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?
SOLUTION:
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.
