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.