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Understanding Haloalkanes and Haloarenes

May 25, 2025

Haloalkanes and Haloarenes

Introduction

  • Replacement of hydrogen in hydrocarbons by halogen forms:
    • Alkyl halides (haloalkanes) in aliphatic hydrocarbons.
    • Aryl halides (haloarenes) in aromatic hydrocarbons.
  • Haloalkanes: Halogen attached to sp3 carbon of alkyl group.
  • Haloarenes: Halogen attached to sp2 carbon of aryl group.
  • Found in nature, used in industries and medicine (e.g., chloramphenicol for typhoid, thyroxine hormone).

Objectives

  • Name haloalkanes and haloarenes using IUPAC nomenclature.
  • Understand preparation, reactions, and structures of haloalkanes and haloarenes.
  • Apply stereochemistry in reaction mechanisms.
  • Recognize environmental effects of polyhalogen compounds.

Classification of Halogenated Compounds

  • Based on Number of Halogen Atoms:
    • Mono, di, or polyhalogen compounds.
  • Based on Structure:
    • Alkyl Halides (RX): Halogen bonded to alkyl group.
      • Primary, secondary, tertiary based on carbon attachment.
    • Allylic Halides: Halogen bonded to sp3 carbon adjacent to double bond.
    • Benzylic Halides: Halogen bonded to sp3 carbon attached to aromatic ring.
    • Vinylic Halides: Halogen bonded to sp2 carbon in double bond.
    • Aryl Halides: Halogen attached directly to aromatic ring's sp2 carbon.

Nomenclature

  • Common names: Alkyl group + halide.
  • IUPAC: Halosubstituted hydrocarbons.
  • Use prefixes for dihalogen derivatives (o-, m-, p- for common names; numbers for IUPAC).

Nature of C-X Bond

  • C-X bond is polarized; carbon bears partial positive charge.
  • Bond length and enthalpy vary with halogen size.

Methods of Preparation

  • From Alcohols: Replace OH group with halogen using halogen acids, phosphorus halides, or thionyl chloride.
  • From Hydrocarbons: Free radical halogenation.
  • From Alkenes: Electrophilic addition.
  • Halogen Exchange: Swarts reaction (for fluorides), Finkelstein reaction (for iodides).

Reactions of Haloalkanes

  • Nucleophilic Substitution:
    • SN2: Bimolecular, inversion of configuration.
    • SN1: Unimolecular, racemization.
  • Elimination Reactions: Formation of alkenes.
  • Reactions with Metals: Formation of organometallic compounds (e.g., Grignard reagents).

Reactions of Haloarenes

  • Less reactive than haloalkanes due to resonance, hybridization, stability.
  • Substitution by nucleophiles and electrophiles.
  • Wurtz-Fittig Reaction: Coupling halides in presence of sodium.

Physical Properties

  • Higher boiling points than hydrocarbons due to strong intermolecular forces.
  • Slightly soluble in water, soluble in organic solvents.

Polyhalogen Compounds

  • DDT: Insecticide, bioaccumulates, banned in some countries.
  • Freons: Used in refrigeration, deplete ozone layer.
  • Carbon Tetrachloride: Solvent, harmful to liver.
  • Chloroform: Once used as anesthetic, CNS depressant.

Summary

  • Haloalkanes and haloarenes are vital in organic chemistry.
  • Their study includes naming, preparation, reactions, and environmental impact.
  • SN1 and SN2 mechanisms are key concepts, influencing stereochemistry.
  • Polyhalogen compounds have significant industrial and environmental roles.

Exercises

  • Practice naming, drawing structures, and predicting reactions of haloalkanes and haloarenes.
  • Explore stereochemical and environmental aspects of these compounds.