Thermodynamic cycles and the calculation of pKa |
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| Affiliation: | 1. Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, United Kingdom;2. Aachener Verfahrenstechnik – Process Systems Engineering, RWTH Aachen University, Aachen, Germany;3. Cambridge Centre for Advanced Research and Education in Singapore Ltd., 1 Create Way, CREATE Tower #05-05, 138602, Singapore;1. Chemical and Biological Pharmaceutical Engineering Research Center, Xuzhou Medical College, No. 209, Tongshan Road, Xuzhou, Jiangsu 221004, China;2. School of Pharmacy, Xuzhou Medical College, No. 209, Tongshan Road, Xuzhou, Jiangsu 221004, China;1. Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, I-87136 Rende (CS), Italy;2. Department of Pharmacognosy and Herbal Medicines, Wroclaw Medical University, Borowska 211A, 50-556 Wroclaw, Poland;3. Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm SE-10691, Sweden |
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| Abstract: | A theoretical equation for the calculation of pKa based on a proton transfer reaction between the acid and one water molecule is derived using the general chemical equilibrium relationship. The present result is compared with two equations recently used that were based on thermodynamic cycles, but predict different pKa’s. It is shown that one thermodynamic cycle is wrong, and its better performance when compared with the correct cycle is due to an erroneous value used for the solvation free energy of the H3O+ ion. In addition, this analysis indicates that the PCM-UAHF solvation model is inconsistently parametrized. |
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