For 1-X-2,4-dinitrobenzenes reacting with piperidine, the reactivity order is $\mathrm{F}>\mathrm{Cl}\approx \mathrm{Br}>\mathrm{I}$. The best explanation is that

1. Afluorine donates electron density into the ring by resonance, making the ring carbon more electrophilic
2. Bthe C-F bond is the weakest of the carbon-halogen bonds and therefore breaks fastest
3. Closs of the halide ion is rate determining and iodide, being most polarisable, leaves slowest
4. Daddition of the nucleophile to form the Meisenheimer complex is rate determining, and the strongly electronegative fluorine stabilises the developing negative charge in that step