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Chapter 09: Alkanes

Chapter 09: Alkanes

The free radical substitution mechanism is as such:


Checklist when drawing FRS Mechanism:

    Name the mechanism

    List the steps in its sequential order

    Insert the necessary half arrows

    Labelling of radicals on the right Carbon atom

Further Substitution

If chlorine is present in excess, further FRS can take place. This is also known as ‘continuous propagation’.


IMPORTANT questions to note:

1) Why is the synthesis of halogenoalkane from alkane not ideal using Free Radical Substitution?


  • Difficult to control the extent of substitution (with presence of excess chlorine)
  • Formation of other by-products
  • Difficult to control the position of substitution of halogen

Production of isomeric products

Isomeric products can be obtained as where the halogen will be substituted will vary.
The different products formed depend on the different chemical environments of the hydrogen atoms. Identifying the different chemical environments in the compound will allow us to calculate the ratio of isomeric products formed.

For example, looking at 2-methylpropane below, it has two groups of hydrogen atoms with different chemical environments, Hᵃ and Hᵇ. Substituting Hᵃ and Hᵇ gives two isomers, which is 1-chloro-2-methylpropane and 2-chloro-2-methylpropane respectively.


This concept is commonly tested in paper one as a multiple-choice-question (MCQ). Hence let’s go through a worked example to better understand this concept.

Worked Example 1:


A mixture of 2-methylbutane and bromine is allowed to react in the presence of sunlight. How many products are formed and what is the ratio in which they are formed?

a)  2 products; 1:3
b)  3 products; 3:6:2
c)  4 products; 6:1:2:3
d)  5 products; 4:3:1:2:2

Answer: c)  4 products; 6:1:2:3


More questions to take note:

1) Why do alkanes not react with I₂?


  • Weak C-I and H-I bonds formed causes the 1st propagation step to be endothermic, preventing the formation of iodoalkanes.

2) Why do alkanes not react with aqueous Cl₂?


  • The presence of polar water molecules in aqueous Cl₂ will polarise Cl₂ to break.
    heterolytically, causing a disproportionation reaction where HOCl and HCl are formed instead of the desired chloroalkane.

Stability of Radicals

More alkyl groups attached to the carbon radical will result in an increase in stability of the carbon radical:


Alkyl groups are generally electron-donating, thus more alkyl groups attached to the carbon radical causes it to be less electron deficient and the radical is more stable.

Continue Reading on Chapter 10: Alkenes