3.3.10.2 - Electrophilic Substitution

Why benzene prefers substitution

Benzene contains a ring of delocalised electrons above and below the plane of the carbon atoms. That delocalised cloud is an area of high electron density, so it attracts species that are short of electrons.

When an electrophile approaches benzene, a pair of electrons from the delocalised ring forms a new bond to the electrophile. At that moment the aromatic ring is temporarily disrupted, so the intermediate is unstable. A proton is then removed from the carbon that now carries the new group, and the delocalised ring is restored.

That final restoration step explains why benzene usually undergoes substitution rather than addition. An addition reaction would leave the ring less stable because the delocalised pi system would not be fully regained. In this specification, we focus on monosubstitution only: one hydrogen on the ring is replaced by one incoming electrophile.

The diagram below shows the shared three-step pattern for electrophilic substitution: attack by the electrophile, formation of the sigma-complex, and loss of $\ce{H^+}$ to restore the aromatic ring.

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A reliable way to tell any electrophilic substitution mechanism is to use the same three-step pattern: make the electrophile, attack the ring, then remove $\ce{H^+}$ to restore aromaticity.

In a drawn mechanism, start the first curly arrow on the aromatic circle (or a drawn pi bond) and end it at the electrophilic atom. In the intermediate the attacked carbon has both H and E and four single bonds. Draw a horseshoe inside the other five carbon atoms, open at the attacked carbon, with a positive charge inside the ring. Do not draw a complete aromatic circle in this intermediate. To restore the ring, start the second curly arrow on the C–H bond of that same carbon and point into the ring; show $\ce{H+}$ lost. An arrow pointing from the ring towards $\ce{H+}$ would wrongly send the bonding electrons away with the proton.

Nitration of benzene

Nitration is one of the most important electrophilic substitution reactions because it introduces a nitro group, $\text{-}\ce{NO2}$, onto the ring. That matters in synthesis because nitrobenzene can later be reduced to an aromatic amine, and nitration steps are also used in the manufacture of explosive nitro compounds.

The reagents are concentrated nitric acid and concentrated sulfuric acid, warmed to about $55\,{}^\circ\mathrm{C}$. Sulfuric acid is the stronger acid, so it helps generate the actual electrophile needed for the reaction: the nitronium ion, $\ce{NO2^+}$.

Once $\ce{NO2^+}$ has formed, a pair of electrons from benzene attacks the nitrogen atom. That gives the usual unstable intermediate. A base in the mixture, commonly $\ce{HSO4^-}$, then removes $\ce{H^+}$ from the ring carbon, and the aromatic ring is restored to give nitrobenzene.

In nitration, sulfuric acid is regenerated after $\ce{H^+}$ reacts with $\ce{HSO4^-}$, so sulfuric acid acts as a catalyst as well as helping generate the electrophile. Keeping the temperature around $55\,{}^\circ\mathrm{C}$ matters because higher temperatures make further substitution more likely, while this specification point is limited to monosubstitution.

The diagram below lays out the nitration mechanism from generation of $\ce{NO2^+}$ to deprotonation and formation of nitrobenzene.