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Direct determination of p K a values of cationic acids conjugated to heterocyclic amine N ‐oxides in polar aprotic and amphiprotic solvents
Author(s) -
Chmurzyński Lech,
Wawrzyniak Grażyna,
Warnke Zygmunt,
Pawlak Ze
Publication year - 1997
Publication title -
journal of heterocyclic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.321
H-Index - 59
eISSN - 1943-5193
pISSN - 0022-152X
DOI - 10.1002/jhet.5570340132
Subject(s) - chemistry , potentiometric titration , protonation , aqueous solution , pyridine , cationic polymerization , amine gas treating , titration , inorganic chemistry , polar , salt (chemistry) , organic chemistry , ion , physics , astronomy , electrode
The applicability of the direct method of p K a determination in the case of protonated heterocyclic amine N ‐oxides in a series of polar non‐aqueous (aprotic and amphiprotic) solvents has been tested. The method is based on the p H determination of the non‐aqueous solution of complex salt (the semiperchlorate in this case) formed by the N ‐oxides studied. The direct method not only provides for quick (one data point per each p K a determined), but also relatively accurate estimates of acidic dissociation constants. It has been experimentally shown on the example of substituted pyridine N ‐oxides that this method is precise enough in all studied non‐aqueous solvents when applied to compounds of not too low basicity (the p K a being of the order of 5 or higher). To prove this, the p K a values of protonated monocyclic N ‐oxides obtained by the direct method have been compared with those resulting from the potentiometric titration curve. The agreement between the results found by using both methods is very good in most cases, the differences being within standard deviations. Based upon this observation it can be inferred that the p K a values of protonated bicyclic N ‐oxides in solvents studied determined by using the direct method can be also considered reliable, especially in the case of polar aprotic solvents.