Overview
This lecture covers haloalkanes, including their naming, classification, nucleophilic substitution reactions, effects of bond strength on reactivity, environmental impacts, and the chemistry of ozone depletion related to halogenated compounds.
Naming and Classification of Haloalkanes
- Haloalkanes are named based on the alkane with prefixes: fluoro (F), chloro (Cl), bromo (Br), and iodo (I).
- Substituents are listed alphabetically in the name.
- Primary haloalkanes have one carbon attached to the carbon bonded to the halogen.
- Secondary haloalkanes have two, and tertiary have three carbons attached to that carbon.
Nucleophilic Substitution Reactions
- A nucleophile is an electron pair donor (:OH⁻, :NH₃, CN⁻).
- Substitution swaps a halogen atom for another group in the molecule.
- The carbon-halogen bond is polar because the halogen is more electronegative than carbon.
- Mechanisms use curly arrows to show electron pair movements.
Factors Affecting Reactivity
- Reaction rate depends on C–X bond strength; weaker bonds react faster.
- C–I: 238 kJ/mol (fastest), C–Br: 276, C–Cl: 338, C–F: 484 (slowest).
- Iodoalkanes are most reactive, and fluoroalkanes are least reactive in substitution.
Nucleophilic Substitution with Hydroxide
- Haloalkanes react with aqueous KOH/NaOH under reflux to form alcohols.
- Ethanolic conditions lead to elimination, not substitution.
Hydrolysis and Comparing Haloalkane Reactivity
- Hydrolysis splits haloalkanes with water; water is a weak nucleophile.
- Aqueous silver nitrate helps compare rates: forms AgX precipitate (AgI = yellow, AgBr = cream, AgCl = white).
- Precipitate forms faster for more reactive haloalkanes.
Environmental Impact and Uses of Haloalkanes
- Many uses of haloalkanes (e.g., CFCs) have stopped due to toxicity and ozone depletion.
- CFCs were used in aerosols and refrigeration due to low reactivity and non-toxicity.
- HFCs (hydrofluorocarbons) are chlorine-free alternatives now used.
Ozone Layer Chemistry
- Ozone (O₃) in the upper atmosphere blocks harmful UV radiation.
- Ozone forms: O₂ + UV → 2O·; O· + O₂ → O₃.
- Ozone breaks down: O₃ + UV → O₂ + O·.
- Natural formation and breakdown rates are balanced, providing constant ozone levels.
Ozone Depletion by CFCs and NOx
- UV light breaks C–Cl bonds in CFCs, forming Cl· radicals.
- Cl· reacts with O₃, catalytically decomposing ozone: Cl· + O₃ → ClO· + O₂, then ClO· + O· → Cl· + O₂.
- Net: O₃ + O· → 2 O₂.
- NOx compounds can also catalyze ozone breakdown.
- CFCs persist in the atmosphere and continue to cause damage.
Key Terms & Definitions
- Haloalkane — Alkane with one or more hydrogens replaced by halogen atoms.
- Nucleophile — Species that donates an electron pair to form a new bond.
- Hydrolysis — Chemical breakdown involving reaction with water.
- CFC (Chlorofluorocarbon) — Halogenated organic compounds that affect the ozone layer.
- HFC (Hydrofluorocarbon) — Chlorine-free halogenated compounds used as safer refrigerants.
- Ozone (O₃) — Triatomic oxygen molecule, absorbs UV light in the stratosphere.
- Free Radical — Atom or molecule with an unpaired electron, highly reactive.
Action Items / Next Steps
- Review examples of naming and identifying haloalkane classes.
- Practice drawing nucleophilic substitution mechanisms with curly arrows.
- Read about environmental impacts of halogenated compounds and current alternatives to CFCs.