New Method to Follow the Dimerisation Reaction Occuring during Oxidation of 4-Ethylguaiacol

Summary 4-Ethylguaiacol is a phenolic model compound used to study the oxidation of lignin. A new method was developed to determine 4,4′-diethyl-6,6′-biguaiacol, an unwanted side-product, in the oxidation of 4-ethylguaiacol. The sample is extracted with ethyl acetate to separate 4,4′-diethyl-6,6′-biguaiacol from the water-soluble oxidation catalyst. No other treatment is required before analysis with a standard UV-Vis spectrophotometer. The spectrophotometer is calibrated to measure concentrations of 4,4′-diethyl-6,6′-biguaiacol up to 0.3 mM. With the new method, the concentrations of 4,4′-diethyl-6,6′-biguaiacol can be reliably determined over a wide alkaline pH range. The performance of the method was demonstrated in experiments investigating the formation of 4,4′-diethyl-6,6′-biguaiacol in the autoxidation and cobalt-sulphosalen catalysed oxidation of 4-ethylguaiacol. In the conditions of the studies (1 bar oxygen pressure and 50°C), a low (0.01 mM) concentration of Co-sulphosalen clearly increased the total conversion of 4-ethylguaiacol. However, relative to autoxidation, the conversions achieved in the presence of the catalyst were just as sensitive to pH, and the yields of the undesired dimeric product, 4,4′-diethyl-6,6′-biguaiacol, were higher.