Lignin methoxyl group determination

Estimating the reactivity of various aromatic nuclei in lignin involves studies to determine the protodedeuteration rates of certain selectively deuterated model compounds. The combined effects of hydroxyl, methoxyl, and side-chain substituents are approximately additive in guaiacyl models while syringyl models show significant deviation from the predicted behavior. The reactivity of the position meta to methoxyl groups is higher while those of the ortho and para positions are lower than anticipated on the basis of combined substituent effects. 

Partial Rate Factors for Protodedeuteration in Perchloric Acid Solution. All lignin model compounds studied contained hydroxyl, methoxyl, and methyl groups as the only substituent groups. Table II summarizes what the authors believe are the most reliable partial rate factors for these substituents in perchloric acid solution. All partial rate factors are based on determinations in 57% perchloric acid solutions. The partial rate factors for ortho and para positions of methyl and methoxyl groups are based on later work by Suzuki (21). The partial rate factors for phenol were calculated from data on 2, 4, 6-trideuterophenol (6) using the value 9.1 X 10-5 hr.-1 for monodeuterobenzene by Suzuki (21) and assuming 1 3.2 ortho to para ratio. 

In a related approach, Ni et al (1990) have developed a procedure for determining lignin in softwood kraft pulp based on the conversion of methoxyl groups in the lignin to methanol when the pulp is chlorinated under a specified... 

Note 6. See Chapter 7.6.3 for determination of methoxyl groups in lignins. 

Note 6. See Chapter 7.6.3 for determination of methoxyl groups in lignins. Note 7. Although the reaction is conducted under nitrogen atmosphere by passing nitrogen gas through the reaction mixture, a small amount of... 

Methoxyl. Methoxyl groups are determined by a modified method (53). Methyl iodide is formed by hydrolysis of the methoxyl groups of wood lignin in hydriodic acid and is distilled under CO2 into a solution of bromine and potassium acetate in glacial acetic acid. Bromine oxidizes iodide to iodate which is then titrated with standard thiosulfate. The method is difficult and time-consuming, and some experience is necessary before satisfactory results can be obtained. Details are in ASTM Standard D 1166 and Tappi Standard T 209 (withdrawn in November 1979). Additional discussion can be found in Reference 54. 

C-L Chen. Determination of methoxyl groups. In SY Lin, CW Dence, eds. Methods in Lignin Chemistry. Berlin Springer-Verlag, 1992, pp. 465 72. 

The aliphatic hydroxyl content of a lignin expressed on a methoxyl content and G> unit basis can be obtained by subtracting the phenolic hydroxyl content reported on a methoxyl or C9 basis from the corresponding total hydroxyl value (see Chap. 7.2 for determination of phenolic hydroxyl groups). 

Chemical methods for analysis of phenolic hydroxyl groups include determination of the increase in methoxyl content resulting from diazomethane methylation (Bjorkman and Person 1957), the increase in phenolic acetyl group content after acetylation (Lenz 1968, Mansson 1983), and low-molecular weight compounds derived from the degradation of phenolic structures (Chap. 5.2). The phenolic acetyl group of acetylated lignin may also be determined by an NMR spectroscopic technique (Lenz 1968, Robert et al. 1986) or by a selective deacetylation in pyrrolidine (aminolysis) (Mansson 1983). 

---