A COMPREHENSIVE INVESTIGATION OF VARIATIONS IN MELTING RANGES AND NMRDATA OF 2,4-DINITROPHENYLHYDRAZINE DERIVATIVES

The well-known characterization method of aldehydes and ketones utiliz ing their 2,4-dinitrophenylhydrazones (DNPs) often gives variable melt ing ranges and conflicting results for a single derivative. We have fo und that the DNPs melting point discrepancies and the irreproducibilit y 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 c onventional methods. A bicarbonate washing of the DNPs removes the aci d, stabilizes the melting range and reproducibly gives higher-melting derivatives. In the DNP preparations of aldehydes and some unsymmetric al ketones only the kinetically favored (and also thermodynamically mo re stable) syn (E) isomer is formed. The six different melting ranges of acetaldehyde DNP (AA-DNP) previously reported by other investigator s are correlated with the amounts of acid present in their DNP crystal s. Derivatives with higher concentrations of acid(s) give lower meltin g ranges. In the presence of small amounts of acid, E-Z isomerization occurs in most solvents but not in methanol or dimethyl sulfoxide (DMS O), Larger amounts of acid cause the isomerization in all solvents, bu t the process is much slower in the aforementioned solvents. Alcohols, but not chlorinated solvents, are suitable for the DNPs recrystalliza tion. 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 equ ilibrium ratio of 2: 1 (E: Z) is reached after 10 minutes. A substanti al decrease (up to 2.05 ppm) is observed for the NMR chemical shifts o f the AA-DNPs protons in benzene and electron rich aromatic solvents c ompared to those obtained in chloroform, acetonitrile, acetone, methan ol, DMSO or even cyclohexane and electron deficient nitrobenzene, An e xplanation is given for the drastic chemical shift changes.