Name: potassium permanganate tests for alkene bonds
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 2 min 21 s
Channel: JoVE

Analogous to the syn dihydroxylation of alkenes with osmium tetroxide, oxidation of alkenes with potassium permanganate follows a similar mechanism to form glycols.
When treated with a dilute solution of cold, alkaline potassium permanganate, alkenes form a cyclic ester, which, on hydrolysis, yields a cis-diol along with manganese dioxide.
The first step of the mechanism involves a syn addition of the permanganate ion across the alkene double bond to form a cyclic manganate ester.
In the second step, the cyclic manganate ester is hydrolyzed by water in the presence of a base. The reaction proceeds with the retention of stereochemistry at the newly formed carbon&#8211;oxygen bond to give a cis-diol as the final product.
Since the permanganate ion is a strong oxidizing agent, it readily accepts electrons from the alkene, which reduces the oxidation state of manganese from +7 to +4.
Potassium permanganate is inexpensive and safer to use compared to osmium tetroxide. However, being a strong oxidizing agent, it can further oxidize the diol to carbonyl compounds.
Thus, syn dihydroxylation is most efficient under mild conditions using cold, basic potassium permanganate. In contrast, hot, basic conditions oxidatively cleave the carbon&#8211;carbon double bond to form ketones or acids depending on the different substituents on the alkene.
For example, under strongly oxidizing conditions, terminal alkenes form carbon dioxide, alkenes with monosubstituted carbons give carboxylic acids, whereas alkenes with disubstituted carbons yield ketones.
The oxidation of alkenes with a cold, basic solution of potassium permanganate, also known as Baeyer&#39;s reagent, provides a qualitative test to detect the presence of olefinic double bonds.
When added to alkenes, the deep purple color of potassium permanganate disappears, transforming into a brown precipitate of manganese dioxide.

oxidation of alkenes: syn dihydroxylation with potassium permanganate

Oxidation of Alkenes: Syn Dihydroxylation with Potassium Permanganate

Alkenes can be dihydroxylated using potassium permanganate. The method encompasses the reaction of an alkene with a cold, dilute solution of potassium permanganate under basic conditions to form a cis-diol along with a brown precipitate of manganese dioxide.
<img alt="Cycloalkene oxidation with cold KMnO4; chemical reaction diagram; organic synthesis process." src="/files/ftp_upload/11782/11782_PT_Figure_1.svg" style="text-align: center; height: 80px;" data-altupdatedat="1754904545000" data-alt="Cycloalkene oxidation to diol via KMnO4, NaOH; diagram shows organic reaction steps and products." />
The mechanism begins with the syn addition of a permanganate ion (MnO4&#8722;) across the same side of the alkene &#960; bond, forming a cyclic manganate ester intermediate. Next, the hydrolysis of the cyclic ester with water gives a cis-diol with the retention of stereochemistry at the newly formed C&#8211;O bonds.
<img alt="Chemical reaction mechanism diagram; organometallic complex formation and product conversion steps." src="/files/ftp_upload/11782/11782_PT_Figure_2.svg" style="text-align: center; height: 220px;" data-altupdatedat="1754904545000" data-alt="Organometallic reaction diagram, Mn complex, NaOH, H2O, showing oxidation process, chemical equations." />
Potassium permanganate is inexpensive and safer compared to osmium tetroxide. However, its strong oxidizing nature leads to over-oxidation of the diol, thereby giving poor yields.
<h4>Syn Dihydroxylation with Hot Basic Potassium Permanganate</h4>
When hot potassium permanganate is used, it oxidatively cleaves the carbon&#8211;carbon double bond, forming ketones or acids depending on the nature of the substituents on the alkene. Thus, terminal alkenes are oxidized to form carbon dioxide, while monosubstituted and disubstituted alkenes give carboxylic acids and ketones, respectively.
<img alt="Organic oxidation reaction, KMnO4, NaOH, heat; diagram showing conversion of alkenes to carboxylic acids." src="/files/ftp_upload/11782/11782_PT_Figure_3.svg" style="text-align: center; height: 140px;" data-altupdatedat="1754904545000" data-alt="Organic oxidation reaction diagram; alkene to carboxylic acids, using KMnO4, NaOH, heat." />
<h4>Qualitative Analysis Using Potassium Permanganate</h4>
The basic potassium permanganate solution is also known as Baeyer's reagent, which is used in qualitative analysis to determine the presence of olefinic double bonds. During the reaction, the deep purple color of potassium permanganate solution decolorizes with the formation of a brown precipitate of manganese dioxide.

Alkenes can be dihydroxylated using potassium permanganate.&#160; The method encompasses the reaction of an alkene with a cold, dilute solution of ...

oxidation-of-alkenes-syn-dihydroxylation-with-potassium-permanganate

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Organic Chemistry

Unit 1

Reactions of Alkenes

KEY TERMS AND CONCEPTS

potassium permanganate tests for alkene bonds

Potassium permanganate can be used to test and change alkene double bonds. When an alkene reacts with cold, dilute potassium permanganate under basic conditions, it forms a cis-diol, which is a molecule with two hydroxyl groups on the same side. A brown precipitate of manganese dioxide also appears during the reaction.

The reaction starts when the permanganate ion adds to the same side of the alkene pi bond. This syn addition forms a cyclic manganate ester intermediate. Water then hydrolyzes the cyclic ester and gives the cis-diol with retention of stereochemistry at the newly formed carbon-oxygen bonds.

Potassium permanganate is inexpensive and safer than osmium tetroxide. Even so, its strong oxidizing power can over-oxidize the diol and lower the yield. The reagent is also known as Baeyer's reagent, and it is used in qualitative analysis to detect olefinic double bonds.

Hot, basic potassium permanganate behaves differently. It oxidatively cleaves the carbon-carbon double bond and forms ketones or acids, depending on the alkene substituents. Terminal alkenes are oxidized to carbon dioxide, while monosubstituted and disubstituted alkenes give carboxylic acids and ketones, respectively. In the test, the deep purple solution decolorizes and a brown manganese dioxide precipitate forms.

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key terms and concepts

See how potassium permanganate reacts with alkene double bonds, forms cis-diols, and changes color in Baeyer's test. Learn the cold and hot reactions.

Potassium Permanganate Tests for Alkene Bonds | JoVE K12

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Potassium Permanganate Tests for Alkene Bonds

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