Halogenoarenes
Halogenoarenes
- A halogenoarene (aryl halide) has a halogen joined to a benzene ring.
- It is made by electrophilic substitution.
- It is far less reactive than a halogenoalkane.
Making one
- An arene reacts with $\text{Cl}_2$ or $\text{Br}_2$ using $\text{AlCl}_3$/$\text{AlBr}_3$ as catalyst — the halogen replaces a ring hydrogen.
- benzene → chlorobenzene; methylbenzene → 2- and 4-chloromethylbenzene.
A halogenoarene is made by reacting an arene with a halogen and:
The halogen-carrier catalyst enables electrophilic substitution of a ring hydrogen.
Why it is unreactive
A halogenoalkane reacts easily by nucleophilic substitution (polar C–Cl). A halogenoarene does not, for two reasons:
- a lone pair on the halogen overlaps the delocalised ring, giving the C–Cl bond partial double-bond character — shorter and stronger, so hard to break.
- the electron-rich ring repels an approaching nucleophile.
So chlorobenzene does not react with $\text{OH}^-$ under normal conditions, while chloroethane does.
A halogenoarene is unreactive towards nucleophiles partly because:
The lone-pair overlap gives partial double-bond character, making C–Cl shorter and stronger.
The second reason a halogenoarene resists nucleophiles is that the ring:
The delocalised, electron-rich ring repels electron-rich nucleophiles.
Under normal conditions, with OH⁻:
The halogenoalkane (chloroethane) undergoes nucleophilic substitution; the halogenoarene (chlorobenzene) does not.
You've got it
- a halogenoarene is made by electrophilic substitution (arene + Cl₂/AlCl₃)
- it is unreactive to nucleophiles: the Cl lone pair strengthens the C–Cl bond, and the ring repels nucleophiles
- chlorobenzene does not react with OH⁻; chloroethane does