Explain electrophilic addition of bromine to ethene
Describe how the alkene π bond attracts a bromine electrophile and forms a dibromoalkane.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where does the first electron-pair arrow start in electrophilic addition?
What you will learn
- Identify the electrophile and electron-pair source
- Show heterolytic bond fission
- Explain formation of a vicinal dibromo product
The π bond attracts an electrophile
The electron-rich π bond in ethene attracts bromine. As Br₂ approaches, its electron cloud becomes polarised: the closer Br atom is δ⁺ and can act as an electrophile.
A full curly arrow shows an electron pair moving from the C=C bond toward Br. The Br–Br bond breaks heterolytically and the other bromine becomes Br⁻. Follow the syllabus mechanism representation and keep charges explicit.
The intermediate reacts with bromide
The first step forms a positively charged carbon intermediate and Br⁻; a lone pair on Br⁻ then forms a C–Br bond.
Overall, each carbon of the original C=C gains one bromine, producing 1,2-dibromoethane. In mechanism drawings, arrows begin at electron pairs or bonds and point to where those electrons go.
Pause and explain
What does heterolytic Br–Br fission form in this mechanism?
Worked example
Outline electrophilic addition of Br₂ to ethene using electron-pair movement.
Show the worked solution
- The alkene π bond polarises nearby Br₂ and attacks δ⁺ Br.
- The Br–Br pair moves to the other Br, forming Br⁻ and a carbon intermediate.
- Br⁻ donates a lone pair to the positively charged carbon.
Answer The π bond attacks polarised Br₂; heterolytic Br–Br fission forms Br⁻ and a positive carbon intermediate, then Br⁻ adds to give CH₂BrCH₂Br.
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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.
Describe the two electron-pair movements in addition of Br₂ to ethene.
Give me a hint
Track the alkene pair, then the bromide lone pair.
Compare my reasoning
- A π pair forms a bond to δ⁺ Br.
- The Br–Br bond pair moves to the second Br.
- Br⁻ donates a lone pair to the carbon intermediate.
The C=C π pair attacks polarised Br₂ and Br–Br fissions heterolytically; Br⁻ then donates a pair to the positively charged carbon to form 1,2-dibromoethane.
Look for these in your work
- First electron source correct
- Br–Br fission shown
- Second attack identified
A student draws a full curly arrow from one dot on a radical to Br₂. Explain the notation error for this electrophilic-addition example.
Give me a hint
Decide whether one electron or a pair moves.
Compare my reasoning
- This mechanism tracks electron pairs, not radical single electrons.
- Full curly arrows represent two-electron movement.
- Begin at the π bond or lone pair and point to the bond-forming destination.
Use full curly arrows from electron pairs for electrophilic addition; fish-hook arrows are for single-electron radical steps.
Look for these in your work
- Mechanism type separated
- Arrow type named
- Arrow source corrected
Identify the product and classify the reaction of Br₂ with ethene.
Give me a hint
Check whether the C=C remains after both bromines add.
Compare my reasoning
- One Br attaches to each former alkene carbon.
- The C=C becomes C–C.
- Two atoms add across the double bond.
1,2-dibromoethane forms by electrophilic addition.
Look for these in your work
- Product connectivity correct
- Bond change stated
- Reaction classified
Common mistakes
- Starting a curly arrow at a positive charge
- Using a fish-hook arrow for an electron pair
- Forgetting the bromide nucleophile in the second step
What can you explain now?
Where does the first electron-pair arrow start in electrophilic addition?
Compare with the explanation
At the alkene π bond
The π electron pair acts as the electron source.
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Make it stick
How do the π bond and bromide ion each contribute to the product in electrophilic addition?
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