See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Concept: Sanger's reagent is a specific chemical used in the identification of the N-terminal amino acid of a peptide or protein chain. It reacts with the free amino group at the N-terminus under mild alkaline conditions via nucleophilic aromatic substitution. Reasoning: Sanger's reagent is defined as 1-fluoro-2,4-dinitrobenzene (FDNB), also written as 2,4-dinitrof luorobenzene (DNFB). It was developed by Frederick Sanger and used in his pioneering work on insulin sequencing. The fluorine atom is an excellent leaving group in nucleophilic aromatic substitution because the C-F bond is highly polarized and fluorine stabilizes the Meisenheimer complex through its strong inductive effect, making the ring highly electrophilic despite fluorine's poor leaving group ability in aliphatic systems. The two nitro groups at positions 2 and 4 further activate the ring toward nucleophilic attack. Why other options fail: - (b) Phenyl isocyanate: This is used for identifying alcohols and amines but is not Sanger's reagent. - (c) 2,4-Dinitrochlorobenzene: Although it can react with amines, it is NOT Sanger's reagent; the chloro derivative is less reactive than the fluoro derivative in this context, and the reagent specifically designated as Sanger's is the fluoro compound. - (d) 2,4-Dinitroi odobenzene: The iodo derivative is also not Sanger's reagent; iodine is a poorer leaving group than fluorine in nucleophilic aromatic substitution with electron-withdrawing groups present. Therefore, 2,4-dinitrof luorobenzene (option a) is correctly identified as Sanger's reagent. Therefore, the correct answer is A.