What you'll learn

• What happens during the acid-catalysed elimination (dehydration) of alcohols.
• The step-by-step mechanism of how an alcohol loses water to form an alkene.
• Why this reaction provides a sustainable route to plastics without using crude oil.
• How to practically prepare and purify an alkene using distillation (Required Practical 5).

Dehydration of alcohols

Alcohols can be converted into alkenes by removing a molecule of water. Because a small molecule is removed from the reactant to form a double bond, this is classified as an elimination reaction. When the small molecule removed is water, the reaction is specifically called dehydration.

To dehydrate an alcohol, you need harsh conditions. The alcohol is heated with a strong acid catalyst — usually concentrated sulfuric acid ($H_2{SO}_4$) or concentrated phosphoric acid ($H_3{PO}_4$).

For example, heating ethanol with concentrated sulfuric acid produces ethene and water:

$${CH}_3{CH}_{2}OH\to {CH}_2{=CH}_2+H_{2}O$$

The environmental importance

You might wonder why we would want to turn alcohols into alkenes. The answer lies in plastics.

Currently, most of the alkenes (like ethene and propene) used to make addition polymers come from the cracking of crude oil fractions. Crude oil is finite and non-renewable.

However, ethanol can be produced from renewable resources by fermenting the sugars found in plants (like sugar cane). If we take this bio-ethanol and dehydrate it using an acid catalyst, we get ethene. This ethene can then be polymerised to make poly(ethene).

The Mechanism

You need to know the exact sequence of electron movements (curly arrows) for this reaction. The catalyst is represented simply as an $H^{+}$ ion.

The mechanism happens in three stages:

1. Protonation: The acid catalyst provides an $H^{+}$ ion. The alcohol accepts it.
2. Loss of water: The carbon–oxygen bond breaks, releasing water and leaving a positively charged carbon atom (a carbocation).
3. Deprotonation: The molecule gets rid of an $H^{+}$ ion to form the double bond. The catalyst is regenerated.

Required Practical 5: Distillation

In the laboratory, you are expected to know how to prepare and purify a liquid alkene from an alcohol. A common example is the dehydration of cyclohexanol to form cyclohexene.

Because organic reactions often produce a mixture of products, unreacted starting materials, and aqueous side-products, you must use distillation to separate the desired alkene as it forms.

1. The reaction and distillation

The alcohol (cyclohexanol) and the acid catalyst (concentrated $H_3{PO}_4$) are placed in a round-bottomed flask along with anti-bumping granules. The mixture is heated.

Cyclohexene has a much lower boiling point than cyclohexanol because cyclohexanol can form strong hydrogen bonds, while cyclohexene only has weak van der Waals forces. As the cyclohexene forms, it boils and turns into a vapour. The vapour travels up the still head, hits the cold Liebig condenser, condenses back into a liquid, and drips into the receiving flask.

2. Purification

The distillate collected is not perfectly pure. It will contain water and likely some unreacted acid and alcohol. To purify it:

• Separating funnel: Pour the mixture into a separating funnel. Water and organic liquids are immiscible (they don't mix). The mixture will form two layers. Run off the lower aqueous layer and keep the upper organic layer (the cyclohexene).
• Neutralisation: You may need to wash the organic layer with a dilute sodium carbonate solution to neutralise any acid that distilled over.
• Drying: Transfer the crude cyclohexene to a conical flask and add a solid drying agent, such as anhydrous calcium chloride (${CaCl}_2$) or anhydrous magnesium sulfate (${MgSO}_4$). Swirl until the liquid goes from cloudy to clear, indicating the water has been absorbed.
• Redistillation: Finally, filter off the drying agent and redistil the liquid, collecting only the fraction that boils at the exact boiling point of cyclohexene.