Effect of the nature of inert solvent on the decay kinetics of the carbocation generated in the photolysis of 1,2-dihydroquinolines in methanol

Abstract

The dependence of the quantum yield and the decay rate constant for the carbocation generated in the photolysis of 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline (6-EtO-DHQ) in methanol on the solvent composition was studied in the mixtures of methanol with isopropanol, acetonitrile, and pentane by pulse photolysis. The addition of these solvents decreases the yield of the carbocation and differently affects the kinetic parameters of its decay. The carbocation decay in the mixtures MeOH-i-PrOH and MeOH-C5H12 is described by the pseudo-first order equation (k 1), with the dependence of k 1 having a maximum at 50 vol % of MeOH in the MeOH-i-PrOH mixtures, and k 1 increasing with a decrease in the MeOH concentration in the MeOH-C5H12 mixtures. In the MeOH-MeCN mixtures, the value of k 1 decreases with a decrease in the MeOH concentration, and, at the concentration of MeOH lower than 50 vol %, the contribution of the second-order reaction (k 2) is observed. The activation energies and preexponential factors were determined in the MeOH-C5H12 mixtures of different compositions, and it was shown that E act practically did not depend on the solvent composition and were close to E act for other carbocations obtained in MeOH. The increase in k 1 with a decrease in the MeOH concentration is caused by an increase in the preexponential factor. The results were discussed on the basis of the reaction mechanism involving the competing reactions of the carbocation combination with two nucleophilic particles, the MeOH molecule and the MeO− anion. The composition of the mixture and the nature of the inert solvent affect strongly the course of these reactions.