The well-known characterization method of aldehydes and ketones utilizing their 2,4- dinitrophenylhydrazones (DNPs) often gives variable melting ranges and conflicting results for a single derivative. We have found that the DNPs melting point discrepancies and the irreproducibility of the method are mainly due to syn–anti (E–Z) isomerization caused by traces of acids which remain in the products when prepared by the conventional methods. A bicarbonate washing of the DNPs removes the acid, stabilizes the melting range and reproducibly gives higher-melting derivatives. In the DNP preparations of aldehydes and some unsymmetrical ketones only the kinetically favored (and also thermodynamically more stable) syn (E) isomer is formed. The six different melting ranges of acetaldehyde DNP (AA-DNP) previously reported by other investigators are correlated with the amounts of acid present in their DNP crystals. Derivatives with higher concentrations of acid(s) give lower melting ranges. In the presence of small amounts of acid, E–Z isomerization occurs in most solvents but not in methanol or dimethyl sulfoxide (DMSO). Larger amounts of acid cause the isomerization in all solvents, but the process is much slower in the aforementioned solvents. Alcohols, but not chlorinated solvents, are suitable for the DNPs recrystallization. E–Z isomerization of AA-DNP also occurs thermally by first order kinetics when its pure E isomer is heated at its melted state. An equilibrium ratio of 2 :1 (E:Z) is reached after 10 minutes. A substantial decrease (up to 2.05 ppm) is observed for the NMR chemical shifts of the AA-DNPs protons in benzene and electron rich aromatic solvents compared to those obtained in chloroform, acetonitrile, acetone, methanol, DMSO or even cyclohexane and electron deficient nitrobenzene. An explanation is given for the drastic chemical shift changes

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