Chemistry Lecture Notes: Haloalkanes and Haloarenes

Overview

• Replacement of hydrogen atoms in hydrocarbons by halogen atoms forms alkyl halides (haloalkanes) and aryl halides (haloarenes).
• Haloalkanes: Halogen attached to sp3 hybridized carbon of an alkyl group.
• Haloarenes: Halogen attached to sp2 hybridized carbon of an aryl group.
• Organohalogen compounds have industrial and clinical applications (e.g., solvents, antibiotics).
• Study includes preparation, properties, and uses of these compounds.

Objectives

• Name haloalkanes and haloarenes using IUPAC nomenclature.
• Describe preparation and reactions of haloalkanes and haloarenes.
• Understand reaction mechanisms using stereochemistry.
• Appreciate organo-metallic compounds and environmental effects of polyhalogen compounds.

Classification

By Number of Halogen Atoms

• Monohalogen, dihalogen, polyhalogen compounds.

By Carbon Hybridization

• Alkyl Halides (RX): Halogen bonded to sp3 carbon.
  • Primary, secondary, tertiary classification.
• Allylic Halides: Halogen on sp3 carbon adjacent to C=C.
• Benzylic Halides: Halogen on sp3 carbon attached to aromatic ring.
• Vinylic Halides: Halogen on sp2 carbon of C=C.
• Aryl Halides: Halogen on sp2 carbon of aromatic ring.

Nomenclature

• Common names: Alkyl group + halide.
• IUPAC: Halosubstituted hydrocarbons.
• Special rules for disubstituted derivatives (o-, m-, p- prefixes in common names).

C-X Bond Nature

• Polar nature due to electronegativity difference.
• Carbon bears partial positive, halogen bears partial negative charge.
• Increase in bond length and decrease in bond enthalpy down the halogen group.

Preparation Methods

From Alcohols

• Alcohols react with halogen acids/phosphorus halides/thionyl chloride.
• Thionyl chloride preferred due to gaseous byproducts (SO2, HCl).

From Hydrocarbons

• Free Radical Halogenation: Produces complex mixtures.
• Electrophilic Substitution: For aryl halides using Lewis acid catalysts.

Chemical Reactions

Haloalkanes

• Nucleophilic Substitution (SN1, SN2): Depend on structure, solvent, and nucleophile.
• Elimination Reactions: Formation of alkenes using alcoholic KOH (β-elimination).
• Reaction with Metals: Formation of Grignard reagents in dry ether.

Haloarenes

• Less reactive in nucleophilic substitution due to resonance and hybridization effects.
• Undergo electrophilic substitution like benzene (halogenation, nitration, etc.).

Stereochemistry

• Optical Activity: Plane polarized light rotation.
• Chirality: Asymmetric carbon (chiral center) leads to optical activity.
• SN2 Reactions: Inversion of configuration.
• SN1 Reactions: Racemization due to planar carbocation intermediate.

Polyhalogen Compounds

• Dichloromethane: Solvent and industrial uses, CNS effects.
• Chloroform: Former anesthetic, CNS depressant.
• Carbon Tetrachloride: Environmental hazard, ozone layer depletion.
• Freons: Refrigerants, stable but ozone-depleting.
• DDT: Insecticide, environmental persistence and toxicity.

Summary

• Alkyl/aryl halides classified by halogen number.
• Polar C-X bond responsible for substitution, elimination, and organometallic reactions.
• Industrial uses and environmental impact of polyhalogen compounds highlighted.