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.

  • 3
    $\begingroup$ 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). $\endgroup$
    – Poutnik
    Apr 2, 2022 at 5:33
  • $\begingroup$ And finally a doubt is it not possible to reduce KMnO4 to Mn(III) salts or to Mn ? $\endgroup$
    – Infinite
    Apr 2, 2022 at 5:42
  • $\begingroup$ What happened between getting the doubt and asking about it? $\endgroup$
    – Poutnik
    Apr 2, 2022 at 5:48
  • $\begingroup$ 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 ? " $\endgroup$
    – Infinite
    Apr 2, 2022 at 5:55
  • $\begingroup$ I have meant the second one. What have you found before asking? $\endgroup$
    – Poutnik
    Apr 2, 2022 at 6:07


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