Dehydration of alcoholsAQA A-Level Chemistry: Revision notes

Alkenes can be made from alcohols by an elimination reaction in which a water molecule is removed. The reaction is also called dehydration. It is catalysed by a concentrated acid, such as concentrated sulfuric acid or phosphoric acid, with heating.

• CH₃CH₂OH → CH₂=CH₂ + H₂O
• cyclohexanol → cyclohexene + H₂O

The OH group leaves from one carbon and an H atom leaves from the adjacent carbon, forming a C=C bond.

The mechanism has three steps, shown with curly arrows. Take propan-2-ol as an example.

1. A lone pair on the oxygen atom of the OH group accepts H⁺ from the acid, forming the protonated alcohol.
2. The C–O bond breaks and the electron pair moves onto the oxygen. A water molecule leaves, forming a carbocation.
3. The electron pair in a C–H bond on an adjacent carbon forms the C=C bond, and H⁺ is lost.

The H⁺ is regenerated, so the acid acts as a catalyst.

Ethanol can be made by fermentation of sugars from plants, which are renewable. Dehydrating this ethanol gives ethene, which can be polymerised:

• n CH₂=CH₂ → (–CH₂–CH₂–)ₙ

So poly(ethene) can be made from biomass without using monomers derived from crude oil. This reduces the use of a finite resource, although growing the crops needs land, water and energy.

A typical preparation is cyclohexene from cyclohexanol using concentrated phosphoric acid.

• Heat the mixture with a mantle (organic liquids are flammable). Add anti-bumping granules for smooth boiling.
• Distillation apparatus: flask, still head with the thermometer bulb level with the side arm, condenser with water in at the bottom and out at the top, and a receiver.
• Collect only the fraction around the boiling point of the product (about 83 °C for cyclohexene).
• The product has a lower boiling point than the alcohol because it cannot form hydrogen bonds, so it distils over first.

The distillate contains the alkene, water and traces of acid. To purify it:

1. Shake with sodium carbonate solution in a separating funnel to remove acid. The organic layer is the upper layer (cyclohexene is less dense than water). Run off the lower aqueous layer.
2. Dry the organic layer with an anhydrous drying agent such as calcium chloride or magnesium sulfate.
3. Decant, then redistil, collecting the fraction at the boiling point of the product.

The percentage yield is the actual mass divided by the theoretical mass, × 100. It is below 100% because the reaction is reversible, side reactions occur and product is lost during transfers.