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.