Haloalkanes and Haloarenes

Haloalkanes and haloarenes are organic compounds in which one or more hydrogen atoms of a hydrocarbon are replaced by halogen atoms. This chapter opens organic chemistry for Class 12 and is worth a reliable 6-9 Board marks and 3-4 JEE/NEET questions a year. We classify these compounds (by number of halogens and by the hybridisation of the C-X carbon), learn IUPAC nomenclature, and understand the polar nature of the C-X bond. We study the methods of preparing haloalkanes (from alcohols, from hydrocarbons by free-radical and electrophilic halogenation and addition, and by halogen exchange via the Finkelstein and Swarts reactions) and haloarenes (electrophilic substitution and the Sandmeyer reaction). The heart of the chapter is the chemical reactivity of haloalkanes: the two nucleophilic substitution mechanisms SN1 and SN2 (with their kinetics, stereochemistry and reactivity orders), the stereochemical ideas of chirality, enantiomers, optical activity, retention, inversion and racemisation, the elimination reactions following Saytzeff's rule, and reactions with metals (Wurtz reaction, Grignard reagents). We contrast the low reactivity of haloarenes (explained by resonance and bond character), and finish with the important polyhalogen compounds (dichloromethane, chloroform, iodoform, carbon tetrachloride, freons and DDT) and their uses and environmental effects.