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Halogenoalkanes and Environmental Impact

Sep 5, 2025

Overview

This lecture covers the naming, structure, and reactivity of halogenoalkanes (haloalkanes), their environmental impact, especially regarding CFCs and the ozone layer, and alternatives to CFCs.

Naming Halogenoalkanes

  • Halogenoalkanes are alkanes with one or more halogen atoms attached.
  • The longest carbon chain forms the base name; halogens use prefixes (fluoro-, chloro-, bromo-, iodo-) in alphabetical order.
  • Number each halogen's carbon position; use di-, tri-, tetra- for multiples of the same halogen.
  • Examples: trifluoromethane, 2-chloropropane, 1-bromo-1,1-dichloro-2-iodoethane.

Bond Polarity and Nucleophiles

  • C–X bonds in halogenoalkanes are polar due to halogen electronegativity, making the carbon δ+ (electron-poor).
  • Nucleophiles (electron pair donors) attack the δ+ carbon; common examples: NH₃, OH⁻, CN⁻, H₂O.
  • Nucleophilic substitution: nucleophile replaces halogen.

Nucleophilic Substitution Reactions

  • With OH⁻ (aqueous sodium hydroxide, warm, reflux): halogenoalkane + NaOH → alcohol + NaX.
  • With H₂O (heated): halogenoalkane + H₂O → alcohol + H⁺ + X⁻ (reaction is slower due to weak nucleophilicity).
  • Reaction involves heterolytic bond fission (both electrons to halogen).

Reactivity and Hydrolysis of Haloalkanes

  • Reactivity increases as the C–X bond strength decreases (from C–F to C–I).
  • Rate order for hydrolysis: iodoalkanes > bromoalkanes > chloroalkanes, seen via silver nitrate test (forms precipitate fastest for I⁻).

CFCs and Ozone Depletion

  • CFCs (chlorofluorocarbons) are halogenoalkanes once widely used as refrigerants and propellants.
  • CFCs release Cl• radicals under UV light, which catalyze ozone (O₃) breakdown in the stratosphere.
  • Mechanism:
    • Initiation: UV splits C–Cl bond to form radicals.
    • Propagation: Cl• reacts with O₃ to form ClO• and O₂; ClO• reacts with O₃ to regenerate Cl• and produce more O₂.
    • Termination: Radicals combine to end the chain.
  • Other radicals (e.g., NO•) from vehicle emissions also destroy ozone.

Environmental Impact and Alternatives

  • CFCs are stable, non-toxic, and were widely used, but banned due to ozone depletion (Montreal Protocol, 1989).
  • Alternatives: HFCs, hydrocarbons, ammonia (for refrigeration), and inert gases for sprays; however, HFCs are potent greenhouse gases.
  • Scientific evidence shows reduced CFCs are helping the ozone layer recover.

Key Terms & Definitions

  • Halogenoalkane (Haloalkane) — An alkane with one or more halogen atoms attached.
  • Nucleophile — Electron pair donor species that attacks electron-deficient carbons.
  • Nucleophilic substitution — Reaction where a nucleophile replaces a halogen on an alkane.
  • CFC (Chlorofluorocarbon) — Compound with chlorine and fluorine used in refrigeration, damaging to ozone.
  • Radical — Atom or molecule with an unpaired electron, highly reactive.
  • Hydrolysis — Breaking a bond using water.

Action Items / Next Steps

  • Practice naming halogenoalkanes using provided rules.
  • Review nucleophilic substitution reaction mechanisms with OH⁻ and H₂O.
  • Summarize the environmental impact of CFCs and list alternatives.
  • Read specification points on halogenoalkanes for your exam board.