Hydrolysis kinetics of photo-excited 6-methoxyquinoline in aqueous ethanol

The kinetics of hydrolysis in the lowest excited single state of 6-methoxyquinoline in aqueous solutions can be evaluated from the pH-dependence of its fluorescence spectrum. In ethanol/water solutions, the rate constant for hydrolysis decreases exponentially with increasing mole fraction of ethanol and hydrolysis is completely inhibited at a mole fraction of ethanol greater than about 0.2. The slope of a plot of the logarithm of the hydrolysis rate constant against the mole fraction of ethanol was found to correlate well with the ratio of the diffusion volume of excited 6-methoxyquinoline to the molecular volume of ethanol. This also appears to explain why the linear plot of the logarithm of the hydrolysis rate constant against the logarithm of the water activity has a slope of 31, suggesting that 30 molecules of water, beside the one that is split in hydrolysis, participate in the reaction.