Determination of Proton Affinities and Acidity Constants of Sugars

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• 作者：Shuting Feng ; Christina Bagia ; Giannis Mpourmpakis
• 刊名：The Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：June 20, 2013
• 年：2013
• 卷：117
• 期：24
• 页码：5211-5219
• 全文大小：374K
• 年卷期：v.117,no.24(June 20, 2013)
• ISSN：1520-5215

文摘

Proton transfer reactions play a key role in the conversion of biomass derived sugars to chemicals. In this study, we employ high level ab initio theoretical methods, in tandem with solvation effects to calculate the proton affinities (PA) and acidity constants (pK_a) of various d-glucose and d-fructose tautomers (protonation鈥揹eprotonation processes). In addition, we compare the theoretically derived pH values of sugar solutions against experimentally measured pH values in our lab. Our results demonstrate that the protonation of any of the O atoms of the sugars is thermodynamically preferred without any significant variation in the PA values. Intramolecular hydrogen transfers, dehydration reactions, and ring-opening processes were observed, resulting from the protonation of specific hydroxyl groups on the sugars. Regarding the deprotonation processes (pK_a), we found that the sugars鈥?anomeric hydroxyls exhibit the highest acidity. The theoretically calculated pH values of sugar solutions are in excellent agreement with experimental pH measurements at low sugar concentrations. At higher sugar concentrations the calculations predict less acidic solutions than the experiments. In this case, we expect the sugars to act as solvents increasing the proton solvation energy and the acidity of the solutions. We demonstrated through linear relationships that the pK_a values are correlated with the relative stability of the conjugate bases. The latter is related to hydrogen bonding and polarization of the C鈥揙^{鈥?/sup> bond. A plausible explanation for the good performance of the direct method in calculating the pK_a values of sugars can be the presence of intramolecular hydrogen bonds on the conjugate base. Both theory and experiments manifest that fructose is a stronger acid than glucose, which is of significant importance in self-catalyzed biomass-relevant dehydration reactions.}