9.1 Preparation of Halogenoarenes

Halogenoarenes are aromatic compounds in which one or more hydrogen atoms on the benzene ring are replaced by a halogen atom (F, Cl, Br, or I). Unlike halogenoalkanes, the C–X bond in halogenoarenes has partial double-bond character due to resonance with the ring, making direct nucleophilic substitution very difficult.

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Method 1: Direct Halogenation (Electrophilic Aromatic Substitution)

Benzene reacts with chlorine or bromine in the presence of a Lewis acid catalyst (halogen carrier) to give chlorobenzene or bromobenzene.

Reaction with Chlorine

${\text{C}}_6{\text{H}}_6+{\text{Cl}}_2\stackrel{{\text{FeCl}}_3{\text{ or AlCl}}_3}{\to }{\text{C}}_6{\text{H}}_5\text{Cl}+\text{HCl}$

Reaction with Bromine

${\text{C}}_6{\text{H}}_6+{\text{Br}}_2\stackrel{{\text{FeBr}}_3{\text{ or AlBr}}_3}{\to }{\text{C}}_6{\text{H}}_5\text{Br}+\text{HBr}$

Role of the Lewis Acid Catalyst

The Lewis acid (e.g., ${\text{FeCl}}_3$) acts as a halogen carrier. It polarises the halogen molecule to generate a powerful electrophile:

${\text{Cl}}_2+{\text{FeCl}}_3\to {\text{Cl}}^{+}\cdots [{\text{FeCl}}_4{]}^{-}$

The ${\text{Cl}}^{+}$ (or the highly polarised ${\text{Cl}}^{\delta +}$–${\text{Cl}}^{\delta -}$) then attacks the electron-rich benzene ring via the standard electrophilic aromatic substitution (EAS) mechanism.

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Method 2: Why Phenol Cannot Be Used to Make Chlorobenzene

Unlike alcohols, phenol cannot be converted to chlorobenzene by reaction with HCl or PCl${}_5$ under normal conditions. This is because:

• The lone pair on the oxygen atom of phenol overlaps with the $\pi$ system of the ring.
• This gives the C–O bond partial double-bond character through resonance.
• The C–O bond is therefore much stronger and shorter than in aliphatic alcohols and cannot be broken easily.

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Method 3: The Sandmeyer Reaction (via Diazonium Salts)

This is the most versatile method for preparing halogenoarenes, especially fluorobenzene and iodobenzene which cannot be made by direct halogenation.

Step 1: Diazotisation of Aniline

Aniline (phenylamine) is converted to benzene diazonium chloride by reaction with sodium nitrite (${\text{NaNO}}_2$) and hydrochloric acid at below 10°C:

${\text{C}}_6{\text{H}}_5{\text{NH}}_2+{\text{NaNO}}_2+2\text{HCl}\stackrel{0\text{–}5°\text{C}}{\to }{\text{C}}_6{\text{H}}_5{\text{N}}_2^{+}{\text{Cl}}^{-}+\text{NaCl}+2{\text{H}}_2\text{O}$

Step 2: Replacement of the Diazonium Group

Sandmeyer Reaction (Cl and Br)

${\text{C}}_6{\text{H}}_5{\text{N}}_2^{+}{\text{Cl}}^{-}+\text{CuCl}\stackrel{\Delta }{\to }{\text{C}}_6{\text{H}}_5\text{Cl}+{\text{N}}_2$

The cuprous salt (${\text{Cu}}^{+}$) acts as a catalyst by facilitating the transfer of the halide to the aryl group.

Iodobenzene Preparation

${\text{C}}_6{\text{H}}_5{\text{N}}_2^{+}{\text{Cl}}^{-}+\text{KI}\stackrel{\Delta }{\to }{\text{C}}_6{\text{H}}_5\text{I}+{\text{N}}_2+\text{KCl}$

No copper catalyst is required because ${\text{I}}^{-}$ is a good enough reducing agent to reduce the diazonium ion directly.

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Summary Table