Electrophilic Aromatic Substitution - Nitration
The electrophile for the nitration reaction, the nitronium cation (NO2+), is generated by reaction of nitric acid (HNO3) with sulfuric acid (H2SO4). Electrophilic aromatic substitution begins with the addition of the electrophile into the aromatic pi?system of the ring. A conjugated, carbocation intermediate is formed, a resonance combination of three forms, concentrating positive charge at three locations, the two ortho positions and one para. After this addition, a proton departs, completing the overall substitution with aromaticity restored.
Consider the case where electrophilic aromatic substitution is attempted on an aromatic ring that already contains an original nitro substituent. If electrophilic aromatic substitution is attempted upon rings that already contain substituents, the location of the next substitution depends on the characteristics of the original substituent. Whether the new substitution occurs ortho, para or meta to the original substituent depends on whether the original substituent either stabilizes or destabilizes a concentration of positive charge upon its carbon at the carbocation intermediate stage. If the substituent already present is electron donating, it will stabilize the carbocation by donating negative charge. New substitutions will occur ortho or para to such electron donating substituents already present on the ring. Electron withdrawing substituents destabilize a carbocation, so the new substitution will most likely be meta. Nitro is such a substituent. Containing only electronegative atoms, nitro is electron-withdrawing by induction. Furthermore, the nitrogen atom in nitro also has no electron pairs capable of donating into the ring by resonance. A nitro group, already present on the ring, is a ring deactivating, meta director for further electrophilic aromatic substitution.
(Some substituents, such as halogen or hydroxide, even though being electronegative (electron withdrawing inductively), will donate electron pairs to the ring by resonance and are still ortho para directing, though deactivating by induction.)