Kinetic study of reactive absorption of same primary amines with carbon dioxide in ethanol solution

The observed pseudo-first-order rate constant (k(o)) values at 278, 283, 288 and 293 K for the reaction of aniline/cyclohexamine/hexamine with carbon dioxide in ethanol are determined using the stopped flow technique. The highest conversion to carbamate ion is detected with hexamine. The results favour the zwitterion intermediate mechanism proposed by Caplow and Danckwerts. The reaction order increases with the basicity of the amine (ranging from 1 to 2). The rate constants determined can predict the k(o) values accurately for the amine concentrations and temperatures studied. The basicity of the amine plays a very important role in the first step of the zwitterion formation while the bulkiness, resonance stabilisation of the zwitterion and temperature mainly affect the zwitterion deprotonation by the amine. Among the amines studied the role of the solvent (ethanol) as a base in the deprotonation step can only be ignored in the case of the cyclohexamine. For hexamine, the rate constant for the zwitterion formation step, is more temperature sensitive than for the other two amines. (C) 2000 Elsevier Science B.V. All rights reserved.