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Reactions in frozen solutions Kiovsky, Thomas Elstun

Abstract

In order to try to explain some of the rather surprising features of reactions in frozen solutions, four different systems are extensively studied. They are the reaction of methyl iodide with triethylamine to form the quaternary ammonium salt in frozen benzene, the base catalyzed decomposition of t-butyl-peroxy formate to carbon dioxide and t-butyl alcohol in frozen p-xylene, the reaction of ethylene chlorohydrin with hydroxide ion to form ethylene oxide in frozen aqueous solution and the mutarotation of glucose in ice. In addition, a demonstration experiment is presented in which iodide ion is oxidized to iodine by arsenic acid in frozen aqueous solution. Several new features of reactions in frozen solutions are reported; including a maximum in the rate - temperature dependence curve, rate enhancements as large as 1000 - fold over reaction in unfrozen solutions and shifts in the equilibrium position. Kinetic equations are developed which correlate all of the results and which also explain some of the observations of other investigators. These equations are based upon the assumptions that (1) when a solution containing reactive species is frozen all of the reactants as well as any other solutes present are rejected by the crystallizing solvent and are concentrated into regions which remain liquid and that (2) the reaction proceeds normally in these regions. The fundamental equation used for correlating the rate data for the second-order reactions studied is, [formula omitted] where Vn is the total volume of the liquid regions, mA is the total moles of reactant A present in the system at any time and Ah and Bh are the concentrations of the reactants A and B in the liquid regions. The ideas developed for the treatment of reactions in frozen solutions are extended to reactions in organic solids which have a melted phase present. This treatment accounts qualitatively for the observations made on mutarotation in solid glucose. The application of the method to the isomerization of 5-norbornene-2,3-endo-dicarboxylic/anhydride to the exo-isomer allows separation of concurrent reactions in the melt and in the solid.

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