Degradation of Lignin with Ozone: Reactivity of Lignin Model Compounds Toward Ozone

In order to elucidate the reactivities of different types of lignin-structural units toward ozone, various lignin model compounds were ozonized. The results obtained are summarized as follows: 1) An α-carbonyl type structure is much more stable against ozone than a benzyl alcohol type structure. 2) A guaiacyl nucleus reacts faster with ozone than a vera try 1 one. 3) Biphenyl and phenylcoumaran structures react readily with ozone. However, a biphenyl structure of veratryl nuclei is very stable against ozone. 4) From the results of molecular orbital calculations, Pz orbitals (LUMO) of ozone form bonding orbitals to those (HOMO) of lignin aromatic nucleus at C3 and C4 positions. Furthermore, the squares of LCAO coefficients of C3 and C4 positions are greater than those of other positions in Pz orbital (HOMO) of lignin aromatic nucleus. This suggests that ozone reacts selectively with the lignin aromatic nucleus at C3-C4 position.