In the reaction of 1,2-dichloro-4-nitrobenzene with sodium ethoxide, why does ethoxide end up substituting the chlorine para to the nitro group rather than the chlorine meta to the nitro group?

Reaction of 1,2-dichloro-4-nitrobenzene with sodium ethoxide

I think that C–2 should be the most electron-poor carbon (and hence prone to nucleophilic attack) since it experiences a combined electron withdrawal from the nitro and chloro groups flanking it. Shouldn't the nucleophilic ethoxide anion attack there instead?


You can obtain an answer by looking at the electronic structure. $\ce{NO2}$ is a meta director for electrophilic aromatic substitution (EAS). The question asks about nucleophilic aromatic substitution (NAS). Aromatic substitutions involve a free pair of electrons resonating around a structure. Electrophiles may add at any resonance site where the pair of electrons are. Nucleophiles may inversely add to any site where electrons are not present shown in the picture below. Therefore if a functional group is meta directing for EAS it will be ortho/para directing for NAS and vice versa.

enter image description here


The nitro group is strongly meta-directing ... for electrophilic substitution. But look a little more closely. Is this really electrophilic substitution?

Once you figure that out, draw the mechanism as suggested in @jerepierre's comment. Now you should see what really happens.


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