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Preparation of Haloarenes

Synthesis via Electrophilic Substitution and Diazonium Salts.

By chemca Team • Updated Jan 2026

Haloarenes (Aryl Halides) are compounds where the halogen atom is directly bonded to an aromatic ring. Due to the partial double bond character of the C-X bond, they are less reactive towards nucleophilic substitution than haloalkanes.

1. From Hydrocarbons (Electrophilic Substitution)

Direct Halogenation

Aryl chlorides and bromides are prepared by reacting arenes with $Cl_2$ or $Br_2$ in the presence of a Lewis Acid catalyst ($FeCl_3$, $FeBr_3$, or anhydrous $AlCl_3$).

$$ C_6H_6 + Cl_2 \xrightarrow{Anhyd. FeCl_3, Dark} \underset{\text{Chlorobenzene}}{C_6H_5Cl} + HCl $$

For Toluene (Ortho/Para Direction):

$$ C_6H_5CH_3 + Cl_2 \xrightarrow{Fe, Dark} \text{o-Chlorotoluene} + \text{p-Chlorotoluene} $$

Important Condition: This reaction must be carried out in the dark to prevent free-radical side chain halogenation. Light favors side chain substitution; Dark + Catalyst favors ring substitution.

2. From Amines (Sandmeyer & Gattermann)

Step 1: Diazotization

Aniline is converted to Benzene Diazonium Chloride by treating with $NaNO_2$ and $HCl$ at low temperature (0-5°C).

$$ C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ C} \underset{\text{Benzene Diazonium Chloride}}{C_6H_5N_2^+Cl^-} + NaCl + 2H_2O $$

Step 2: Sandmeyer Reaction

Mixing the diazonium salt with cuprous chloride ($Cu_2Cl_2$) or cuprous bromide ($Cu_2Br_2$) yields aryl halides.

$$ C_6H_5N_2^+Cl^- \xrightarrow{Cu_2Cl_2/HCl} C_6H_5Cl + N_2 $$ $$ C_6H_5N_2^+Cl^- \xrightarrow{Cu_2Br_2/HBr} C_6H_5Br + N_2 $$

Alternative: Gattermann Reaction

A modification using Copper powder ($Cu$) and halo-acid instead of cuprous halide.

$$ C_6H_5N_2^+Cl^- \xrightarrow{Cu/HCl} C_6H_5Cl + N_2 + CuCl $$

3. Preparation of Iodobenzene

Reaction with KI

Direct iodination of benzene is reversible and difficult. Iodobenzene is best prepared by warming the diazonium salt with aqueous Potassium Iodide ($KI$).

$$ C_6H_5N_2^+Cl^- + KI \xrightarrow{\text{Warm}} C_6H_5I + N_2 + KCl $$

4. Preparation of Fluorobenzene

Balz-Schiemann Reaction

Direct fluorination is explosive. Fluorobenzene is prepared by treating diazonium chloride with Fluoroboric acid ($HBF_4$), followed by heating.

$$ C_6H_5N_2^+Cl^- + HBF_4 \rightarrow \underset{\text{Precipitate}}{C_6H_5N_2^+BF_4^-} \xrightarrow{\Delta} C_6H_5F + BF_3 + N_2 $$

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