Syringyl alcohol reaction

Bis-4-hydroxy-3,5-dimethoxyphenylmethane. Table I shows, the reference compound, bis-4-hydroxy-3,5-dimethoxyphenylmethane, had an Rf value of 0.41 and was blue when sprayed with the location sprays. Table I also shows that the syringyl alcohol reaction mixture contained a... 

This research was undertaken to study aqueous alkaline reactions of monomeric structures, similar to polymeric wood constituents, from which color would be formed. Since hardwood pulps were of the greatest immediate interest, the reaction of syringyl alcohol, representing the hardwood lignin structure, in aqueous alkaline solution at room temperature has been studied extensively up to the present time, but the reactions of vanillyl alcohol and a-methylvanillyl alcohol, representing the softwood lignin structure, have also been studied to some extent under the same reaction conditions. 

Effect of Oxygen on the Reaction of Syringyl Alcohol with Alkali. Ethanolic solutions of syringyl alcohol (0.0545M in 5 ml.) and sodium hydroxide (0.0923M in 5 ml.) were placed separately in each leg of an inverted Y-tube. After the system was exhaustively evacuated, the solutions were mixed and allowed to react at room temperature. For comparison, an identical experiment was conducted in the presence of air. 

Preparation of Reaction Solutions. In general, the reaction solutions of the aromatic alcohols (syringyl alcohol, vanillyl alcohol, and a-methylvanillyl alcohol and their ethers were prepared by adding aromatic alcohol or ether (usually 2.5 X 10-4 mole) to the solvent (water or ethanol) in a 10-ml. volumetric flask. After the model compound was dissolved, the calculated amount of a sodium hydroxide solution was added to make the reaction solution 1 1 molar (model compound to alkali). The solution was then made up to the 10 ml. mark by adding solvent. These solutions were allowed to react at room temperature for given periods. 

A modification of Sawicki s method (22) was used to help prove that 2,6-dimethoxyquinone was a product of the reaction of syringyl alcohol with aqueous alkali. The coupling of the quinone and 1-ethylquinaldinium iodide to form a dye, measured by spectral absorption in the 725 m/x region, was done as follows. 

Color Characteristics of the Reaction Mixtures. Initial experiments in air involving the reactions of syringyl alcohol, vanillyl alcohol, and a-methylvanillyl alcohol with alkali showed both visually and spectro-photometrically that the reaction mixture of syringyl alcohol, the hardwood lignin model, and alkali was more intensely colored than either of the reaction mixtures of the guaiacyl compounds and alkali. 

The color characteristics of the reaction mixtures in aqueous and ethanolic solutions were also interesting. In ethanol, the mixture of syringyl alcohol and alkali turned pale violet within 15 minutes after preparation, and the color slowly became more intense with time until a deep violet color was formed, which persisted for approximately 2 months after this time, the violet color slowly faded, and the solution became brown. Concomitant with the appearance of the brown color, a brown polymeric substance settled out of solution. The same reaction mixture in aqueous medium turned yellow in about 15 minutes and then slowly changed to... 

Effect of Oxygen on the Reaction of Syringyl Alcohol with Alkali. Preliminary experiments 18) on the reaction of white birchwood meal with alkali in the presence and absence of oxygen indicated that in the presence of oxygen a red-brown color was formed while in its absence (total pressure 1 X 10-4 mm.), the reaction mixture was yellow-brown. 

Products from the Reaction of Syringyl Alcohol with Aqueous Alkali. Table I shows the locations on the paper chromatogram of products from the reaction of syringyl alcohol with alkali in aqueous medium. The chromatograms were developed using a modified FR-L II system (see Experimental), and the Rf values were compared with the Rf values of reference compounds also shown in Table I. 

Table I. Reaction of Syringyl Alcohol with Alkali...

After the reaction product at the Rf value of 0.40 was eluted with water from the chromatogram, its spectrum in neutral solvent and its ionization Ac curve were recorded. The neutral spectrum and the ionization Ac curve of bis-4-hydroxy-3,5-dimethoxyphenylmethane in 47.5% ethanol were also recorded the wavelengths of the ultraviolet absorption maxima of the two compounds—eluted product and reference compound—are given in Table II. The spectra of the compounds possess maxima at nearly identical wavelengths. Thus, paper chromatography and ultraviolet spectroscopy indicate positively that bis-4-hydroxy-3,5-dimethoxyphenylmethane is a product of the reaction of syringyl alcohol with alkali in aqueous solution. 

Dimethoxyquinone. Table I shows that the reference compound, 2,6-dimethoxyquinone, had an Rf value of 0.73, and the color of the spot on the chromatogram was yellow. A product from the reaction mixture of syringyl alcohol and alkali had a similar Rf value and color (Table I). 

