Haloalkanes

Naming Haloalkanes

• Derived from original alkane, prefix indicates halogen:
  • Fluoro (F), Chloro (Cl), Bromo (Br), Iodo (I)
• Example:
  • 1-bromopropane: C3H7Br
  • 2-chloro-2-methylbutane: C5H11Cl
• Substituents listed alphabetically

Classifying Haloalkanes

• Based on the number of carbon atoms attached to C-X functional group:
  • Primary Haloalkane: One carbon attached
  • Secondary Haloalkane: Two carbons attached
  • Tertiary Haloalkane: Three carbons attached

Nucleophilic Substitution Reactions

• Nucleophile: Electron pair donator (e.g., :OH-, :NH3, CN-)
• Substitution: Swapping halogen atom for another atom/group
• Reaction rate depends on C-X bond strength:
  • Weaker bond = Faster reaction
  • Bond Enthalpies:
    • C-I: 238 kJ/mol
    • C-Br: 276 kJ/mol
    • C-Cl: 338 kJ/mol
    • C-F: 484 kJ/mol
• Iodoalkanes react fastest, fluoroalkanes slowest due to bond strength

Reaction Mechanism

• Use curly arrows to show electron pair movement
• Arrows start from lone pairs or bond center
• Mechanism shows detail of reaction process

Nucleophilic Substitution with Aqueous Hydroxide Ions

• Functional Group Change: Haloalkane to alcohol
• Reagent: Potassium/Sodium hydroxide
• Conditions: Aqueous solution, heat under reflux
• Mechanism: Nucleophilic substitution
• Reagent Type: Nucleophile, OH-
• Aqueous conditions crucial; ethanol leads to elimination reaction

Hydrolysis Reactions

• Water as a poor nucleophile, needs heat or reflux
• Hydrolysis: splitting by reaction with water
• Example: CH3CH2X + H2O → CH3CH2OH + X- + H+
• Silver nitrate test for reactivity:
  • Precipitate forms with halide leaving group
  • Rate of precipitate shows reactivity
• Iodoalkane forms precipitate fastest, indicating weakest bond

Environmental Concerns

• Toxicity of haloalkanes and impact on ozone layer
• CFCs used in aerosols, refrigerants, now reduced due to environmental impact
• HFCs (Hydrofluorocarbons) as safer alternatives (no C-Cl bond)
• CO2 used as a blowing agent instead of CFCs

The Ozone Layer

• Beneficial in filtering UV radiation in upper atmosphere
• Ozone formation and depletion:
  • Formed by UV light splitting O2 into radicals
  • Depleted by reactions catalyzed by CFC-derived radicals
• Continuous cycle of formation and depletion maintains atmospheric balance
• CFCs and NOx catalyze ozone breakdown
• Legislation supports CFC reduction, development of safer alternatives
• CFCs still pose a concern due to long atmospheric lifetime and ongoing emissions

These notes cover the main points about haloalkanes, their reactions, environmental impacts, and the chemistry of the ozone layer, providing a comprehensive overview for studying these topics.