Haloalkanes and Haloarenes

Introduction

• Replacement of hydrogen in hydrocarbons by halogens forms alkyl halides (haloalkanes) and aryl halides (haloarenes).
• Haloalkanes: Halogen attached to sp3 hybridised carbon in alkyl groups.
• Haloarenes: Halogen attached to sp2 hybridised carbon in aryl groups.
• Widely used in industry as solvents and starting materials.
• Important medical applications include chloroquine for malaria and halothane as an anesthetic.
• Study covers preparation, properties, and uses of organohalogen compounds.

Objectives

• Name haloalkanes and haloarenes using IUPAC nomenclature.
• Understand reactions for preparation and transformation of haloalkanes and haloarenes.
• Use stereochemistry to understand reaction mechanisms.
• Correlate structures with reaction types and assess environmental impacts.

Classification

• By Number of Halogen Atoms: Mono, di, or polyhalogen compounds.
• Carbon-Halogen Bond:
  • Alkyl halides (primary, secondary, tertiary): Halogen on sp3 hybridised carbon.
  • Allylic halides: Halogen on carbon adjacent to double bond.
  • Benzylic halides: Halogen on sp3 carbon attached to aromatic ring.
  • Vinylic halides: Halogen on sp2 carbon in a double bond.
  • Aryl halides: Halogen on sp2 carbon in aromatic ring.

Nomenclature

• IUPAC naming uses halosubstituted hydrocarbons.
• Common and IUPAC names differ for dihalogen derivatives.
• Examples: sec-Butyl chloride is 2-Chlorobutane; allyl bromide is 3-Bromopropene.

Nature of C-X Bond

• Polarised due to electronegativity difference; carbon is partially positive.
• Bond length and polarity vary with halogen size and type.
• Alkyl halides best prepared from alcohols.

Methods of Preparation

• From Alcohols: Reaction with halogen acids, phosphorus halides, or thionyl chloride.
• From Hydrocarbons:
  • Free-radical halogenation for alkanes.
  • Electrophilic substitution for arenes.
  • Sandmeyer’s reaction for amines.

Physical Properties

• Boiling points higher than hydrocarbons; vary with halogen type and molecule size.
• Low solubility in water due to weak interactions compared to hydrogen bonds.
• Denser than water; density increases with number of halogen atoms.

Chemical Reactions

• Nucleophilic Substitution:

  • SN1: Favors tertiary halides; carbocation intermediate.
  • SN2: Favors primary halides; direct displacement.
  • Reactivity influenced by steric and electronic effects.

• Elimination Reactions:

  • Formation of alkenes by b-elimination.
  • Zaitsev’s rule for more substituted product.

• Reactions with Metals:

  • Formation of organometallic compounds like Grignard reagents.

Haloarenes Reactions

• Less reactive towards nucleophilic substitution due to resonance and bond strength.
• Undergo electrophilic substitution like halogenation and nitration.

Polyhalogen Compounds

• Dichloromethane: Solvent, toxic effects on CNS.
• Chloroform: Used for refrigerants, toxic to liver.
• Carbon Tetrachloride: Solvent, ozone layer depletion.

Summary

• Organohalogen compounds have diverse industrial and environmental impacts.
• Their polar nature contributes to unique chemical behaviors.
• Awareness of environmental implications is crucial for sustainable use.