JEE Advanced Chemistry Question (2026) — Solution

Step-by-step solution

For the reaction in (P), 2-bromo-5-nitrobenzaldehyde oxime reacts with aqueous NaOH. The strong base deprotonates the oxime group. The resulting oximate anion undergoes intramolecular nucleophilic aromatic substitution (S_NAr), attacking the ortho-carbon and displacing the bromide ion to form a 1,2-benzisoxazole intermediate. Since it is derived from an aldoxime, this intermediate has a hydrogen atom at the C3 position. In the presence of NaOH, it undergoes base-catalyzed Kemp elimination (ring opening) to form 2-hydroxy-5-nitrobenzonitrile, which corresponds to structure (1). Thus, P 1. In reaction (Q), the aldoxime is treated with acetic anhydride, which acetylates the oxime group to form an O-acetate. Subsequent treatment with the mild base Na_2CO_3 causes the elimination of acetic acid, converting the aldoxime acetate into a nitrile. The bromide is not displaced under these mild conditions. The product is 2-bromo-5-nitrobenzonitrile, which corresponds to structure (2). Thus, Q 2. For the reaction in (R), 2-bromo-5-nitroacetophenone oxime reacts with aqueous NaOH. Similar to (P), the oximate anion is formed and displaces the ortho-bromide via S_NAr to form a 1,2-benzisoxazole ring. However, because it is derived from a ketoxime, there is a methyl group at the C3 position instead of a hydrogen. Therefore, Kemp elimination cannot occur, and the reaction stops at the stable 3-methyl-5-nitro-1,2-benzisoxazole, which corresponds to structure (4). Thus, R 4. In reaction (S), the O-acetate of the ketoxime is treated with aqueous Na_2CO_3. Since it is a ketoxime derivative, it cannot undergo elimination to form a nitrile. The mild base simply hydrolyzes the acetate group back to the oxime. The base is not strong enough to promote the subsequent S_NAr ring closure. The product is 2-bromo-5-nitroacetophenone oxime, which corresponds to structure (5). Thus, S 5. Combining these results gives the sequence P 1, Q 2, R 4, S 5. Answer: P 1; Q 2; R 4; S 5