Preparation of Haloarenes

Haloarenes are prepared by two principal routes: direct halogenation of an arene ring, and diazotization of an aromatic amine followed by displacement of the diazonium group.

(i) Electrophilic Substitution on the Arene Ring

Aryl chlorides and aryl bromides can be obtained directly by treating an arene with chlorine or bromine in the presence of a Lewis acid catalyst such as iron or iron(III) chloride, with the reaction carried out in the dark. The halogen acts as an electrophile toward the aromatic ring, and substitution occurs at positions activated by any existing substituent.

For toluene, the methyl group directs the incoming halogen mainly to the ortho and para positions, so the reaction gives a mixture of $o$-halotoluene and $p$-halotoluene:

Because the para isomer is more symmetric and packs more efficiently in the solid state, it has a distinctly different (higher) melting point than the ortho isomer, which makes the two easy to separate.

Three points worth noting about this route:

• Reactions with iodine are reversible and only proceed usefully in the presence of an oxidising agent — such as $\text{HNO}_3$ or $\text{HIO}_4$ — that consumes the HI formed as a by-product and pulls the equilibrium toward the aryl iodide.
• Fluoroarenes are not made this way, because elemental fluorine is far too reactive to control safely in a direct substitution.
• The catalyst (Fe or FeCl$_3$) generates the electrophilic halogen species; without it the ring is not reactive enough toward $X_2$.

(ii) From Aromatic Amines: Diazotization and Sandmeyer's Reaction

A second, more versatile route starts from a primary aromatic amine rather than the hydrocarbon itself.

Step 1 — Diazotization. When a primary aromatic amine, dissolved or suspended in a cold aqueous mineral acid, is treated with sodium nitrite at a low temperature (273–278 K), it is converted into a diazonium salt:

This benzene diazonium halide is unstable and is generated fresh, immediately before it is used in the next step.

Step 2 — Displacement of the diazonium group. Mixing the freshly prepared diazonium salt solution with cuprous chloride or cuprous bromide replaces the $-\overset{+}{\text{N}}{}_2$ group with $-\text{Cl}$ or $-\text{Br}$, releasing nitrogen gas:

This substitution of the diazonium group using a cuprous halide is known as Sandmeyer's reaction, and it is a reliable way to place chlorine or bromine at a specific ring position — namely, wherever the amino group originally was.

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