Q.What happens when ethylene is passed through cold dilute alkaline potassium permanganate?

Oxidation and Ozonolysis of Alkenes

Alkenes can be oxidised in two importantly different ways, plus cleaved by ozone -- and all three are also standard tests for unsaturation, since each discharges a strongly coloured reagent.

Cold, dilute, alkaline KMnO4 (Baeyer's reagent) oxidises the double bond WITHOUT breaking the carbon skeleton, adding two -OH groups across it to give a vicinal diol (glycol); the purple MnO4- is reduced through green (Mn6+) to a brown MnO2 precipitate (Mn4+).

Hot, acidified KMnO4 is a much stronger oxidant and CLEAVES the double bond entirely, converting each alkene carbon into a carbonyl-based product depending on how many alkyl groups it originally carried: a carbon bearing two alkyl groups (no H) becomes a KETONE; a carbon bearing one alkyl group and one H is oxidised all the way to a CARBOXYLIC ACID; and a terminal =CH2 carbon (two H's) is fully oxidised to CO2 + H2O. This fragmentation pattern is a useful structural-proof tool, since the identity of the products reveals exactly how substituted each original alkene …