JEE Main · 2021 · 20 Jul 2021 · Shift 1 · p-Block Elements

Disproportionation demands an element in an intermediate oxidation state, so that part of it can be oxidised while the rest is reduced. Bromine is $+1$ in $\mathrm{Br}{\mathrm{O}}^{-}$, $+3$ in $\mathrm{Br}{\mathrm{O}}_2^{-}$ and $+5$ in $\mathrm{Br}{\mathrm{O}}_3^{-}$, each well below the group maximum, so every one of these can move part of its bromine up to a higher oxyanion and part down to bromide. In $\mathrm{Br}{\mathrm{O}}_4^{-}$ bromine already holds its highest oxidation state, $+7$, and cannot be oxidised further, so it can only act as an oxidant. The species that does not show disproportionation is $\mathrm{Br}{\mathrm{O}}_4^{-}$.