Free Radical Chain Reactions

Free radical chain reactions involve three key steps: initiation, propagation, and termination. These reactions are characterized by the generation and use of free radicals, which are highly reactive species with unpaired electrons.

Steps in Free Radical Chain Reactions

1. Initiation

• Purpose: Produces free radicals by breaking a covalent bond homolytically.
• Example: UV light breaks the Cl-Cl bond in chlorine, creating two chlorine radicals (Cl·).
• Mechanism: One electron from the bond goes to each chlorine atom, forming radicals.
• Notation: Can be written as 2Cl· (dot denotes a radical).
• Electron Movement: Arrows show the movement of electrons, with single-headed arrows representing the movement of one electron.

2. Propagation

• Purpose: Uses free radicals to react with stable molecules, forming new molecules and other radicals.
• Example with Chlorine and Methane:
  • First Propagation Reaction:
    • A chlorine radical reacts with methane, forming hydrogen chloride (HCl) and a methyl radical (CH3·).
  • Second Propagation Reaction:
    • The methyl radical reacts with Cl2, forming chloromethane (CH3Cl) and a new chlorine radical.
• Outcome: Continues the chain reaction by producing and using radicals.

3. Termination

• Purpose: Consumes free radicals to form stable molecules, ending the chain reaction.
• Examples:
  • Two chlorine radicals form Cl2.
  • A methyl and a chlorine radical form methyl chloride (CH3Cl).
  • Two methyl radicals form ethane (C2H6).
• Complexity: Produces a mixture of products, some of which can enter further reactions.

Example: Reaction Between Ethane and Bromine

• Overall Reaction: Ethane reacts with Br2 in a substitution reaction, producing bromoethane (C2H5Br) and HBr.
• Initiation: UV light splits Br2 to form two bromine radicals.
• Propagation:
  • Bromine radical extracts hydrogen from ethane, forming HBr and an ethyl radical (C2H5·).
  • Ethyl radical reacts with Br2, producing bromoethane and a bromine radical.
• Termination:
  • Two bromine radicals form Br2.
  • An ethyl radical and a bromine radical form bromoethane.
  • Two ethyl radicals form butane.

Importance and Impact

• UV Light: Drives initiation by splitting halogen bonds, found in sunlight.
• Environmental Impact: Chlorofluorocarbons (CFCs) decompose in the atmosphere, forming radicals that deplete the ozone layer.

Conclusion

• Free radical chain reactions are highly reactive and can produce various products, some of which have significant environmental impacts.

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