Chemical Kinetics in Water-Lean Solvents

Nevertheless, the nature of this relationship is not entirely clear. Both Sada et al. and Park et al. have proposed that such a decrease is typically exponential. However, there are exceptions to this rule. One remarkable exception is the case of MEA. Aqueous MEA 0.5–2 M has a k₂ = 7740 L·mol^–1·s^–1 and k_b/k_–1 = ∞ L·mol^–1 (meaning that the conversion from zwitterion to carbamate is infinitely faster than its conversion back to MEA). Meanwhile, in mixtures of MEA 0.5–2 M with methanol, ethanol, and 2-propanol, k₂ = 8330 L·mol^–1·s^–1 and k_b/k_–1 assume, respectively, the values of 0.78, 0.35, and 0.27 L·mol^–1. In other words, while an exponential decrease with ε is observed for k_b/k_–1, an actual increase is observed for k₂, an increase that could perhaps be attributed to the difficulties of fitting both of these parameters at the same time to experimental data. Regardless of the reasons, this indicates that the effects of ε on kinetic rate constants should (i) be considered on a case-by-case basis with regard to each individual amine and (ii) not be taken entirely at face value but merely as interpolations of available experimental data. Therefore, any extrapolations should be carried out with caution.