Determination of thermodynamic p K a values of benzimidazole and benzimidazole derivatives by capillary electrophoresis

Department of Chemistry, Thompson Rivers University, Kamloops, BC, Canada.
Journal of Separation Science (Impact Factor: 2.74). 04/2009; 32(7):1087-95. DOI: 10.1002/jssc.200800482
Source: PubMed


Thermodynamic pK(a) values for benzimidazole and several substituted benzimidazoles were determined by CE. Electrophoretic mobilities of benzimidazoles were determined by CE at different pH levels and ionic strengths. The dependence of electrophoretic mobilities on pH was used to obtain pK(a) values at different ionic strengths. Extrapolations to zero ionic strength were used to determine the thermodynamic pK(a) values. Using this method the thermodynamic pK(a) values of 15 benzimidazoles were determined and found to range from 4.48 to 7.38. Results from the CE measurements were compared with spectrophotometric measurements which were evaluated at wavelengths where the highest absorbance difference for varying pH was recorded. The two analytical techniques were in good agreement.

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    • "This suggests that one proton will be tightly held between the pyridyl and the imidazolyl nitrogen in weakly acidic solutions while the second proton is only accommodated in strongly acidic solutions. The pK 1 value is intermediate between that of imidazole (6.95) and pyridine (5.26) (Pettit and Powell, 2007), and is closer to that of benzimidazole (5.49) (Jerez et al., 2009). "
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