Topic 4 · page 15 of 17

Alkanes and halogens

In ultraviolet light, an alkane and a halogen react by substitution. A hydrogen atom is replaced by a halogen atom, and a hydrogen halide is made. Without light there is no reaction.

9 min read4.174.17(ii)

Read first: Combustion of alkanes

Substitution by a halogen 4.174.17(ii)

Alkanes are not reactive towards most reagents, because their bonds are strong and almost non-polar. They do burn (Combustion of alkanes), and they react with the halogens chlorine and bromine in the presence of ultraviolet (UV) light.

Methane reacts with chlorine in ultraviolet light to give chloromethane and hydrogen chloride. One hydrogen atom of methane is replaced by a chlorine atom, shown in orange.
One hydrogen atom of methane is replaced by a chlorine atom.
CHX4+ClX2→UV lightCHX3Cl+HCl \ce{CH4 + Cl2 ->[UV light] CH3Cl + HCl}
CX2HX6+BrX2→UV lightCX2HX5Br+HBr \ce{C2H6 + Br2 ->[UV light] C2H5Br + HBr}

This is a substitution reaction (Types of reaction): an H atom is replaced by a halogen atom. The organic product is a halogenoalkane, named from the alkane with a halogen prefix: chloromethane, bromoethane. The other product is a hydrogen halide.

Evidence: the bromine test

PracticalBromine and hexane in light and dark

This is an illustration of the observation that the specification expects you to be able to describe; it is not one of the listed core practicals.

  1. Put 2 cm³ of hexane into each of two test tubes.
  2. Add a few drops of bromine to each. Both tubes are orange-brown. (Bromine is toxic and corrosive. Work in a fume cupboard, wear gloves and eye protection. Hexane is flammable.)
  3. Put one tube in a dark cupboard. Stand the other in bright light, such as UV light or strong sunlight.
  4. Compare the tubes after several minutes.

The tube kept in the dark stays orange-brown. The tube in UV light slowly becomes colourless, because the bromine has been used up to make a colourless halogenoalkane and hydrogen bromide.

Check yourself

  1. State the condition needed for ethane to react with chlorine.
  2. Write an equation for the reaction of methane with bromine, and name the organic product.
  3. Classify the reaction in question 2.
  4. A tube containing hexane and bromine is kept in the dark. State what you would observe, and explain.
  5. Write an equation for the reaction of propane with chlorine in which one hydrogen atom is replaced, and give the two possible names of the organic product.
  6. Why do alkanes react with chlorine only in the presence of UV light?

Answers cover these until you have tried the questions

  1. Ultraviolet light (or sunlight).
  2. CH₄ + Br₂ → CH₃Br + HBr (UV light over the arrow). The organic product is bromomethane.
  3. Substitution: a hydrogen atom of methane is replaced by a bromine atom.
  4. The orange-brown colour stays. There is no reaction in the dark, because the light is needed to supply the energy to break the Br–Br bond.
  5. C₃H₈ + Cl₂ → C₃H₇Cl + HCl (UV light over the arrow). The organic product may be 1-chloropropane or 2-chloropropane, depending on which hydrogen is replaced.
  6. Alkane molecules are not polar enough to react with chlorine, and the Cl–Cl bond is not broken at room temperature in the dark. UV light supplies the energy to break the Cl–Cl bond, forming chlorine free radicals that start the reaction.