Halogenation of Alkanes (SL IB Chemistry)

Stability of alkanes

• Alkanes are relatively stable / unreactive due to the strengths of the C–C and C–H bonds and their non-polar nature

Lack of polarity

• The electronegativities of the carbon and hydrogen atoms in alkanes are almost the same
• This means that both atoms share the electrons in the covalent bond almost equally

 Pauling electronegativity values for the elements

The Pauling Scale shows that the difference in electronegativity between carbon and hydrogen is only 0.4

• As a result of this, alkanes are nonpolar molecules and have no partial positive or negative charges (δ⁺ and δ^–  respectively)

Structural formula of ethane showing bond polarities

Ethane is an example of an alkane that lacks polarity due to almost similar electronegativities of the carbon and hydrogen atoms

• Alkanes, therefore, do not react with polar reagents
  • They have no electron-deficient areas to attract nucleophiles
  • And also lack electron-rich areas to attract electrophiles
• Alkanes only react in combustion reactions and undergo substitution by radicals

Free-radical substitution of alkanes

• Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)
• Since alkanes are very unreactive, ultraviolet light (sunlight) is needed for this substitution reaction to occur

Proving that energy from UV light is required for radical reactions with halogens

The fact that the bromine colour has disappeared only when mixed with an alkane and placed in sunlight suggests that the ultraviolet light is essential for the free radical substitution reaction to take place

• The free-radical substitution reaction consists of three steps:
  • In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals
    • For more information about the initiation step, see our revision note about Homolytic Fission
  • These radicals create further radicals in a chain reaction called the propagation step
  • The reaction is terminated when two radicals collide with each other in a termination step

Propagation step

• The propagation step refers to the progression (growing) of the substitution reaction in a chain reaction
  • 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 halogen molecule to form the halogenoalkane and regenerate the halogen radical
  • This radical can then repeat the cycle
• For example, the chlorination of ethane is:

CH3CH3	+	Cl•	→	•CH2CH3	+	HCl
ethane		chlorine radical		alkyl (ethyl) radical
•CH2CH3	+	Cl2	→	CH3CH2Cl	+	Cl•
ethyl radical		chlorine molecule		halogenoalkane (chloroethane)		chlorine radical regenerated

• This reaction is not very suitable for preparing specific halogenoalkanes as a mixture of substitution products is formed
• If there is enough halogen present, all the hydrogens in the alkane will eventually get substituted
• For example, the chlorination of ethane could continue:

CH3CH2Cl	+	Cl•	→	•CH2CH2Cl	+	HCl
halogenoalkane (chloroethane)		chlorine radical		radical
•CH2CH2Cl	+	Cl2	→	ClCH2CH2Cl	+	Cl•
radical		chlorine molecule		disubstituted halogenoalkane		chlorine radical regenerated

• This process can repeat until hexachloroethane, C₂Cl₆, is formed

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
• For example, the single substitution of ethane by chlorine can form: 

•CH2CH3	+	Cl•	→	CH3CH2Cl
ethyl radical		chlorine radical		chloroethane
•CH2CH3	+	•CH2CH3	→	CH3CH2CH2CH3
ethyl radical		ethyl radical		butane
Cl•	+	Cl•	→	Cl2
chlorine radical		chlorine radical		chlorine molecule