Identify nucleophiles, electrophiles and electron-pair arrows
Classify electron-pair donors and acceptors and draw curly arrows from their electron source.
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 does a nucleophile donate?
What you will learn
- Define nucleophile and electrophile
- Recognise electron-rich and electron-poor sites
- Start a curly arrow at a bond or lone pair
Nucleophiles donate electron pairs
A nucleophile is an electron-pair donor. It may be a negative ion, such as OH⁻, or a neutral molecule with a lone pair, such as NH₃.
An electrophile accepts an electron pair. Positive ions and electron-deficient atoms can be electrophiles; H⁺ is a simple example.
Curly arrows track electron-pair movement
In an organic mechanism, a full curly arrow represents movement of a pair of electrons. It begins at the source of that pair: a lone pair or a bond.
The arrow head points to the atom or bond receiving the electrons. Curly arrows show electron movement, not atom movement. Radical steps use single-headed arrows instead.
Pause and explain
Where should a curly arrow start?
Worked example
Classify OH⁻ and H⁺ in a reaction where OH⁻ attacks H⁺, and describe the arrow.
Show the worked solution
- OH⁻ has an available lone pair and donates it.
- H⁺ accepts the electron pair.
- The curly arrow begins at an OH⁻ lone pair and points to H⁺.
Answer OH⁻ is the nucleophile, H⁺ is the electrophile, and the curly arrow starts at the hydroxide lone pair and points to H⁺.
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Test this topic →From guided practice to a new situation
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.
Classify NH₃ and BF₃ in a simple lone-pair donation model.
Give me a hint
Ask which species supplies and which accepts the electron pair.
Compare my reasoning
- NH₃ has a lone pair and can donate it.
- BF₃ has an electron-deficient boron that can accept a pair.
- NH₃ is the nucleophile; BF₃ is the electrophile.
NH₃ is the nucleophile and BF₃ the electrophile.
Look for these in your work
- Lone pair found
- Electron deficiency identified
- Roles stated
Describe the curly-arrow direction when OH⁻ forms a bond to a carbon atom in an electrophile.
Give me a hint
Start at the electron pair that forms the new bond.
Compare my reasoning
- The electron pair is on oxygen in OH⁻.
- The curly arrow begins at the oxygen lone pair.
- It points to the electrophilic carbon atom.
Draw a full curly arrow from an OH⁻ lone pair to the electrophilic carbon.
Look for these in your work
- Source identified
- Full pair arrow
- Target identified
A student draws an arrow from an electrophilic carbon toward a nucleophile with no electron source shown. Explain the correction.
Give me a hint
Arrows show electrons moving from their source to their destination.
Compare my reasoning
- The nucleophile supplies the electron pair.
- Start the arrow at its lone pair or bond.
- Point the arrow toward the electrophilic centre.
Reverse the arrow so it starts at the nucleophile's electron pair and points to the electrophilic carbon.
Look for these in your work
- Electron source named
- Arrow origin corrected
- Arrow target correct
Common mistakes
- Starting a curly arrow at an atom with no electron source
- Calling an electron-pair donor an electrophile
- Using a full curly arrow for one radical electron
What can you explain now?
What does a nucleophile donate?
Compare with the explanation
An electron pair
Nucleophiles are electron-pair donors.
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Make it stick
What information does the starting point of a curly arrow encode?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.