Kinetic investigation on carbamate formation from the reaction of carbon dioxide with amino acids in homogeneous aqueous solution

Abstract

The kinetics of carbamate formation from the reaction of carbon dioxide with α‐amino acids in D2O was first investigated by means of nuclear magnetic resonance spectroscopy. Potassium carbonate was used as the CO2 source. For each amino acid, the maximum carbamate yield, the apparent rate constant for the carbamate formation kapp, and the rate constants for the formation k1 and the breakdown k−1 of the carbamate were estimated. Plots of log k1 or log k−1 versus pKa of amino acids indicated that the formation rate k1 increased with the basicity (pKa) 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 pKa 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 (EN) at the nitrogen atom of the amino group were calculated. Although log k1 correlated with σ and EN, log k−1 was unrelated with both of these parameters. Considering that the carbamate formation (k1) is not only base‐catalyzed but should also be promoted by the nucleophilicity of the amino nitrogen, its correlation with σ and EN in addition to pKa is rational. The irrelevance of log k−1 to σ and EN is not surprising, because σ and EN are not a direct measure of [H+] of the solution. Copyright © 2011 John Wiley & Sons, Ltd.