Reactions of Alkenes with KMnO4
A concise revision guide to the oxidation of alkenes using potassium manganate(VII), including the formation of diols, the purple to colourless observation and how this reaction can be used as a test for the alkene functional group.
- 6A.5-d
- 6C.22v
What these spec points say
- 6A.5-d be able to classify reactions as oxidation
- 6C.22v understand the addition reactions of alkenes with: potassium manganate(VII), in acid conditions, to oxidise the double bond and produce a diol
GCSE Recap: Saturated, Unsaturated and a Positive Test
Before you start, check the GCSE ideas this page builds on: what makes a hydrocarbon unsaturated, and what a positive test looks like.
The Overall Reaction
Alkenes react with KMnO4, potassium manganate(VII), in acidified solution to form diols.
A diol is an organic compound containing two alcohol, -OH, groups. In this reaction, the C=C double bond opens and an -OH group is added to each carbon atom from the original double bond.
For example, propene is oxidised to form propane-1,2-diol.
Key idea: KMnO4 oxidises the alkene functional group, converting the C=C double bond into a diol.
The displayed reaction shows the alkene functional group in propene changing into a diol after oxidation with KMnO4.
Quick Check: Name the Diol
Apply the reaction to an alkene that this page does not use.
Conditions and Observation
This reaction is carried out using acidified KMnO4 at room temperature.
The important observation is that the purple colour of the MnO4– ion decolourises to colourless.
This colour change is useful because it provides a simple chemical test for the presence of the alkene functional group.
Exam focus: State the full observation as purple to colourless. Do not write only “goes clear”.
Quick Check: Two Unlabelled Bottles
Drag the words into place to complete the account of the two tests.
Why It Tests for Alkenes
The reaction can be used as a test for alkenes because alkenes contain a C=C double bond that can be oxidised under these conditions.
Alkanes do not contain a carbon-carbon double bond. They are saturated hydrocarbons, so they do not react in the same way with acidified KMnO4 at room temperature.
Therefore, an alkene gives the purple to colourless change, but an alkane would show no colour change.
Diol: An organic compound containing two alcohol, -OH, groups. In this alkene oxidation reaction, the two -OH groups form on the two carbon atoms from the original C=C double bond.
Remember: The test depends on oxidation of the alkene double bond, not substitution or hydrogenation.
Both tests decolourise with an alkene: bromine water from orange, manganate(VII) from purple.
Quick Check: Which Samples React?
Click every hydrocarbon that makes the purple colour fade.
Quick Check: The Limits of the Test
Write a short explanation, then compare it with the mark points and the model answer.
Common Exam Mistakes
Students often lose marks by forgetting that the product is a diol, not a single alcohol.
Another common error is to describe the reaction as addition only. The expected reaction type here is oxidation.
For observation questions, the safest answer should include both the starting colour and the final colour.
Exam focus: Link the colour change to the reaction of MnO4– with the alkene C=C double bond.
For the bromine water test too, give both colours: orange to colourless, never just “clear”.
Quick Check: Oxidising an Alkene from Start to Finish
Each round covers one part of the page, from the product to the conditions. Choose the one accurate statement.
FAQs
These questions cover the manganate(VII) points that cost marks.
What is the colour change with an alkene?
Purple to colourless, as the MnO4– ion is reduced. Give both colours.
What does ethene give with cold, dilute acidified KMnO4?
Ethane-1,2-diol, HOCH2CH2OH. One -OH group adds to each carbon of the double bond.
Is this reaction addition or oxidation?
Name it oxidation. The alkene gains oxygen, and the manganate(VII) is the oxidising agent.
Why is bromine water the more common alkene test?
Both tests work, but manganate(VII) also oxidises other groups, such as aldehydes and some alcohols, so a colour change is less specific.
What does the diol product have in common with ethanol?
Both are alcohols. A diol has two -OH groups, on adjacent carbons here.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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