Reaction mechanisms — AQA A-Level Chemistry
Test yourself on Reaction mechanisms with AQA A-Level practice questions.
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Reaction mechanisms explained
A mechanism details how a reaction happens: which bonds break, which form, and where electrons go.
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Homolytic fission splits a bond so one electron goes to each atom, producing radicals. Heterolytic fission gives both electrons to one atom, producing ions. This dictates notation: dots for radicals, curly arrows for electron pairs. Named mechanisms include free-radical substitution, electrophilic addition, nucleophilic substitution, and nucleophilic addition-elimination. Naming the mechanism often earns credit. While most names require two parts (attacking species and reaction type, e.g., electrophilic addition), 'elimination' is a complete one-word mechanism name and is an exception.
Free-radical mechanisms: the unpaired electron in a radical is represented by a dot the use of curly arrows is not required for radical mechanisms.
A radical is a species with an unpaired electron, shown by a dot placed next to the atom carrying it, as in Cl• or •CH₃. The dot is essential notation. Missing a radical dot is penalised, although usually only once within a set of linked questions. Curly arrows are not required for radical mechanisms. This is a deliberate concession: an ordinary curly arrow shows a pair of electrons moving, which is the wrong picture for a single electron. While half-headed arrows may be used, they are not expected. Crucially, omitting curly arrows does not earn a mark; they are simply ignored if absent. However, using ordinary double-headed curly arrows in a radical mechanism is incorrect and will be penalised.
Students should be able to: write balanced equations for the steps in a free- radical mechanism.
There are three kinds of step and each has a defining test. Initiation makes radicals from a non-radical and needs ultraviolet light: Cl₂ → 2Cl•. Propagation steps each have one radical in and one radical out, so the radical count is unchanged and the chain continues: Cl• + CH₄ → •CH₃ + HCl, then •CH₃ + Cl₂ → CH₃Cl + Cl•. Adding the two propagation steps together gives the overall equation, with the radicals cancelling. Termination joins two radicals to give a non-radical, removing radicals from the system: Cl• + Cl• → Cl₂, or •CH₃ + Cl• → CH₃Cl, or •CH₃ + •CH₃ → C₂H₆. The same pattern describes ozone depletion, where Cl• + O₃ → ClO• + O₂ and ClO• + O₃ → Cl• + 2O₂.
Other mechanisms: the formation of a covalent bond is shown by a curly arrow that starts from a lone electron pair or from another covalent bond the breaking of a covalent bond is shown by a curly arrow starting from the bond.
A curly arrow tracks a pair of electrons, and where it starts matters more than where it points. To make a bond, the arrow starts on a lone pair or on an existing bond such as a C=C, and ends at the atom being bonded to; AQA allows the head to reach the atom, sit alongside it, or go more than half way towards it. To break a bond, the arrow starts on the bond itself and ends on the atom that keeps both electrons. Arrows that start on a negative charge, on an atom, or in mid-air gain no credit and are penalised. Skeletal formulae are acceptable in mechanisms, but where a C–H bond breaks both that hydrogen and its bond must be drawn, or the arrow has nothing legitimate to start from.
Students should be able to: outline mechanisms by drawing the structures of the species involved and curly arrows to represent the movement of electron pairs.
Outlining a mechanism earns marks in identifiable pieces, so draw all of them. One mark is usually for the structures of the reactant species, drawn correctly with any relevant lone pairs. One mark goes to each curly arrow, judged on where it starts. A further mark goes to the structure of the intermediate: in electrophilic addition that is the carbocation, and a mark scheme will refuse the mark if you have drawn the primary carbocation where the secondary or tertiary one is required for the major product. Partial charges are usually ignored unless they are wrong, in which case they cost you. Displayed or other structural formulae are both allowed, so choose whichever makes the electron movement clearest and draw it large.
Your focus
- Name the mechanism of a given organic reaction using both the attacking species and the reaction type, or the single word 'elimination'.
- Distinguish homolytic from heterolytic fission and state the type of intermediate each produces.
- Identify the nucleophile, electrophile or radical in a given reaction step.
Show all 15 objectives
- Draw a radical with the dot placed on the atom that carries the unpaired electron.
- Complete a propagation step by deducing the missing radical, so that the atoms balance and one unpaired electron is shown on each side.
- Distinguish a radical from an ion by showing an unpaired electron rather than a charge.
- Write balanced initiation, propagation and termination equations for the chlorination of an alkane.
- Classify a given radical equation as initiation, propagation or termination by counting radicals on each side.
- Combine two propagation steps to produce the balanced overall equation for the reaction.
- Draw the bond-forming arrow for a named step, taking it from the lone pair on the oxygen of OH⁻ to the δ⁺ carbon of a haloalkane.
- Draw the bond-breaking arrow for that same step, from the C–Br bond to the bromine, so that Br⁻ leaves with both electrons.
