Question

Acetone gives positive test with:
A. Fehling's solution
B. Schiff's solution
C. $2,4 - DNP$
D. all of the above

Answer 1

Before solving the question, the first thing to keep in mind is that acetones belong to the ketone functional group. Ketones do not have the hydrogen (attached to carbon – oxygen double bond) which is present in aldehydes.

Complete step by step answer:
-Fehling’s solution is a reagent which is used to differentiate between reducing and non-reducing sugars, to determine whether a carbonyl compound is an aldehyde or ketone.
-Schiff’s solution is a reagent which is used to detect presence of aldehydes.
-$2,4 - DNP$ is a qualitative test to check for the presence of carbonyl groups.
We know that acetone belong to the ketone functional group category having the following structure:

-Acetones in general do not respond to Fehling’s test , because they do not have the hydrogen (attached to carbon – oxygen double bond which is present in aldehydes) which will undergo oxidation, hence it is difficult to oxidise them with this solution. Ketones are very less reactive towards oxidation.
-Similarly Schiff’s reagents do not bring about oxidation in ketones. Thus option A and B are incorrect.
-Acetone, however reacts with $2,4 -$ $dinitrophenylhydrazine$ ( $DNP$ ) to form the compound called as $2,4 - dinitrophenylhydrazone$ which is a yellow coloured precipitate. Reaction is given below:

Therefore Option C is correct.

Additional Information:
Fehling’s solution is actually a deep blue alkaline solution prepared by combining two different solutions. First is Fehling’s A which is copper (II) sulphate solution, deep blue in colour and the other is Fehling’s B which is a solution of aqueous sodium potassium tartrate (also known by the name Rochelle salt) which is colourless. These two are mixed and made strongly alkaline by adding potassium hydroxide ( $KOH$ ).
Schiff’s solution is Fuchsine or rosaniline which is a magenta coloured dye having the chemical formula ${C_{20}}{H_{20}}{N_3}.HCl$ and it is decolourised by a sulphurous acid.
$2,4 - dinitrophenylhydrazone$ is red to orange coloured dye with formula ${C_6}{H_3}{(N{O_2})_2}NHN{H_2}$

Note:
We need to be careful with Fehling’s test as some $\alpha - hydroxy$ ketones do give positive tests as these can undergo tautomerization to get converted to aldehyde form which gives a positive test towards the Fehling’s test.