Dehydrate an alcohol to form an alkene
Remove H₂O using a heated catalyst or concentrated acid, then locate the new C=C 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
What small molecule is removed during alcohol dehydration?
What you will learn
- Recall dehydration conditions
- Write an alkene and water product
- Distinguish dehydration from oxidation
Eliminate water from an alcohol
Heating an alcohol with a catalyst such as Al₂O₃ or with concentrated acid can remove H₂O and form an alkene. This is dehydration, an elimination reaction.
Ethanol → ethene + water: CH₃CH₂OH → CH₂=CH₂ + H₂O. The two carbon atoms stay in the molecule and form a double bond.
Show the condition and product bond
State a heated catalyst or concentrated H₂SO₄. Do not confuse dehydration with oxidation by acidified dichromate, which changes the carbon bonded to OH into a carbonyl or carboxyl group.
For a larger alcohol, examine adjacent carbon atoms and place the C=C where hydrogen and OH are removed; there may be more than one possible alkene if the structure allows it.
Pause and explain
Which condition is suitable for alcohol dehydration?
Worked example
Show dehydration of butan-2-ol to one possible alkene.
Show the worked solution
- Remove the OH group from carbon 2 and H from an adjacent carbon.
- Form a C=C between those carbons.
- Include water and state suitable heating conditions.
Answer CH₃CH(OH)CH₂CH₃ → CH₃CH=CHCH₃ + H₂O; heat with Al₂O₃ or 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 one dehydration equation for propan-1-ol.
Give me a hint
Remove OH from carbon 1 and H from the adjacent carbon.
Compare my reasoning
- Keep three carbon atoms.
- Form a terminal C=C bond.
- Add H₂O as the eliminated molecule.
CH₃CH₂CH₂OH → CH₃CH=CH₂ + H₂O; heat with Al₂O₃ or concentrated H₂SO₄.
Look for these in your work
- Alkene named
- Water included
- Condition stated
Compare dehydration of ethanol with oxidation of ethanol.
Give me a hint
One route removes water; the other adds oxygen or removes hydrogen.
Compare my reasoning
- Dehydration forms ethene and water.
- Oxidation forms ethanal or ethanoic acid depending on conditions.
- Name the distinct reagents and apparatus.
Dehydration gives ethene using heat/catalyst or concentrated acid; oxidation uses acidified oxidant and gives ethanal or ethanoic acid.
Look for these in your work
- Products distinguished
- Condition sets separated
- Reaction types correct
A proposed product from ethanol dehydration is CH₃CH₃. Identify the missing structural feature.
Give me a hint
The product should be unsaturated after water is removed.
Compare my reasoning
- Dehydration forms a C=C bond.
- Ethane has only single C–C bonds and would require hydrogenation.
- The product is ethene, CH₂=CH₂.
The alkene product is ethene, CH₂=CH₂; the proposed ethane is saturated and incorrect.
Look for these in your work
- Product class checked
- Double bond added
- Formula corrected
Common mistakes
- Calling dehydration oxidation
- Omitting water
- Writing an alkene with no C=C
What can you explain now?
What small molecule is removed during alcohol dehydration?
Compare with the explanation
Water
Dehydration eliminates H₂O as the alkene forms.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which bond is formed when an alcohol is dehydrated, and what molecule is lost?
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.