Producing Halogenoalkanes (Cambridge (CIE) AS Chemistry): Revision Note

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

• A free-radical substitution reaction involves three main steps: initiation, propagation, and termination

• This reaction occurs between an alkane and a halogen, such as chlorine (Cl₂) or bromine (Br₂), in the presence of ultraviolet (UV) light

  • Initiation:

    • UV light provides energy to break the Cl–Cl or Br–Br bond by homolytic fission, producing two identical halogen free radicals (Cl• or Br•)

  • Propagation:

    • The halogen radicals react with alkane molecules in a chain reaction, producing new radicals and continuing the substitution of hydrogen atoms with halogen atoms

  • Termination:

    • The reaction stops when two free radicals combine to form a stable molecule, ending the chain process

Free-radical substitution reactions of alkanes

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

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

Different methods of forming 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

Primary, secondary and tertiary halogenoalkanes