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

 

Substitution of alcohols

  • In the substitution of alcohols an alcohol group is replaced by a halogen to form a halogenoalkane
  • The subustition of the alcohol group for a halogen can be achieved by reacting the alcohol with:
  • HX (or KBr with H2SO4 or H3PO4 to make HX)
  • PCl3 and heat
  • PCl5 at room temperature
  • SOCl2

Halogen Compounds Substitution of Alcohols, downloadable AS & A Level Chemistry revision notes

Substitution of alcohols to produce 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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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