All unsaturated compounds except those which contains delocalisation of $\pi$-electrons, decolourises alkaline $KMnO_{4}$ . While acidic $KMnO_{4}$ is decolourised by reducing agents
The acidic/alkaline $KMnO_{4}$ is not decolourised by the aromatic compounds due the
delocalisation of $\pi$ electrons in the nucleus
Aromatic compounds which have $ = $ or $\equiv$ bond in the side chain decolourise acidic/alkaline $KMnO_{4}$ Benzene does not decolourise the acidic/alkaline $KMnO_{4}$ due to the delocalisation of $\pi$ electrons
While propene decolourised the alkaline $ KMnO_{4}$ due to the presence of $=$ bond. Ferrous ammonium sulphate and oxalic acid decolourised the $KMnO_{4}$ in acidic medium
$2KMnO_{4}+10FeSO_{4}+8H_{2}SO_{4} \to K_{2}SO_{4} + 2MnSO_{4}+5Fe_{2}(SO
_{4})_{3} +8 H_{2}O$
$2KMnO_{4}+5C_{2}H_{2}O_{4}+3H_{2}SO_{4} \to K_{2}SO_{4}+2MnSO_{4}+10CO_{2}+8H_{2}O$