PROTOLYTIC EQUILIBRIUM OF THE ISOMERIC PHENYL-1,2,4-TRIAZINES

Abstract

The protonation constants for the first and second stages (pK^BH+, pK^BH2+) of a series of 1,2,4-triazines with a phenyl substituent at various positions in the ring were determined in aqueous solution by a spectrophotometric method. The values of the basicity constants characterizing the first protonation of the heterocycles investigated was in the range of acidity of the medium of pH 3.5 to H⁰ -2, and the second from H₀ -7.3 to H₀ -8.7. The position of the phenyl substituent proved to have a significant effect on the size of pK_BH+. According to the results of ab initio calculations using HF/6-31G** for the heterocycles investigated the 1H⁺ form is thermodynamically most stable among the monocations, with the exception of 6-phenyl-1,2,4-triazine for which the existence of the  monocation in the 1H⁺ and 2H⁺ forms are equally probable. In the case of the dications of all the triazines the 2,4-H,H²⁺ tautomer is the most preferred. The aromaticity of the 1,2,4-triazine ring is changed insignificantly on mono- and diprotonation.

How to Cite
Khabibulina, I. V.; Volovodenko, A. P.; Trifonov, R. E.; Yashukova, G. V.; Mochul'skaya, N. N.; Charushin, V. N.; Rusinov, G. L.; Beresnev, D. G.; Itsikson, N. A.; Ostrovskii, V. A. Chem. Heterocycl. Compd. 2003, 39, 616. [Khim. Geterotsikl. Soedin. 2003, 712.]

For this article in the English edition, see DOI https://doi.org/10.1023/A:1025198100025