why is a little acid added to form the DNP derivative of carbonyl compounds?

Question

what is the mechanism of the reaction?

what role does the acid play?

hey!can u please give me the link to understand the mechanism?
BTW,why does the question of keto enol tautomerism arise if i only consider a simple ketone?

Answer 1

If you are talking about Brady's reagent (2,4-dinitrophenyl hydrazine), then the acid is a catalyst to help change the C=O bond of the aldehyde or ketone into a single C-O bond. It also helps in the elimination of water to form the final hydrazone (with its C=N bond).

The site below does show a mechanism which comes close (it is the best I could find).

The lone pair of electron on the terminal Nitrogen of the hydrazine interacts with the partial positive of the carbonyl C. As this N-C bond forms the Pi bonding electrons of the C=O move (localize) on the oxygen atom.

This gives an intermediate species with a + on N and an - on the O. The acid can provide the proton forming the O-H as the N-H bond is severed. This seems to be the step which is acid catalyzed. It is not a matter of the H from the N moving to the O atom.

Answer 2

You need to look at keto-enol tautomerism, and the role that H+ ions can play in it!