Part 2. The chemistry of ALKENES - unsaturated hydrocarbons

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My advanced A level organic chemistry index of notes on alkenes

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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.

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.

(i) 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,

(ii) The simplified equation for propene is:

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

The product is called propane-1,2-diol, ,

(iii) The simplified 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

(iv) The simplified equation for 2-methylbut-2-ene is:

(CH₃)₂C=CHCH₃  +  [O]  +  H₂O  ===>  (CH₃)₂C(OH)CH(OH)CH₃

The product is 2-methylbutane-2,3-diol

 

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 E^θ = +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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