• 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 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 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)
• PCl_3?and heat
• PCl_5?at room temperature
• SOCl₂

Substitution of alcohols to produce 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