- Identify and correct curly arrows that start from an atom or from a charge.
- Outline a named mechanism by drawing reactant structures, curly arrows, the intermediate and the product.
- Select the correct carbocation intermediate for a stated major product and justify the choice by stability.
- Draw lone pairs in the positions where the mechanism requires an arrow to originate.
Reaction mechanisms exam tips
Marking Points
- Naming the mechanism correctly, providing both the attacking species and reaction type where applicable, or just 'elimination'.
- Identifying the bond that breaks and whether the fission is homolytic or heterolytic.
- Identifying the attacking species as a radical, a nucleophile or an electrophile.
- Drawing the correct structure of the intermediate, such as a carbocation, where the mechanism forms one.
- Showing the unpaired electron as a dot on every radical in the mechanism.
- Identifying the correct radical species at each stage, such as Cl• in initiation and •CH₃ in propagation.
- Placing the dot on the correct atom of a larger radical to show where the unpaired electron resides.
- Writing a balanced equation for a propagation step showing one radical reacting to form a new radical.
- one mark for an initiation equation forming two radicals from a covalent molecule, with ultraviolet light stated
- one mark for each propagation step having exactly one radical on each side
- one mark for a termination step combining two radicals to give a single non-radical product
- one mark for a balanced overall equation obtained by adding the propagation steps
- one mark for each curly arrow starting from a lone pair or from a bond rather than from an atom or a charge
- one mark for a bond breaking arrow that starts on the bond and ends on the atom taking both electrons
- one mark for the correct structure of the intermediate produced by those arrows
- one mark for drawing the C–H bond and the hydrogen explicitly when that bond is the one that breaks
- one mark for drawing the structures of the reacting species correctly, including relevant lone pairs
- one mark for each curly arrow drawn with a valid origin and a clear destination
- one mark for the correct structure of the intermediate, such as the more stable carbocation
- one mark for the organic product that follows from that intermediate
Examiner Tips
- 💡Give both words of the mechanism name for most reactions, but remember 'elimination' is a valid single-word name.
- 💡Decide first whether the reaction is radical or ionic, since that fixes whether you use dots or curly arrows.
- 💡Put the dot on every radical as you write each equation rather than adding them all at the end.
- 💡Leave curly arrows out of radical mechanisms entirely; while omitting them doesn't earn a mark, including double-headed ones will lose you one.
- 💡Count radicals on each side: one in and one out is propagation, two in and none out is termination.
- 💡Write ultraviolet light or sunlight beside the initiation step, because the condition often carries the mark.
- 💡State symbols are generally ignored, so spend your time balancing rather than on phases.
- 💡Draw the lone pair in before you draw the arrow, so that the arrow has a legitimate origin.
- 💡Take the arrowhead at least half way to the target atom, because short arrows are read as ambiguous.
- 💡If a C–H bond is involved, abandon skeletal notation for that part and draw the hydrogen in.
- 💡Draw large, because cramped mechanisms lose arrow marks for ambiguity rather than for chemistry.
- 💡Check which carbocation the required product needs before committing to an intermediate.
- 💡Leave partial charges off rather than guess them, since wrong ones are penalised and right ones are often ignored.
Common Mistakes
- Naming a two-part mechanism with only half the label, such as 'addition' instead of 'electrophilic addition'.
- Calling a species a nucleophile when it is accepting rather than donating an electron pair.
- Describing homolytic fission as producing ions instead of radicals.
- Omitting the radical dot, which is penalised even though usually only once per set of linked questions.
- Drawing ordinary double-headed curly arrows in a radical mechanism, which is penalised.
- Writing Cl₂ with a dot instead of showing two separate Cl• radicals after homolysis.
- Giving a radical a charge, since a radical is neutral and a dot is not a charge.
- writing a propagation step with no radical on one side, so the chain cannot continue
- giving Cl• + CH₄ → CH₃Cl + H• as a propagation step, which breaks the wrong bond and produces the wrong radical
- using Cl₂ rather than Cl• as the attacking species in a propagation step
- labelling •CH₃ + •CH₃ → C₂H₆ as propagation when it removes radicals and is termination
- starting the arrow at the negative sign of an ion such as OH⁻ instead of at the lone pair on the oxygen
- starting an arrow at an atom rather than at a lone pair or a bond, which is penalised each time it occurs
- using a skeletal formula and then drawing an arrow from a C–H bond that has not been shown
- drawing a bond breaking arrow pointing towards the bond rather than away from it
- drawing the primary carbocation when the major product requires the secondary or tertiary one, which loses that mark outright
- showing partial charges in the wrong direction, which is penalised even though correct ones are usually ignored
- drawing arrows so small or so short that the examiner cannot tell which atom they reach
- omitting the lone pair on the intermediate when the mark scheme requires it to be shown