3.4.4 Epoxyethane

3.4.4 Epoxyethane: Describe how epoxyethane is made from ethene, explain why its strained ring makes it reactive, and write equations and mechanisms for its ring-opening reactions with water and alcohols.

Epoxyethane, C₂H₄O, is a cyclic ether in which two carbon atoms and one oxygen atom form a three-membered ring.

Manufacture

Ethene is partially oxidised by oxygen over a silver catalyst: 2C₂H₄ + O₂ → 2C₂H₄O.

Ring strain

The C–C–O angles are about 60°, far smaller than the 109.5° preferred around a saturated atom, so the ring is strained.

Reactivity

Bonds in the strained ring break easily and relieve the strain, so epoxyethane reacts much more readily than an ordinary ether.

Polar C–O bonds

Oxygen is more electronegative than carbon, so each ring carbon carries a partial positive charge and attracts nucleophiles.

Hazards

It is toxic, flammable and can react violently, which makes its manufacture and storage hazardous.

Epoxyethane reacts with water in the presence of an acid catalyst to give a diol.

A hydrogen ion from the acid catalyst bonds to a lone pair on the ring oxygen, making the oxygen positively charged.

A ring carbon is then attacked by the lone pair on the oxygen of a water molecule acting as the nucleophile.

The C–O bond to the ring oxygen breaks, opening the ring and relieving the strain.

Loss of an H⁺ ion from the attacking water molecule regenerates the catalyst and gives the product.

The overall equation is C₂H₄O + H₂O → HOCH₂CH₂OH, ethane-1,2-diol.

Reaction with Alcohols and Further Products

With an alcohol

ROH attacks the ring carbon in the same way and opens the ring to give ROCH₂CH₂OH, a compound that is both an ether and an alcohol.

Chain growth

The new –OH group can attack another epoxyethane molecule, so with excess epoxyethane a chain of repeated –OCH₂CH₂– units can form.

Short chains

One unit with water gives ethane-1,2-diol and two units give diethylene glycol, both used in coolants and in polyester making.

Long chains

Many units give polyethylene glycol type products that are used as surfactants in detergents, paints and cosmetics.

Uses and Importance

Antifreeze

Ethane-1,2-diol lowers the freezing point of water in engine coolants, so the coolant does not freeze and crack the engine.

Surfactants

Products with a water-loving chain and a hydrocarbon tail loosen grease and oil, so they are used in washing products.

Environmental care

Ethane-1,2-diol is toxic, so spills and used coolant must be disposed of safely to avoid contaminating water.

Biodegradability

Modern surfactants are designed to be biodegradable so that they do not persist and cause foaming in rivers.

Question: Epoxyethane reacts with methanol in the presence of an acid catalyst. Write the equation and name the functional groups in the product.

The nucleophile is methanol, CH₃OH, so R is CH₃.
The ring opens and the product is CH₃OCH₂CH₂OH.
The equation is C₂H₄O + CH₃OH → CH₃OCH₂CH₂OH.
The product contains an ether (C–O–C) and an alcohol (–OH) group.

Answer: C₂H₄O + CH₃OH → CH₃OCH₂CH₂OH; the product contains an ether group and an alcohol group.

3.4.4 – Key Points for the Exam

Epoxyethane: A cyclic compound C₂H₄O with a three-membered ring containing one oxygen atom.
Ring strain: Extra energy in a ring caused by bond angles much smaller than the preferred angle.
Nucleophile: A species that donates a lone pair to a partially positive carbon atom.
Ethane-1,2-diol: The diol HOCH₂CH₂OH, used as antifreeze.
Surfactant: A substance with a water-loving part and a water-hating part that helps to remove grease.

Common Mistakes

Calling the bond angle in the ring 109.5°; it is about 60°, which is the cause of the strain.
Drawing the product of the reaction with water with the ring still intact; the ring always opens.
Forgetting that the acid catalyst is regenerated, so H⁺ appears at the start and the end of the mechanism.
Stating that epoxyethane is made by complete oxidation of ethene, which would give carbon dioxide and water instead.