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Communication | Special issue | Vol 15, No. 2, 1981, pp.697-708
Published online, 1st January, 1970
DOI: 10.3987/S-1981-02-0697
Substituent Effects on Acidities and Tautomeric Structures of 1-Aryl-3-ethoxycarbonyl-4-pyrazolones and Their 5-Arylazo Derivatives

Ahmad S. Shawali,* Hamdi M. Hassaneen, and Mokntar A. Hanna

*Department of Chemistry, Faculty of Science, University of Cairo, Giza, Egypt

Abstract

Two series of 1-aryl-3-ethoxycarbonyl-4-hydroxypyrazoles (1) and 1-(m-chlorophenyl)-3-ethoxycarbonyl-4-hydroxy-5-arylazopyrazoles (2) have been prepared and their acidities determined spectrophotometrically at 25°C and ionic strength of 0.1 in 60 vol.% ethanol-water medium. The pKa data were correlated with the simple and extended Hammett equations. The regression lines are: pKa(1) =9.34 - 0.794 δX ; pKa(1) = 9.78 - 0.794 δXC6H4 ; PKa(2) = 7.49 - 1.158 δX ; and PKa(2) = 7.49 -1.245 δI,X - 1.075 δR,X. It is concluded that such linear relations exclude the presence of the keto tautomers (1A) and (2A) in equilibrium with the hydroxy tautomers (1B) and (2B) respectively, in agreement with the spectral data of the compounds investigated. The electrical effect, E, of value 0.86 determined for series (2) indicates that such compounds exist in the cis-chelated hydroxyazo structure (2B).