Halogenoarenes
| English | Chinese | Pinyin |
|---|---|---|
| halogenoarene | 卤代芳烃 | lǔ dài fāng tīng |
| aryl halide | 芳基卤 | fāng jī lǔ |
| nucleophilic substitution | 亲核取代 | qīn hé qǔ dài |
| persistent | 持久 | chí jiǔ |
| pesticide | 农药 | nóng yào |
| bioaccumulate | 生物累积 | shēng wù lěi jī |
Halogens on a ring
- A halogenoarene 卤代芳烃 (aryl halide 芳基卤) has a halogen joined to a benzene ring.
- It is made by electrophilic substitution.
- It is far less reactive than a halogenoalkane.
Halogenoarene lab
Compare halogenoarenes with halogenoalkanes.
The C–halogen bond in a halogenoarene is less reactive than in a halogenoalkane.
The ring's delocalised electrons strengthen the bond.
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.
Benzene + Cl₂ with an AlCl₃ catalyst → chlorobenzene, by electrophilic substitution
A halogenoarene is made by reacting an arene with a halogen and:
The halogen-carrier catalyst enables electrophilic substitution of a ring hydrogen.
Match each term to its meaning.
Each item links the term to its correct meaning.
Why it is unreactive
A halogenoalkane reacts easily by nucleophilic substitution 亲核取代 (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.
Why a halogenoarene is much less reactive than a halogenoalkane
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.
Useful but persistent 持久
- That same stability makes halogenoarenes useful — in pesticides 农药 (e.g. DDT), dyes and many synthesis intermediates.
- But being unreactive, they do not break down in the environment.
- So they persist and bioaccumulate 生物累积 up food chains — which is why several, including DDT, have been banned.
Many pesticides and herbicides are halogenoarenes — chlorine atoms bonded to a benzene ring.
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
- that stability makes them useful (pesticides, dyes) but persistent pollutants (DDT was banned)