Question

Acetaldehyde and acetylene can be distinguished by:
A.Tollen’s reagent
B.Fehling solution
C.$\,2,4 - D.N.P\,$
D.Iodoform test

Answer 1

Hint: Acetaldehyde and acetylene belong to the family of aldehydes and alkynes respectively. When we look into the chemical reactions used to distinguish them, we can find the solution to this question.

Complete step by step answer:
Let us analyse each reagents one by one;
A.The Tollens test uses a reagent known as the Tollens reagent which is ammoniacal silver nitrate, is a colourless, fundamental, aqueous solution containing ammonia-coordinated silver ions. When we test acetaldehyde with tollen’s reagent it gives a positive test that is it forms a silver mirror, But, when we react acetylene with this it gives negative result as acetylene cannot reduce this reagent. Hence, this test can be used to distinguish the two compounds.
B.Fehling 's solution is a complex of cupric ions with tartaric acid. In colour, it is blue. A red precipitate of cuprous oxide is obtained as it is heated with acetaldehyde. Acetaldehyde transforms the cupric ions to cuprous ions in this reaction. However, acetylene doesn’t form this precipitate and hence, this test can also be used to distinguish acetaldehyde and acetylene.
C.Acetaldehyde reacts to form yellow , orange, or reddish-orange precipitates with $\,2,4 - \,$ dinitrophenylhydrazine reagents, whereas acetylene does not respond. Hence, this can also be used to distinguish the two.
D.If an aldehyde receives a positive iodoform examination it must be acetaldehyde, because it is the only aldehyde with a $\,C{H_3}C = O\,$ group. Here a pale-yellow precipitate of iodoform/ triiodomethane is formed. Whereas, this test is negative for acetylene. Hence, this test can also be used to distinguish between the two.
So, for the above question, all the options are correct.

Note:
Here, in all the reactions, acetaldehyde gives positive test whereas acetylene gives negative test. This is because acetylene is unable to reduce any of the reagents above. It is because of this reason that we use all the above reagents to distinguish between the two.