FREE-RADICAL INDUCED OXIDATION OF PHLOROGLUCINOL - A PULSE-RADIOLYSISAND EPR STUDY

Phloroglucinol-derived radicals have been studied using pulse radiolys is and EPR spectroscopy. Phloroglucinol 1a (pK(a) = 8.0) and its anion 1b (pK(a) = 9.2) have phenolic structures while the 3,5-dihydroxycycl ohexa-2,5-dienone structure predominates in the dianion 2c. The neutra l OH-adduct radicals (lambda(max) = 345 nm) rapidly eliminate water (k = 2 x 10(5) s-1) yielding the 3,5-dihydroxyphenoxyl radical 4a (lambd a(max) = 495 nm, pK(a) = 6.5). This radical as well as its monoanion 4 b (lambda(max) = 550 nm, pK(a) = 8.6), its isomer 5b, derived from 2c (lambda(max) > 800 nm), and the dianion 4c (lambda(max) = 640 nm) can be generated directly with the N3 radical (k = 1.4 x 10(9) dm3 Mol-1 s -1 at pH 6). All four radicals have been characterized by EPR spectros copy. Radical 5b reacts with the phloroglucinol monoanion 1 b with a r ate constant of 2 x 10(7) dm3 mol-1 s-1. Formation of an adduct is exc luded by EPR. Therefore, electron transfer from the phloroglucinol mon oanion 1b to radical 5b is favoured as an explanation for this reactio n. While 4a does not react with O2 (k < 4 x 10(5) dm3 mol-1 s-1), the anions 4b and 5b do so quite rapidly (k = 2.1 x 10(8) and 1.9 x 10(8) dM3 Mol-1 s-1, respectively) and at pH 7, O2 is consumed with G = 15 x 10(-7) mol J-1. Although Br2.- mainly produces radicals 4a and b, bro mination occurs with an efficiency of at least 10%.