Hydrogenate alkenes and hydrate them with steam
Compare alkene addition of hydrogen with acid-catalysed addition of steam.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What product forms when ethene is hydrogenated?
What you will learn
- Describe alkene hydrogenation
- Describe hydration by steam
- Distinguish alkane and alcohol products
Hydrogenation adds hydrogen
H₂(g) adds across C=C with Pt or Ni catalyst and heat to form the corresponding alkane. For ethene: CH₂=CH₂ + H₂ → CH₃CH₃.
The π bond is replaced by a C–C single bond and each carbon gains H.
Hydration adds steam
Steam adds across C=C in the presence of an H₃PO₄ catalyst to form an alcohol. For ethene: CH₂=CH₂ + H₂O(g) → CH₃CH₂OH.
Hydrogenation and hydration both use addition at the double bond, but the reagent and product differ: H₂ gives an alkane; steam gives an alcohol.
Pause and explain
Which catalyst is specified for hydration of an alkene with steam?
Worked example
Compare hydrogenation and hydration of ethene, including reagents, conditions and products.
Show the worked solution
- For hydrogenation, add H₂ across C=C and name Pt/Ni with heat.
- For hydration, add H and OH from steam across C=C.
- Name H₃PO₄ catalyst for the steam route.
Answer Hydrogenation: CH₂=CH₂ + H₂ → CH₃CH₃, Pt or Ni and heat. Hydration: CH₂=CH₂ + H₂O(g) → CH₃CH₂OH, H₃PO₄ catalyst.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the hydration equation for propene to the major alcohol product in the standard electrophilic-addition orientation.
Give me a hint
Add H and OH across the double bond; state the acid catalyst.
Compare my reasoning
- Propene has three carbon atoms.
- In the usual major product, OH bonds to the more substituted carbon.
- Name the alcohol and write H₂O as steam.
CH₃CH=CH₂ + H₂O(g) → CH₃CH(OH)CH₃, H₃PO₄ catalyst; propan-2-ol is the major product.
Look for these in your work
- C₃ skeleton retained
- Alcohol product identified
- Steam and catalyst stated
For one mole of ethene, compare the product from one mole of H₂ with one mole of steam.
Give me a hint
Use the addition equations and 1:1 alkene ratios.
Compare my reasoning
- Ethene and H₂ form ethane in a 1:1 ratio.
- Ethene and steam form ethanol in a 1:1 ratio.
- The products differ because the added molecule differs.
One mole ethene plus one mole H₂ forms one mole ethane; one mole ethene plus one mole steam forms one mole ethanol.
Look for these in your work
- Hydrogenation ratio correct
- Hydration ratio correct
- Difference explained
A student uses H₂ and Pt to convert ethene directly to ethanol. Correct the reagents.
Give me a hint
Ethanol contains an added oxygen atom; identify its source.
Compare my reasoning
- Hydrogenation adds only H₂ and gives ethane.
- To form ethanol, add steam across C=C.
- Use H₃PO₄ catalyst for hydration.
Use steam, H₂O(g), with H₃PO₄ catalyst to form ethanol; H₂/Pt hydrogenates ethene to ethane.
Look for these in your work
- Wrong product explained
- Hydration reagent supplied
- Routes distinguished
Common mistakes
- Giving an alcohol as the hydrogenation product
- Using H₂SO₄ as the stated steam catalyst instead of H₃PO₄
- Confusing catalyst and added reagent
What can you explain now?
What product forms when ethene is hydrogenated?
Compare with the explanation
Ethane
H₂ adds across the double bond to form the alkane.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How do reagent, catalyst and product differ between hydrogenation and steam hydration?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.