Distinguish nucleophilic substitution and nucleophilic addition
Compare mechanisms in which a nucleophile replaces a group or adds to an electrophilic multiple bond.
Section 13 · An introduction to AS Level organic chemistry
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the published functional-group scope and nomenclature limits; draw formula and mechanism evidence with correct connectivity, electron movement and stereochemistry. Written practice is self-checked
Before you begin
- Use covalent structures, valency, molecular formulas, lone pairs, and single, double and triple bonds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens in nucleophilic substitution?
What you will learn
- Recognise nucleophilic substitution
- Recognise nucleophilic addition
- Use electron-pair arrows from a nucleophile to an electrophile
Nucleophilic substitution replaces a group
A nucleophile donates a pair of electrons to an electron-deficient carbon and a leaving group departs. The nucleophile replaces the leaving group, so the reaction is nucleophilic substitution.
For a halogenoalkane, the C–X bond electron pair moves to X as a nucleophile forms a new bond to carbon. The arrows start at the nucleophile pair and the C–X bond.
Nucleophilic addition changes a carbonyl
A nucleophile can add to the electrophilic carbon in a C=O bond. The π electron pair moves toward oxygen as the new carbon–nucleophile bond forms.
The overall pattern is addition because the nucleophile adds to the carbonyl carbon and the oxygen accepts the C=O pair. Do not confuse this with substitution, where a group leaves and is replaced.
Pause and explain
In nucleophilic addition to C=O, where does the π pair move?
Worked example
Compare the defining product change in nucleophilic substitution and nucleophilic addition.
Show the worked solution
- Substitution replaces a leaving group with a nucleophile.
- Addition forms a new bond to an electrophilic multiple-bond carbon.
- In carbonyl addition the C=O π pair moves to oxygen.
Answer Nucleophilic substitution replaces a group; nucleophilic addition adds a nucleophile to a multiple-bond carbon, with the C=O π pair moving to oxygen.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the two key changes in nucleophilic substitution at a halogenoalkane carbon.
Give me a hint
Track a new C–nucleophile bond and the leaving group.
Compare my reasoning
- The nucleophile donates an electron pair to carbon.
- The C–X bond pair moves to X.
- X leaves and is replaced by the nucleophile.
A nucleophile forms a bond to carbon while X leaves, so substitution occurs.
Look for these in your work
- Nucleophile action
- C–X pair tracked
- Replacement recognized
Describe electron movement in nucleophilic addition to a carbonyl carbon.
Give me a hint
Show the nucleophile pair forming a bond and the C=O pair moving.
Compare my reasoning
- The nucleophile donates a lone pair to carbonyl carbon.
- The C=O π pair moves onto oxygen.
- The nucleophile adds across the carbonyl bond.
A nucleophile adds to the carbonyl carbon and the C=O pair moves to oxygen.
Look for these in your work
- Electron source
- Arrow destination
- Overall type
A reaction replaces Br in a halogenoalkane with OH⁻. Why is nucleophilic substitution the better classification than nucleophilic addition?
Give me a hint
Check whether a group is replaced or whether a group adds across a multiple bond.
Compare my reasoning
- OH⁻ is the nucleophile.
- Br leaves as the C–Br pair moves to it.
- OH replaces Br; no multiple bond is being added across.
It is nucleophilic substitution because OH replaces the leaving Br atom.
Look for these in your work
- Nucleophile identified
- Leaving group identified
- Reaction type justified
Common mistakes
- Calling substitution addition because a new bond forms
- Starting an arrow at the electrophilic carbon
- Forgetting to move the C=O π pair in nucleophilic addition
What can you explain now?
What happens in nucleophilic substitution?
Compare with the explanation
A nucleophile replaces a leaving group
Replacement defines substitution.
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Make it stick
What product change separates nucleophilic substitution from nucleophilic addition?
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