Absorption Spectra of Some 2,4-Dinitrophenylhydrazones

It has been pointed out by Redemann that the crystalline 2,4-dinitrophenylhydrazones of α,β-unsaturated carbonyl compounds are usually bright red in contrast to the yellow colors of the corresponding saturated derivatives. In view of the widespread interest in the use of 2,4-dinitrophenylhydrazones as derivatives for the identification and characterization of carbonyl compounds, coupled with the fact that a number of exceptions are known to the above-mentioned color rule, we have undertaken a study of the absorption spectra of a number of dinitrophenylhydrazones in neutral and alkaline solutions to see if more reliable criteria for the structure and degree of unsaturation of the original compounds could be obtained.