CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.3.1 Production of Halogenoalkanes

Production of Halogenoalkanes: Substitution & Addition Reactions

  • Halogenoalkanes are alkanes that have one or more halogens
  • They can be produced from:
    • Free-radical substitution of alkanes
    • Electrophilic addition of alkenes
    • Substitution of an alcohol

Free-radical substitution of alkanes

  • Ultraviolet light (UV) is required for the reaction to start off
  • A free-radical substitution reaction is a three-step reaction consisting of initiation, propagation and termination steps
  • In the initiation step the halogen bond is broken by energy from the UV light to produce two radicals in a homolytic fission reaction
  • The propagation step refers to the progression (growing) of the substitution reaction in a chain type reaction
  • The termination step is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule

 

Halogen Compounds Free-Radical Substitution, downloadable AS & A Level Chemistry revision notes

Free-radical substitution reactions of alkanes produce halogenoalkanes

Electrophilic addition

  • Halogenoalkanes can also be produced from the addition of hydrogen halides (HX) or halogens (X2) at room temperature to alkenes
  • In hydrogen halides, the hydrogen acts as the electrophile and accepts a pair of electrons from the C-C bond in the alkene
    • The major product is the one in which the halide is bonded to the most substituted carbon atom (Markovnikov’s rule)
  • In the addition of halogens to alkenes, one of the halogen atoms acts as an electrophile and the other as a nucleophile

Halogen Compounds Electrophilic Addition to Alkenes, downloadable AS & A Level Chemistry revision notes

Electrophilic addition of hydrogen halides or hydrogen at room temperatures to alkenes results in the formation of halogenoalkanes

 

Halogen Compounds Summary, downloadable AS & A Level Chemistry revision notes

Overview of the different ways to produce halogenoalkanes

Classifying Halogenoalkanes

  • Depending on the carbon atom the halogen is attached to, halogenoalkanes can be classified as primary, secondary and tertiary
    • A primary halogenoalkane is when a halogen is attached to a carbon that itself is attached to one other alkyl group
    • A secondary halogenoalkane is when a halogen is attached to a carbon that itself is attached to two other alkyl groups
    • A tertiary halogenoalkane is when a halogen is attached to a carbon that itself is attached to three other alkyl groups

 

Halogen Compounds Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

Primary, secondary and tertiary halogenoalkanes

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