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

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.