Reactions of Alkenes with Sulfuric Acid
A concise revision guide to the electrophilic addition of concentrated sulfuric acid to alkenes, the alkyl hydrogensulfate intermediate, hydrolysis to an alcohol, and how carbocation stability decides the major product with unsymmetrical alkenes.
The Overall Reaction
Alkenes react with cold, concentrated sulfuric acid in an addition reaction. The C=C double bond opens, a hydrogen atom adds to one carbon atom and the hydrogensulfate group, OSO2OH, adds to the other.
The product is an alkyl hydrogensulfate. With ethene the product is ethyl hydrogensulfate, CH3CH2OSO2OH.
CH2=CH2 + H2SO4 → CH3CH2OSO2OH
Key idea: This is the same addition pattern as HBr. Sulfuric acid supplies H+ as the electrophile and the hydrogensulfate ion, HSO4–, adds to the carbocation.
The hydrogen atom and the hydrogensulfate group add across the double bond to give ethyl hydrogensulfate.
Quick Check: Where the Two Groups Go
Apply the addition pattern to an alkene whose two double-bond carbons are identical.
Why Sulfuric Acid Acts as an Electrophile
Sulfuric acid contains two O-H bonds. Oxygen is much more electronegative than hydrogen, so each O-H bond is polar and the hydrogen atom carries a partial positive charge, Hδ+.
The electron-rich π bond of the alkene is attracted to this Hδ+. The hydrogen atom accepts a pair of electrons from the π bond, so it is the electrophile.
Electrophile: An electron-pair acceptor. In this reaction the Hδ+ of an O-H bond in sulfuric acid is attracted to the π bond and starts the electrophilic addition mechanism.
Exam focus: Draw the whole sulfuric acid molecule, H-O-SO2-O-H, in the mechanism and show the dipole on the O-H bond that reacts. The electrophile is the hydrogen atom, not the sulfur atom.
Quick Check: Concentrated or Strong?
Decide whether the student's reason for using concentrated acid is the right one.
The Electrophilic Addition Mechanism
The π bond donates a pair of electrons to the Hδ+ of an O-H bond, forming a new C-H bond. At the same time the O-H bond breaks by heterolytic fission: both electrons move to the oxygen atom, producing the hydrogensulfate ion, HSO4–.
One carbon atom from the original double bond is left with a positive charge, giving a carbocation intermediate. A lone pair on an oxygen atom of the hydrogensulfate ion then forms a bond to the carbocation, giving the alkyl hydrogensulfate.
Step 1: the π bond attacks Hδ+
A curly arrow goes from the C=C bond to the hydrogen atom of an O-H bond in H2SO4.
Step 2: the O-H bond breaks heterolytically
A curly arrow goes from the O-H bond to the oxygen atom, forming HSO4– and leaving a carbocation.
Step 3: HSO4– attacks the carbocation
A curly arrow goes from a lone pair on the negatively charged oxygen atom to C+, forming the C-O bond of the alkyl hydrogensulfate.
Exam focus: AQA asks for this mechanism by name. Start every curly arrow at an electron pair and show the carbocation with its positive charge on the correct carbon atom.
Electrophilic addition of sulfuric acid to ethene. The carbocation intermediate is attacked by the hydrogensulfate ion.
Quick Check: Find the Errors
Read the account against the mechanism and click every single word that is wrong.
Hydrolysis to an Alcohol
Alkyl hydrogensulfates are not usually the final product. Adding water and warming hydrolyses the alkyl hydrogensulfate to an alcohol and regenerates sulfuric acid.
CH3CH2OSO2OH + H2O → CH3CH2OH + H2SO4
Because the sulfuric acid is used in the first step and released again in the second, it acts as a catalyst for the overall conversion of an alkene into an alcohol. The overall change is a hydration of the alkene.
| Stage | Reagents and conditions | Product |
|---|---|---|
| Addition | Concentrated H2SO4, cold | Alkyl hydrogensulfate |
| Hydrolysis | Water, warm | Alcohol and regenerated H2SO4 |
| Overall | H2O with H2SO4 catalyst | Alcohol |
Key idea: Industrially, ethanol is made by the direct hydration of ethene with steam and a phosphoric acid catalyst, which AQA covers in 3.3.5.1. The sulfuric acid route is the laboratory version of the same overall reaction.
Warming with water hydrolyses the alkyl hydrogensulfate to the alcohol and returns the sulfuric acid.
Quick Check: Follow the Acid Through
In each round, choose the one statement that is accurate.
Unsymmetrical Alkenes and the Major Product
With an unsymmetrical alkene such as propene, H+ can add to either carbon atom of the C=C bond, so two different carbocations are possible. As with HBr, the reaction proceeds mainly through the more stable carbocation.
Adding H+ to the CH2 end of propene gives a secondary carbocation, which is stabilised by two electron-releasing alkyl groups. Adding it to the middle carbon gives a primary carbocation. The secondary carbocation is more stable, so the hydrogensulfate group ends up on the middle carbon and the major product is 1-methylethyl hydrogensulfate, which hydrolyses to propan-2-ol. The minor product hydrolyses to propan-1-ol.
| Carbocation type | Alkyl groups attached to C+ | Relative stability |
|---|---|---|
| Primary | One alkyl group | Less stable |
| Secondary | Two alkyl groups | More stable |
| Tertiary | Three alkyl groups | Most stable |
Exam focus: Explain the major product by comparing the stability of the two carbocation intermediates. Alkyl groups release electron density towards the positive carbon and stabilise it.
The secondary carbocation is more stable, so the major product after hydrolysis is propan-2-ol.
Quick Check: Which Alcohol Wins?
Compare the two possible carbocations before you choose.
Quick Check: Five Alkenes to Work Through
Work each one out on paper, then turn the card over to check your reasoning against the full working.
Common Exam Mistakes
- Using dilute sulfuric acid. The addition needs concentrated acid; water would compete for the carbocation.
- Forgetting the hydrolysis step when asked how an alkene is converted into an alcohol using sulfuric acid.
- Drawing the curly arrow from the hydrogen atom rather than from the π bond, or starting the second arrow at the oxygen atom instead of the O-H bond.
- Attaching the hydrogensulfate group through sulfur. It bonds to the carbon atom through an oxygen atom.
Exam focus: Write the product as CH3CH2OSO2OH or draw the full structure. The name is ethyl hydrogensulfate.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
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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