The pH dependence of photochemical reactions between methoxyphenols with Fe(III)-oxalates

The changes in optical properties and chemical compositions of four methoxyphenols, including guaiacol (GU), vanillin (VA), 4-nitroguaiacol (NGU) and 5-nitrovanillin (NVA) were investigated during the photochemical reaction with Fe(III)-oxalates. The reaction pathways of methoxyphenols with Fe(III)-oxalates were varied with solution pH. Four methoxyphenols were significantly decomposed by .OH produced from Fe(III)-oxalates under irradiation at a solution pH of 3, followed by the formation of phenols, ketones and ethers. The degradation rate of methoxyphenols was closely correlated to electron-withdrawing groups on the benzene ring, such as NO2 and CHO groups. Under the neutral condition, the absorbance of methoxyphenols varied or remained unchanged, depending on the pKa values of methoxyphenols. The changes in the light absorption of VA and NGU solutions were contributed to the deprotonation of OH group in the presence of dissolved O2.