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Properties of Haloarenes

Reactivity Comparison, Substitution Reactions, and Metal Interactions.

By chemca Team • Updated Jan 2026

Haloarenes (Aryl Halides) exhibit distinct chemical behavior compared to haloalkanes. The presence of the aromatic ring significantly alters the reactivity of the halogen atom.

1. Why are Haloarenes Less Reactive towards Nucleophilic Substitution?

Key Reasons

• Resonance Effect: The lone pair on the halogen atom participates in resonance with the benzene ring. This gives the $C-X$ bond partial double bond character, making it stronger and harder to break than a single bond in haloalkanes.
• Hybridization: The carbon attached to the halogen is $sp^2$ hybridized (more electronegative, shorter bond length) compared to $sp^3$ in haloalkanes.
• Instability of Phenyl Cation: The ionization of haloarenes would yield a phenyl cation, which is highly unstable, ruling out the $S_N1$ mechanism.

2. Nucleophilic Substitution (Drastic Conditions)

Dow's Process

Substitution requires drastic conditions (high temperature and pressure).

$$C_6H_5Cl + NaOH \xrightarrow{623K, 300 atm} C_6H_5O^-Na^+ \xrightarrow{H^+} \underset{\text{Phenol}}{C_6H_5OH}$$

Effect of Electron Withdrawing Groups (EWG): Presence of EWGs like $-NO_2$ at ortho and para positions increases reactivity.

$$p\text{-Nitrochlorobenzene} + NaOH \xrightarrow{443K} p\text{-Nitrophenol}$$ $$2,4,6\text{-Trinitrochlorobenzene} + H_2O \xrightarrow{\text{Warm}} \text{Picric Acid}$$

3. Electrophilic Substitution Reactions

Ortho-Para Directing Nature

The halogen atom is deactivating (due to -I effect) but ortho-para directing (due to +R effect). Electrophiles attack the ring, usually yielding the para isomer as the major product.

A. Halogenation:

$$C_6H_5Cl + Cl_2 \xrightarrow{Anhyd. FeCl_3} \underset{\text{1,4-Dichlorobenzene (Major)}}{p\text{-Isomer}} + \text{o-Isomer}$$

B. Nitration:

$$C_6H_5Cl + HNO_3 \xrightarrow{H_2SO_4} p\text{-Nitrochlorobenzene} + o\text{-Isomer}$$

C. Sulphonation:

$$C_6H_5Cl + H_2SO_4(fuming) \xrightarrow{\Delta} \text{4-Chlorobenzenesulphonic acid}$$

D. Friedel-Crafts Reactions:

Alkylation: $$C_6H_5Cl + CH_3Cl \xrightarrow{Anhyd. AlCl_3} p\text{-Chlorotoluene}$$

Acylation: $$C_6H_5Cl + CH_3COCl \xrightarrow{Anhyd. AlCl_3} p\text{-Chloroacetophenone}$$

4. Reaction with Metals

Coupling Reactions

A. Wurtz-Fittig Reaction: Reaction between an aryl halide and an alkyl halide with Sodium.

$$C_6H_5Br + 2Na + CH_3Br \xrightarrow{\text{Dry Ether}} \underset{\text{Toluene}}{C_6H_5CH_3} + 2NaBr$$

B. Fittig Reaction: Reaction involving two aryl halide molecules.

$$2C_6H_5Cl + 2Na \xrightarrow{\text{Dry Ether}} \underset{\text{Diphenyl}}{C_6H_5-C_6H_5} + 2NaCl$$

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