How can we change ethanol...

...to ethene...

...and water?

We need an elimination reaction.

The reaction is also called dehydration because water is eliminated from the ethanol.

The ethanol must be heated to about 180 °C with concentrated sulphuric acid or phosphoric acid.

Alternatively, ethanol vapour can be passed over alumina, Al2O3, at 350 °C.

Although the reaction looks simple, it happens in more than one step.

This is the start of the mechanism for the elimination of water from ethanol using concentrated sulphuric acid.

A covalent bond begins to form between the oxygen atom in the hydroxyl group and ahydrogen atom in a sulphuric acid.

The bond between the hydrogen atom and the rest of the sulphuric acid molecule begins to break.

A protonated alcohol is formed.

The pair of electrons in the C-O bond is attracted to the positive charge and the bond begins to break.

When the C-O bond breaks a molecule of water is eliminated.

The remainder of the ethanol molecule is now a positively charged ion, called a carbocation.

A new bond begins to form between the two carbon atoms.

A bond begins to form between the hydrogensulphate ion and one of the hydrogen atoms.

The products of the reaction are ethene...

... and water.

Notice that, overall, sulphuric acid has not been used up in the reaction so it behaves as a catalyst.

This is the start of the simplified mechanism for the elimination of water from ethanol.

A covalent bond begins to form between the oxygen atom in the hydroxyl group and a hydrogen ion.

A protonated alcohol is formed.

The pair of electrons in the C-O bond is attracted to the positive charge and the bond begins to break.

When the C-O bond breaks a molecule of water is eliminated.

The remainder of the ethanol molecule is now a positively charged ion, called a carbocation.

A new bond begins to form between the two carbon atoms.

The product of the reaction are ethene…

…and water.

Notice that, overall, the hydrogen ion has not been used up in the reaction so it behaves as a catalyst.

This is butan-2-ol, a secondary alcohol.

If it is heated to 180 °C with concentrated sulphuric acid, two alkenes form.

In the first stage of the reaction mechanism a protonated alcohol forms.

The C-O bond breaks.

A secondary carbocation is formed and water is eliminated.

There are two hydrogen atoms next to the positively charged carbon atom in the carbocation.

This hydrogen atom can be removed.

And so can this hydrogen atom.

Two different alkenes can be produced.

But-2-ene…

...and…

...But-1-ene.