Free radical substitution

Free radical substitution is the reaction of an alkane with a halogen, such as methane with chlorine, in ultraviolet light. A hydrogen atom on the alkane is replaced (substituted) by a halogen atom. The mechanism is a chain reaction in three stages: initiation makes radicals, propagation uses and remakes them, and termination removes them.

What is a free radical?

A free radical (or just radical) is an atom or group of atoms with an unpaired electron. The unpaired electron is shown by a dot: Cl• is a chlorine radical and •CH₃ is a methyl radical. Radicals are very reactive, because the unpaired electron readily pairs up with an electron from another species to form a bond.

Radicals form by homolytic fission: a covalent bond breaks evenly, and each atom takes one of the two shared electrons. (In heterolytic fission, one atom takes both and ions form.)

Free radical substitution of methane

Methane and chlorine don't react in the dark at room temperature. In ultraviolet light, they react to form chloromethane and hydrogen chloride:

This overall equation hides a chain reaction. Here is the mechanism, step by step.

Initiation

UV light provides the energy to break the Cl-Cl bond by homolytic fission, making two chlorine radicals:

The Cl-Cl bond breaks, not a C-H bond, because it is much weaker.

Propagation

A chlorine radical takes a hydrogen atom from methane, making hydrogen chloride and a methyl radical. The methyl radical then takes a chlorine atom from a chlorine molecule, making chloromethane and a new chlorine radical:

Each propagation step uses up one radical and makes another, so the chlorine radical made in the second step can start the first step again. This is why it is a chain reaction: one initiation can lead to thousands of molecules of product. Add the two propagation steps together and the radicals cancel, giving the overall equation.

Termination

The chain stops when two radicals meet and join, so no radicals are left:

The last step explains why a little ethane is found in the products, even though no ethane was put in.

Check your understanding.

Why you get a mixture of products

Free radical substitution is hard to control. Once some chloromethane has formed, chlorine radicals can take a hydrogen from it too, and the substitution carries on:

So the product is a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane, plus ethane from termination. With longer alkanes the halogen can replace a hydrogen on different carbons, giving isomers too. Using a large excess of methane makes chloromethane the main product, because a chlorine radical is then much more likely to meet a methane molecule than a chloromethane molecule. The mixture is then separated by fractional distillation.

Worked example: ethane and bromine

Write the mechanism for the reaction of ethane with bromine in UV light to form bromoethane.

1. Initiation: Br₂ → 2Br•
2. Propagation: Br• + C₂H₆ → HBr + •C₂H₅, then •C₂H₅ + Br₂ → C₂H₅Br + Br•
3. Termination: Br• + Br• → Br₂; •C₂H₅ + Br• → C₂H₅Br; •C₂H₅ + •C₂H₅ → C₄H₁₀

Notice the last termination step: two ethyl radicals join to make butane, C₄H₁₀. Overall: C₂H₆ + Br₂ → C₂H₅Br + HBr.

Free radicals and the ozone layer

Ozone, O₃, high in the atmosphere absorbs ultraviolet radiation, protecting life on Earth. Chlorofluorocarbons (CFCs), once used as refrigerants and aerosol propellants, are very unreactive, so they last long enough to reach the upper atmosphere. There, UV radiation breaks their C-Cl bonds and releases chlorine radicals, which destroy ozone in a chain reaction:

Add the steps and the overall reaction is 2O₃ → 3O₂. The chlorine radical comes out of the second step unchanged, which makes it a catalyst, and one chlorine radical can break down a very large number of ozone molecules. Research by different groups of scientists provided the evidence that led to laws restricting the use of CFCs.

Common mistakes

• Forgetting UV light. It is needed for initiation; say so in the mechanism.
• Breaking a C-H bond in initiation. The Cl-Cl bond breaks, because it is the weakest.
• Leaving the dots off. Every radical needs its dot, in every step.
• Writing H• as a propagation product. The chlorine radical removes a hydrogen atom to make HCl; no free hydrogen radicals form.
• Missing the chain idea. State that the second propagation step regenerates Cl•.
• Calling it electrophilic or nucleophilic substitution. It is a radical mechanism.

Practice questions

1. What is a free radical?
2. What conditions are needed for methane to react with chlorine?
3. Write the two propagation steps for the reaction of methane with bromine.
4. Explain why some ethane is formed when methane reacts with chlorine.
5. Explain why a chlorine radical can destroy many ozone molecules.

Answers: 1) A species with an unpaired electron. 2) Ultraviolet light. 3) Br• + CH₄ → HBr + •CH₃, then •CH₃ + Br₂ → CH₃Br + Br•. 4) Two methyl radicals join in a termination step: •CH₃ + •CH₃ → C₂H₆. 5) It comes back out of the second step unchanged, so it works as a catalyst and goes round again.

Practise with a short timed paper

Alkanes, halogenoalkanes and reaction mechanisms are in the first year of A-level Chemistry. The daily A-level Chemistry: Year 1 paper is a short timed paper of 8 questions, with a new one every day. Compare this mechanism with electrophilic addition and nucleophilic substitution.

Read next

Chemistry · Organic chemistryNucleophilic substitution: mechanism, reactions, SN1 and SN2Nucleophilic substitution for A-level Chemistry: the mechanism with curly arrows, halogenoalkanes with OH⁻, CN⁻ and NH₃, reaction rates, and SN1 vs SN2.