CONTRIBUTION OF LIGNIN SUB-STRUCTURES TO NITROBENZENE OXIDATION-PRODUCTS

Nitrobenzene oxidation of a series of lignin model compounds with varying aromatic nuclei and sidechain structures, including diphenylmethane compounds, gave benzaldehydes and benzoic acids in a range of yields. The yields were dependent on the structure of the aromatic nucleus, and were in the order syringyl > guaiacyl > 4-hydroxyphenyl. The yields were also dependent on the benzylic group, in the order C=O > CHOH > CH2. Veratryl compounds afforded low yields of oxidation products. Lignin model 1-1'-, 1,5'-and 5,5'-diphenylmethanes gave rise to moderate yields of nitrobenzene oxidation products. However, 1,6'-diphenylmethanes gave only low yields of oxidation products, and no oxidation products were obtained from a nonphenolic 1,1'-diphenylmethane. This suggests that the assumption in the nucleus exchange-nitrobenzene oxidation method for determining lignin structures, that diphenylmethanes do not give rise to oxidation products, should be reappraised. It is proposed that a key influence on the yield of nitrobenzene oxidation products is the ability of the aromatic ring to supply electrons to the benzylic carbon atoms. Alternatively, condensations at the ortho positions of the guaiacyl and 4-hydroxyphenyl compounds could affect the yield of oxidation products.