Haloarenes

1.0Introduction

Haloarenes are halogen derivatives of aromatic hydrocarbons in which a halogen atom is directly attached to a carbon atom of the aromatic ring. They are also referred to as aryl halides. Haloarenes are produced when a halogen atom replaces a hydrogen atom attached to an aromatic ring.

Examples: Chlorobenzene, Bromobenzene
General Formula: Ar–X, where Ar represents an aryl group, and X is a halogen atom.

2.0Classification of Haloarenes

3.0Nomenclature of Haloarenes

The systematic names are derived by adding prefixes like fluoro, chloro, bromo, or iodo to the aromatic hydrocarbon's name.
Example: Chlorobenzene, Bromobenzene

In disubstituted or trisubstituted compounds, the relative positions of substituents are indicated by Arabic numerals or prefixes:

• Ortho (o-): Substituents at adjacent positions (1,2).
• Meta (m-): Substituents at alternate positions (1,3).
• Para (p-): Substituents at opposite positions (1,4).

Example: o-Dichlorobenzene, p-Dibromobenzene

4.0Methods of Preparation of Haloarenes

1. From Arenes:

Benzene reacts with Cl₂ or Br₂ in the presence of FeCl₃ to form haloarenes, yielding ortho and para isomers.

Example: Benzene + Cl₂ → Chlorobenzene (catalyzed by FeCl₃).

Iodination requires an oxidizing agent (HNO₃ or HIO₄). Due to fluorine's reactivity, fluoro compounds are not prepared this way. 

2. From Diazonium Compounds (Sandmeyer’s Reaction):

Treating primary aromatic amines with nitrous acid forms diazonium salts. In Sandmeyer's reaction, these salts react with CuCl or CuBr to replace the diazonium group with chlorine or bromine. Iodine substitution occurs with KI, replacing the diazonium group with iodine, yielding haloarenes.

5.0Properties of Haloarenes

Physical Properties:

State and Color: Typically colorless liquids or crystalline solids.

• Solubility: Insoluble in water but soluble in organic solvents like alcohol and ether due to their low polarity.
• Density: Heavier than water, increasing with the atomic number of the halogen.
• Melting and Boiling Points: Increase with the size of the halogen atom.

Chemical Properties:

Low Reactivity in Nucleophilic Substitution Reactions:

Reasons:

Resonance stabilization: Partial double-bond character strengthens the C–X bond.

Hybridization: sp² hybridized carbon increases bond strength due to high electronegativity.

Low polarity: Reduces the release of halide ions.

2. Reactions of Haloarenes:

Electrophilic Substitution: Common at ortho and para positions due to halogen's electron-withdrawing and electron-donating effects.

Examples:

Halogenation: Formation of dihaloarenes.

Nitration: Introduction of nitro groups.

Sulfonation: Reaction with sulfuric acid.

Friedel-Crafts Reactions: Alkylation and acylation.

3. Reactions with Metals:

Fitting Reaction: Formation of diaryl compounds (e.g., diphenyl).

Wurtz-Fitting Reaction: Reaction with alkyl halides to form alkylated aromatic compounds.

Reduction Reactions: Conversion of haloarenes to hydrocarbons using reducing agents like Ni-Al alloy in alkali.

6.0Uses of Haloarenes

• Manufacturing of phenol, picric acid, and DDT.
• Picric acid: Used as a dye and antiseptic.
• DDT: An insecticide for controlling mosquitoes, flies, and agricultural pests.