Overview
This lecture covers the properties, naming, reactions (nucleophilic substitution and elimination), and environmental impact of halogenoalkanes (haloalkanes), with a focus on A-level chemistry concepts and mechanisms.
Introduction to Halogenoalkanes
- Halogenoalkanes are alkanes where one or more hydrogens are replaced by halogen atoms.
- They do not occur naturally and are important in the manufacture of products like PVC, Teflon, solvents, and anesthetics.
- General formula: CₙH₂ₙ₊₁X, where X is a halogen.
- Functional group is represented as R–X, with R as a carbon chain and X as a halogen.
Naming of Halogenoalkanes
- Name by identifying the longest carbon chain and adding prefixes for halogens (bromo-, chloro-, fluoro-, iodo-).
- Use locants (numbers) to indicate which carbon the halogen is attached to; keep numbers as low as possible.
- Multiple halogens use prefixes: di-, tri-, tetra-, etc.
- Multiple different halogens are listed alphabetically.
Properties of Halogenoalkanes
- Bond Polarity: Carbon-halogen bonds are polar due to differences in electronegativity, most polar for C–F and least for C–I.
- Solubility: Insoluble in water (polar solvent) due to large nonpolar regions; soluble in hydrocarbons.
- Boiling Point: Increases with chain length and size of halogen (C–I > C–Br > C–Cl > C–F); branching lowers boiling point.
Reactivity of Halogenoalkanes
- Reactivity depends on the ease of breaking the C–X bond (bond enthalpy is most important).
- C–I bonds break most easily, making iodoalkanes the most reactive.
- Tested by hydrolysis with silver nitrate: iodoalkane reacts fastest, then bromo-, then chloroalkane.
Nucleophilic Substitution Reactions
- Substitution where a nucleophile replaces the halogen.
- Nucleophiles have a lone pair on an electronegative atom (e.g. OH⁻, CN⁻, NH₃).
- OH⁻ → alcohol; CN⁻ → nitrile; NH₃ (needs 2 molecules) → amine + ammonium halide.
- Mechanisms require curly arrows to show electron movement.
- Reaction conditions: aqueous solution, warming, sometimes with ethanol to help mixing.
Elimination Reactions
- Elimination forms an alkene, halide ion, and water from a halogenoalkane.
- Requires potassium/sodium hydroxide dissolved in ethanol and heating.
- Hydroxide ion acts as a base, removing a proton from an adjacent carbon.
- Multiple products/isomers (E/Z) can form if different hydrogens are available.
- Test for alkene by decolorizing bromine water (yellow to colorless).
CFCs and Ozone Depletion
- CFCs (chlorofluorocarbons) are halogenoalkanes with chlorine and fluorine, no hydrogen.
- UV light breaks C–Cl bonds, generating chlorine radicals that catalyze ozone (O₃) breakdown.
- Overall effect: 2O₃ → 3O₂, depleting the ozone layer which protects from UV radiation.
- Legislation and alternative compounds have reduced CFC use, but ozone monitoring continues.
Key Terms & Definitions
- Halogenoalkane — Alkane with one or more hydrogens replaced by halogen atoms.
- Nucleophile — Species with a lone pair of electrons that attacks electron-deficient atoms.
- Bond enthalpy — Energy needed to break a covalent bond.
- Elimination — Reaction where atoms/groups are removed, forming a double bond.
- CFC — Chlorofluorocarbon, a halogenoalkane with only chlorine and fluorine.
- Ozone layer — Region in the atmosphere with high O₃ concentration absorbing UV light.
Action Items / Next Steps
- Practice naming halogenoalkanes with various halogens and chain lengths.
- Draw and practice nucleophilic substitution and elimination mechanisms with curly arrows.
- Review the environmental impact of CFCs and regulations related to ozone depletion.