Part 1 of 6Structure and Manufacture
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.
Part 2 of 6Reaction with Water
Epoxyethane reacts with water in the presence of an acid catalyst to give a diol.
1
A hydrogen ion from the acid catalyst bonds to a lone pair on the ring oxygen, making the oxygen positively charged.
2
A ring carbon is then attacked by the lone pair on the oxygen of a water molecule acting as the nucleophile.
3
The C–O bond to the ring oxygen breaks, opening the ring and relieving the strain.
4
Loss of an H⁺ ion from the attacking water molecule regenerates the catalyst and gives the product.
5
The overall equation is C₂H₄O + H₂O → HOCH₂CH₂OH, ethane-1,2-diol.
Part 3 of 6Reaction with Alcohols and Further Products
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.
Part 4 of 6Uses and Importance
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.
Part 5 of 6Worked Example
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.
Part 6 of 6Exam Checklist & Tips
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.