Determination of Total and Aliphatic Hydroxyl Groups

Abstract

As discussed in Chapter 1.1, lignins are produced in vivo by an enzyme-initiated dehydrogenative polymerization of three monomeric precursors, p-hydroxycinnamyl alcohol (1), coniferyl alcohol (2) and sinapyl alcohol (3). These compounds all have a terminal aliphatic hydroxyl group at C-γ on the side chain in addition to a phenolic hydroxyl group at C-4 of the aromatic ring. In contrast to the phenolic hydroxyl group, approximately only 20–25 mol% of the aliphatic hydroxyl groups of the monomeric precursors is involved in the lignification process based on results from functional group analyses of lignins and investigations on the in vitro dehydrogenative polymerization of lignin (Freudenberg 1965, Freudenberg and Neish 1968, Lai and Sarkanen 1971, Adler 1977). This statement implies that lignins in wood tissue should contain a minimum total aliphatic hydroxyl content of approximately 0.75/C9 unit. Indeed, as shown by the values in Table 7.1.1, milled wood and bamboo lignins (MWL and MBL) have total aliphatic hydroxyl contents of more than 1.1 mol/C9 unit in contrast to total phenolic hydroxyl contents which are in the 0.2–0.4 mol/C9 unit range (Freudenberg et al. 1964, Robert and Brunow 1984, Pan et al. 1990, Tai et al. 1990). Moreover, the total phenolic hydroxyl content of lignins in wood is usually less than 0.1mol/C9 unit (Yang and Goring 1980, Winston et al. 1986).