Discussion
Which of the following is most reactive towards nucleophilic aromatic substitution?
Answer: E
The most reactive is the ring with NO2 both ortho and para to Cl (1-chloro-2,4-dinitrobenzene).
Nucleophilic aromatic substitution goes through a negatively charged intermediate formed when the nucleophile adds at the carbon bearing Cl. A nitro group ortho or para to that carbon delocalises the negative charge onto its oxygens by resonance, stabilising the intermediate and speeding the reaction. Two such groups stabilise it more than one.
A single ortho or para nitro group helps, but less. A meta nitro group cannot take the charge by resonance and helps only weakly by induction. A second chlorine at the meta position gives almost no activation.
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