Reactivity of carbocations from 1,2-dihydroquinolines in binary mixtures of methanol with pentane and acetonitrile

The rate constants and activation parameters of the reactions of the carbocation resulting from 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline photolysis with methanol (k1) and the methoxide ion (k2) have been measured by flash photolysis in binary mixtures of methanol with inert solvents (nonpolar pentane and polar acetonitrile) in wide composition ranges. The changes in the activation parameters for k1 at different solvent compositions show that the increase in the rate constant in the pentane mixtures is mainly deter-mined by the increase in the preexponential factor. The decrease in k1 in the acetonitrile mixtures is deter-mined by the decrease in the methanol concentration and by the increase in the activation energy. The different roles of the methoxide ion in the reaction are demonstrated. They depend on the nature of the inert solvent in the mixture. The results of this study are considered in terms of methanol clustering in pentane and acetonitrile, the different solubilities of 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline in the components of the binary mixtures, and the difference in distribution and solvation between the carbocation and the methoxide ion in the mixtures.