THE REACTION BETWEEN UNSYMMETRICAL ALKENES AND SULPHURIC ACID

 

This page gives you the facts and a simple, uncluttered mechanism for the electrophilic addition reactions between sulphuric acid and unsymmetrical alkenes like propene. If you want the mechanism explained to you in detail, there is a link at the bottom of the page.
The electrophilic addition reaction between propene and sulphuric acid

The facts

Alkenes react with concentrated sulphuric acid in the cold to produce alkyl hydrogensulphates. In the case of propene, the equation is:

Note:  If you are confused about the structure of the product, a similar structure is described in more detail in the page about the reaction between sulphuric acid and symmetrical alkenes.

This is in line with Markovnikov's Rule which says:

When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.

In this case, the hydrogen becomes attached to the CH2 group, because the CH2 group has more hydrogens than the CH group.

Notice that only the hydrogens directly attached to the carbon atoms at either end of the double bond count. The ones in the CH3 group are totally irrelevant.

The mechanism

This is an example of electrophilic addition.

The addition is this way around because it is easier to produce the secondary carbocation (carbonium ion) than the primary one which would be formed if the hydrogen became attached to the centre carbon atom and the rest of the sulphuric acid to the end one.

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© Jim Clark 2000