Product of aldehyde and DNP gives imine? What functional groups/ structural features makes the absorption of colour?

Answer 1

First of all, DNP is 2,4-dinitrophenylhydrazine, a vivid orange aromatic derivative of hydrazine which is a very effective reagent for characterizing both aldehydes and ketones by splitting out water and introducing a C=N. This imine-like functional group is a part of the structure of the resultant formed orange-red 2,4-dinitrophenylhydrazones. These can typically be purified by recrystallization from 95% ethanol and give characteristically sharp melting points which help identify the aldehyde or ketone.

Secondly, these 2,4-dinitrophenylhydrazone derivatives are very strong UV absorbers (as indicated by their color) due to the presence of two strongly electron-withdrawing nitro groups on the benzene ring, which enable the pi electrons of the aromatic ring to be delocalized over both groups. This feature creates resonance forms which are both conjugated and now contain additional C=N portions, all of which affect both UV absorption and resultant color.