Читать книгу Adhesives for Wood and Lignocellulosic Materials - R. N. Kumar - Страница 26

The following methods based on classical organic chemistry, namely, degradation and synthesis, led to the conclusion, already by 1940, that lignin is built up by phenylpropane units [11].

1 Permanganate oxidation (methylated spruce lignin) affords veratric acid (3,4-dimethoxybenzoic acid) and minor amounts of isohemipinic (4,5-dimethoxyisophthalic) acid and dehydrodiveratric acid. The formation of isohemipinic acid supports occurrence of condensed structures (e.g., β-5 or γ-5). See structures 1 to 3 in Figure 1.6.

2 Nitrobenzene oxidation of softwoods in alkali results in the formation of vanillin (4-hydroxy-3-methoxybenzaldehyde). Oxidation of hardwoods and grasses results respectively in syringaldehyde (3,5 dimethoxy-4-hydroxybenzaldehyde) and p-hydroxybenzaldehyde. See structures 4 to 6 in Figure 1.6.

3 Hydrogenolysis yields propylcyclohexane derivatives. See structure 7 in Figure 1.6.

4 Ethanolysis yields so-called Hibbert ketones. See structures 8 to 11 of Figure 1.6.

Figure 1.6 Various degradation products of lignin.