Department of Chemistry

Abstract

To investigate the substituent effect on the distribution of the forms of the ground-state species of 8-substituted 7-hydroxyquinolines, UV-absorption and resonance Raman expts. were performed for 8-chloro-7-hydroxyquinoline (CHQ-OAc) and 8-cyano-7-hydroxyquinoline (CyHQ-OAc) in acetonitrile (MeCN), in NaOH-H₂O/MeCN (60:40, vol./vol., pH 11-12), and in H₂O/MeCN (60:40, vol./vol., pH 6-7) solns., and these results were compared to those previously reported for the 8-bromo-7-hydroxyquinoline (BHQ-OAc) compd. Swapping a bromine atom in BHQ-OAc for a chlorine atom in CHQ-OAc causes the amt. of the tautomeric species to become larger, although the neutral species is still the predominant species for both systems in water-rich solns. The absorption spectra and the resonance Raman spectra of CyHQ-OAc suggest that, because of the strong electron-withdrawing nature of the cyano substituent, a measurable amt. of the anionic species is present and the tautomeric species cannot be easily detected in water-rich solns. The results reported here reveal large substituent effects on the distribution of the different forms of the XHQ-OAc compds. in largely aq. solns. The steric effect of the 8-substituted group and competitive hydrogen bonding between the 8-substituted group and water mols. hinders the formation of a cyclic BHQ-OAc-water complex, and the electron-withdrawing property of the 8-substituted group enhances the deprotonation of the phenol group while disfavoring the formation of the pos. charged quinoline nitrogen. We briefly discuss the implications of the substituent effects for using these compds. as phototriggers.