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2,4-Dinitrophenylhydrazine is an important laboratory reagent for the detection of Carbonyl (C=O) group. It reacts with Carbonyl group by the typical Nucleophilic Addition-Elimination reaction and if positive the test results in the formation of bright yellow orange coloured derivatives. 2,4-DNP

I was reading from a source and it says the reason for showing color is -"Extended conjugation". Okay that may be the reason but what exactly happens that they look such beautiful bright yellow, orange solids ?

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    $\begingroup$ It would be useful if you could explicitly say at which level you want an explanation. It can be quickly done as "it depends on the electronic energy levels of the molecule and the allowed transition between them" or it can require much more effort. I would assume that someone doing such a test can more or less answer himself, or not pose this question at all. $\endgroup$
    – Alchimista
    Jul 26, 2017 at 12:02
  • $\begingroup$ Do not forget the more important value of 2,4-dinitrophenyl hydrazones (or of the 3,5-dinitrobenzoates, [for alcohols]; the benzenesulfonamides and picrates [for amines], just to mention a few examples of preparing derivatives) is / was lesser about a little varying colour and determination of the absorption maximum (which may require a spectrometer). It is / was to obtain a crystalline compound of sharp melting point, which subsequently may be compared with reference tables. On the other hand, instead of preparing several derivatives of X, modern spectroscopic techniques have own benefits. $\endgroup$
    – Buttonwood
    Jul 26, 2017 at 20:52

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