52467-44-4

Upstream product

• 63125-16-6 4-iodophenoxyl radical 
• 16885-71-5 C₇H₄O₃^(2-) 
• 99-96-7 4-hydroxy-benzoic acid 

Downstream Products

• 63125-16-6 4-iodophenoxyl radical 
• 16885-71-5 C₇H₄O₃^(2-) 

Relevant academic research and scientific papers

The Origin of Base Catalysis in the OH Oxidation of Phenols in Water

Time-resolved Raman and transient absorption studies on hydroxyl ( OH) radical reactions with aqueous 4-carboxyphenol ( -O2C-PhOH), the substrate in p-hydroxybenzoate hydroxylase, have led to the first direct observation of the basic form of a dihydroxycyclohexadienyl (OH-adduct of phenolate anion) radical and hydration-induced intramolecular electron transfer in this species. The base-catalyzed phenoxyl radical formation in this model system has been quantitatively described in terms of pKa of the phenolic proton (9.05) of the OH-adduct and the rate of OH - elimination (2.9 × 106 s-1) from its basic form. A small fraction, 12(+2)%, of the phenoxyl radical is formed via water elimination from the OH-adduct at the ipso position of the hydroxyl group (rate > 107s-1). About 3% OH addition is seen at the carboxylic ipso position in basic solutions, which produces the p-benzosemiquinone radical anion (rate ～ 106s-1). This work provides spectroscopic and kinetic evidence of the early chemical steps in the phenoxyl radical formation by OH oxidation and establishes the precise relationship between the formation rate and pH. A relationship between the rates of OH- elimination from the OH adducts of phenolate anions and pK a of the corresponding phenols is given.

Quenching of triplet-excited flavins by flavonoids. Structural assessment of antioxidative activity

(Figure Presented) The mechanism of flavin-mediated photooxidation of flavonoids was investigated for aqueous solutions. Interaction of triplet-excited flavin mononucleotide with phenols, as determined by laser flash photolysis, occurred at nearly diffusion-controlled rates (k～1.6x10 9 Lmol-1 s-1 for phenol at pH 7, 293 K), but protection of the phenolic function by methylation inhibited reaction. Still, electron transfer was proposed as the dominating mechanism due to the lack of primary kinetic hydrogen/ deuterium isotope effect and the low activation enthalpy (-1) for photooxidation. Activation entropy worked compensating in a series of phenolic derivatives, supporting a common oxidation mechanism. Anortho-hydroxymethoxy pattern was equally reactive (k～2.3x109Lmol-1 s-1 for guaiacol at pH 7) as compounds with ortho-dihydroxy substitution (k～2.4x109 L mol-1 s-1 for catechol at pH 7), which are generally referred to as good antioxidants. This refutes the common belief that stabilization of incipient phenoxyl radicals through intramolecular hydrogen bonding is the driving force behind the reducing activity of catechol-like compounds. Instead, such bonding improves ionization characteristics of the substrates, hence the differences in reactivity with (photo)oxidation of isolated phenols. Despite the similar reactivity, radicals from ortho-dihydroxy compounds are detected in high steady-state concentrations by electron paramagnetic resonance (EPR) spectroscopy, while those resulting from oxidation of ortho-hydroxymethoxy (or isolated phenolic) patterns were too reactive to be observed. The ability to deprotonate and form the corresponding radical anions at neutral pH was proposed as the decisive factor for stabilization and, consequently, for antioxidative action. Thus, substituting other ionizable functions for the ortho- or para-hydroxyl in phenolic compounds resulted in stable radical anion formation, as demonstrated for para-hydroxybenzoic acid, in contrast to its methyl ester. 2009 American Chemical Society.

The one-electron reduction potential of 4-substituted phenoxyl radicals in water

By means of pulse radiolysis the one-electron reduction potentials of twelve 4-substituted phenoxy radicals have been determined. The main reference used was the ClO2./ClO2- couple. By combining the redox potentials of phenoxyl radicals with the aqueous acidities of phenols the bond strength of the phenolic O-H bond was calculated. These values were found to be in good agreement with O-H bond dissociation enthalpies measured in the gas phase.