Cupric oxide oxidation of lignin

The cupric oxidation of lignins is optimally performed by heating a mixture of lignin preparation, cupric oxide, and sodium hydroxide m an approximate weight ratio of 1 6 2 9 m an autoclave at 170°C for 2-5h, the concentration of sodium hydroxide being 1 5-2 M (Pearl and Beyer 1950, Pepper et al 1967) In the laboratory, cupric oxide oxidation of lignins is usually conducted according to the procedure of Pepper and co-workers (1967) The constituents of the oxidation mixture are qualitatively and quantitatively determined as described for the compounds m nitrobenzene oxidation mixtures... 

Nitrobenzene and Cupric Oxide Oxidations of Lignins in Grass Tissues... 

As stated above, nitrobenzene and cupric oxide oxidations of lignins are simple but by no means perfect methods for the characterization of lignins However, the methods are effective in the characterization of lignins if they are... 

Biphenyl coupling occurs to some extent on cupric oxide oxidation of lignin model compounds [55,56]. This indicates that the value derived from permanganate oxidation studies is slightly too high. On the basis C NMR examinations, Drumond et al. [57] arrived at a higher value for the frequency of biphenyl units (24-26%). The accuracy of this estimate can be questioned, since no separate signals from biphenyl units can be discerned in the spectra. 

Alkaline cupric oxidation of coals produced phenolic acids (p-hydroxy and 3,4 hydroxy-benzoic acids, 4 hydroxy l,2 and 4-hydroxy l,3 benzenedicarboxylic acids), which are known as characteristic lignin oxidation products. The results indicate that lignin-like polymers are incorporated into macromolecules of coals and still are identifiable in lower rank coals. 

Kogel, I. and Bochter, R. (1985). Characterization of lignin in forest humus layers by high-performance liquid chromatography of cupric oxide oxidation products. Soil Biology and Biochemistry 17 637-640. 

Fusion with Alkali and Cupric Oxide in Nonaqueous Solvents. Alkali lignin was fused with potassium hydroxide and cupric oxide in methanol under conditions suggested by Tiemann (20) and in n-amyl alcohol as suggested by Klages (4). These procedures were very effective in earlier model compound studies in our laboratories (12). Ether extracts obtained were less than those from corresponding experiments in aqueous solution, and qualitative compositions were essentially the same. In the case of the amyl alcohol experiments, artifacts with the cupric oxide were obtained. Again, experiments were conducted under more dilute conditions in a bomb under superatmospheric conditions, but results were no better. 

Hedges, J.I. and Ertel, J.R. (1982) Characterization of lignin by gas capillary chromatography of cupric oxide oxidation products. Anal. Chem. 54, 174—178. 

Fig. 6.2.1. Nitrobenzene and cupric oxide oxidation products of lignins...

Table 6.2.5. Conversion factors (C) and HPLC retention times for the major saponification, nitrobenzene, and cupric oxide oxidation products of lignins...

As mentioned previously (Chap. 2.2.2.4.1), the major purpose of the GC-MS analysis of a nitrobenzene or cupric oxide oxidation mixture is to verify the identity of the oxidation products established previously by GC or HPLC analysis, and to elucidate the structure of unknown constituents. For example, GC-MS analysis of the nitrobenzene oxidation mixture of milled bamboo lignin from Phyllostachys pubescence showed unequivocally that compounds (l)-(3) in the total ion chromatogram of the oxidation mixture (Fig. 6.2.2) are indeed p-hydroxybenzaldehyde, vanillin, and syringaldehyde, respectively (Tai et al. 1990) (see Chap. 9.1 for a discussion of the GC-MS technique). In addition, the unknown compound in the chromatogram was identified as p-hydroxyazobenzene (15) (Fig. 6.2.1), one of the phenolic reduction products of nitrobenzene. 

For the characterization of lignins, nitrobenzene oxidation has the following advantages over cupric oxide oxidation (a) the constituents of the oxida-... 

Pepper JM, Casselman BW, Karapally JC (1967) Lignin oxidation Preferential use of cupric oxide Can J Chem 45 3009-3012... 

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