Haloalkanes and Haloarenes

• Introduction & Classification

  • Definition: Replacement of hydrogen atom(s) in hydrocarbons by halogen atom(s).
  • Classification by Number of Halogens: Mono-, Di-, Polyhalogen compounds.
  • Classification based on $sp^3$ $C-X$ bond:
    • Primary ($1^\circ$), Secondary ($2^\circ$), Tertiary ($3^\circ$) alkyl halides.
    • Allylic halides: $X$ bonded to $sp^3$ carbon next to $C=C$.
    • Benzylic halides: $X$ bonded to $sp^3$ carbon next to an aromatic ring.
  • Classification based on $sp^2$ $C-X$ bond:
    • Vinylic halides: $X$ bonded to $sp^2$ carbon of $C=C$.
    • Aryl halides: $X$ bonded to $sp^2$ carbon of an aromatic ring.

• Nomenclature

  • IUPAC System: Prefixes used (chloro, bromo, iodo).
  • Common System: Named as alkyl halides.

• Nature of C-X Bond

  • Polarity: Halogens are more electronegative than carbon.
  • Bond length and bond enthalpy trends down the halogen group ($F$ to $I$).

• Methods of Preparation

  • From Alcohols:
    • Using halogen acids ($HX / ZnCl_2$).
    • Using phosphorus halides ($PCl_3, PCl_5, PBr_3$).
    • Using Thionyl chloride ($SOCl_2$) - best method.
  • From Hydrocarbons:
    • Free radical halogenation of alkanes.
    • Electrophilic substitution of arenas.
    • Addition of $HX$ to alkenes (Markovnikov's and Anti-Markovnikov's rule).
  • Halogen Exchange:
    • Finkelstein reaction (preparation of alkyl iodides).
    • Swarts reaction (preparation of alkyl fluorides using $AgF/Hg_2F_2$).

• Physical Properties

  • Melting and Boiling Points: Higher than parent hydrocarbons due to dipole-dipole and van der Waals forces; increases with molecular mass.
  • Solubility: Slightly soluble in water, soluble in organic solvents.
  • Density: Polyhalogen compounds and bromides/iodides are heavier than water.

• Chemical Reactions

  • Nucleophilic Substitution Reactions ($S_N$):
    • $S_N2$ Mechanism: Bimolecular, inversion of configuration, favored by primary halides.
    • $S_N1$ Mechanism: Unimolecular via carbocation intermediate, racemization, favored by tertiary halides.
    • Stereochemical aspects: Retention, Inversion, and Racemization.
  • Elimination Reactions ($\beta$-elimination):
    • Dehydrohalogenation ($HX$ removal).
    • Saytsev's rule: More substituted alkene is the major product.
  • Reaction with Metals:
    • Reaction with Magnesium: Formation of Grignard Reagents ($RMgX$).
    • Wurtz reaction: Coupling of two alkyl halides with sodium.

• Haloarenes

  • Methods of Preparation:
    • Electrophilic substitution (chlorination/bromination of benzene).
    • Sandmeyer reaction (from diazonium salts using $Cu_2Cl_2 / HCl$).
  • Physical Properties: Colourless liquids or crystalline solids; insoluble in water, soluble in organic solvents.
  • Chemical Reactions:
    • Low reactivity towards nucleophilic substitution due to resonance, $sp^2$ hybrid carbon, and phenyl cation instability.
    • Nucleophilic Substitution under drastic conditions (e.g., Dow's process).
    • Electrophilic Aromatic Substitution: Halogenation, Nitration, Sulphonation, Friedel-Crafts reaction (ortho and para directing).
    • Reaction with Metals: Fittig reaction and Wurtz-Fittig reaction.

• Polyhalogen Compounds

  • Dichloromethane (Methylene chloride): Uses and health effects.
  • Trichloromethane (Chloroform): Preparation, uses, and oxidation to toxic phosgene.
  • Triiodomethane (Iodoform): Antiseptic properties.
  • Tetrachloromethane (Carbon tetrachloride): Uses and environmental impact.
  • Freons: Chlorofluorocarbons (CFCs) and ozone depletion.
  • DDT: Dichlorodiphenyltrichloroethane (potent insecticide, environmental persistence).