3.3.5.3 - Elimination (Alcohols)

From alcohol to alkene

An alcohol contains a hydroxyl group, -$\ce{OH}$. In an elimination reaction, a small molecule is removed from a larger molecule. For alcohols, the small molecule removed is water, so this particular elimination is also called dehydration.

For water to be produced, two pieces must be removed from the alcohol:

• the -$\ce{OH}$ group from one carbon
• a hydrogen atom from the carbon next to it

Those atoms combine to make $\ce{H2O}$, and the two carbons become joined by a double bond. That is why the organic product is an alkene.

For ethanol, the overall reaction is:

For an unsymmetrical alcohol, inspect both adjacent carbons. Butan-2-ol can lose H from C1 to give but-1-ene, or from C3 to give but-2-ene. But-2-ene has E and Z forms. In pentan-3-ol the two neighbouring positions are equivalent, so both routes give pent-2-ene; pentan-2-ol instead gives pent-1-ene and pent-2-ene. Check connectivity before counting different drawings as different products.

The same pattern works for larger alcohols. If propan-1-ol is dehydrated, the product is propene because the carbon carrying the -$\ce{OH}$ group and the adjacent carbon form the new $\ce{C=C}$ bond.

A quick check is to ask yourself where the water comes from. If you can point to the hydroxyl group and a hydrogen on the next carbon along, you are reading the reaction correctly.

Why the reaction is acid-catalysed

The reaction is described as acid-catalysed because a strong acid such as concentrated sulfuric acid supplies $\ce{H+}$ ions that help the alcohol lose water. The acid is not used up overall.

This matters because -$\ce{OH}$ on its own is a poor leaving group. Once the oxygen atom is protonated by $\ce{H+}$, the group becomes -$\ce{OH2+}$, which can leave as water. Water is much easier to remove than hydroxide, so protonation makes elimination possible.

You will usually see the conditions written as heat with concentrated sulfuric acid or concentrated phosphoric acid. You may also meet dehydration over heated aluminium oxide, but when the specification says acid-catalysed elimination, the key idea is the role of $\ce{H+}$ in the mechanism.

The hydrogen removed does not come from just anywhere in the molecule. It must come from the carbon next to the one bearing the hydroxyl group. That is what allows the electrons from the $\ce{C-H}$ bond to help form the new $\ce{C=C}$ bond.

Outlining the mechanism

A clear outline mechanism for ethanol shows the pattern for alcohol dehydration in general.

First, the lone pair on the oxygen atom bonds to $\ce{H+}$. This protonates the alcohol:
$\ce{CH3CH2OH}$ + $\ce{H+}$ → $\ce{CH3CH2OH2+}$