In \(S_{N} 2\) reactions of alkyl halides, the order of reactivity is \(\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}\). Alkyl iodides are considerably more reactive than alkyl fluorides, often by factors as great as \(10^{6}\). All 1-halo-2,4-dinitrobenzenes, however, react at approximately the same rate in nucleophilic aromatic substitutions. Account for this difference in relative reactivities.

The reactivity of alkyl halides in S\(_{N} 2\) reactions follows the order: \(\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}\). This order is due to the strength of the carbon-halogen bond. Generally, the weaker the carbon-halogen bond, the more reactive the alkyl halide is in S\(_{N} 2\) reactions. The bond strength decreases as we move from fluorine to iodine in the periodic table (C-F > C-Cl > C-Br > C-I), causing iodine to be a better leaving group. Hence, alkyl iodides are more reactive than other alkyl halides.

In nucleophilic aromatic substitutions, all 1-halo-2,4-dinitrobenzenes react at approximately the same rate. This is because the nucleophilic aromatic substitution reaction is governed by the stability and reactivity of the intermediate Meisenheimer complex, which is stabilized by the electron-withdrawing nitro groups attached to the benzene ring. As a result, the reaction rate is dictated more by the resonance stabilization provided by the nitro groups and less by the carbon-halogen bond strength of the starting halo compound.

The difference in relative reactivities between alkyl halides in S\(_{N} 2\) reactions and 1-halo-2,4-dinitrobenzenes in nucleophilic aromatic substitutions is due to the factors controlling the reaction rates. In S\(_{N} 2\) reactions, the reaction rate is primarily determined by the strength of the carbon-halogen bond, making alkyl iodides considerably more reactive than alkyl fluorides. In nucleophilic aromatic substitutions, however, the reaction rate is largely controlled by the stability of the Meisenheimer complex intermediate, which is influenced by the electron-withdrawing nitro groups. As a result, all 1-halo-2,4-dinitrobenzenes react at approximately the same rate, regardless of the carbon-halogen bond strength.