Reaction kinetics of some secondary alkanolamines with carbon dioxide in aqueous solutions by stopped flow technique

The kinetics of the reaction between aqueous solutions of carbon dioxide and 2-(methylamino)ethanol (MMEA), 2-(n-butylamino)ethanol (NBAE) and 2-(tert-butylanaino)ethanol (TBAE) was investigated over a temperature range of 283-308 K by using the direct stopped flow technique. These secondary alkanolamines differ only in the nature of the alkyl substituent on the amino group. At each of the temperatures studied, MMEA reacts faster than NBAE, which in turn, reacts faster than TBAE. Reactions of MMEA and NBAE can be explained by the zwitterion mechanism. However, the reaction mechanism of the TBAE, a severely hindered amine, is suggested to be similar to that for tertiary amines, but at a faster rate. The rate constants required to predict reaction rates of these three amines have been calculated and presented in this work. Water is found to stabilize and extensively deprotonate the zwitterion formed in the case of both MMEA and NBAE; the zwitterion of the former amine is found to be more stable than that of the latter one and the zwitterion stability for both amines decreases with temperature. At the temperatures studied, steric factors cause TBAE to react slower than its unhindered constitutional isomer, NBAE, but with increase in temperature, the detrimental effect of these steric factors on the reaction rates is found to decrease. (C) 2002 Elsevier Science B.V. All rights reserved.