Question

The unknown in question 4 causes Schiff’s reagent to turn a magenta color. Draw the structure of a compound that would give this result.

(#4 being "An unknown sample produces a precipitate upon reaction with 2,4-dinitrophenylhydrazine reagent. Draw the structure of a compound that would give this result.")

Answer 1

Solution:

2,4-dinitrophenylhydrazine is used for the qualitative analysis of the compound having the carbonyl functional group. Both ketones and aldehydes have this carbonyl group. Hence

Clue 1: Precipitate formation

Unknown sample + 2,4-dinitrophenylhydrazine reagent → precipitate, which is a positive result indicating the presence of either aldehyde or ketone

The ketone or aldehyde is converted to its hydrazone by reaction with- 2,4 dinitrophenylhydrazine

The colour of the precipitate indicates whether it is aliphatic or aromatic. An yellow precipitate is given by aliphatic compound and red precipitate is given by aromatic compound.

Clue 2:color of the solution

Unknown sample (if aldehyde) + Schiff’s reagent → magenta coloration.

Unknown sample (if ketone) + Schiff’s reagent → no magenta coloration.

Hence the given compound could be benzaldehyde (C₆H₅CHO) with the following structure.