Hydrate an alkene to make an alcohol
Use steam and phosphoric acid catalyst to add H and OH across an alkene double bond.
Section 16 · Hydroxy compounds
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State reagents and conditions, follow the stated reaction pathway, and distinguish each alcohol class and oxidation product. Written practice is self-checked
Before you begin
- Use functional-group formulae and reaction types from Topic 13, alkene reactions from Topic 14, and halogenoalkane substitution from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which catalyst is used to add steam to an alkene?
What you will learn
- Recognise electrophilic addition of steam
- State H₂O(g) and H₃PO₄ catalyst
- Predict the alcohol product
Add the elements of water across C=C
Steam adds across an alkene double bond in the presence of an H₃PO₄ catalyst. One carbon gains H and the other gains OH; the π bond is consumed and an alcohol forms.
Ethene + steam → ethanol. Propene can form propan-2-ol as the major product in the standard acid-catalysed hydration model; use the product convention and starting structure given in a question.
State the actual reagent and catalyst
Write water as steam, H₂O(g), and name phosphoric acid, H₃PO₄, as the catalyst. This is hydration, an addition reaction, not hydrolysis of an ester.
The syllabus specifies the reagent and catalyst; do not add an unsupported catalyst or confuse hydration with cold dilute manganate(VII), which makes a diol.
Pause and explain
What functional group forms when steam adds across an alkene?
Worked example
Write the product and conditions for hydrating ethene.
Show the worked solution
- The C=C bond becomes a single C–C bond.
- One alkene carbon gains H and the other gains OH.
- State steam and the phosphoric acid catalyst.
Answer 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.
Show the hydration equation for propene and name one alcohol product.
Give me a hint
Add H and OH to the two carbons of the double bond.
Compare my reasoning
- Propene has three carbons and one C=C bond.
- The product retains three carbons and gains one OH group.
- Use steam and H₃PO₄ catalyst.
CH₃CH=CH₂ + H₂O(g) → CH₃CH(OH)CH₃; propan-2-ol (H₃PO₄ catalyst).
Look for these in your work
- Carbon skeleton preserved
- Alcohol product named
- Reagent and catalyst stated
A learner predicts propane from ethene and steam. Correct the product class and explain the bond change.
Give me a hint
Hydration adds the elements of water rather than hydrogen alone.
Compare my reasoning
- Hydrogenation would add H₂ and form an alkane.
- Hydration adds H and OH.
- The product therefore contains C–OH.
Steam hydration produces an alcohol, ethanol, because H and OH add across ethene's C=C bond.
Look for these in your work
- Hydration distinguished
- H/OH addition identified
- Product class corrected
Choose between steam/H₃PO₄ and cold dilute acidified KMnO₄ to prepare an alcohol versus a diol from an alkene.
Give me a hint
Count how many OH groups enter the product.
Compare my reasoning
- Steam hydration gives one alcohol group.
- Cold dilute acidified manganate(VII) oxidises the alkene to a diol.
- Match the requested product to the route.
Use steam/H₃PO₄ for an alcohol; use cold dilute acidified KMnO₄ for a diol.
Look for these in your work
- Both routes compared
- Product groups counted
- Correct conditions selected
Common mistakes
- Using aqueous liquid water instead of the specified steam
- Naming NaOH as the hydration catalyst
- Predicting a diol instead of an alcohol
What can you explain now?
Which catalyst is used to add steam to an alkene?
Compare with the explanation
H₃PO₄
Phosphoric acid catalyses alkene hydration in this syllabus route.
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Make it stick
How do steam/H₃PO₄ and cold dilute manganate(VII) differ as alkene-to-alcohol routes?
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.