Electrophilic Addition Mechanism
A concise revision guide to addition reactions of alkenes, electrophiles and why the carbon-carbon double bond, a centre of high electron density, is the reactive centre in alkene chemistry.
- 3.3.4.1ii-b
What these spec points say
- 3.3.4.1ii-b know that bonding in alkenes involves a centre of high electron density
Electrophilic Addition in Motion
Choose a step to watch HBr and then Br₂ add across the π bond of ethene, one curly arrow at a time.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: What Alkenes Do
Three quick questions on the reactions of alkenes that you met at GCSE.
Addition Reactions of Alkenes
An addition reaction is a reaction in which two reactant molecules combine to form a single product molecule.
Alkenes commonly undergo addition reactions because their carbon-carbon double bond can be broken and replaced by new single bonds.
This is why the C=C bond is the functional group that controls the characteristic reactions of alkenes.
Key idea: In an alkene addition reaction, atoms add across the C=C double bond and only one organic product molecule is formed.
Addition across a double bond
The double bond opens up, allowing new atoms or groups to attach to the two carbon atoms from the original C=C bond.
Quick Check: Addition or Not?
Decide quickly whether each equation shows an addition reaction.
Why the C=C Double Bond Reacts
In an alkene, the C=C double bond holds four electrons, two shared pairs, between the two carbon atoms.
So the C=C double bond is a centre of high electron density (the π bond) and attracts the electrophile: a species that is short of electrons and accepts a pair of them.
Alkanes have only single bonds and no centre of high electron density, so electrophiles are not attracted to them.
Exam focus: Do not stop at “alkenes have a double bond”. Say that the C=C double bond is a centre of high electron density and attracts the electrophile.
A centre of high electron density
The four electrons of the C=C double bond, drawn here above and below the plane of the molecule, attract electrophiles.
Quick Check: A New Electrophile
Explain why a different reagent, iodine monochloride, attacks the C=C double bond of but-1-ene.
Electrophiles
An electrophile is an electron pair acceptor.
Electrophiles are attracted to regions of high electron density. In alkenes, this means the electrophile is drawn towards the C=C double bond, a centre of high electron density.
This attraction begins the electrophilic addition mechanism.
Electrophile: A species that accepts an electron pair from an electron-rich region during a chemical reaction.
An electrophile is an electron-pair acceptor: a positive or δ+ species attracted to the electron-rich C=C double bond.
Quick Check: Find the Electrophile
Identify the electrophile in a reagent that this page has not used.
Watch: the first two steps of electrophilic addition
The pi bond breaks, H–Br splits, and a carbocation is left on the board. What happens next?
The Electrophilic Addition Mechanism
In electrophilic addition, part of the C=C double bond breaks: the two carbon atoms stay joined by a single bond, and each forms a new single bond. The electrophile accepts an electron pair from the alkene.
At this stage, students must be careful with curly arrows. A curly arrow shows the movement of an electron pair, so the arrow should start from a bond or lone pair, not from a positive charge.
1. Attraction
The electrophile is attracted to the high electron density of the C=C double bond.
2. Bond breaking
An electron pair from the C=C double bond moves to the electrophile, and a carbocation forms.
3. New bonds form
New single bonds form to give an addition product.
Remember: Electrophilic addition happens because the C=C double bond is a centre of high electron density and attracts electrophiles.
The electrophile is attracted to the C=C double bond, two bonds break heterolytically to give a carbocation, then the negative ion forms the second new bond.
Quick Check: Order the Mechanism
Put the events of an electrophilic addition you have not seen into the correct order.
The Three AQA Mechanisms
You need the electrophilic addition mechanism of alkenes with three reagents: HBr, H2SO4 and Br2.
The pattern is the same each time: the C=C double bond attacks the electrophile, a carbocation forms, and a negative ion then attacks the carbocation.
| Reagent | Electrophile | Negative ion formed | Product with ethene |
|---|---|---|---|
| HBr | Hδ+ of the polar H-Br bond | Br– | Bromoethane |
| H2SO4 (concentrated) | Hδ+ of an O-H bond | HSO4– (hydrogensulfate ion) | Ethyl hydrogensulfate, then ethanol on hydrolysis |
| Br2 | Brδ+ of the induced dipole | Br– | 1,2-dibromoethane |
Bromine is a non-polar molecule, so the electrophile is created when the alkene approaches: the high electron density of the C=C double bond repels the electrons in the Br-Br bond and induces a dipole.
The nearer bromine atom becomes Brδ+ and the Br-Br bond then breaks heterolytically.
Exam focus: Draw every curly arrow from an electron pair: the C=C double bond to the electrophile, the polar bond to the atom that becomes the negative ion, and the lone pair on the negative ion to the carbocation.
Show the carbocation with its positive charge on the correct carbon atom.
Quick Check: Three Reagents, New Alkenes
Apply the three mechanisms to but-2-ene and cyclohexene.
Exam Link: Why Alkenes React This Way
Exam answers should connect three ideas: the alkene contains a C=C double bond, the double bond is a centre of high electron density, and the electrophile is attracted to it and accepts a pair of electrons from it.
It is not enough to write that alkenes are reactive because they have a double bond. The explanation must say that the double bond is a centre of high electron density and name the electrophile.
| Exam phrase | Why it matters | Common mistake to avoid |
|---|---|---|
| C=C is a centre of high electron density | Explains why an electrophile is attracted to the alkene. | Only saying that the double bond is reactive. |
| Electrophile accepts an electron pair | Links the definition of electrophile to the mechanism. | Calling the electrophile an electron donor. |
| C=C becomes a single bond as two new bonds form | Explains why the reaction is an addition reaction. | Describing substitution instead of addition. |
Quick Check: Explain Why the Alkene Reacts
Write a short explanation, then compare it with the mark points and the model answer.
Quick Check: Hydrogen Iodide and Cyclohexene
In each round, pick the one accurate statement about this addition.
FAQs
These questions secure the curly-arrow and electrophile ideas that every alkene mechanism uses.
Does the first curly arrow start at the electrophile or at the C=C bond?
At the C=C bond. A curly arrow shows an electron pair moving, so it starts at the C=C double bond and points to the electrophile. An arrow drawn from a positive atom shows nothing moving and loses the mark.
Is an electrophile always a positive ion?
No. An electrophile is an electron pair acceptor. It can be a positive ion such as H+, or the δ+ atom of a polar or polarised molecule, such as Hδ+ in HBr or Brδ+ in Br2.
Why is the carbocation called an intermediate and not a product?
It forms part-way through the mechanism and is used up straight away when the negative ion attacks it. It is never isolated, so it does not appear in the overall equation.
Why do alkenes react with electrophiles when alkanes do not?
Alkanes contain only single bonds, so they have no centre of high electron density. In an alkene the C=C double bond is a centre of high electron density, so it attracts electrophiles.
If a bond breaks, why is the reaction called addition?
Only part of the C=C double bond breaks: the carbons stay joined by a single bond and two new single bonds form. All the atoms of the reagent end up in one product. Nothing is removed from the alkene, so it is addition, not substitution.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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