nitration
/ ny-TRAY-shun /
Nitration is how you stamp a nitro group (-NO2) onto a benzene ring. It may sound exotic, but it is one of the most important reactions in the whole field, because the nitro group it installs is a doorway: it can later be reduced to an amino group (-NH2), which then opens up amines, dyes, drugs, and the diazonium chemistry that lets you put almost anything onto a ring.
The electrophile here is the nitronium ion, NO2(+), a small, linear, very reactive cation. You generate it on the spot by mixing concentrated nitric acid with concentrated sulfuric acid: the sulfuric acid protonates the nitric acid, water leaves, and NO2(+) is born (HNO3 + 2 H2SO4 -> NO2(+) + H3O(+) + 2 HSO4(-)). Once formed, the nitronium ion is electrophilic enough to be attacked by benzene's pi cloud. The usual EAS two-step follows: the ring bonds to the nitrogen to give an arenium ion, then loses a proton to restore aromaticity, yielding nitrobenzene.
Nitration shows up constantly in synthesis precisely because of that reduction trick. To make aniline (aminobenzene), you nitrate benzene and then reduce the -NO2 to -NH2. Because -NO2 is a strong deactivating, meta-directing group, it also makes the ring much less reactive toward a second EAS step and steers any further substitution to the meta position — a fact chemists exploit when ordering the steps of a multi-step aromatic synthesis.
Benzene + HNO3 / H2SO4 -> nitrobenzene + H2O. The sulfuric acid is not just a solvent; it manufactures the real electrophile, the nitronium ion NO2(+).
Nitric acid plus sulfuric acid makes NO2(+), the actual attacker.
The single most useful thing about nitration is that the nitro group is a masked amino group: -NO2 is easy to put on and easy to reduce to -NH2, so nitration-then-reduction is the standard route to anilines, even though you could never install -NH2 directly by EAS.