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Reaction Mechanisms: Free Radical Substitution

Explore the mechanism of free radical substitution during monochlorination of methane and ethane, revealing chain reactions initiated by ultraviolet light and leading to chlorinated products. Evidence presented includes the formation of trace amounts of ethane and butane.

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Reaction Mechanisms: Free Radical Substitution

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  1. Reaction Mechanisms: Free Radical Substitution

  2. Monochlorination of methane • CH4 + Cl2→ CH3Cl + HCl methane chloromethane

  3. Initiation • Ultraviolet light causes splitting of chlorine molecules into chlorine atoms Cl - Cl → Cl● + Cl● Chlorine Chlorine atoms molecule (free radicals)

  4. Propagation CH4 + Cl● = CH3● + HCl methane Methyl radicalHydrogen chloride CH3● + Cl2 = CH3Cl+ Cl ● Chloromethane Chlorine atom free to react with another methane molecule

  5. Termination Cl● + Cl● = Cl2 CH3● + Cl ●= CH3Cl CH3● + CH3● = C2H6

  6. Evidence for this mechanism Use of ultraviolet light even for a very short period causes a chain reaction Formation of trace quantities of ethane Reaction speeded up by sources of free radicals such as tetraethyl lead

  7. Monochlorination of ethane • C2H6 + Cl2→ C2H5Cl + HCl ethane chloroethane

  8. Initiation • Ultraviolet light causes splitting of chlorine molecules into chlorine atoms Cl - Cl → Cl● + Cl● Chlorine Chlorine atoms molecule

  9. Propagation C2H6 + Cl● = C2H5● + HCl ethane ethyl radicalhydrogen chloride C2H5● + Cl2 = C2H5Cl+ Cl ● Chloroethane Chlorine atom free to react with another ethane molecule

  10. Termination Cl● + Cl● = Cl2 C2H5● + Cl ●= C2H5Cl C2H5● + C2H5● = C4H10

  11. Evidence for this mechanism Use of ultraviolet light even for a very short period causes a chain reaction Formation of trace quantities of butane Reaction speeded up by sources of free radicals such as tetraethyl lead

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