${\mathrm{MnO}}_{4}^{2-}$ undergoes disproportionation reaction in acidic medium but ${\mathrm{MnO}}_{4}^{-}$ does not. Give reason.

In ${\mathrm{MnO}}_{4}^{2-}$, the oxidation number of Mn is +6. It can increase its oxidation number (to +7) or decreases its oxidation number (to +4, +3, +2, 0).
Hence, it undergoes disproportionation reaction in acidic medium.
In ${\mathrm{MnO}}_{4}^{2-}$, Mn is in its highest oxidation state, i.e., +7. It can only decrease its oxidation number. Hence, it cannot undergo disproportionation reaction.