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Kinetic investigation on carbamate formation from the reaction of carbon dioxide with amino acids in homogeneous aqueous solution
Author(s) -
Yamamoto Yuki,
Hasegawa Junya,
Ito Yoshikatsu
Publication year - 2012
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1900
Subject(s) - chemistry , carbamate , reaction rate constant , amino acid , nucleophile , aqueous solution , carbon dioxide , medicinal chemistry , catalysis , inorganic chemistry , kinetics , stereochemistry , organic chemistry , biochemistry , physics , quantum mechanics
The kinetics of carbamate formation from the reaction of carbon dioxide with α‐amino acids in D 2 O was first investigated by means of nuclear magnetic resonance spectroscopy. Potassium carbonate was used as the CO 2 source. For each amino acid, the maximum carbamate yield, the apparent rate constant for the carbamate formation k app , and the rate constants for the formation k 1 and the breakdown k −1 of the carbamate were estimated. Plots of log k 1 or log k −1 versus p K a of amino acids indicated that the formation rate k 1 increased with the basicity (p K a ) of amino acid, while the decomposition rate k −1 decreased. A Brϕnsted β value of 0.39 was obtained from the former plot, being in good agreement with the previously reported ones (0.26–0.43). The observed negative p K a dependence of log k −1 (Brϕnsted α = 0.34) is reasonable, because the carbamate decomposition is acid‐catalyzed and the steady‐state concentration of H + should be higher for weaker basic amines. The charge (σ) and the lone‐pair energy ( E N ) at the nitrogen atom of the amino group were calculated. Although log k 1 correlated with σ and E N , log k −1 was unrelated with both of these parameters. Considering that the carbamate formation ( k 1 ) is not only base‐catalyzed but should also be promoted by the nucleophilicity of the amino nitrogen, its correlation with σ and E N in addition to p K a is rational. The irrelevance of log k −1 to σ and E N is not surprising, because σ and E N are not a direct measure of [H + ] of the solution. Copyright © 2011 John Wiley & Sons, Ltd.