1H‐NMR‐spektroskopische Untersuchungen zur Azo‐Hydrazon‐Tautomerie in substituierten 1‐Phenylazo‐2‐naphtholen

Abstract

1H‐N.M.R. Spectroscopic Investigations of the Azo‐Hydrazon‐Equilibrium in Substituted 1‐Phenylazo‐2‐naphtholsIn substituted 1‐phenylazo‐2‐naphthols an equilibrium between the azoic and the quinoid hydrazone form is observed. It is possible to obtain the equilibrium constants by means of 1H‐n.m.r.‐spectroscopy in CDCl3. Both chemical shifts δH−3 and δH−8 in the naphthalene ring system can be used to determine independently the equilibrium constants. 1‐Phenylazo‐2‐naphthylamine and 1‐(pyrid‐2‐ylazo)‐2‐naphthole were used as reference substances for the pure azoic and hydrazone form, respectively. An appropriate correction is introduced to compare the different influence of OH‐ und NH2‐groups, respectively, on the protons H‐3 and H‐8.All equilibrium constants measured show a linear correlationship with respect to Hammett‐σ‐constants of the substituents attached to the phenyl ring. The standard reaction entropy for the conversion of the azoic form into the hydrazone form is always negative and of nearly identical magnitude. Obviously, no substituent effect to this value exists. One possibility to interprete these results is to assume that the quinoid hydrazone form shows a higher tendency to aggregation.