DETERMINATION OF AZO-HYDRAZONO TAUTOMERISM OF SOME 2-PYRAZOLIN-5-ONE DYES BY MEANS OF NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY AND 15N-LABELED COMPOUNDS

Through use of 16N labeling and nmr spectroscopy we showed that 3-methyl-l-phenyl-4-phenylazo-2-pyrazolin-5-one (1) exists entirely in the hydrazone form in chloroform solution, in agreement with some of the earlier ir and nmr results with unlabeled compounds. In dimethyl sulfoxide (DMSO) and pyridine solutions a mixture of the hydrazono and the enol-azo forms is proposed. In a similar fashion we found that 3-anilino-1-phenyl-4-phenylazo-2-pyrazolin-5-one (2) exists entirely in the hydrazone form in chloroform and DMSO at 38° and in pyridine at -40°. At 38° in pyridine, however, a mixture of tautomeric forms is proposed. © 1969, American Chemical Society. All rights reserved.