# Products formed on the reduction of KMnO4 in aqueous solution

The Latimer diagram for $$\ce{MnO^-_4}$$ is given below:

$$\ce{MnO^{-}_4 ->[0.56V] MnO^{2-}_4 ->[2.26V] MnO_2 ->[0.95V] Mn^{3+} ->[1.51V] Mn^{2+} ->[-1.18V] Mn}$$

Then I have calculated the standard reduction potential values for the convertion of $$\ce{MnO^-_4}$$ to $$\ce{MnO^{2-}_4,MnO_2,Mn^{3+},Mn^{2+}and Mn}$$.

The values are found to be :

$$\ce{MnO^{-}_4 ->[0.56V] MnO^{2-}_4}$$ $$\ce{MnO^{-}_4 ->[1.693V] MnO_2}$$ $$\ce{MnO^{-}_4 ->[1.5075V] Mn^{3+}}$$ $$\ce{MnO^{-}_4 ->[1.508V] Mn^{2+}}$$ $$\ce{MnO^{-}_4 ->[0.74V] Mn}$$

So according to the $$\mathrm{SRP}$$ values the feasibility for the formation of respective products follows the order:

$$\ce{ MnO_2 > Mn^{2+} > Mn^{3+} > Mn > MnO^{2-}_4 }$$

So my question is which is the favourable product formed on the reduction of $$\ce{KMnO4}$$ in acidic solutions.
The values suggests that the formation of $$\ce{MnO_2}$$ is preferred over others.
But it is not observed in experimental reactions. Most of the reaction suggests that $$\ce{Mn^{2+}}$$ is the favorable product.
For example:

1. In oxidation of toulene to benzoic acid, the total redox reaction is: $$\ce{5 C6H5CH3 + 6 KMnO4 + 9 H2SO4 -> \\5 C6H5COOH + 14 H2O + 3 K2SO4 + 6 MnSO4}$$
2. In oxidation of ferrous ions to ferric ions, that is: $$\ce{10FeSO4 + 2KMnO4 + 8H2SO4 -> \\5Fe2(SO4)3 + 2MnSO4 + K2SO4 + 8H2O}$$

And there are many more reactions that are known to us in which formation of $$\ce{Mn^{2+}}$$ takes place. So why there is such disprency between experimental and observed facts. Which other factors influences here?
Note: There are some questions related to this topic but none of them has an appropriate answer to this question.

• Mn^2+ is preferred in strongly acidic solutions, MnO2 in neutral or mildly acidic/basic solutions and MnO4^2- in strongly basic solutions. You may want to calculate also a chart of E=f(pH). Apr 2, 2022 at 5:33
• And finally a doubt is it not possible to reduce KMnO4 to Mn(III) salts or to Mn ? Apr 2, 2022 at 5:42
• What happened between getting the doubt and asking about it? Apr 2, 2022 at 5:48
• My first doubt was mentioned in question that MnO2 should form instead of Mn(+2) . Then I got a clarity on it that it also depends on the pH of the solution . Then my second doubt (which wasn't mentioned in the question ) " is it not possible to get Mn(III) salts or Mn by the reduction of KMnO4 in aq soln under any condition like strongly acidic or basic etc ? " Apr 2, 2022 at 5:55
• I have meant the second one. What have you found before asking? Apr 2, 2022 at 6:07