The rates of nucleophilic substitution of nitrohalogenobenzenes depend on the halogen displaced. The order F>> Cl> Br> I, is the opposite of that observed for fitness as a leaving group in most other substitution reactions. How is this information consistent with the two-step mechanism for nucleophilic aromatic substitution?
The slow step of nucleophilic aromatic substitution by addition elimination is the nucleophilic attack on the carbon of the halogen. This stage is favored with electronegative groups that generate a high positive polarity on carbon, which explains the higher rate of reaction with fluorine.