Ethanol can be produced by fermentation of sugar solutions using yeast. Ethanol can also be produced by the hydration of ethene. Explain why ethene is needed as a starting material for the hydration reaction, and describe how ethene is obtained industrially from crude oil fractions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethene is an important feedstock for the petrochemical industry. It is produced on a large scale from crude oil.
Model answer (5 marks)
Ethene is an alkene, containing a C=C double bond, so it is unsaturated and can undergo addition reactions such as hydration.
The hydration of ethene to ethanol proceeds by electrophilic addition of water across the double bond, giving ethanol as the only product.
Industrial ethene is produced from crude oil by cracking. Long‑chain hydrocarbons (larger alkanes) are heated to vapourise them and then passed over a hot powdered aluminium oxide catalyst (catalytic cracking) or mixed with steam and heated to a very high temperature (steam cracking). The high temperature breaks the C–C bonds, producing smaller, shorter‑chain molecules, including alkenes such as ethene.
Thus ethene is required as the starting material because it contains the C=C bond that reacts with water, and it is obtained industrially by cracking long‑chain hydrocarbons from crude oil.
The hydration of ethene to ethanol proceeds by electrophilic addition of water across the double bond, giving ethanol as the only product.
Industrial ethene is produced from crude oil by cracking. Long‑chain hydrocarbons (larger alkanes) are heated to vapourise them and then passed over a hot powdered aluminium oxide catalyst (catalytic cracking) or mixed with steam and heated to a very high temperature (steam cracking). The high temperature breaks the C–C bonds, producing smaller, shorter‑chain molecules, including alkenes such as ethene.
Thus ethene is required as the starting material because it contains the C=C bond that reacts with water, and it is obtained industrially by cracking long‑chain hydrocarbons from crude oil.
Examiner tips
- Use the term "alkene" and mention the C=C double bond to justify the addition reaction.
- Explain the cracking process (catalytic or steam) and that it yields ethene as a product.
- Show the reaction pathway from ethene to ethanol to demonstrate the relevance of the starting material.
Common mistakes
- Saying ethene is a saturated compound; it is unsaturated. Forgetting to mention the cracking step or the catalyst. Using the wrong terminology (e.g., calling ethene a "alkane" or "alkyl" instead of "alkene").
Mark scheme (5 marks)
- Ethene is an alkene / contains a carbon–carbon double bond / is unsaturated
- Ethene is produced by cracking (long-chain hydrocarbons / larger alkanes)
- The hydrocarbons are heated to vaporise them
- Vapour is passed over a hot powdered aluminium oxide catalyst (catalytic cracking) OR mixed with steam and heated to a very high temperature (steam cracking)
- Cracking produces smaller / shorter-chain molecules including alkenes (such as ethene)
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Aromatic chemistry (A-Level only) questions
- Crude oil contains many hydrocarbons with different chain lengths. Describe how …
- Explain why cracking is carried out in the petrochemical industry and describe t…
- Describe how fractional distillation separates crude oil into fractions, and exp…
- A student tests two hydrocarbon samples, X and Y, with bromine water. Sample X d…