Question

A gas decolourises alkaline Kmno4 solution but does not give precipitation with ammonia AgNo3 solution is?

Answer 1

Alkenes

Alkanes, because of the presence of only sigma bonds and the absence of pi bonds, are quite stable and hence do not react with KMnO4. However alkaline KMnO4 oxidises 3 degree hydrogen to - OH group.

E.g. Me3CH->Me3COH (with KMnO4)

Alkenes are readily oxidised by cold dilute neutral or alkaline KMnO4 solution to give vicinal or 1,2-glycols while KMnO4 is itself reduced to MnO2. This reaction is called hydroxylation. During this reaction, the pink colour of KMnO4 is discharged and a brown precipitate of MnO2 is obtained. This reaction is used as a test of unsaturation under the name Baeyer's test.

It does not precipitate AgNO3 because due to its non polar nature it does not reacts to acquire or get adsorb in AgNO3 solution