A step-by-step account of how a reaction happens, showing how bonds break and form.

What is a free radical?

A species with an unpaired electron.

What is homolytic fission?

Bond breaking in which each atom takes one electron from the shared pair, forming radicals.

What is heterolytic fission?

Bond breaking in which both electrons go to one atom, forming ions.

What forms radicals from chlorine?

Ultraviolet light, which breaks the Cl–Cl bond: Cl₂ → 2Cl•.

What are the three stages of a free-radical chain reaction?

Initiation, propagation and termination.

Write the two propagation steps for methane and chlorine.

Cl• + CH₄ → CH₃• + HCl and CH₃• + Cl₂ → CH₃Cl + Cl•

Give two termination steps.

Cl• + Cl• → Cl₂ and CH₃• + CH₃• → C₂H₆

Are curly arrows required in free-radical mechanisms?

No. Balanced equations for each step are used.

What does a curly arrow show?

The movement of a pair of electrons.

Where does a curly arrow start?

At a lone pair of electrons or at a covalent bond.

What is shown by a curly arrow from a bond to an atom?

The bond breaks and both electrons go onto that atom.

Reaction mechanismsAQA A-Level Chemistry: Flashcards

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What is a mechanism?

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What is a mechanism?: A step-by-step account of how a reaction happens, showing how bonds break and form.
What is a free radical?: A species with an unpaired electron.
How is a radical shown?: With a dot, for example Cl•.
What is homolytic fission?: Bond breaking in which each atom takes one electron from the shared pair, forming radicals.
What is heterolytic fission?: Bond breaking in which both electrons go to one atom, forming ions.
What forms radicals from chlorine?: Ultraviolet light, which breaks the Cl–Cl bond: Cl₂ → 2Cl•.
What are the three stages of a free-radical chain reaction?: Initiation, propagation and termination.
Write the two propagation steps for methane and chlorine.: Cl• + CH₄ → CH₃• + HCl and CH₃• + Cl₂ → CH₃Cl + Cl•
Give two termination steps.: Cl• + Cl• → Cl₂ and CH₃• + CH₃• → C₂H₆
Are curly arrows required in free-radical mechanisms?: No. Balanced equations for each step are used.
What does a curly arrow show?: The movement of a pair of electrons.
Where does a curly arrow start?: At a lone pair of electrons or at a covalent bond.
What is shown by a curly arrow from a bond to an atom?: The bond breaks and both electrons go onto that atom.
Which species forms when the C–Br bond in bromoethane breaks heterolytically with OH⁻?: The bromide ion, Br⁻.

Exam questions on Reaction mechanisms

Methane reacts with chlorine in the presence of ultraviolet light to form chloromethane and hydrogen chloride. The reaction does not occur in the dark. It is a free-radical reaction.
Write equations for the two propagation steps of this reaction.2 marks
An organic chemist is drawing the mechanism for the reaction of bromoethane with hydroxide ions: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. The C–Br bond is polar, with the carbon atom δ+.
Describe the two curly arrows needed to show this mechanism.2 marks
In the stratosphere, ultraviolet radiation breaks a C–Cl bond in the CFC dichlorodifluoromethane, CCl₂F₂, to form chlorine atoms. These chlorine atoms then take part in reactions that destroy ozone, O₃.
Write an equation for this bond-breaking step, name the type of bond fission involved and state what happens to the pair of electrons in the C–Cl bond.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).