Fluoride ion is usually a poor leaving group because it is not very polarizable. Fluoride serves as the leaving group in the Sanger reagent (2, 4-dinitrofluorobenzene), used in the determination of peptide structures. Explain why fluoride works as a leaving group in this nucleophilic aromatic substitution, even though it is a poor leaving group in the SN1 and SN2 mechanisms.

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NO2 NO2 +HF NO NO 2,4-dinitrofluorobenzene amine 2,4-dinitrophenyl derivative (Sanger reagent) ai.