Triple proton transfer of excited 7-hydroxyquinoline along a hydrogen-bonded water chain in ethers: secondary solvent effect on the reaction rate

A large secondary solvent effect on the reaction rate has been experimentally observed in the excited-state tautomerization of a 7-hydroxyquinoline (7HQ) molecule complexed cyclically with two water molecules in ethers. The proton acceptance of a water molecule from the enolic group of 7HQ is the rate-determining step while the proton donation of a water molecule to the imino group of 7HQ is followed rapidly to complete the triple proton transfer of the 7HQ·(H₂O)₂ complex in both diethyl ether and di-n-propyl ether. The rate constant of the tautomerization is larger in diethyl ether than in di-n-propyl ether due to the more polar environment around the complex in diethyl ether. Although the activation energies of the proton transfer are similar in both ethers, the kinetic isotope effect of the rate constant is larger in di-n-propyl ether than in diethyl ether. We attribute these kinetic differences to dissimilarity in the polarities of the two secondary solvents.