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Advanced Level Organic Chemistry: Part 2. Reaction of alkenes with KMnO4(aq)

Part 2. The chemistry of ALKENES - unsaturated hydrocarbons

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Part 2.7 The reaction of alkenes with aqueous potassium manganate(VII) - oxidation to form diol alcohols

A redox reaction involving oxidation of an organic compound and the reduction of an inorganic compound.

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2.7 The reaction of alkenes with aqueous potassium manganate(VII)

If alkenes are bubbled into, or the liquid shaken with, a neutral aqueous potassium manganate(VII), KMnO4(aq), an oxidation reaction occurs and a diol alcohol is formed at room temperature.

The potassium manganate(VII) solution decolourises and a brown/black precipitate of manganese dioxide, MnO2 forms.

This is an oxidation reaction of an organic compound.

The Mn in KMnO4 is reduced from a +7 oxidation state to a +4 oxidation state in MnO2.

The simplified equation for ethane is shown below, which assumes the oxygen atom originates from the oxidising agent.

(1) H2C=CH2  +  [O]  +  H2O  ===>  HOCH2-CH2OH

The product is called ethane-1,2-diol, diols triols and cyclo-alcohols structure and naming (c) doc b

The equation for propene is:

(2) CH3CH=CH2  +  [O]  +  H2O  ===>  CH3CH(OH)CH2OH

The product is called propane-1,2-diol, diols triols and cyclo-alcohols structure and naming (c) doc b , diols triols and cyclo-alcohols structure and naming (c) doc b

The equations for but-1-ene and but-2-ene are:

(3) CH3CH2CH=CH2  +  [O]  +  H2O  ===> CH3CH2CH(OH)CH2OH

(3) CH3CH=CHCH3  +  [O]  +  H2O  ===> CH3CH(OH)CH(OH)CH3

The products are butane-1,2-diol  and  butane-2,3-diol respectively.

See also the structure and naming of alcohols

 

The half-reaction equation for the potassium manganate(VII) is ...

MnO4-(aq)  +  4H+(aq)  + 3e-  ===>  MnO2(s)  +  H2O(l)

The half-cell potential for this half-reaction is Eθ = +1.67 V, so the manganate(VII) ion is a strong oxidising agent.

See also the chemistry of the 3d block transition metal manganese


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