Question

An organic compound A burns with a sooty flame. It is negative towards Tollen’s reagent test and positive for Borsche’s reagent test. The compound A is:
(A) Benzaldehyde
(B) Acetophenone
(C) Acetone
(D) Salicylic acid

Answer 1

Hint: The compounds which have high carbon % can burn with sooty flame. Tollen’s reagent is an ammoniacal silver nitrate solution. Borsche’s reagent is 2,4-dinitrophenyl hydrazine solution.

Complete Step-by-Step Solution:
We will make some inferences from the observations given in the question and find the compound from it.
- It is given that the compound A burns with sooty flame. We know that when we burn the organic compound, it gets converted to carbon dioxide and water is oxidized completely. In case of aromatic compounds, they give sooty flame because carbon content is high. Aliphatic compounds generally do not give sooty flames because they have less carbon content.
- So, we can say from the above observation that Acetone cannot give sooty flame as it is an aliphatic compound. Thus, it cannot be a true answer.
- Tollen’s reagent is ammoniacal silver nitrate solution. It is a mild oxidizing agent because of silver metal. It oxidized aldehyde functional groups to carboxylic acid functional groups. It cannot oxidize alcohol or ketones.
- It is given that the compound A does not give Tollens test. Thus, benzaldehyde cannot be a true answer because it gives a positive Tollens test.
- Borsche’s reagent is 2,4-dinitrophenyl hydrazine. It reacts with aldehyde and ketones to give shiny precipitates.
- Now, salicylic acid does not have an aldehyde or ketone functional group. Thus, acetophenone fulfils all the given criteria. So, it is the correct answer.

- Therefore, the correct answer is (B).

Note: Note that Borsche’s reagent is also known as Brady’s reagent. The nucleophilic nitrogen of this reagent attacks the electrophilic carbonyl carbon and forms a compound having C=N bond which gives shiny precipitates.