Chemistry Reaction Schemes

Free-radical substitution: chlorination of methane

In UV light, chlorine reacts with methane by a radical chain reaction. Radicals have an unpaired electron, so their mechanism is drawn with half-headed 'fishhook' arrows, each showing one electron moving.

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Free-radical substitution (chain reaction)

UV light splits Cl₂ into two chlorine radicals (initiation). A chlorine radical takes a hydrogen from methane, and the methyl radical formed takes a chlorine atom from Cl₂, regenerating a chlorine radical (propagation). The chain continues until two radicals meet (termination).

Overall reaction

methane
chlorine
UV light
chloromethane
+
hydrogen chloride
byproduct

Mechanism — 3 steps

  1. 1Initiation: homolytic fission of Cl₂initiation (homolysis)
    chlorine
    UV
    chlorine radical
    +
    chlorine radical

    Electron movement

    1. One electron from the Cl–Cl bond goes to the left chlorine atom.
    2. The other electron goes to the right chlorine atom.

    UV light provides the energy to break the relatively weak Cl–Cl bond evenly (homolytic fission), giving two radicals, each with an unpaired electron.

  2. 2Propagation 1: Cl• takes a hydrogenpropagation
    chlorine radical
    +
    methane
    hydrogen chloride
    +
    methyl radical

    Electron movement

    1. The unpaired electron on chlorine moves to form a new H–Cl bond.
    2. One electron from the C–H bond pairs with it to complete the H–Cl bond.
    3. The other C–H electron stays on carbon, leaving a methyl radical.

    A chlorine radical removes a hydrogen atom (not H⁺) from methane, forming HCl and a methyl radical.

  3. 3Propagation 2: CH₃• takes a chlorinepropagation
    methyl radical
    +
    chlorine
    chloromethane
    +
    chlorine radical

    Electron movement

    1. The unpaired electron on carbon moves to form a new C–Cl bond.
    2. One electron from the Cl–Cl bond completes the C–Cl bond.
    3. The other Cl–Cl electron stays on the far chlorine, regenerating a chlorine radical.

    The chlorine radical formed here goes on to react with another methane molecule, so each initiation step can produce many product molecules (a chain reaction).

Stereochemistry

There is no stereochemistry with methane. With larger alkanes, radical substitution gives a mixture of isomers because any hydrogen can be replaced.

Key concepts

  • Homolytic fission and radicals
  • Fishhook (single-electron) arrows
  • Initiation, propagation and termination
  • Why a mixture of products forms (further substitution: CH₂Cl₂, CHCl₃, CCl₄)

Common mistakes

  • Using full double-headed arrows for radical steps
  • Writing H⁺ or Cl⁻ (radical reactions don't involve ions)
  • Forgetting that propagation steps regenerate a radical
  • Termination: two radicals combine, e.g. Cl• + •CH₃ → CH₃Cl, which isn't drawn here

More mechanisms