Reactions of Haloarenes – Electrophilic Substitution Reaction 

1.0Introduction to Haloarenes

Haloarenes are aromatic compounds in which one or more hydrogen atoms of benzene are substituted by a halogen atom (Cl, Br, I, F).

• Example: Chlorobenzene, Bromobenzene, Iodobenzene.
• These compounds show Electrophilic Aromatic Substitution (EAS) reactions, but their reactivity is modified by the halogen substituent present on the aromatic ring.

2.0Structure and Nature of Haloarenes

• Haloarenes consist of a benzene ring directly bonded to a halogen atom.
• The C–X bond (X = halogen) is polar, due to the electronegativity difference.
• Resonance occurs as the lone pair of halogen interacts with the π-electrons of the benzene ring, donating electron density to ortho and para positions.

This dual nature of halogen — inductive effect (–I, withdrawing) and resonance effect (+R, donating) — controls the substitution reactions.

3.0General Mechanism of Electrophilic Substitution in Haloarenes

Like benzene, haloarenes follow the three-step mechanism of EAS:

Step 1: Formation of Electrophile

• Strong reagents like HNO₃ + H₂SO₄ form NO₂⁺ electrophile (for nitration).
• Cl₂/FeCl₃ generates Cl⁺ electrophile (for halogenation).

Step 2: Attack on the Aromatic Ring

• Electrophile attacks the π-electron cloud of the haloarene, preferably at ortho or para positions.

Step 3: Formation of Arenium Ion

• A resonance-stabilized carbocation intermediate forms (sigma complex or arenium ion).

Step 4: Deprotonation

• Loss of H⁺ regenerates aromaticity, yielding substituted haloarene.

4.0Directive Influence of the Halogen Atom

Halogen atoms exert two opposite effects:

• Inductive effect (–I): Withdraws electron density from the ring, making it less reactive than benzene.
• Resonance effect (+R): Donates lone pair electrons, activating the ortho and para positions.

5.0Types of Electrophilic Substitution Reactions in Haloarenes

Nitration of Haloarenes

• Reagent: HNO₃ + H₂SO₄
• Example: chlorobenzene nitration yields mainly ortho-nitrochlorobenzene and para-nitrochlorobenzene. The reaction is slow, often requiring elevated temperature and strong acid.
• Ratio: para product usually dominates due to steric hindrance at ortho positions.

Sulfonation of Haloarenes

• Reagent: fuming H₂SO₄ (SO₃/H₂SO₄)
• Similar pattern: ortho- and para-sulfonation, with para predominating under milder conditions. High temperatures can shift the equilibrium.

Friedel–Crafts Alkylation and Acylation of Haloarenes

• Reagent: R–Cl + AlCl₃ (alkylation) or RCOCl + AlCl₃ (acylation)
• Because haloarenes are deactivated, Friedel–Crafts reactions are generally very difficult or do not occur (especially with strongly deactivated rings). For example, chlorobenzene resists alkylation.
• In rare cases with very activated electrophiles, some reaction may occur, but yield is poor.

Halogenation and Other Special Cases

• Reagent: X₂ + FeX₃ (e.g., Br₂/FeBr₃)
• Haloarenes undergo halogenation even more sluggishly than benzene. The new halogen adds to ortho/para positions.
• Special case: halogen exchange (like Sandmeyer reaction) is not EAS but involves diazonium chemistry—outside the scope here.

6.0Examples of Electrophilic Substitution Reactions in Haloarenes

• Halogenation: Involves the addition of a halogen (e.g., chlorine) in the presence of a Lewis acid catalyst (e.g., FeCl₃) to produce ortho and para substituted products. 
• Nitration: An electrophile, the NO₂⁺ ion (formed from nitric and sulfuric acids), substitutes a hydrogen on the ring at the ortho and para positions. 
• Sulfonation: Involves introducing a sulfonic acid group (-SO₃H) onto the ring. 
• Friedel-Crafts Reaction: An acyl group or alkyl group is introduced onto the ring using an acyl halide or alkyl halide, respectively, with a Lewis acid catalyst. For example, the Friedel-Crafts acylation of chlorobenzene produces p-chloroacetophenone as a major product. 

7.0Factors Affecting Reactivity in Haloarene Substitution

• Type of halogen: Iodobenzene reacts faster than chlorobenzene.
• Temperature and catalyst: Higher temp increases substitution rate.
• Nature of electrophile: Strong electrophiles attack faster.
• Resonance stabilization: Ortho and para products dominate due to resonance donation.

8.0Why Electrophilic Substitution in Haloarenes is Important

• Haloarenes undergo electrophilic substitution instead of addition, because the aromatic ring resists loss of aromaticity.
• Substitution maintains the stability of the benzene ring.
• For JEE, knowing the mechanism, orientation (ortho/para/meta), and examples is key for problem-solving.

9.0Advantages and Limitations of Electrophilic Substitution in Haloarenes

Advantages:

• Produces commercially valuable intermediates.
• Helps in synthesis of pharmaceuticals, pesticides, and polymers.

Limitations:

• Reaction rates are slower than benzene.
• Formation of mixtures (ortho and para) requires separation.
• Not as versatile as benzene in substitution reactions.