Question

Distinguish between the following pair of compounds by the test given within the brackets:
Ethane and ethene (using alkaline permanganate solution)

Answer 1

Hint: As we all know that ethane is an alkane with formula ${C_2}{H_6}$ having single bond between two carbon atoms and ethane is an alkyne with formula ${C_4}{H_4}$ having double bond between two carbon atoms. These two can be distinguished on the basis of the reactivity they show towards different compounds in chemical reactions.

Complete step by step answer:
Alkaline permanganate solutions contain permanganate ions having formula $MnO_4^ - $ . It is a strong oxidizing agent and its solutions are purple in colour. It reacts with unsaturated carbon compounds to give hydroxyl compounds. Ethene reacts with permanganate solution in alkaline medium to give hydroxyl compounds whereas ethane having no unsaturated bond could not react with permanganate ions. Thus, the purple colour of alkaline permanganate solution disappears or the solution turns colourless in the presence of ethene and not in the presence of ethane.
Additional information: The oxidation state of Mn in $MnO_4^ - $ is $ + 7$ , thus, it acts as a strong oxidizing agent in alkaline and neutral media. Another test which is used to distinguish between alkane and alkene is bromine water test in which the alkenes turn brown bromine water colourless but alkanes cannot. Bromine reacts with carbon-carbon double bonds to give additional products. In other words we can also say that unsaturated carbon compounds give additional reactions which can be used to distinguish them from saturated carbon compounds which cannot show additional reactions.
Note:
Alkanes, alkenes and alkynes are simple hydrocarbon chains with no functional groups. The simplest organic compounds are the alkanes. Alkanes have only single bonds between carbon atoms and are called saturated hydrocarbons. Alkenes have at least one carbon-carbon double bond.The permanganate ions also react with alkynes to give carboxylic acids. Thus, this reaction can also be used to distinguish between alkynes and alkanes.