Alkanes

What happens

• In the presence of ultraviolet (UV) light, an alkane reacts with a halogen (chlorine or bromine) by a substitution reaction
• A hydrogen atom on the alkane is swapped for a halogen atom; the displaced hydrogen leaves as a hydrogen halide (HCl or HBr)
• General word equation:

alkane + halogen → halogenoalkane + hydrogen halide

• UV light is needed to break the halogen–halogen bond and start the reaction; the reaction does not occur in the dark

Example — ethane and bromine

• Ethane reacts with bromine in UV light to give bromoethane and hydrogen bromide:

C₂H₆(g) + Br₂(g) → C₂H₅Br(g) + HBr(g)

ethane + bromine → bromoethane + hydrogen bromide

• Visible clue: the orange-brown bromine vapour fades as the reaction proceeds and the bromine is consumed

Example — ethane and chlorine

• Ethane reacts with chlorine in UV light to give chloroethane and hydrogen chloride:

C₂H₆(g) + Cl₂(g) → C₂H₅Cl(g) + HCl(g)

• The same pattern: one H atom on the ethane swaps with one Cl atom from the chlorine, releasing HCl

Products: halogenoalkanes

• The organic product of an alkane–halogen substitution is called a halogenoalkane (or haloalkane) — an alkane with one (or more) hydrogen atoms replaced by halogen atoms
• Naming follows the same stem-plus-ending rule as the parent alkane, but the halogen is named as a prefix:
  • Methane + Cl₂ → chloromethane (CH₃Cl)
  • Ethane + Br₂ → bromoethane (C₂H₅Br)
  • Propane + I₂ → iodopropane (C₃H₇I)
• If more than one halogen has been substituted, prefixes like di-, tri- and a position number show how many and where (for example, 1,2-dichloroethane has one Cl on each of its two carbons)

Why this reaction matters

• Halogenoalkanes are useful as solvents, refrigerants, propellants and starting materials for pharmaceuticals
• A historically important sub-family was the chlorofluorocarbons (CFCs), once used in fridges and aerosol propellants
• CFCs were phased out under the Montreal Protocol because they migrate to the upper atmosphere and damage the ozone layer