The spectra (1H nmr, ir and uv) of a series of 3‐arylazo‐4‐hydroxycoumarins 1a‐m indicate that such compounds exist predominantly in the keto hydrazone form 1A both in solid state and in solution. The acid dissociation constants of the series studied were determined spectrophotometrically in 80 vol% ethanol‐water mixture at 27° and ionic strength of 0.1. The results of the correlations of these constants by Hammett and Yukawa‐Tsuno equations exclude the presence of the hydroxyazo form 1B in equilibrium with 1A in agreement with the spectral data. Also, the HMO method has been used to study tautomerism in such compounds. The results are also in full agreement with both the spectral and linear free energy correlations, the hydrazone form 1A being the most stable. It is further shown that both the intermolecular and intramolecular hydrogen bonding favor the hydrazone tautomer.

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Für die Kupplungsprodukte aus 4‐Hydroxy‐cumarin und den entsprechenden Diazoniumsalzen wird überwiegend die Keto‐hydrazon‐Struktur (I) bewiesen.