Writing Free Radical Substitution Mechanism

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When writing mechanisms:

• It is a good practice to write the name of the mechanism whether or not the question asks for it.
• You just need to show the series of equations/ arrows. There is no need for descriptions i.e. the chlorine molecule undergoes homolytic cleavage to give two chlorine atoms/ radicals.

 

Common Errors:

1. Sloppy naming e.g. ‘substitution’, ‘free radical reaction’ or ‘free substitution’ or ‘FRS’

    

2. Wrong illustration of arrows

   

    

3. Forgot to put conditions i.e UV light or heat

   

    

4. Use of molecular formula

   

    

5. Wrong halogen.

   Under exam conditions, students sometimes use the wrong halogen (i.e. use Cl₂ when qns asks for Br₂) This happens when they blindly regurgitate what they have memorised.

 

NOTE:

• C–H bond is relatively strong;  it does NOT break during Free Radical Substitution.
• Hence, H · and H₂ will never appear in any step of the mechanism.

The second point is often tested in MCQs e.g.

Which of the following species would be present during the free radical chlorination of ethane?

 

  1  · CH₂CH₃

  2  · CH₂CH₂CH₂CH₃

  3  H₂