Interference of carbohydrates in the determination of the methoxyl content of lignin in woody samples
2006
Goto, H. | Koda, K. | Tong, G. | Matsumoto, Y. | Meshitsuka, G.
To evaluate the effect of the presence of carbohydrates on the determination of the methoxyl content of lignin, the mechanism of acid-catalyzed reaction of lignin methoxyl groups with iodide ion to form methyl iodide was evaluated using carbohydrate and lignin model compounds. Not only the iodide concentration but also the acid concentration was found to significantly affect the rate of formation of methyl iodide. This fact and Hammett plots of the relative reaction rates observed for several model compounds suggest an SN2cA reaction mechanism for methyl iodide formation. Carbohydrates interfered with the rate of formation of methyl iodide, probably by acting as Lewis bases. Interestingly, the study also revealed that a certain amount of methyl iodide could arise from carbohydrates even when the carbohydrates did not contain methoxyl groups as potential precursors to methyl iodide. These sources of error were significant when methoxyl content was determined for samples with low lignin content.
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