Halogen Compounds

· Halogen compounds in this topic = mainly halogenoarenes, where a halogen atom is bonded directly to a benzene ring.
· General structure of a halogenoarene: Ar–X, where Ar = aryl group and X = Cl or Br.
· Key examples: chlorobenzene, bromobenzene, methyl-substituted chlorobenzenes.
· Main CIE focus: how halogenoarenes are made and why they are less reactive than halogenoalkanes.

Chlorobenzene is a halogenoarene because the chlorine atom is directly bonded to the benzene ring. This differs from chloroalkanes, where chlorine is bonded to an alkyl carbon. Source

Production of Halogenoarenes

· Halogenoarenes are produced by electrophilic substitution of an arene.
· Benzene reacts with Cl₂ or Br₂ in the presence of a halogen carrier catalyst.
· Catalyst for chlorination: AlCl₃.
· Catalyst for bromination: AlBr₃.
· General reaction: C₆H₆ + X₂ → C₆H₅X + HX.
· Example: benzene + chlorine → chlorobenzene + hydrogen chloride.
· Equation: C₆H₆ + Cl₂ → C₆H₅Cl + HCl.
· Conditions: Cl₂ / AlCl₃ or Br₂ / AlBr₃, usually room temperature.
· Reaction type: electrophilic substitution, not addition, because the benzene ring retains aromatic stability.

This page shows how benzene undergoes halogenation by electrophilic substitution. The catalyst helps generate a stronger electrophile so the benzene ring can react with Cl₂ or Br₂. Source

Electrophilic Substitution: Exam Mechanism Idea

· The benzene ring contains a delocalised π electron system.
· AlCl₃ / AlBr₃ polarises the halogen molecule, helping to generate an electrophile.
· Example electrophile: Cl⁺ or Br⁺.
· The π electrons in benzene attack the electrophile.
· One H atom on the ring is replaced by Cl or Br.
· The catalyst is regenerated, so it is not used up overall.
· Key exam phrase: benzene undergoes substitution rather than addition to preserve aromatic stability.

The diagram summarises the overall pattern of electrophilic aromatic substitution. It shows that an electrophile replaces a hydrogen atom on the aromatic ring while aromaticity is restored at the end. Source

Halogenation of Methylbenzene

· Methylbenzene can also form halogenoarenes by reaction with Cl₂ or Br₂ and AlCl₃ or AlBr₃.
· The methyl group directs substitution mainly to the 2-position and 4-position.
· Products from chlorination include 2-chloromethylbenzene and 4-chloromethylbenzene.
· These are often described as ortho- and para-substituted products.
· Exam point: make sure the halogen is shown on the benzene ring, not on the methyl side-chain, when using AlCl₃ / AlBr₃.
· If conditions involve UV light, halogenation may occur in the side-chain instead; this is not the same as halogenoarene formation.

This mechanism shows how an aromatic ring reacts with bromine in the presence of a Lewis acid catalyst. It is useful for understanding why halogenoarenes form by substitution, not addition. Source

Halogenoalkanes vs Halogenoarenes

· Halogenoalkane example: chloroethane, CH₃CH₂Cl.
· Halogenoarene example: chlorobenzene, C₆H₅Cl.
· Chloroethane reacts more readily in nucleophilic substitution because the C–Cl bond is polar and the carbon bonded to chlorine is accessible to nucleophiles.
· Chlorobenzene is much less reactive towards nucleophilic substitution.
· In chlorobenzene, a lone pair on chlorine overlaps with the delocalised π system of benzene.
· This gives the C–Cl bond partial double bond character.
· The C–Cl bond in chlorobenzene is stronger and shorter than in chloroethane.
· The benzene ring’s high electron density also repels incoming nucleophiles.
· Key comparison: halogenoalkanes are generally more reactive than halogenoarenes in nucleophilic substitution.

Why Chlorobenzene Is Less Reactive Than Chloroethane

· In chloroethane, the C–Cl bond is a normal single bond.
· Nucleophiles can attack the δ⁺ carbon bonded to chlorine.
· In chlorobenzene, the carbon bonded to chlorine is part of an sp² aromatic ring.
· The chlorine lone pair can interact with the π electrons of benzene.
· This makes the C–Cl bond stronger due to partial double bond character.
· A stronger bond means the C–Cl bond is harder to break.
· The benzene ring also makes direct nucleophilic attack difficult because of electron-rich π density.
· Therefore, chlorobenzene does not readily undergo nucleophilic substitution under normal conditions.

Common Exam Traps

· Do not confuse halogenoarenes with aryl-substituted halogenoalkanes. The halogen must be directly attached to the benzene ring.
· Do not write UV light for electrophilic substitution of benzene; use AlCl₃ or AlBr₃ catalyst.
· Do not say chlorobenzene is unreactive only because “benzene is stable”; explain partial double bond character and stronger C–Cl bond.
· Do not draw an addition product for benzene halogenation; benzene usually undergoes substitution to retain aromatic stability.
· For methylbenzene, remember the major substitution positions are 2- and 4-.