Nitration of aromatic groups
Description: Aromatic groups can be nitrated by the action of nitric acid and an acid catalyst
Notes: A common acid for this reaction is sulfuric acid, H2SO4, although other acids can also be used. Note that water is a byproduct of this reaction.
Notes: Note that in the first example the NO2 group on the aromatic group directs the NO2 to the meta position. In the second example the Br directs the NO2 group to the para position (and also to the ortho, albeit less so due to steric hindrance). Finally in the third example the CH3 is a stronger directing group than the Br, so the NO2 is predominantly directed to the para position (but also to the position ortho to the CH3).
Mechanism: This is an electrophilic aromatic substitution. It proceeds in two phases, an “activation” phase, and the “substitution” phase.
In the first step of the activation phase, nitric acid is protonated to give an H2O(+) group attached to nitrogen (Step 1, arrows A and B). Elimination of water(Step 2, arrows C and D) lead to the nitronium ion NO2(+) which is a very good electrophile.
In the substitution phase, a π bond from the aromatic ring attacks the nitrogen of the nitronium ion, breaking aromaticity and forming a carbocation (Step 3, arrows E and F). A proton is then removed from the aromatic ring, re-forming the π bond and forming the aromatic product (Step 4, arrows G and H).
Notes: It’s also reasonable to show water as the base in step 4. HSO4(-) was shown here to demonstrate that the process is catalytic in acid.
Note that the π bond is broken in step 3 but re-formed in step 4.