Radical Substitution of Halogen into Alkanes

Introduction

• Free radical substitution is a type of chemical reaction where one or more hydrogen atoms in an alkane are replaced by halogen atoms (e.g., chlorine or bromine).
• This reaction involves the use of free radicals, highly reactive species with unpaired electrons.

Initiation

• The reaction begins with the initiation step, where free radicals are generated. This can be achieved by exposing a halogen molecule (e.g., Cl₂ or Br₂) to UV light or heat.
• UV light or heat energy breaks the diatomic halogen bond (X-X), creating two highly reactive halogen radicals (X·).

Propagation

• Once initiated, the reaction proceeds through a series of propagation steps:
  • A halogen radical (X·) reacts with an alkane molecule (RH).
  • This reaction forms an alkyl radical (R·) and a hydrogen halide molecule (HX).
  • The alkyl radical (R·) can then react with another halogen molecule (X₂), producing a new halogen radical (X·) and a halogenated alkane (R-X).
  • The process continues as long as there are alkane molecules available.

Termination

• Free radical reactions can also terminate when two radicals combine to form stable molecules:
  • For example, two alkyl radicals (R·) can combine to form a dimer (R-R).
  • Alternatively, an alkyl radical (R·) and a halogen radical (X·) can combine to form a halogenated alkane (R-X).

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Free radical substitution of halogen into alkanes

Examples

• An example of free radical substitution is the chlorination of methane (CH₄):
  • Initiation: Cl₂ → 2Cl• (UV light or heat)
• Propagation:
  • Cl• + CH₄ → CH₃Cl + H•
  • CH₃• + Cl₂ → CH₃Cl₂ + Cl•
  • CH₃Cl• + Cl₂ → CHCl₃ + Cl•
  • CHCl₃ + Cl• → CCl₄ + H•
  • Termination: Various possible combinations of radicals.

Conclusion:

• Free radical substitution of halogen into alkanes is a chemical reaction involving the replacement of hydrogen atoms in alkanes with halogen atoms.
• The reaction proceeds through initiation, propagation, and termination steps.
• This reaction has applications in the synthesis of halogenated organic compounds and is essential in understanding organic chemistry reactions involving alkanes and halogens.