The Free Radical Substitution Mechanism (Edexcel International A Level Chemistry)

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Free Radical Substitution - Mechanism

  • The free-radical substitution reaction consists of three steps:

Initiation step

  • In the initiation step, the Cl-Cl or Br-Br is broken by energy from the UV light
  • This produces two radicals in a homolytic fission reaction

Hydrocarbons Initiation Step, downloadable AS & A Level Chemistry revision notes

The first step of the free-radical substitution reaction is the initiation step, in which two free radicals are formed by sunlight

Propagation step

  • The propagation step refers to the progression (growing) of the substitution reaction in a chain reaction
    • Free radicals are very reactive and will attack the unreactive alkanes
    • A C-H bond breaks homolytically (each atom gets an electron from the covalent bond)
    • An alkyl free radical is produced
    • This can attack another chlorine/bromine molecule to form the halogenoalkane and regenerate the chlorine/bromine free radical
    • This free radical can then repeat the cycle

Hydrocarbons Propagation Step, downloadable AS & A Level Chemistry revision notes

The second step of the free-radical substitution reaction is the propagation step in which the reaction grows in a chain reaction

    • This reaction is not very suitable for preparing specific halogenoalkanes as a mixture of substitution products are formed
    • If there is enough chlorine/bromine present, all the hydrogens in the alkane will eventually get substituted (eg. ethane will become C2Cl6/C2Br6)

Hydrocarbons Side-Products, downloadable AS & A Level Chemistry revision notes

The free-radical substitution reaction gives a variety of products and not a pure halogenoalkane

Termination step

  • The termination step is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule
    • Multiple products are possible

Hydrocarbons Termination Step, downloadable AS & A Level Chemistry revision notes

The final step in the substitution reaction to form a single unreactive molecule

Limitations of the free radical mechanism

  • In the termination step, there are a number of possibilities 
  • Remember that termination involves any free radical bonding with another free radical
  • If we have two CHradicals, they can bond to form ethane, CH3CH3
    • CHCH3 → CH3CH3
    • If we are trying to form a chloroalkane, then ethane is an impurity

Further substitution

  • Excess chlorine present when reacted with methane in the presence of UV light will promote further substitution and could produce CH2Cl2, CHCl3, CCl4
  • Further substitution can occur as follows
    • CH3Cl + Cl → HCl + CH2Cl
    • CH2Cl + Cl2 → CH2Cl+ Cl
  • These reactions could occur
    • CH3Cl + Cl2 → CH2Cl+ HCl 
    • CH2Cl2 + Cl2 → CHCl3 + HCl
    • CHCl3 + Cl2 → CCl4 + HCl

Substitution of different carbon atoms 

  • If we have an alkane with a middle carbon such as propane, substitution can occur here
  • Propagation steps for the substitution of propane with excess bromine in the presence of UV light on the middle carbon are as follows:
    • CH3CH2CHBr → CH3CHCH+ HBr
    • CH3CHCH+ Br2 → CH3CH(Br)CH3Br
  • If the question asks for the halogen to be substituted onto a middle carbon, you must show the radical dot in the correct place, so on the electron-deficient carbon

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Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.