Skip to main content
Chemistry LibreTexts

Reactions of Alkenes with Sulfuric Acid

This page gives you the facts and a simple, uncluttered mechanism for the electrophilic addition reactions between sulfuric acid and alkenes like ethene and cyclohexene.

The electrophilic addition reaction between ethene and sulfuric acid

Alkenes react with concentrated sulfuric acid in the cold to produce alkyl hydrogensulphates. For example, ethene reacts to give ethyl hydrogensulphate.

\[ \ce{CH_2=CH_2 + H_2SO_4 \rightarrow CH_3CH_2OSO_2OH}\]

The structure of the product molecule is sometimes written as \(CH_3CH_2HSO_4\), but the version in the equation is better because it shows how all the atoms are linked up. You may also find it written as \(CH_3CH_2OSO_3H\).

All you need to do is to learn the structure of sulfuric acid, and after that the mechanism is exactly the same as the one with hydrogen bromide. As you will find out, the formula of the product follows from the mechanism in an inevitable way.

The mechanism for the reaction between ethene and sulfuric acid

The hydrogen atoms are attached to very electronegative oxygen atoms which means that the hydrogens will have a slight positive charge while the oxygens will be slightly negative. In the mechanism, we just focus on one of the hydrogen to oxygen bonds, because the other one is too far from the carbon-carbon double bond to be involved in any way.

 

Look carefully at the structure of the product so that you can see how it relates to the various formulae given earlier (CH3CH2OSO2OH etc).

The electrophilic addition reaction between cyclohexene and sulfuric acid

This time we are going straight for the mechanism without producing an initial equation. This is to show that you can work out the structure of obscure products provided you can write the mechanism.

 

 

Having worked out the structure of the product, you could then write a simple equation for the reaction if you wanted to.

 

Contributors

Jim Clark (Chemguide.co.uk)