Electrochemistry of Caffeic Acid Aqueous Solutions with pH 2.0 to 8.5

[摘要] Caffeic acid (H3CAF), 3-(3,4-dihydroxyphenyl)-2-propenoic acid (Figure 1), is the phenylpropenoid most encountered in nature and has proven medicinal properties, especially as an antioxidant agent.1,2 Despite this, few studies have been dedicated to the oxidation mechanism of this substance. The enzymatic oxidation is the most important reaction of H3CAF in the presence of polyphenoloxidase.3 However, non-enzymatic oxidation can take place in the presence of oxygen, particularly in alkaline medium.4 Previous studies3,5 showed that the chemical oxidation of H3CAF promoted by sodium periodate leads to the formation of the corresponding o-quinone (o-HCAF), 3-(cyclohex-1,5-dien-3,4-dione)-2-propenoic acid. In acidic conditions a disproportion of o-HCAF occurs, leading to the formation of two isomers of 2,5-(3',4'-dihydroxyphenyl) tetrahydrofuran-3,4-dicarboxylic acid. The electro-oxidation of H3CAF in non-aqueous solution5 - acetonitrile - occurs in two stages, leading to the formation of the semi-quinone and then to the corresponding o-quinone and involves the transfer of two electrons and two protons. In aqueous solution (pH 4.0) the electro-oxidation also involves the transfer of two electrons but there is little information about the products formed. As can be deduced from the structural formula (Figure 1), the electro-oxidation of H3CAF is pH-dependent, and this aspect has been poorly explored in the literature. Electro-oxidation of organic compounds is carried out almost always in non-aqueous solutions due to solubility characteristics. However, this is not the

[学科分类] 化学（综合）