Haloalkanes and Haloarenes — Class 12 Chemistry

Haloalkanes and haloarenes are formed when hydrogen atoms of aliphatic or aromatic hydrocarbons are replaced by halogen atoms. In haloalkanes the halogen is attached to an carbon of an alkyl group, while in haloarenes it is attached to an carbon of an aryl ring. They are named by the IUPAC system and classified by the number of halogen atoms and by whether the carbon bearing the halogen is primary, secondary or tertiary. They are prepared from alcohols, hydrocarbons (by free radical halogenation or addition to alkenes), and by halogen exchange. The carbon-halogen bond is polar, making the carbon a site for nucleophilic attack. Haloalkanes undergo nucleophilic substitution by two mechanisms: , which is unimolecular and proceeds through a carbocation favouring tertiary halides, and , which is bimolecular with inversion of configuration favouring primary halides. They also undergo elimination to form alkenes. Optical isomerism arises in molecules with a chiral carbon. Haloarenes are far less reactive toward nucleophilic substitution because of resonance and the partial double bond character of the C-X bond. Many organohalogen compounds, such as chloroform, freons and DDT, are industrially and environmentally important, though some are pollutants.