Free radical induced oxidation of phloroglucinol. A pulse radiolysis and EPR study

Abstract

Phloroglucinol-derived radicals have been studied using pulse radiolysis and EPR spectroscopy. Phloroglucinol 1a(pKa= 8.0) and its anion 1b(pKa= 9.2) have phenolic structures while the 3,5-dihydroxycyclohexa-2,5-dienone structure predominates in the dianion 2c. The neutral OH-adduct radicals (λmax= 345 nm) rapidly eliminate water (k= 2 × 105 s–1) yielding the 3,5-dihydroxyphenoxyl radical 4a(λmax= 495 nm, pKa= 6.5). This radical as well as its monoanion 4b(λmax= 550 nm, pKa= 8.6), its isomer 5b, derived from 2c(λmax > 800 nm), and the dianion 4c(λmax= 640 nm) can be generated directly with the N3 radical (k= 1.4 × 109 dm3 mol–1 s–1 at pH 6). All four radicals have been characterized by EPR spectroscopy. Radical 5b reacts with the phloroglucinol monoanion 1b with a rate constant of 2 × 107 dm3 mol–1 s–1. Formation of an adduct is excluded by EPR. Therefore, electron transfer from the phloroglucinol monoanion 1b to radical 5b is favoured as an explanation for this reaction.While 4a does not react with O2(k < 4 × 105 dm3 mol–1 s–1), the anions 4b and 5b do so quite rapidly (k= 2.1 × 108 and 1.9 × 108 dm3 mol–1 s–1, respectively) and at pH 7, O2 is consumed with G= 15 × 10–7 mol J–1.Although Br2˙– mainly produces radicals 4a and b, bromination occurs with an efficiency of at least 10%.