Chapter 6 — Haloalkanes and Haloarenes

Chapter 6 of the Class 12 Chemistry NCERT textbook, "Haloalkanes and Haloarenes", covers 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.

• Two families defined by hybridisation — The chapter distinguishes haloalkanes, where halogen sits on sp3 carbon, from haloarenes, where it sits on sp2 aromatic carbon — a structural difference that governs everything from classification into primary/secondary/tertiary halides to reactivity.
• Making and characterising halides — It gathers preparation routes from alcohols, alkenes and halogen-exchange reactions and links physical trends like boiling point and density to molecular structure, giving a systematic way to obtain and identify these compounds.
• Substitution, elimination and stereochemistry — Core reactivity is framed around competing SN1 and SN2 substitution, elimination following Zaitsev's rule, and metal reactions like Grignard and Wurtz — all interpreted through stereochemical ideas of chirality, inversion and racemisation.
• Why haloarenes resist and why some halides harm — The chapter explains the low reactivity of haloarenes through resonance and bond effects, and closes on the environmental significance of persistent polyhalogen compounds such as 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.