Hydrogenate an alkene to produce an alkane
Use hydrogen, a platinum or nickel catalyst and heat to convert a carbon–carbon double bond into a single bond.
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
Which reagent adds across C=C in hydrogenation?
What you will learn
- Recognise hydrogenation as an alkane preparation
- Select H₂ and Pt or Ni with heat
- Balance the addition across C=C
Hydrogen adds across the alkene bond
Hydrogenation adds H₂ across a carbon–carbon double bond. The two alkene carbons each gain one hydrogen, and the C=C becomes C–C.
Use H₂(g), a Pt or Ni catalyst and heat. For example, CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃. The carbon skeleton stays the same; the product is the corresponding alkane.
Check the formula and conditions
A catalyst provides an alternative route with a lower activation energy; it is written over the reaction arrow and is not consumed in the equation.
Do not confuse hydrogenation with hydration: hydrogenation adds H₂, whereas hydration adds H and OH from steam or water under the specified acid catalyst.
Pause and explain
Which set of conditions matches the syllabus hydrogenation route?
Worked example
Write the equation and conditions for converting propene to propane.
Show the worked solution
- Add one H atom to each carbon of the C=C bond.
- Write H₂ as a reactant and propane as the product.
- State Pt or Ni catalyst and heat.
Answer CH₃CH=CH₂ + H₂(g) → CH₃CH₂CH₃; Pt or Ni catalyst, heat.
Try a topic test
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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.
Complete the hydrogenation of but-1-ene and state the conditions.
Give me a hint
Add H₂ across the C=C without changing the four-carbon chain.
Compare my reasoning
- But-1-ene is CH₂=CHCH₂CH₃.
- The two alkene carbons gain one hydrogen each.
- Name the product and give the catalyst and heat.
CH₂=CHCH₂CH₃ + H₂ → CH₃CH₂CH₂CH₃; Pt or Ni catalyst, heat.
Look for these in your work
- Carbon chain preserved
- H₂ added across C=C
- Conditions stated
A learner converts propene to propane using steam and phosphoric acid. Correct the preparation.
Give me a hint
Identify whether the target is hydrogenation or hydration.
Compare my reasoning
- Propane is the alkane product, so hydrogen must add across C=C.
- Replace steam with H₂(g).
- Use Pt or Ni catalyst and heat.
Use H₂(g) with a Pt or Ni catalyst and heat; steam/H₃PO₄ would form an alcohol instead.
Look for these in your work
- Reaction type corrected
- Reagent corrected
- Conditions distinguished
A 0.40 mol sample of propene reacts completely with excess hydrogen. How many moles of propane form?
Give me a hint
Use the 1:1 equation ratio for alkene to alkane.
Compare my reasoning
- Write C₃H₆ + H₂ → C₃H₈.
- One mole of propene forms one mole of propane.
- Apply the ratio to 0.40 mol.
0.40 mol propane.
Look for these in your work
- Equation ratio identified
- Limiting amount used
- Unit stated
Common mistakes
- Using steam instead of hydrogen
- Changing the number of carbon atoms
- Writing the catalyst as a stoichiometric reactant
What can you explain now?
Which reagent adds across C=C in hydrogenation?
Compare with the explanation
H₂
Hydrogen supplies one hydrogen atom to each alkene carbon.
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Make it stick
What changes at the C=C bond during hydrogenation, and what stays the same?
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.