The Properties and Assignment of Isomers of 2,4-Dinitrophenylhydrazones of Some Aliphatic α-Keto Acids

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Visible and infrared spectra and some other properties of cis and trans isomers of 2,4-dinitrophenylhydrazones of some aliphatic α-keto acids have been studied. It was observed that absorption maximum of the cis isomer in visible region occurred at a longer wavelength than that of the trans isomer in ethyl acetate as well as in bicarbonate solution and vice versa in sodium hydroxide solution. In infrared region, N–H and C=O stretching bands of the cis isomer were found at longer wavelengths than the corresponding bands of the trans isomer. It was concluded that a hydrogen bond is formed between C=O and N–H groups in the cis isomer. Further evidence which supports the above conclusion was given by the difference between both isomers in the nickel complex forming reaction, sodium carbonate reaction and in the properties of sodium salt. Instability of the trans isomer appeared to increase as the number of carbon atoms in the keto acid molecule increased. 2,4-Dinitrophenylhydrazones of some other α-keto acids were found to give polymorphs but not geometrical isomers.
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