Class 12 Chemistry Haloalkanes and Haloarenes

Class 12 Chemistry Haloalkanes and Haloarenes

6.1 Classification

  • Haloalkanes: Organic compounds containing alkyl groups bonded to halogen (F, Cl, Br, I)
    • Primary (1°), Secondary (2°), Tertiary (3°) based on carbon attached to halogen
  • Haloarenes (Aryl halides): Halogen attached to aromatic ring

6.2 Nomenclature

IUPAC Rules

  1. Identify the longest carbon chain containing halogen
  2. Number chain from end nearest to halogen
  3. Halogen named as prefix: fluoro-, chloro-, bromo-, iodo-
  4. Multiple halogens: Use prefixes di-, tri-, tetra-

Example: CH₃CHClCH₃ → 2-Chloropropane

6.3 Nature of C–X Bond

  • Polar bond due to difference in electronegativity
  • Bond strength: C–F > C–Cl > C–Br > C–I
  • Reactivity: C–I > C–Br > C–Cl > C–F (opposite of bond strength)

6.4 Methods of Preparation of Haloalkanes

  1. From Alcohols
    • RX = R–OH + HX (or PCl₅, SOCl₂)
  2. From Alkanes
    • Free radical halogenation: CH₄ + Cl₂ → CH₃Cl + HCl
  3. From Alkenes
    • Addition of HX or X₂ across double bond

6.5 Preparation of Haloarenes

  1. From Arenes
    • Electrophilic substitution: Benzene + Cl₂/Br₂ (with AlCl₃/FeBr₃) → Chlorobenzene/Bromobenzene
  2. From Diazonium Salts
    • Ar–N₂⁺X⁻ → Ar–X + N₂ (Sandmeyer reaction)

6.6 Physical Properties

  • Colorless, inflammable liquids or gases
  • Insoluble in water, soluble in organic solvents
  • Higher boiling point than corresponding alkanes due to polar C–X bond

6.7 Chemical Reactions

Haloalkanes

  1. Nucleophilic Substitution (SN1 and SN2)
    • RX + OH⁻ → ROH
  2. Elimination (dehydrohalogenation)
    • RX + alcoholic KOH → Alkene + HX

Haloarenes

  • Less reactive due to resonance stabilization of C–X bond
  • Undergo electrophilic substitution

6.8 Polyhalogen Compounds

  • Compounds containing more than one halogen attached to the same carbon or carbon chain
  • Example: CHCl₃ (chloroform), CCl₄ (carbon tetrachloride), CHBr₃ (bromoform)
  • Important in industry and pharmaceuticals

Important Exam Points

  • C–X bond polarity and bond strength influence reactivity
  • SN1 → tertiary haloalkanes, SN2 → primary haloalkanes
  • Haloarenes less reactive due to resonance
  • Sandmeyer reaction useful for preparation of haloarenes