Question

Which of the following is used to distinguish ethylene and acetylene?
A.Alkaline ${\text{KMn}}{{\text{O}}_4}$
B.Bromine water
C.Ammoniacal Cuprous chloride
D.Concentrated ${{\text{H}}_2}{\text{S}}{{\text{O}}_4}$

Answer 1

Hint: The systematic name for ethylene is ethene and that for acetylene is ethyne. To answer this question, you must recall the difference in properties between alkenes and alkynes and methods to distinguish them.

Complete step by step solution:
Acetylene and ethylene are the simplest alkynes and alkenes respectively.
When the compounds are reacted with alkaline ${\text{KMn}}{{\text{O}}_4}$, they undergo oxidation.
Acetylene reacts with alkaline ${\text{KMn}}{{\text{O}}_4}$ and gives oxalic acid while ethylene on reaction with alkaline ${\text{KMn}}{{\text{O}}_4}$ gives two moles of formic acid as product. Although the products formed are different, there are no visible changes that can help us distinguish between the two compounds immediately after the reaction.
On reaction with bromine water, anti-addition of bromine takes place over the multiple bonds resulting in the discoloration of the bromine water. The bromine water test does provide visual evidence of change and detects unsaturation in a compound. But both the compounds are unsaturated and give the bromine water test. Thus it cannot be used.
On reaction with ammoniacal cuprous chloride, ethyne forms a red precipitate of copper acetylide. Ethyne can react with cuprous chloride since it has acidic hydrogen atoms. The carbon in ethyne is $sp$ hybridized and is highly electronegative. Thus the hydrogen atoms attached to it are acidic. Ethene on the other hand does not have acidic hydrogen atoms and does not react with ammoniacal cuprous chloride solution.

Thus, the correct option is C.

Note: In order to distinguish between two compounds we must use such a reaction which is given by only one compound and also involves changes that can be observed physically like colour change, formation of precipitate or evolution of a gas.