Halogen compounds

A halogenoarene 卤代芳烃 (also called an aryl halide) is formed when an arene 芳烃 reacts with $\text{Cl}_2$ or $\text{Br}_2$, using $\text{AlCl}_3$ or $\text{AlBr}_3$ as a catalyst 催化剂. This is the electrophilic substitution from the arenes topic — the halogen replaces a hydrogen on the ring.

• benzene gives chlorobenzene.
• methylbenzene gives 2-chloromethylbenzene and 4-chloromethylbenzene (the methyl group directs the chlorine to positions 2 and 4).

Making a halogenoarene by electrophilic substitution: with an AlCl₃ catalyst, a chlorine replaces a hydrogen on the ring (giving HCl)

Many pesticides and herbicides are halogenoarenes — chlorine atoms bonded to a benzene ring

Compare chloroethane (a halogenoalkane 卤代烷) with chlorobenzene (a halogenoarene).

A halogenoalkane reacts easily by nucleophilic substitution 亲核取代: its C–Cl bond is polar, so a nucleophile can attack the slightly positive carbon and push the halogen out.

A halogenoarene is very unreactive towards nucleophilic substitution. There are two reasons:

• a lone pair 孤对电子 on the chlorine overlaps sideways with the delocalised 离域 ring of electrons. This gives the C–Cl bond partial double-bond character, making it shorter and stronger, so it is much harder to break.
• the electron-rich ring repels an approaching nucleophile 亲核试剂.

So chlorobenzene does not react with nucleophiles such as $\text{OH}^-$ under normal conditions, while chloroethane does.

Chloroethane reacts (a nucleophile attacks the $\delta+$ carbon), but chlorobenzene does not: the Cl lone pair strengthens the C–Cl bond and the ring repels nucleophiles

• A halogenoarene is less reactive than a halogenoalkane: the lone pair delocalises into the ring, giving the $\text{C-X}$ bond partial double-bond character.
• Distinguish a halogen on the ring (needs a catalyst, unreactive to substitution) from one on a side chain (reacts like a halogenoalkane).