9 min readPathway sheet1-mark MCQ on catalysts and conditions; 2-mark equations for ring halogenation and Sandmeyer's reaction; conversions in 3-mark answers

Preparation of haloarenes

Aryl chlorides and bromides form by ring halogenation with a Lewis acid in the dark. Diazonium salts give aryl halides by Sandmeyer's reaction (Cu₂X₂) or with KI.

Catalyst for ring halogenationA Lewis acid: iron, or anhydrous FeCl₃ (for Cl₂) or FeBr₃ (for Br₂), in the dark

DiazotisationC₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O, at 273 to 278 K

Sandmeyer's reactionArN₂⁺X⁻ → Ar-X + N₂, with Cu₂X₂ written over the arrow (X = Cl or Br)

IodobenzeneC₆H₅N₂⁺Cl⁻ + KI → C₆H₅I + KCl + N₂; no copper salt needed

By electrophilic substitution

An **electrophile** (an electron-seeking species) replaces a hydrogen on the ring. The **Lewis acid** (an electron-pair acceptor) polarises the halogen molecule to make the electrophile.

1
C₆H₆ + Cl₂ → C₆H₅Cl + HCl
Anhydrous FeCl₃ (or iron), in the dark
2
CH₃C₆H₅ + Cl₂ → 2-chlorotoluene or 4-chlorotoluene, + HCl
Iron, dark. The -CH₃ group sends Cl to the ortho and para positions. The two isomers are easy to separate because their melting points are very different
3★
C₆H₆ + I₂ ⇌ C₆H₅I + HI
Reversible. An oxidising agent such as HNO₃ or HIO₄ removes the HI as it forms, so the reaction goes forward
4
Fluorine reacts too violently with arenes, so fluoroarenes are not made by direct fluorination.

★ key step

Sandmeyer's reaction

A primary aromatic amine is first turned into a **diazonium salt** (Ar-N₂⁺X⁻). The -N₂⁺ group then leaves as nitrogen gas and a halogen takes its place.

1
C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O
Diazotisation: keep the mixture at 273 to 278 K in an ice bath, because the diazonium salt breaks down when warm
2★
C₆H₅N₂⁺Cl⁻ → C₆H₅Cl + N₂
Cuprous chloride (Cu₂Cl₂) in HCl
3★
C₆H₅N₂⁺Br⁻ → C₆H₅Br + N₂
Cuprous bromide (Cu₂Br₂) in HBr
4
C₆H₅N₂⁺Cl⁻ + KI → C₆H₅I + KCl + N₂
For iodobenzene, simply shake the diazonium salt with KI solution; no copper salt is needed

★ key step

Ring chlorination vs side-chain chlorination of toluene

Dark, with FeCl₃Cl replaces H on the ring: 2-chlorotoluene and 4-chlorotoluene. These are haloarenes.

Sunlight or heat, no catalystCl replaces H on the CH₃ group by free radicals: C₆H₅CH₂Cl (benzyl chloride), a benzylic halide, not a haloarene.

Same reactants, different conditions, different place of attack.

Why phenol is not used

Alcohols give haloalkanes with HX or PCl₅, but phenol does not give a haloarene this way. In phenol the oxygen's lone pair is shared with the ring, so the C-O bond has partial double bond character. It is stronger than a single bond and hard to break.

In real life

Many medicines carry a halogen on an aromatic ring, for example chloroquine (used against malaria) and the iodine-containing thyroid hormone thyroxine, whose lack causes goitre.

Exam angle

2 marks: “How will you convert (i) benzene to chlorobenzene and (ii) aniline to chlorobenzene? Give equations with conditions.” (i) C₆H₆ + Cl₂ (anhydrous FeCl₃, dark) → C₆H₅Cl + HCl. (ii) C₆H₅NH₂ + NaNO₂ + 2HCl (273 to 278 K) → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O; then C₆H₅N₂⁺Cl⁻ (Cu₂Cl₂/HCl) → C₆H₅Cl + N₂.

1 marks: “Why is iodination of benzene carried out with an oxidising agent such as HNO₃?” The reaction is reversible; HNO₃ oxidises the HI formed, so the equilibrium moves towards iodobenzene.

3 marks: “What is Sandmeyer's reaction? Write equations to make chlorobenzene, bromobenzene and iodobenzene from benzenediazonium salts.” A diazonium salt with cuprous halide gives an aryl halide and N₂. C₆H₅N₂⁺Cl⁻ (Cu₂Cl₂/HCl) → C₆H₅Cl + N₂; C₆H₅N₂⁺Br⁻ (Cu₂Br₂/HBr) → C₆H₅Br + N₂; C₆H₅N₂⁺Cl⁻ + KI → C₆H₅I + KCl + N₂.

Remember it

CCC: Cold, Catalyst, Core (ring). SSS: Sunlight, Strong heat, Side chain.