Dehydration of ethanol

The idea

Heating ethanol at 443 K with excess concentrated sulphuric acid, which acts as a dehydrating agent removing water, dehydrates ethanol to the unsaturated hydrocarbon ethene.

A change in formula

Ethanol is C₂H₆O and ethene is C₂H₄. Heating ethanol with excess concentrated sulphuric acid turns one into the other.

What do you think the acid takes out of ethanol?

• Water
• Carbon dioxide
• Only hydrogen

Removing water

Press Next step. A teacher does this in the lab.

A dehydrating agent

The acid does not become part of the product. It pulls water out of ethanol, and what is left has a double bond.

Heating ethanol at 443 K with excess concentrated sulphuric acid, which acts as a dehydrating agent removing water, dehydrates ethanol to the unsaturated hydrocarbon ethene.

Notes

Heating ethanol at 443 K with excess concentrated sulphuric acid, a dehydrating agent that removes water, dehydrates ethanol to the unsaturated hydrocarbon ethene.

Check yourself

At what temperature, in kelvin, is ethanol heated with excess concentrated sulphuric acid to give ethene?

Answer: 443 K

The book gives 443 K.

What does concentrated sulphuric acid do to ethanol here?

What unsaturated hydrocarbon is formed?

Which substance leaves the ethanol molecule?

• It adds oxygen to it. Adding oxygen is what oxidising agents do. Concentrated sulphuric acid here removes water.
• It removes water, acting as a dehydrating agent — correct. Yes!
• It adds hydrogen to it. Adding hydrogen to unsaturated hydrocarbons needs palladium or nickel. Here the acid removes water.
• Ethane. Ethane is saturated. The product has a double bond and is ethene.
• Ethyne. Ethyne has a triple bond. The product here is ethene.
• Ethene — correct. Yes!
• Water — correct. Yes!
• Hydrogen gas. Hydrogen gas is evolved when ethanol reacts with sodium.
• Carbon dioxide. Carbon dioxide is not formed in this reaction. Water is removed.