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(pK_a= 8.0) and its anion 1b(pK_a= 9.2) have phenolic structures while the 3,5-dihydroxycyclohexa-2,5-dienone structure predominates in the dianion 2c. The neutral OH-adduct radicals (λ_max= 345 nm) rapidly eliminate water (k= 2 × 10⁵ s^–1) yielding the 3,5-dihydroxyphenoxyl radical 4a(λ_max= 495 nm, pK_a= 6.5). This radical as well as its monoanion 4b(λ_max= 550 nm, pK_a= 8.6), its isomer 5b, derived from 2c(λ_max > 800 nm), and the dianion 4c(λ_max= 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%.