Dehydrate an alcohol to prepare an alkene
Remove water from an alcohol using a heated catalyst or concentrated acid to form an alkene.
Section 14 · Hydrocarbons
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 small molecule is removed during alcohol dehydration?
What you will learn
- Define dehydration as water removal
- State a syllabus route and conditions
- Write an alcohol-to-alkene equation
Dehydration removes the elements of water
An alcohol can lose H and OH from neighboring carbon atoms to form water and a C=C bond. This is an elimination route to an alkene.
Ethanol can be dehydrated over heated aluminium oxide or using concentrated sulfuric acid. A representative equation is CH₃CH₂OH → CH₂=CH₂ + H₂O.
Track atoms and conditions
The carbon skeleton stays the same while one hydrogen and the hydroxy group leave as H₂O.
Do not confuse alcohol dehydration with hydration of an alkene: the former removes water from an alcohol, while the latter adds steam to an alkene to make an alcohol.
Pause and explain
Which condition can dehydrate ethanol to ethene?
Worked example
Show how ethanol can form ethene by dehydration.
Show the worked solution
- Remove the OH group and one H from adjacent carbon atoms.
- Join the carbon atoms with a double bond.
- Write water as the other product and state a permitted condition.
Answer CH₃CH₂OH → CH₂=CH₂ + H₂O; heat with Al₂O₃ catalyst or use concentrated H₂SO₄.
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 equation for dehydrating propan-2-ol to propene.
Give me a hint
Remove OH and a neighboring H while retaining three carbons.
Compare my reasoning
- Keep the C₃ skeleton.
- Form a double bond between adjacent carbons.
- Add water as the second product.
CH₃CH(OH)CH₃ → CH₃CH=CH₂ + H₂O.
Look for these in your work
- Carbon skeleton retained
- C=C formed
- Water included
Give two syllabus condition options for preparing ethene from ethanol.
Give me a hint
Recall the heated solid catalyst or concentrated acid route.
Compare my reasoning
- One route uses heated aluminium oxide.
- Another uses concentrated sulfuric acid.
- Both remove water from the alcohol.
Heat ethanol over Al₂O₃ catalyst, or use concentrated H₂SO₄.
Look for these in your work
- First route correct
- Second route correct
- Dehydration stated
A student writes CH₂=CH₂ + H₂O → CH₃CH₂OH and calls it dehydration. Correct the name and direction.
Give me a hint
Track whether water is added or removed.
Compare my reasoning
- Water is a reactant in the written equation.
- The equation forms ethanol from ethene.
- That is hydration; dehydration is the reverse direction.
The written reaction is hydration of ethene. Dehydration is CH₃CH₂OH → CH₂=CH₂ + H₂O.
Look for these in your work
- Water role identified
- Reaction named
- Reverse equation supplied
Common mistakes
- Calling dehydration addition
- Forgetting that water is a product
- Using the alkene hydration conditions for alcohol dehydration
What can you explain now?
What small molecule is removed during alcohol dehydration?
Compare with the explanation
Water
Dehydration removes the elements of H₂O.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which atoms leave an alcohol during dehydration, and what new bond appears?
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.