Producing Halogenoalkanes (CIE AS Chemistry)

• 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

Free-radical substitution reactions of alkanes

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

Electrophilic addition to alkenes

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 substitution 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

Substitution of alcohols

Substitution of alcohols produces halogenoalkanes

Different methods of forming halogenoalkanes

Overview of the different ways to produce 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

Primary, secondary and tertiary halogenoalkanes

The number of alkyl groups attached determines if the halogenoalkane is primary, secondary or tertiary