See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: The Hofmann rearrangement (Hofmann degradation) converts a primary amide (R-CO-NH2) to a primary amine (R-NH2) with loss of CO2, using NaOCl (or Br2/NaOH) in aqueous base. When applied to a cyclic imide (phthalimide derivative), the reaction opens the imide ring and converts one of the carbonyl-nitrogen bonds via Hofmann rearrangement. Step 1 – Identify the starting material: The starting material is 4-nitrophthalimide. This is a phthalimide (isoindole-1,3-dione) with a nitro group at the 4-position of the benzene ring. The imide has two C=O groups and one NH. Step 2 – Reaction with NaOCl/H2O: NaOCl in water is the classic Hofmann rearrangement reagent. Under basic aqueous conditions, NaOCl reacts with the NH of the imide. The mechanism involves: (i) N-chlorination of the imide NH to give N-chloro intermediate; (ii) base abstracts the remaining NH proton (already done); (iii) Hofmann rearrangement occurs — one acyl group migrates, forming an isocyanate intermediate; (iv) the isocyanate hydrolyzes in water to give a carbamic acid, which loses CO2 to yield a primary amine. Simultaneously, the other carbonyl (ester/acid) remains as a carboxylic acid after ring opening. Step 3 – Determine regiochemistry of the product: In 4-nitrophthalimide, the nitro group is at position 4 of the benzene ring. The two carbonyl groups are at positions 1 and 3 of the isoindole system. The Hofmann rearrangement converts one C=O-NH unit to NH2, and the other C=O becomes CO2H after hydrolysis. The carbon bearing the nitro group is adjacent to one of the carbonyl carbons. The product will have NH2 and CO2H groups on adjacent carbons of the benzene ring (ortho to each other), with the NO2 group also on the ring at the position adjacent to CO2H. Step 4 – Assign the product structure: The product has: NH2 ortho to CO2H on the benzene ring, with NO2 at the position adjacent to CO2H (i.e., NH2 and NO2 are on either side of CO2H, or specifically NH2 at one position, CO2H next, NO2 next). Based on the numbering of 4-nitrophthalimide, the NO2 is at position 4 (adjacent to the C3 carbonyl carbon). After Hofmann rearrangement, C1-carbonyl becomes CO2H and C3-carbonyl-NH becomes NH2 (or vice versa). The result is that NH2 is at the carbon adjacent to NO2, and CO2H is between NH2 and the ring junction — giving NH2 at C1 of the benzene, CO2H at C2, NO2 at C3 (i.e., option b pattern). Step 5 – Eliminate other options: (a) has CO2H and NH2 swapped relative to the correct regiochemistry given the NO2 position — incorrect regiochemistry. (c) gives a dicarboxylic acid with no amine — this would be simple hydrolysis, not Hofmann rearrangement. (d) places NO2 para to NH2, which does not match the 4-nitrophthalimide starting material connectivity. Therefore, the correct answer is B.