Haloarenes: Preparation, Chemical Properties & Reaction Charts
Aryl halides present a fascinating contrast to alkyl halides. Their remarkable stability towards nucleophilic substitution makes their chemical behavior a high-yield topic for JEE, NEET, and Board exams.
1. Preparation of Haloarenes
Aryl halides are primarily prepared by direct halogenation of aromatic rings (Electrophilic Aromatic Substitution) or via the highly versatile Diazonium salt pathways.
Key Preparation Methods
- Direct Halogenation: Benzene reacts with $Cl_2$ or $Br_2$ in the dark, utilizing a Lewis acid catalyst ($FeCl_3$, $AlCl_3$).
- Sandmeyer Reaction: Treating Benzene Diazonium Chloride ($C_6H_5N_2^+Cl^-$) with $CuCl/HCl$ or $CuBr/HBr$.
- Gattermann Reaction: Modifies Sandmeyer by using Copper powder and halogen acid instead of Cuprous salts.
- Balz-Schiemann Reaction: Specific method for synthesizing Fluorobenzene using Fluoroboric acid ($HBF_4$).
Figure: Major Synthetic Routes to Haloarenes
2. Chemical Properties & Reactivity
Unlike haloalkanes, haloarenes are extremely unreactive towards Nucleophilic Substitution ($S_N$ reactions). This is because the lone pairs on the halogen atom are in resonance with the benzene ring, imparting a partial double bond character to the $C-X$ bond.
Major Chemical Reactions
- Nucleophilic Substitution (Dow's Process): Only occurs under extreme conditions ($623\text{ K}, 300\text{ atm}$) to convert Chlorobenzene to Phenol. The presence of electron-withdrawing groups (like $-NO_2$) at ortho/para positions makes this easier.
- Electrophilic Substitution: Halogens are ortho/para directing due to resonance (+R effect), but are overall deactivating due to strong inductive withdrawal (-I effect).
- Wurtz-Fittig Reaction: Aryl Halide + Alkyl Halide + $Na$ (in dry ether) $\rightarrow$ Alkylbenzene.
- Fittig Reaction: Aryl Halide + Aryl Halide + $Na$ (in dry ether) $\rightarrow$ Biphenyl.
Figure: Chemical Reactivity and Transformations of Chlorobenzene
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