Which of the following reactions are disproportionation reactions? (1) $$Cu^+ \rightarrow Cu^{2+} + Cu$$ (2) $$3MnO_4^{2-} + 4H^+ \rightarrow 2MnO_4^- + MnO_2 + 2H_2O$$ (3) $$2KMnO_4 \rightarrow K_2MnO_4 + MnO_2 + O_2$$ (4) $$2MnO_4^- + 3Mn^{2+} + 2H_2O \rightarrow 5MnO_2 + 4H^+$$. Choose the correct answer from the options given below:

Identify which reactions are disproportionation reactions.

A disproportionation reaction is one where the same element in a single oxidation state is simultaneously oxidised and reduced to two different oxidation states.

Reaction (1): $$Cu^+ \rightarrow Cu^{2+} + Cu$$

$$Cu^+$$ (+1) is oxidised to $$Cu^{2+}$$ (+2) and reduced to $$Cu$$ (0). The same element (Cu) in the same oxidation state (+1) undergoes both oxidation and reduction. This IS a disproportionation reaction.

Reaction (2): $$3MnO_4^{2-} + 4H^+ \rightarrow 2MnO_4^- + MnO_2 + 2H_2O$$

Mn in $$MnO_4^{2-}$$ is in +6 state. It goes to +7 in $$MnO_4^-$$ (oxidation) and +4 in $$MnO_2$$ (reduction). Same element, same initial state, both oxidised and reduced. This IS a disproportionation reaction.

Reaction (3): $$2KMnO_4 \rightarrow K_2MnO_4 + MnO_2 + O_2$$

Mn goes from +7 (in $$KMnO_4$$) to +6 (in $$K_2MnO_4$$) and +4 (in $$MnO_2$$). Both are reductions. The oxygen goes from -2 to 0 (oxidation). Here Mn is only reduced (to two different states), while O is oxidised. This is NOT disproportionation of a single element in one state.

Reaction (4): $$2MnO_4^- + 3Mn^{2+} + 2H_2O \rightarrow 5MnO_2 + 4H^+$$

Mn starts in two different oxidation states (+7 and +2) and goes to one state (+4). This is a comproportionation (reverse of disproportionation), NOT a disproportionation reaction.

The correct answer is Option A: 1, 2.