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Oximes of 2-bromo-5-nitroaryl carbonyls in base and after O-acetylation: intramolecular SNAr, the Kemp elimination, dehydration to the nitrile and ester hydrolysis

JEE Advanced 2026 Paper 1 · Question 16 Chemistry Reactions of oximes - intramolecular nucleophilic aromatic substitution, Kemp elimination and dehydration of O-acyl aldoximes to nitriles Difficult

Four reactions of one ring - Br ortho to an oxime, NO2 para to the Br - run at two levels at once. A 3-D reach scan turns the side chain about the ring-C bond and shows that only a Z oxime's oxygen can come within contact of the C-Br carbon; then each flask runs with every electron pair pushed as a curved arrow: deprotonation, intramolecular SNAr through a Meisenheimer complex, bromide loss, the Kemp elimination of the 3-H benzisoxazole to 2-cyano-4-nitrophenoxide (P); O-acetylation and anti E2 to 2-bromo-5-nitrobenzonitrile (Q); SNAr to 3-methyl-5-nitro-1,2-benzisoxazole with no Kemp elimination (R); and hydrolysis of the E O-acetyl ketoxime to the E oxime (S). An analysis bench (A380, AgBr, IR C#N) and a graph match to List-II give option (B).

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Sub-topics covered

E/Z isomerism of oximes and its consequences
Intramolecular SNAr of o-halo aryl oximes to 1,2-benzisoxazoles
Activation of SNAr by a para-nitro group (Meisenheimer complex)
Kemp elimination of 3-unsubstituted 1,2-benzisoxazoles
O-Acetylation of oximes and anti elimination of O-acyl aldoximes to nitriles
Hydrolysis of O-acyl ketoximes
Beckmann rearrangement (distractor)

What it teaches

C=N does not rotate: only a Z oxime O (syn to the ring) can reach the ortho C-Br carbon; an E oxime O never can
Intramolecular SNAr needs the ring activated: the NO2 para to Br takes the Meisenheimer charge
A 1,2-benzisoxazole with H on C3 is opened by base (Kemp elimination) to a 2-cyanophenoxide; with CH3 on C3 it survives
O-Acetylation removes the nucleophile; an O-acetyl aldoxime loses AcOH by anti E2 to the nitrile
An O-acetyl ketoxime in aqueous carbonate is only hydrolysed; the C=N geometry is kept
The amide (3) is a Beckmann product - it needs acid, not base

Simulation ID

ADV-2026-P1-CHE-Q16 · revision 1

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