Radical Substitution & Ozone Impact

Overview

This lecture covers the free-radical substitution mechanism for forming halogenoalkanes from alkanes, including stepwise reaction details and the environmental impact on the ozone layer.

Free Radicals and Their Properties

• Free radicals are atoms or molecules with an unpaired electron, represented by a dot (•).
• Common free radicals include those of halogens (Cl•, Br•) and organic molecules (methyl, ethyl radicals).
• The dot must be placed on the specific atom with the unpaired electron.
• Free radicals are highly reactive due to their unpaired electron.

Formation of Halogenoalkanes

• Alkanes react with halogens under ultraviolet (UV) light to form halogenoalkanes and hydrogen halides.
• No reaction occurs without UV light.
• Example: hexane + bromine (with UV) → bromohexane + hydrogen bromide (HBr).
• Structural formulas are preferred for clarity in exams.

Free-Radical Substitution Mechanism

• The reaction involves three steps: initiation, propagation, and termination.
• Initiation: UV light splits a halogen (e.g., Cl2) into two free radicals (homolytic fission).
• Propagation:
  • Step 1: Cl• + CH4 → HCl + CH3•
  • Step 2: CH3• + Cl2 → CH3Cl + Cl•
  • The reaction is a chain reaction as the free radicals regenerate.
• Termination: Two free radicals combine to form a stable molecule (e.g., Cl• + Cl• → Cl2).

Reaction Variations and Further Substitution

• Substitution can continue: monohalogenated → dihalogenated → trihalogenated → tetrahalogenated products.
• Multiple products form if the original alkane or halogenoalkane has more than one type of hydrogen atom.
• Mixtures of products are separated using fractional distillation.

Ozone Layer Depletion by Free-Radical Substitution

• Ozone (O3) absorbs harmful UV radiation and protects life on Earth.
• CFCs (chlorofluorocarbons) released into the atmosphere generate Cl• radicals under UV light.
• Stepwise ozone depletion:
  • Initiation: CFC + UV → Cl•
  • Propagation 1: Cl• + O3 → ClO• + O2
  • Propagation 2: ClO• + O3 → 2O2 + Cl•
  • Net reaction: 2O3 → 3O2 (Cl• acts as a catalyst).
  • Termination: Cl• + Cl• → Cl2

Key Terms & Definitions

• Free Radical — Atom or molecule with an unpaired electron, highly reactive.
• Homolytic Fission — Bond breaking where each atom takes one electron, forming two radicals.
• Propagation Step — Stage where radicals react to create new radicals, continuing the chain.
• Termination Step — Reaction where two radicals combine, stopping the chain.
• Halogenoalkane — Alkane with one or more hydrogen atoms replaced by halogen(s).
• CFC (Chlorofluorocarbon) — Compound containing chlorine, fluorine, and carbon, involved in ozone depletion.

Action Items / Next Steps

• Practice writing initiation, propagation, and termination steps for different alkanes and halogens.
• Review examples of product mixtures and how to separate them by fractional distillation.
• Prepare for further study of halogenoalkanes and alkenes in upcoming lectures.