Haloalkanes and Haloarenes

Introduction

• Haloalkanes (Alkyl halides): Halogen atoms attached to sp3 hybridized carbon of an alkyl group.
• Haloarenes (Aryl halides): Halogen atoms attached to sp2 hybridized carbon of an aryl group.

Objectives

• Learn IUPAC nomenclature for haloalkanes and haloarenes.
• Understand methods of preparation and reactions.
• Link structure with reactions using stereochemistry.
• Study organo-metallic compound applications.
• Consider environmental effects of polyhalogen compounds.

Classification

By Number of Halogen Atoms

• Mono, di, or polyhalogen: Based on number of halogen atoms.

By Hybridization

• Alkyl halides: Halogen bonded to sp3 carbon (primary, secondary, tertiary).
• Allylic halides: Halogen bonded to sp3 carbon adjacent to C=C bond.
• Benzylic halides: Halogen bonded to sp3 carbon attached to aromatic ring.

By Bond Type

• Vinylic halides: Halogen bonded to sp2 carbon of C=C bond.
• Aryl halides: Halogen bonded to sp2 carbon of aromatic ring.

Nomenclature

• Alkyl halides named as halosubstituted hydrocarbons.
• Use "o-", "m-", "p-" prefixes for common names; use numerals for IUPAC.

Nature of C-X Bond

• Polarity: Carbon-halogen bond is polar; carbon is partially positive.
• Bond Length & Strength: Varies with halogen size (smallest: F, largest: I).

Preparation Methods

1. From Alcohols: Replace OH in alcohols with halogen using acids or reagents.
   • E.g., Thionyl chloride for pure alkyl halides.
2. From Hydrocarbons:
   • Free radical halogenation: For alkanes.
   • Electrophilic substitution: For aryl halides.
3. Halogen Exchange: E.g., Finkelstein and Swarts reactions.

Physical Properties

• Boiling Points: High due to strong intermolecular forces.
• Solubility: Low in water, better in organic solvents.
• Density: Increases with more carbon or halogen atoms.

Chemical Reactions

1. Nucleophilic Substitution

   • SN1 and SN2 Mechanisms:
     • SN1: Unimolecular, forms carbocation intermediate.
     • SN2: Bimolecular, involves inversion of configuration.
   • Factors: Reactivity influenced by steric hindrance and nucleophile strength.

2. Elimination Reactions

   • b-Elimination: Forms alkenes; Zaitsev's rule applies.

3. Reactions with Metals

   • Grignard Reagents (RMgX): Involve carbon-metal bonds.
   • Wurtz Reaction: Combines alkyl halides with sodium.

Reactions of Haloarenes

1. Nucleophilic Substitution: Less reactive than haloalkanes due to resonance.
2. Electrophilic Substitution: Ortho- and para-directing.
3. With Metals: Includes Wurtz-Fittig and Fittig reactions.

Polyhalogen Compounds

• Dichloromethane, Chloroform, Carbon Tetrachloride, Freons, DDT.
• Applications in industry but pose environmental risks, e.g., ozone layer depletion.

Exercises

• Practice IUPAC naming, chemical reactions, and conversions.
• Explore stereochemistry, reaction mechanisms, and environmental implications.

Summary

• Haloalkanes and haloarenes are crucial in organic chemistry with broad applications.
• Understanding preparation, properties, and reactions is essential for practical applications and environmental considerations.