Abstract
The protonation constants for the first and second stages (pKBH+, pKBH2+) 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 H 0 -2, and the second from H 0 -7.3 to H 0 -8.7. The position of the phenyl substituent proved to have a significant effect on the size of pKBH+. 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.
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Khabibulina, I., Volovodenko, A.P., Trifonov, R.E. et al. Protolytic Equilibrium of the Isomeric Phenyl-1,2,4-triazines. Chemistry of Heterocyclic Compounds 39, 616–623 (2003). https://doi.org/10.1023/A:1025198100025
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DOI: https://doi.org/10.1023/A:1025198100025