AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Reactions of organic compounds can be explained using mechanisms.
Full specification wording
Reaction mechanisms: Reactions of organic compounds can be explained using mechanisms.
A mechanism details how a reaction happens: which bonds break, which form, and where electrons go.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A mechanism details how a reaction happens: which bonds break, which form, and where electrons go. 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.
What earns the marks
- 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.
Common mistakes and how to put them right
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.
Exam 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.
Try the lesson
The start of the Reaction mechanisms: Reactions of organic compounds can be explained using mechanisms. lesson, made from checked teaching notes for this specification point.
Have a go first
In a reaction mechanism, what does a curly arrow show?
Listen
A mechanism details how a reaction happens: which bonds break, which form, and where electrons go. 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. A strong answer earns marks for these points.
See it
Key words
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homolytic fission
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heterolytic fission
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reaction mechanisms
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reactive intermediates
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See every definition
- homolytic fission
- A bond splits so that one electron goes to each atom, producing radicals.
- heterolytic fission
- A bond splits so that both electrons go to one atom, producing ions.
- reaction mechanisms
- A mechanism details how a reaction happens: which bonds break, which form, and where electrons go.
- reactive intermediates
- A species formed during a mechanism, such as a carbocation, whose correct structure must be drawn where the mechanism forms one.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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