Aromatic Substitution: Nucleophilic (SNAr)

(a) Explain why 1-fluoro-2,4-dinitrobenzene reacts readily with NaOH at room temperature, while fluorobenzene does not. (b) Rank the following in order of reactivity toward SNAr: 1-chloro-4-nitrobenzene, 1-chloro-2,4-dinitrobenzene, 1-chloro-2,4,6-trinitrobenzene (picryl chloride). (c) Why is fluorine a better leaving group than iodine in SNAr, but worse in aliphatic SN2? (d) Draw the Meisenheimer complex intermediate.