14C labeling. Step 1 - Identify the starting material: Chlorobenzene with 14C (*) at C1, the carbon bearing the chlorine substituent. Step 2 - Benzyne formation: NaNH2 abstracts a proton adjacent to the C-Cl carbon. Two benzyne intermediates can form: - Benzyne with the triple bond between C1(*) and C2 (by loss of H from C2) - Benzyne with the triple bond between C1(*) and C6 (by loss of H from C6, equivalent by symmetry to C2) Both form equally, so effectively we have one type of benzyne: triple bond between C1(*) and C2. Step 3 - Amide ion addition to benzyne: NH2- can attack either carbon of the triple bond: - Attack at C1(*): gives aniline with NH2 at C1* (labeled carbon bears NH2) — this is product (a), with * at the amino-bearing carbon - Attack at C2: gives aniline with NH2 at C2 and * at C1 (adjacent to NH2) — this is product (b) Step 4 - Product ratio analysis: The problem states products form in 1:2:1 ratio. By symmetry of the benzyne (C1*-C2 triple bond), attack at C2 gives the product where * is at the carbon adjacent to NH2. Since both C1*-C2 and C1*-C6 benzynes form equally, and attack can occur at either end: - Product where NH2 is on unlabeled carbon and * is adjacent (ortho to NH2): formed from attack at C2 of both benzynes — this accounts for 2 out of 4 possible pathways (the major product in 2/4 = 1/2 fraction) - Product where NH2 is on * carbon: formed from the remaining pathways The major product (formed in ratio 2, i.e., 50%) is aniline where NH2 is at the unlabeled carbon and 14C (*) is at the adjacent carbon — corresponding to option (b): the structure showing * at a ring carbon adjacent to the NH2 group. Step 5 - Why other options fail: (a) NH2 directly on the 14C carbon — this is a minor product (ratio 1) (c) Shows NH2 and * in a meta relationship — not possible from benzyne addition (benzyne puts NH2 at one of the two triple-bond carbons, which are always adjacent) (d) Shows * at para position to NH2 — not consistent with benzyne mechanism from this starting material Therefore, the correct answer is B.