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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >The Origin of Base Catalysis in the ~·OH Oxidation of Phenols in Water
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The Origin of Base Catalysis in the ~·OH Oxidation of Phenols in Water

机译：水中苯酚~~ OH氧化中碱催化的起源

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摘要

Time-resolved Raman and transient absorption studies on hydroxyl (~·OH) radical reactions with aqueous 4-carboxyphenol (~-O_2C-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 pK_a 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 > 10~7s~(-1)). About 3% ~·OH addition is seen at the carboxylic ipso position in basic solutions, which produces the p-benzosemiquinone radical anion (rate ～ 10~6s~(-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 pKa of the corresponding phenols is given.
机译：时间分辨拉曼光谱和与羟基4-苯酚（〜-O_2C-PhOH）（对羟基苯甲酸酯羟化酶的底物）的羟基（〜·OH）自由基反应的瞬态吸收研究已导致首次直接观察到其基本形式二羟基环己二烯基（酚盐阴离子的〜·OH加合物）自由基和水合引起的该物种内分子内电子转移。该模型系统中碱催化的苯氧基自由基的形成已通过〜·OH加合物的酚质子（9.05）的pK_a和〜·OH〜-消除速率（2.9 * 106 s〜（ -1））。通过从羟基加成基的ipso位置的OH加合物中除去水，可形成一小部分苯氧基自由基（12（+2）％）（速率> 10〜7s〜（-1））。在碱性溶液中，在羧基位置上可以看到约3％〜·OH的加成，生成对苯并半醌醌阴离子（速率约为10〜6s〜（-1））。这项工作提供了通过〜·OH氧化形成苯氧基自由基的早期化学步骤的光谱和动力学证据，并建立了形成速率和pH之间的精确关系。给出了酚盐阴离子〜OH加合物中〜OH消除速率与相应酚的pKa之间的关系。

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