Chapter 6 — Haloalkanes and Haloarenes

Class 12 Chemistry Chapter 6 covers Haloalkanes and Haloarenes — organic compounds formed by replacing hydrogen atoms in hydrocarbons with halogen atoms — explaining their classification, nomenclature, preparation methods, physical properties, and key reactions including nucleophilic substitution (SN1 and SN2), elimination, and reactions with metals.

Haloalkanes and Haloarenes (NCERT Class 12 Chemistry Part II, Chapter 6) deals with organic compounds where halogen atoms replace hydrogen in aliphatic or aromatic hydrocarbons. Haloalkanes carry halogens on sp3-hybridised carbon while haloarenes carry them on sp2-hybridised carbon. The chapter covers classification into primary, secondary and tertiary halides; IUPAC nomenclature; preparation from alcohols, alkenes and by halogen exchange (Finkelstein and Swarts reactions); physical properties such as boiling point trends and density; and chemical reactions including SN1, SN2, elimination (Zaitsev's rule), Grignard reagent formation, Wurtz and Wurtz-Fittig reactions. Stereochemical concepts — chirality, enantiomers, inversion and racemisation — are discussed in the context of substitution mechanisms. The chapter concludes with the environmental and industrial significance of polyhalogen compounds such as chloroform, carbon tetrachloride, freons and DDT.

1. 01Haloalkanes have halogen bonded to sp3-hybridised carbon; haloarenes have halogen bonded to sp2-hybridised carbon of an aromatic ring.
2. 02Alkyl halides are best prepared from alcohols using thionyl chloride (preferred, gives pure product), phosphorus halides or halogen acids; aryl halides are made by electrophilic substitution or Sandmeyer's reaction.
3. 03The C–X bond length and bond enthalpy trend follows C–F > C–Cl > C–Br > C–I in bond strength, while boiling points follow RI > RBr > RCl > RF due to increasing van der Waals forces.
4. 04SN2 reactions proceed with inversion of configuration and favour primary halides; SN1 reactions proceed via a planar carbocation intermediate leading to racemisation and favour tertiary halides.
5. 05Haloarenes are far less reactive than haloalkanes towards nucleophilic substitution due to resonance (partial C–Cl double-bond character), shorter C–X bond length (169 pm vs 177 pm), and instability of the phenyl cation.
6. 06Polyhalogen compounds including freons (CCl2F2) and DDT persist in the environment, deplete the ozone layer, and accumulate in fatty tissues, making them significant environmental hazards.