Interpret electrophilic addition to alkenes
Recognise electrophilic addition as a mechanism and use curved arrows to track electron-pair movement at C=C.
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
Which part of an alkene can donate an electron pair?
What you will learn
- Identify an alkene as an electron-rich reaction site
- Classify electrophilic addition
- Draw curly arrows from the π bond or lone pair
The π bond attracts an electrophile
The electron-rich π bond in an alkene can donate an electron pair to an electrophile. The electrophile accepts the pair and a new bond forms.
In electrophilic addition, atoms or groups add across C=C and the π component is used to form new sigma bonds. The carbon framework remains connected.
Curly arrows must begin at an electron source
A full curly arrow may begin at the alkene π bond and point toward the electrophile. Later electron-pair movements are shown from a bond or lone pair, as required by the mechanism.
The detailed intermediate depends on the electrophile and substrate. At this level, identify the mechanism type and follow electron-pair sources and destinations without inventing an unsupported product.
Pause and explain
Where does a curly arrow begin in electrophilic attack on an alkene?
Worked example
Classify addition of HBr to ethene and state where the first electron-pair arrow begins.
Show the worked solution
- Ethene's C=C π bond is electron-rich.
- The π bond donates an electron pair to H⁺ in the polar reagent.
- The reaction is electrophilic addition; the arrow starts at the π bond.
Answer It is electrophilic addition. The first curly arrow starts at the C=C π bond and points to the electrophile H⁺.
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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.
Explain why an alkene can act as a nucleophile toward an electrophile.
Give me a hint
Relate the π electrons to electron-pair donation.
Compare my reasoning
- The C=C π bond contains electron density.
- A nucleophile donates an electron pair.
- The π bond can donate toward an electron-pair acceptor.
An alkene can act as a nucleophile because its π bond can donate an electron pair to an electrophile.
Look for these in your work
- π bond identified
- Definition applied
- Interaction explained
In electrophilic addition, what happens overall to the alkene double bond?
Give me a hint
Separate the sigma and pi components of C=C.
Compare my reasoning
- The π bond is used in forming new bonds.
- The C–C sigma connection remains.
- The final product has groups added across the former double bond.
Groups add across C=C; the π bond is replaced by new bonds while the carbon chain remains connected.
Look for these in your work
- π bond role
- Carbon connectivity
- Product pattern
An arrow in an electrophilic-addition mechanism starts at H⁺ and points to the alkene. Diagnose the arrow.
Give me a hint
An arrow begins where its electrons originate.
Compare my reasoning
- H⁺ is the electrophile and accepts electrons.
- The alkene π bond supplies the pair.
- Reverse the arrow so it begins at C=C and points to H⁺.
The arrow is reversed: start at the alkene π bond and point toward the electrophile.
Look for these in your work
- Electrophile identified
- Electron source identified
- Arrow corrected
Common mistakes
- Starting the arrow at H⁺
- Calling it nucleophilic addition
- Leaving a carbon double bond after adding across it
What can you explain now?
Which part of an alkene can donate an electron pair?
Compare with the explanation
The π bond
The π electrons are the electron-rich region.
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Make it stick
Why is electrophilic addition associated with alkenes rather than saturated alkanes?
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.