Halogenation of Alkanes (DP IB Chemistry): Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

Halogenation of alkanes

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

Strength of bonds

  • Alkanes consist of carbon and hydrogen atoms which are bonded together by single bonds

  • Unless a lot of heat is supplied, it is difficult to break these strong C-C and C-H covalent bonds

  • This decreases the alkanes’ reactivities in chemical reactions

Lack of polarity

  • The electronegativity 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

Periodic table of elements showing electronegativity values on a Pauling scale, arranged by atomic number. Elements with values range from 0.7 to 4.0.
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

Diagram of an alkane with nonpolar C-H and C-C bonds, showing electron sharing and explaining the molecule's nonpolarity with text annotations.
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

Flowchart showing reaction of hexane and bromine: sunlight causes bromine colour to disappear, dark conditions result in no reaction.
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

Initiation step

  • In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals

  • The covalent Cl-Cl bond is broken by energy from the UV light

  • Each atom takes one electron from the covalent bond

  • This produces two radicals in a homolytic fission reaction

Cl–Cl rightwards arrow with UV on top 2Cl

  • For more information about the initiation step, see our revision note about homolytic fission

Propagation step

  • The halogen free radicals are very reactive and will attack the unreactive alkanes

  • One of the methane C-H bond breaks homolytically to produce an alkyl radical

CH4 + ClCH3 + HCl

  • The alkyl radical can attack another chlorine molecule to form a halogenoalkane

  • This also regenerates the chlorine free radical

CH3 + Cl2 → CH3Cl + Cl 

  • The regenerated chlorine free radical can then repeat the cycle

  • For example, the chlorination of ethane is:

ethane + chlorine radical → ethyl radical + hydrogen chloride

CH3CH3 + ClCH2CH3 + HCl

ethyl radical + chlorine molecule → chloroethane + regenerated chlorine radical

CH2CH3 + Cl2 → CH3CH2Cl + Cl

  • 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:

chloroethane + chlorine radical → radical + hydrogen chloride

CH3CH2Cl + ClCH2CH2Cl + HCl

radical + chlorine molecule → 1,2-dichloroethane + regenerated chlorine radical

CH2CH2Cl + Cl2 → CH2Cl2 + Cl

  • 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: 

ethyl radical + chlorine radical → chloroethane

CH2CH3 + Cl → CH3CH2Cl

ethyl radical + ethyl radical → butane

CH2CH3 + CH2CH3 → CH3CH2CH2CH3

chlorine radical + chlorine radical → chlorine molecule

Cl + Cl → Cl2

Examiner Tips and Tricks

  • Make sure you practice and are able to write out these equations, especially the propagation steps

  • Students frequently get the propagation steps wrong, by showing the formation of a hydrogen radical produced in propagation

    • This step (CH3CH3 + Cl → CH3CH2 Cl + H) does not happen:

  • Do not fall into this trap!

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Richard Boole

Author: Richard Boole

Expertise: Chemistry Content Creator

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.

Philippa Platt

Reviewer: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener