EXPLAINING THE REACTION BETWEEN SYMMETRICAL ALKENES AND SULPHURIC ACID

This page guides you through the mechanism for the electrophilic addition of sulphuric acid to symmetrical alkenes like ethene or cyclohexene. Unsymmetrical alkenes are covered separately, and you will find a link at the bottom of the page.

The electrophilic addition reaction between ethene and sulphuric acid

This reaction looks more complicated than the reaction between ethene and hydrogen bromide, but it isn't! The only problem is that H₂SO₄ is a more complicated structure than HBr. The mechanisms are exactly the same.

The structure of sulphuric acid

Compare the structure of sulphuric acid with that of hydrogen bromide:

We are focussing on only one of the hydrogens in the sulphuric acid because the other one will be pointing away from the double bond in the alkene as the molecules approach each other.

In each case, the hydrogen is attached to a more electronegative element, and so carries a slight positive charge. That means that the hydrogen atoms will serve as electrophiles.

When the sulphuric acid reacts, the whole of the shaded part of the molecule remains as a complete unit. What happens to that unit is exactly the same as happens to the bromine in the reaction involving HBr.

When you write the mechanisms involving sulphuric acid, keep that shaded part unchanged throughout - apart from where you would change the bromine. For example, you will need to put a lone pair and a negative charge on the oxygen atom in the middle of the mechanism. That's exactly what you had to do with the bromine in the HBr case.

The mechanism

The structure of ethene is shown in the diagram on the right. The pi bond is an orbital above and below the plane of the rest of the molecule, and relatively exposed to things around it. The two electrons in this orbital are highly attractive to anything which is positively charged.

The slightly positive hydrogen atom in the sulphuric acid acts as an electrophile, and is strongly attracted to the electrons in the pi bond.

The electrons from the pi bond move down towards the slightly positive hydrogen atom.

In the process, the electrons in the hydrogen-oxygen bond are repelled down until they are entirely on the oxygen atom, producing a negative ion.

So the first stage of the reaction is:

The ion with a positive charge on the carbon atom is called a carbocation or carbonium ion (an older term).

Why is there a positive charge on the carbon atom? The pi bond was originally made up of an electron from each of the carbon atoms. Both of those electrons have been used to make a new bond to the hydrogen. That leaves the right-hand carbon an electron short - hence positively charged.

In the second stage of the mechanism, the lone pair of electrons on the oxygen atom is strongly attracted to the positive carbon and moves towards it until a bond is formed.

The overall mechanism is therefore

The electrophilic addition reaction between cyclohexene and sulphuric acid

Once again

• the pi bond breaks and the pair of electrons is used to form a bond with the hydrogen atom;

• the electrons in the hydrogen-oxygen bond are pushed on to the oxygen atom giving it a full negative charge;

• the lower carbon atom in the original C=C bond becomes positively charged because the electron it originally supplied to the pi bond has been moved away to form the new bond.

In the second stage, the lone pair on the negatively charged oxygen is attracted towards the positively charge carbon and forms a bond with it.

The overall mechanism is therefore

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Look at the same reactions involving unsymmetrical alkenes . . .

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