Oxidise an alkene into a vicinal diol
Use cold, dilute acidified potassium manganate(VII) to add two neighbouring hydroxy groups.
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 product class forms from an alkene in cold dilute acidified KMnO₄?
What you will learn
- State cold dilute acidified KMnO₄ conditions
- Predict a diol
- Locate the two neighbouring OH groups
Two OH groups form across a double bond
Cold, dilute acidified potassium manganate(VII), KMnO₄, reacts with an alkene to form a diol. The two carbon atoms that shared C=C each gain an OH group.
Ethene forms ethane-1,2-diol: CH₂=CH₂ → HOCH₂CH₂OH. The carbon skeleton stays intact in this cold, dilute dihydroxylation route.
Conditions distinguish other oxidation pathways
Hot, concentrated acidified KMnO₄ can cleave a C=C bond. That is a different alkene oxidation covered in Topic 14; the cold dilute route forms the diol.
State cold, dilute and acidified, and show two adjacent –OH groups in the product. Avoid treating all manganate(VII) conditions as equivalent.
Pause and explain
How many OH groups are added across one C=C bond in this route?
Worked example
Predict the product when cyclohexene reacts with cold dilute acidified KMnO₄.
Show the worked solution
- Identify the two carbons of the C=C bond.
- Replace the double bond with a single bond.
- Add one OH group to each of those carbons.
Answer Cyclohexane-1,2-diol forms; the two OH groups are on the former alkene carbons.
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 structural change for propene to its cold dilute manganate(VII) oxidation product.
Give me a hint
Add one OH to each carbon of the original double bond.
Compare my reasoning
- Keep all three carbon atoms.
- Convert C=C to C–C.
- Place OH on the former C1 and C2 alkene carbons.
CH₃CH=CH₂ → CH₃CH(OH)CH₂OH; propane-1,2-diol.
Look for these in your work
- Carbon skeleton retained
- Double bond consumed
- Two OH positions correct
A student writes CH₃C(OH)₂CH₃ from but-2-ene. Identify the structural error.
Give me a hint
Check which atoms originally formed the double bond.
Compare my reasoning
- In but-2-ene, C2 and C3 form C=C.
- Each of those two carbons receives one OH.
- The product is butane-2,3-diol, not a geminal diol.
The OH groups belong on adjacent C2 and C3: CH₃CH(OH)CH(OH)CH₃.
Look for these in your work
- Original alkene carbons located
- OH placement corrected
- Product named
Why must a question's temperature and concentration be read before predicting the manganate(VII) product?
Give me a hint
Different conditions can change the oxidation pathway.
Compare my reasoning
- Cold dilute acidified reagent forms a diol.
- Hot concentrated reagent can cleave the double bond.
- State the condition before drawing the product.
Cold dilute conditions give a diol; hot concentrated conditions may cleave C=C, so the conditions select the product route.
Look for these in your work
- Both conditions distinguished
- Different products linked
- Conclusion justified
Common mistakes
- Putting both OH groups on one carbon
- Using hot concentrated conditions for the diol route
- Breaking the carbon skeleton in the cold dilute reaction
What can you explain now?
What product class forms from an alkene in cold dilute acidified KMnO₄?
Compare with the explanation
A diol
Both former alkene carbons gain an OH group.
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
Which alkene bond atoms gain OH in the cold dilute manganate(VII) route?
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.