Syringaldehyde. Table I shows that the reference compound, syringaldehyde, is located at an Rf value of 0.58 and that one of the products of the reaction of syringyl alcohol with aqueous alkali is also located at the same Rf value. These compounds were located on the chromatograms most easily using a long wave ultraviolet lamp after applying the location sprays, and they appeared as violet spots under these conditions. 

In addition to the products discussed above, Table I shows that there are other reference compounds for which there are no corresponding reaction products. Experimental evidence has indicated that 3,3, 5,5 -tetra-methoxy-4,4 -diphenoquinone (coerulignone), 4,4 -dihydroxy-3,3, 5,5 -tetramethoxystilbene and 2,6-dimethoxyphenol are not products of the reaction of syringyl alcohol with alkali while additional experimental work... 

At present the only colored product that has been identified in the reaction of syringyl alcohol with aqueous alkali is 2,6-dimethoxyquinone. No evidence exists that dimeric chromophoric structures such as a dipheno-quinone or a stilbenequinone, which have been found to be products in the oxidative reactions of phenols in other studies (/, J, I5y 23), are also products of this reaction. However, with the identification of 2,6-dimethoxyquinone and the two colorless products, bis-4-hydroxy-3,5-dimethoxyphenylmethane and syringaldehyde, one of the logical pathways for the reaction is suggested and discussed. 

The same mechanism is proposed for the reaction between syringyl alcohol and aqueous sodium hydroxide to form bis-4-hydroxy-3,5-dimeth-oxyphenylmethane under the conditions used in this study. This mechanism is shown in Scheme I. 

The validity of the reaction presented in Scheme II has been partially proved at this time. When bis-4-hydroxy-3,5-dimethoxyphenylmethane reacts with aqueous alkali under conditions similar to the reaction between syringyl alcohol and aqueous alkali in this study, 2,6-dimethoxyquinone and syringaldehyde are found amTong the reaction products. 

We noted earlier that the color reactions of an ethanolic reaction solution of syringyl alcohol and sodium hydroxide (Figure 1) and also an aqueous reaction solution of the same components are similar to the color reaction noted by Kharasch and Joshi in their study. In other words, the longer wavelength colors of the alkaline reaction solutions are discharged with the formation of a yellow color when the solutions are neutralized. 

Thus, Structure A in Scheme II, which might be representative of unknowns IV-VII, could be responsible for the color changes of the reaction mixture of syringyl alcohol and alkali under varying pH conditions. 

Studies indicate that AHQ retards lignin-like condensation reactions of vanil-lyl and syringyl alcohols [94,105,106] and of an isolated lignin [105], The effect was attributed to AHQ ions transferring electrons to intermediate quinone methide structures that were incapable of fragmenting. The transfer results in an ion radical that can undergo further reactions that lead to destruction of the QM and, thus. 

FIGURE 10.20 Syringyl alcohol/AHQ reduction reaction. (Smith, D.A. and Dimmel, D.R., J Wood Chem Technol, 14, 297, 1994.)... 

DA Smith, DR Dimmel. Electron Transfer Reactions in Pulping Systems (IX) Reactions in Syringyl Alcohol with Typical Pulping Reagents. J Wood Chem Technol 14 297, 1994. 

The quinones and ring cleavage products found in the reaction of unstabilized peroxide with lignin model compounds like a-methyl syringyl alcohol (Figure 12.1 VII) [25] are a result of hydroxyl radical attack [28,30]. 

Typical reaction conditions are shown in Figure 5 for syringyl alcohol as substrate. In the presence of the catalysts in Figure 4, 2,6-dimethoxy-benzoquinone is formed in 71 and 88% yield, respectively. 

Takahama, U. Oxidation of hydroxycinnamic acid and hydroxycinnamyl alcohol derivatives by laccase and peroxidase—interactions among p-hydroxyphenyl, guaiacyl and syringyl groups during the oxidation reactions. Physiol. Plantarum 1995, 93, 61-68. 

Condensation reactions. The nature of phenyl units and reaction conditions has been shown to influence lignin condensation reaction [311-313]. Syringyl nuclei condensed more readily than guaiacyl nuclei with vanillyl alcohol [311]. Yasuda et al. [312] observed the formation of benzyl chloride on treatments of (3-0-4 dimers in hydrochloric acid this would reduce condensation at the benzyl position. They also observed an intramolecular condensation product of a phenylcoumaran type [313]. This intramolecular condensation was shown to be dominant in an 85% formic acid solution [314,315] and was insignificant in 50% aqueous ethanol containing 0.2 M HCl [306]. 

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