Haloalkanes and
Haloarenes

Haloalkanes (alkyl halides) are compounds in which one or more hydrogen atoms of an alkane are replaced by halogen atoms (F, Cl, Br, or I). The carbon bearing the halogen is sp3-hybridised, and the C–X bond is polar, making these compounds highly reactive intermediates in organic synthesis. Classification is based on the number of halogen atoms (mono-, di-, tri-, polyhalogen) and the degree of the carbon to which the halogen is attached (primary, secondary, tertiary).

The chapter covers two major types of nucleophilic substitution: SN1 (unimolecular, proceeds via a planar carbocation intermediate, favours tertiary halides, leads to racemisation) and SN2 (bimolecular, concerted backside attack, favours primary halides, leads to Walden inversion / complete inversion of configuration). Elimination reactions (E1 and E2) compete with substitution, and Saytzeff's rule governs the preferred product. Named reactions — Wurtz, Finkelstein, Swartz, and the Grignard reagent preparation — are frequently tested.

Haloarenes (aryl halides) have the halogen directly attached to a benzene ring. Resonance delocalises the halogen's lone pairs into the ring, shortening the C–X bond and making nucleophilic substitution much harder than in haloalkanes. Electrophilic aromatic substitution (EAS) is their characteristic reaction. Halogen is an ortho/para director in EAS despite being a deactivator. The chapter also covers polyhalogen compounds of industrial significance — DDT, BHC (lindane), chloroform, iodoform — and their uses and environmental concerns.