Halogen Compounds

Specification Reference Organic Chemistry, Halogen Compounds 31.1

Quick Notes

Halogenoarenes are formed by electrophilic substitution of arenes with Cl₂ or Br₂ in the presence of a halogen carrier acid catalyst (AlCl₃ or AlBr₃).
Halogenoarenes (e.g. chlorobenzene) are less reactive towards nucleophilic substitution than halogenoalkanes (e.g. chloroethane).
- This is due to delocalisation of the lone pair on the halogen into the aromatic ring and partial double bond character in the C–Cl bond of halogenoarenes.

Full Notes

Formation of Halogenoarenes

Halogenoarenes are aromatic compounds where a halogen atom (Cl or Br) is directly bonded to a benzene ring.

They are formed in electrophilic substitution reactions of benzene with a halogen:

Reagents: Cl₂ or Br₂
Catalyst: AlCl₃ or AlBr₃ (a Lewis acid to generate the electrophile)
Mechanism: Electrophilic substitution
Conditions: Room temperature, anhydrous

Example: Bromination of Benzene

CIE A-Level Chemistry diagram showing electrophilic substitution of benzene with bromine in the presence of AlBr3 catalyst, forming bromobenzene.

Benzene + Br₂ (with AlBr₃) → Bromobenzene + HCl

The AlBr₃ catalyst polarises Br₂ to generate a Br⁺ electrophile.

Example: Chlorination of Methylbenzene

CIE A-Level Chemistry diagram showing chlorination of methylbenzene producing 2-bromomethylbenzene and 4-bromomethylbenzene.

Methylbenzene + Br₂ → 2-bromomethylbenzene and 4-bromomethylbenzene

The methyl group is an electron-donating group.
It activates the ring and directs substitution to the 2nd and 4th positions.

Why is a Catalyst Needed?

The aromatic ring is very stable due to the delocalised π-electron system. A halogen molecule alone isn't reactive enough, so the Lewis acid catalyst helps generate a stronger electrophile (Cl⁺ or Br⁺).

Comparing Reactivity: Halogenoalkanes vs. Halogenoarenes

Halogenoalkanes (e.g. chloroethane) readily undergo nucleophilic substitution because the polar C–Cl bond allows the nucleophile to attack the partially positive carbon.

CIE A-Level Chemistry diagram showing polarised C–Cl bond in halogenoalkane where nucleophile attacks the carbon atom.

Halogenoarenes (e.g. chlorobenzene) are less reactive towards nucleophiles.

This is due to several reasons:

Resonance (Delocalisation): The lone pair on the chlorine can delocalise into the benzene ring, creating a partial double bond character between C–Cl. This strengthens the bond and makes it harder to break.
Electron Density: The benzene ring is electron-rich, so it repels nucleophiles.
Bond Strength: The C–Cl bond in halogenoarenes is shorter and stronger due to partial double bond character, making cleavage more difficult.

CIE A-Level Chemistry diagram comparing bond strength of C–Cl in halogenoalkane and halogenoarene.

Examples

Chloroethane reacts easily with aqueous NaOH (nucleophilic substitution).
Chlorobenzene does not react under the same conditions, and requires much more severe conditions (e.g. high pressure and temperature, stronger nucleophiles).

CIE A-Level Chemistry comparison diagram showing reactivity differences between chloroethane and chlorobenzene with nucleophiles.

Summary

Halogenoarenes are formed by electrophilic substitution with halogens in the presence of AlCl₃/AlBr₃.
They are less reactive towards nucleophilic substitution compared to halogenoalkanes.
This reduced reactivity is due to resonance delocalisation, electron density effects, and stronger C–Cl bonds.
Halogenoalkanes readily undergo nucleophilic substitution, but halogenoarenes require much harsher conditions.