31.1 Halogen compounds
- Syllabus
- 9701–2028–2029
- Topic
- 31.1
- Level
- A2
Learning objectives
31.1.1Preparation of halogenoarenes• Recall the reactions by which halogenoarenes can be produced: substitution of an arene with C l₂ or Br₂ with a catalyst, Al Cl₃ or Al Br₃ to form a halogenoarene, exemplified by benzene to form chlorobenzene and methylbenzene to form 2-chloromethylbenzene and 4-chloromethylbenzene31.1.2Difference in reactivity• Explain the difference in reactivity between a halogenoalkane and a halogenoarene as exemplified by chloroethane and chlorobenzene
Lewis-acid catalysis prepares halogenoarenes from arenes
Benzene or methylbenzene reacts with Cl2 or Br2 in the presence of anhydrous AlCl3 or AlBr3. The halogen carrier generates a strong electrophile and enables electrophilic substitution on the ring.
The arene loses H+ after electrophile attack, restoring the delocalised pi system. Methylbenzene gives mainly the 2- and 4-substituted products because the methyl group directs incoming electrophiles to those positions.
| Starting arene | Reagents | Halogenoarene products | Other product |
|---|---|---|---|
| benzene | Cl2, anhydrous AlCl3 | chlorobenzene | HCl |
| benzene | Br2, anhydrous AlBr3 | bromobenzene | HBr |
| methylbenzene | Cl2, anhydrous AlCl3 | 2-chloromethylbenzene and 4-chloromethylbenzene | HCl |
Without the Lewis-acid halogen carrier, simply mixing benzene and chlorine is not the syllabus preparation. Do not confuse catalyst-driven ring halogenation with UV-initiated substitution in an alkyl side-chain.
Lone-pair delocalisation makes the C-Cl bond in chlorobenzene harder to break
Chloroethane readily undergoes nucleophilic substitution, whereas chlorobenzene does not under ordinary halogenoalkane conditions. The difference follows from the electronic environment of the carbon-chlorine bond, not merely from both compounds containing chlorine.
| Feature | Chloroethane, C2H5Cl | Chlorobenzene, C6H5Cl |
|---|---|---|
| carbon bonded to Cl | sp3 carbon in an alkyl group | sp2 carbon in the aromatic ring |
| electron interaction | polar sigma C-Cl bond; no overlap with a neighbouring aromatic pi system | a Cl lone pair overlaps with the ring pi system and becomes partly delocalised |
| C-Cl character | ordinary single bond | partial double-bond character, so shorter and stronger |
| nucleophilic substitution | OH- can attack the delta-positive carbon and displace Cl- on heating | C-Cl is much harder to break, so ordinary aqueous-OH- conditions do not readily substitute it |
Overlap of a chlorine lone pair with adjacent ring p orbitals spreads electron density into the aromatic pi system. This gives the C-Cl bond partial double-bond character; the stronger bond requires much more energy to break, so chlorobenzene is far less reactive than chloroethane towards nucleophilic substitution.
Chlorobenzene can react with hydroxide only under very harsh conditions, commonly described as about 200 °C and 200 atm. This exception reinforces rather than removes the comparison: it does not behave like an ordinary halogenoalkane.
Do not explain chlorobenzene's low reactivity by saying only that benzene is stable or that chlorine is attached to a ring. The scored causal link is lone-pair delocalisation -> partial double-bond character -> stronger C-Cl bond -> difficult bond breaking.