Reaction mechanismsAQA A-Level Chemistry: Revision notes
Section 1
What is a mechanism?
A mechanism is a step-by-step account of how a reaction happens, showing which bonds break and form, and in what order. Covalent bonds can break in two ways. In homolytic fission each atom takes one electron from the shared pair, forming two radicals. In heterolytic fission both electrons go to one atom, forming a positive ion and a negative ion.
Section 2
Free radicals
A free radical is a species with an unpaired electron. It is shown with a dot, for example Cl• or CH₃•. Radicals are very reactive. They are formed when ultraviolet (UV) light breaks a covalent bond by homolytic fission, such as Cl₂ → 2Cl•.
In free-radical mechanisms curly arrows are not required. You write balanced equations for each step instead.
Leaving out the dot. Cl• is a radical, but writing Cl does not show the unpaired electron.
Section 3
A free-radical mechanism: methane and chlorine
In UV light, methane and chlorine react. The steps are:
Initiation: Cl₂ → 2Cl•
Propagation: Cl• + CH₄ → CH₃• + HCl, then CH₃• + Cl₂ → CH₃Cl + Cl•
Termination: Cl• + Cl• → Cl₂, CH₃• + Cl• → CH₃Cl or CH₃• + CH₃• → C₂H₆
In propagation a radical is used and another is made, so a chain reaction occurs. In termination two radicals combine and no radical is left.
Section 4
Curly arrows
In other mechanisms a curly arrow shows the movement of an electron pair.
- To form a covalent bond: start the arrow at a lone pair or at a covalent bond, and point it to the atom or the gap where the bond forms.
- To break a bond: start the arrow at the bond and point it to the atom that takes the electrons.
Arrows never start at an atom or at a charge. Show lone pairs and partial charges (δ+, δ−) on the structures.
Section 5
Worked examples with curly arrows
Hydroxide ion with bromoethane: the carbon in the C–Br bond is δ+. One arrow goes from a lone pair on the oxygen of OH⁻ to the carbon. Another goes from the C–Br bond to Br. The products are CH₃CH₂OH and Br⁻.
Ethene with HBr: one arrow goes from the C=C double bond to the δ+ hydrogen of HBr, and another from the H–Br bond to Br. This gives CH₃CH₂⁺ and Br⁻. A third arrow goes from a lone pair on Br⁻ to the C⁺, giving CH₃CH₂Br.
Check that each arrow starts at a lone pair or bond, and that the arrowheads end where the electrons go.
That's the notes covered.
Carry on to the next subtopic.
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
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).