1. Structure of Epoxyethane
- Epoxyethane, C₂H₄O, is a three-membered ring of two carbon atoms and one oxygen atom.
- It is a cyclic ether.
- The bond angles in the ring are about 60°. This is much smaller than the normal tetrahedral angle of 109.5°.
2. Production by Partial Oxidation of Ethene
\[ 2\text{CH}_2\text{=CH}_2 + \text{O}_2 \rightarrow 2\text{C}_2\text{H}_4\text{O} \]
Conditions
- Silver catalyst.
- About 250 to 300 °C.
- 1 to 2 MPa pressure.
It is called partial oxidation because complete combustion to CO₂ and H₂O must be avoided.
3. Hazards of the Process
- Explosion risk: mixtures of ethene and oxygen are flammable and can explode. The oxygen concentration is carefully controlled.
- Runaway reaction: the reaction is exothermic. The side reaction (complete combustion) gives out even more heat. The temperature must be controlled to prevent a runaway reaction.
- The product: epoxyethane is toxic and carcinogenic. It is highly flammable. It is a gas at room temperature (boiling point 11 °C), so leaks spread easily.
4. Why Epoxyethane Is So Reactive
Ring strain
The 60° bond angles strain the ring. The C-O bonds are weak and break easily, because breaking them releases the strain.
Polar C-O bonds
Oxygen is more electronegative than carbon, so the ring carbons are δ+. Nucleophiles with a lone pair, such as H₂O and alcohols, attack them.
5. Reaction with Water: Ethane-1,2-diol
\[ \text{C}_2\text{H}_4\text{O} + \text{H}_2\text{O} \rightarrow \text{HOCH}_2\text{CH}_2\text{OH} \]
The product is ethane-1,2-diol. An acid catalyst, or heat and pressure, is used.
Uses of ethane-1,2-diol
- Antifreeze in car cooling systems. It lowers the freezing point of water.
- Making polyester (Terylene, PET) with benzene-1,4-dicarboxylic acid. See Polymers in Unit 4.
6. Reaction with Alcohols
\[ \text{C}_2\text{H}_4\text{O} + \text{CH}_3\text{OH} \rightarrow \text{CH}_3\text{OCH}_2\text{CH}_2\text{OH} \]
The product is 2-methoxyethanol. It is used as a solvent.
The product still has an -OH group. So it can react with more epoxyethane, and a chain builds up:
\[ \text{ROH} + n\,\text{C}_2\text{H}_4\text{O} \rightarrow \text{R(OCH}_2\text{CH}_2)_n\text{OH} \]
When R is a long hydrocarbon chain (for example C₁₂H₂₅), the product is a non-ionic surfactant. The long chain is hydrophobic. The (OCH₂CH₂)ₙOH end is hydrophilic. These surfactants are used in detergents and cleaning products.
7. Mechanism: Ring Opening by Nucleophilic Attack
- A lone pair on the O of H₂O (or ROH) attacks a δ+ ring carbon. At the same time, a curly arrow from the C-O ring bond moves that pair onto the ring oxygen. The ring opens.
- This gives an intermediate with -O⁻ on one carbon and -O⁺H₂ (or -O⁺HR) on the other.
- A proton transfers from the O⁺ to the O⁻. This gives HOCH₂CH₂OH (or ROCH₂CH₂OH).
Overall this is an addition reaction: two molecules give one product. In the mechanism, the nucleophile attacks a ring carbon and the C-O bond breaks, so the ring opens.
Beyond the OxfordAQA specification
With an acid catalyst, the ring oxygen is protonated first. This makes the ring carbon even more δ+, so the nucleophile attacks it more easily.
8. Economic and Environmental Importance
- Both reactions have 100% atom economy, because all the atoms end up in the product.
- Ethane-1,2-diol antifreeze is valuable in cold climates. But it is toxic: spills harm wildlife, and it must be disposed of safely.
- Surfactants made from epoxyethane are needed for detergents. Some persist in waterways, so biodegradable surfactants are preferred.
- Production is a large-scale industry, because so many products come from one intermediate.
9. Practice Questions
Show Answer & Mark Scheme
- Ring strain: bond angles of about 60° instead of 109.5°. (1 mark)
- The weak C-O bonds break easily to relieve the strain. (1 mark)
- The polar C-O bonds make the ring carbons δ+ and open to attack by nucleophiles. (1 mark)
Show Answer & Mark Scheme
- C₂H₄O + H₂O → HOCH₂CH₂OH (1 mark)
- Antifreeze, or making polyesters. (1 mark)
Show Answer & Mark Scheme
C₂H₄O + CH₃CH₂OH → CH₃CH₂OCH₂CH₂OH (1 mark)
Show Answer & Mark Scheme
- Curly arrow from a lone pair on the O of water to a ring carbon. (1 mark)
- Curly arrow from the ring C-O bond to the O. (1 mark)
- Intermediate with O⁻ and O⁺H₂. (1 mark)
- Proton transfer to give the diol. (1 mark)
Show Answer & Mark Scheme
Any two of these (1 mark each):
- Ethene and oxygen mixtures can explode. Controlled by keeping the oxygen concentration low.
- The reaction is exothermic, with a risk of a runaway reaction. Controlled by cooling and temperature control.
- The product is toxic and flammable. It is handled in sealed systems.