B.Sc. 3rd Semester: Halogen Derivatives of Arenes (Haloarenes)
Haloarenes, also known as aryl halides, are compounds where one or more hydrogen atoms of an aromatic hydrocarbon (arene) are replaced by halogen atoms (F, Cl, Br, I).
Their general formula is Ar-X, where Ar is the aryl group and X is a halogen.
Structure and Bonding
The halogen atom is bonded to an sp2-hybridized carbon of the aromatic ring. The C–X bond in haloarenes has partial double bond character due to resonance between the lone pair electrons of the halogen and the aromatic π-system. This resonance makes the bond shorter, stronger, and less reactive compared to alkyl halides.
Preparation of Haloarenes
Haloarenes are mainly prepared by the following methods:
- Direct Halogenation: Aromatic hydrocarbons like benzene react with chlorine or bromine in the presence of a Lewis acid catalyst (e.g., FeCl3, AlCl3) to form haloarenes.
- Sandmeyer Reaction: Aromatic amines (anilines) are converted to diazonium salts by treatment with nitrous acid, which then react with cuprous halides (CuCl, CuBr) to give haloarenes.
- Gattermann Reaction: A modification of Sandmeyer reaction using copper powder and hydrogen halides to prepare chloro- and bromoarenes.
- From Phenols: Phenols react with phosphorus halides (PCl5, SOCl2) to form haloarenes, though this method is less common.
➤ Methods of Preparation in details
Classification
- Monohaloarenes: One halogen atom attached.
- Dihaloarenes, Trihaloarenes, etc.: Multiple halogens attached at different positions.
Chemical Properties
- Nucleophilic Substitution: Less reactive due to resonance stabilization; requires harsh conditions or activating groups.
- Electrophilic Substitution: The halogen is ortho/para-directing but deactivates the ring group, making reactions slower than benzene.
- Replacement by Hydroxyl Group: Haloarenes can be converted to phenols using aqueous NaOH under high temperature and pressure.
- Reactions with Metals: They participate in reactions like the Wurtz-Fittig reaction forming substituted arenes.
➤ Chemical Properties in details
➤ Mechanism of Substitution Reactions
➤ Mechanism of Elimination Reactions
Uses and Applications
Haloarenes are important intermediates in organic synthesis, pharmaceuticals, and agrochemical industries due to their ability to undergo various substitution reactions.