Question

Which of the following does not disproportionate?

• A. ${\mathrm{BrO}}^{-}$
• B. ${{\mathrm{BrO}}_2}^{-}$
• C. ${{\mathrm{BrO}}_3}^{-}$
• D. ${{\mathrm{BrO}}_4}^{-}$

Answer 1

The correct option is D

${{\mathrm{BrO}}_4}^{-}$

Explanation for correct option:

Option (D):

• In a disproportionation reaction, one of the species of intermediate oxidation state gets oxidized as well as reduced.
• Both oxidation and reduction reactions can take place when the oxidation state of the atom is in between its highest and lowest oxidation state.
• The minimum oxidation state of Bromine ($\mathrm{Br}$) is $-1$ and the maximum is $+7$.
• In ${\mathrm{BrO}}_4^{-}$, the oxidation state of Bromine is $+7$ which is its highest oxidation state.
• Since Bromine is already in its highest oxidation state, it cannot get oxidised any further. This is why ${\mathrm{BrO}}_4^{-}$ does not exhibit a disproportionation reaction.

Explanation for incorrect options:
Option (A): In ${\mathrm{BrO}}^{-}$, Bromine's oxidation state is $+1$ which is between its maximum and minimum possible oxidation states. Thus, ${\mathrm{BrO}}^{-}$ can undergo disproportionation reactions.
Option (B): In ${\mathrm{BrO}}_2^{-}$, Bromine is in a $+3$ oxidation state which is between its maximum and minimum possible oxidation states. Thus ${\mathrm{BrO}}_2^{-}$ can undergo disproportionation reactions.
Option (C): In ${\mathrm{BrO}}_3^{-}$, Bromine is in a $+5$ oxidation state which is between its maximum and minimum possible oxidation states. Thus, ${\mathrm{BrO}}_3^{-}$ can undergo disproportionation reactions.

Hence, option (D) is the correct answer.