Substitution Reactions of Alkanes

This lesson covers: 

  1. How halogens undergo radical substitution reactions with alkanes
  2. The reaction mechanism for the halogenation of alkanes

Photochemical halogenation of alkanes

Halogens react with alkanes in special light-induced reactions called photochemical reactions.


For the reaction to occur:

  • Ultraviolet (UV) light must be present.
  • This UV light provides the activation energy to start the reaction.


The overall reaction is a substitution, where a hydrogen atom in the alkane molecule is replaced by a halogen atom like chlorine or bromine.

Free radical substitution mechanism

Photochemical halogenation of alkanes follows a three-step free radical substitution mechanism:

  1. Initiation - UV light produces reactive radicals
  2. Propagation - Radicals react in a chain reaction
  3. Termination - Radicals join to form stable molecules


Methane reacts vigorously with chlorine gas in the presence of UV light, as represented by the equation:

CH4 + Cl2 ➔ CH3Cl + HCl

The three-stage mechanism for this reaction is detailed below.


Stage 1 - Initiation

  • UV light breaks the Cl-Cl bond in chlorine via homolytic fission
  • This gives two chlorine radicals (Cl•):

Cl2 ➔ 2Cl•

  • The unpaired electron makes Cl• highly reactive.


Stage 2 - Propagation

  • The chlorine radical attacks a methane molecule in a substitution reaction:

Cl• + CH4 ➔ CH3• + HCl

  • The methyl radical (CH3•) attacks another chlorine molecule:

CH3• + Cl2 ➔ CH3Cl + Cl•

  • This propagation cycle continues until reagents are used up.


Stage 3 - Termination

  • Two radicals join to form a stable covalent bond:

Cl• + CH3• ➔ CH3Cl

  • Other combinations like 2CH3• ➔ C2H6 or 2Cl• ➔ Cl2 are also possible.


The end products of the termination step depend on which reagent is in excess:

  • Excess chlorine - Further substitution on products like chloromethane occurs, producing a mixture of products such as CH3Cl, CH2Cl2, CHCl3, and CCl4.
  • Excess methane - Predominantly single substitution occurs to form chloromethane.