Redox Reactions of 8-Hydroxyquinoline. A Pulse Radiolysis Study

Abstract

Pulse radiolysis technique was used to study the reactions of eaq-, H atoms, and other reducing radicals such as COO-/COOH, (CH3)2COH, CH3CHOH, and CH2OH radicals with 8-hydroxyquinoline (8HQ) at various pH's. eaq- reacts with 8HQ with high rate constants (k2 = 3 × 1010 at pH 7 and 2.6 × 1010 at pH 12). Absorption spectra of the transient species were recorded in the wavelength region 300−800 nm. Semireduced species formed by the reaction of eaq- with 8HQ as well as that formed by H atom reaction were found to be strong reductants and were found to transfer an electron to methylviologen. COO-/COOH and alcohol radicals were found to react with 8HQ by addition reactions. The rate constants for the reactions were much higher at acidic pH's (≈109 dm3 mol-1 s-1), where 8HQ is in the protonated form, as compared to those at neutral pH (≈108 dm3 mol-1 s-1), where it is present in the neutral form. These adduct radicals were also found to be reducing in nature. The rate constant for OH radical reaction with 8HQ was estimated to be 7.6 × 109 dm3 mol-1 s-1 at pH 7 and 1.3 × 1010 dm3 mol-1 s-1 at pH 0; a cyclohexadienyl type of radical is formed in both the cases. These adducts react with oxygen (k = 3.2 × 109 dm3 mol-1 s-1 at pH 7) to give peroxyl radicals. Br2- and N3 radicals react with 8HQ by electron transfer at pH 13 giving a phenoxyl-type radical. At pH 7, N3 and SO4- radicals react with 8HQ by electron transfer whereas the reaction of the Br2- radical is quite slow. Only SO4- is able to oxidize 8HQ in its protonated form existing at acidic pH.