Determination of nitrobenzene oxidation products by GC and H-1-NMR spectroscopy using 5-iodovanillin as a new internal standard

The nitrobenzene oxidation method was modified to obtain more reproducible data and more structural information about lignin, not only by gas chromatography (GC) but also by proton nuclear magnetic resonance (H-1-NMR:) spectroscopy for quantitative determination of the oxidation products and to simplify the procedures. The nitrobenzene oxidation mixture was directly extracted after acidification without preextraction of by-products. The direct extraction made the extractive step easy and gave reproducible data. 5-Iodovanillin was selected as a new internal standard. The reason for this selection was that 5-iodovanillin did not exist in the nitrobenzene oxidation products from any plant species and had an aldehyde group whose peak did not overlap with the other aldehyde peaks on an H-1-NMR spectrum. Thus, the use of 5-iodovanillin enabled us to quantify p-hydroxybenzaldehyde, vanillin, and syringaldehyde in oxidation products on the basis of H-1-NMR analysis as well as GC. Furthermore, more information about the condensed structure of lignin was derived by comparing the H-1-NMR and GC analyses. syringaldehyde (4a) (Fig. 1); and instrumental analysis. The conventional method was first proposed using lignin model compounds' and was modified mainly after advances in analytical instruments! The use of acetovanillone (5a) as an internal standard focuses on an important issue of quantitative analysis because the compound can be derived from lignin as a reaction product.(5) Alternatively, m-meconin has been used as an internal standard because of its absence of reaction products, but it is not commercially available. Furthermore, extraction procedures are lengthy and may cause incomplete recovery of products because of the differences in partition coefficients of products. In this paper, we report on the improved reproducibility of the nitrobenzene oxidation method by introduction of an alternative internal standard, 5-iodovanillin (la), which does not exist in reaction products. It is done simplifying the extraction steps and using proton nuclear magnetic resonance (H-1-NMR) spectroscopy together with conventional gas chromatography (GC).