Rate constants, AHS and AS3 have been measured by the conductimetric stopped-flow technique for the reaction of carbon dioxide in aqueous solution with the primary amines 2-methoxyethylamine, 2-aminoethano1, 3-aminopropan-l-ol, 2-aminopropan-2-ol, ~~-aminopropan-2-01 and the secondary am i nes 1 ,I 'im i nodi pro pa n -2 -01, 2 -amino -2 -( h yd roxymet h y I) pro pane -1 , 3 -diol, 2,2'-imi nodiet ha nol, 2,2,6,6-tetramethylpiperidin-4-01, and morpholine. The observed first-order rate constants fit theThe much-quoted Danckwerts mechanism is shown to be unlikely.The reaction of carbon dioxide with aqueous amines is of great technical importance as providing the best way of stripping carbon dioxide from gas streams. It has therefore been much studied, but with confusing and conflicting results. Blauwhoff et ul.' give a review of the earlier literature. Fourteen studies of the reaction with 2-aminoethanol all found the rate to be proportional to the first power of the amine concentration. Eleven studies of the reaction with 2,2'-iminodiethanol, quoting data from seven techniques, found the rate to be proportional to the first power, whereas five studies, quoting data from four techniques, found the rate to be proportional to the second power of the amine concentration. Two more studies,233 using the stopped-flow technique which should give the clearest data, subsequent to this review are clearly in favour of a first-order dependence on amine concentration. A study4 in which the rate of the reaction was measured indirectly from the rate of absorption from the gas phase gave fractional orders of reaction, between unity and two, for the reaction of carbon dioxide with 3-aminopropan-1-01 in ethanol solution. In an attempt to obtain a clearer picture, we have studied the reaction in anhydrous ethanol using the novel technique of conductimetric stopped-flow, and found an unambiguous dependence of the rate on the second power of the 2,2'-iminodiethanol c~ncentration.~ In this paper we report on the kinetics of the reaction with ten amines in aqueous solution studied using the same apparatus.

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The rate of formation of N,N-dialkylcarbamate from diethanolamine and carbon dioxide in anhydrous ethanol has been found to depend on the square of the amine concentration. The rate and activation data are consistent with the two-step Danckwerts mechanism, in which a zwitterion intermediate is present at very low concentration and reacts with a second molecule of amine at the diffusion-controlled limit to give a final salt in the rate-determining step.

The proton-transfer reactions between acids and bases in aprotic solvents of low relative permittivity lead to the production of ion-pair complexes via hydrogen-bonded reaction intermediates. For most systems the rate-limiting step is the formation of the hydrogen-bonded intermediate. The rates of these reactions, as measured by the temperature-jump technique, agree well with those calculated by a refined theory of diffusion-controlled rates which takes into account rotation within the encounter complex. The reactions of Bromophenol Blue with pyridine basis are anomalously slow, the rate-limiting step being proton-transfer within the hydrogen-bonded complex to form the ion-pair. Values of the enthalpy, entropy and volume of activation, and the primary kinetic isotope effect, enable the structure of the transition state to be deduced.

Ensuring that solute movement is due solely to diffusion (and not bulk movement) by holding the liquid in a gel.