Estimation of Phenolic Hydroxyl Groups

Another factor which may influence the yield of acids obtained from lignin in wood is the presence of easily hydrolyzable aryl ether linkages such as those represented by phenylpropane-a-aryl ether structures In phenolic units, such structures are easily cleaved even under very mild alkaline conditions (Adler et al 1968) Therefore, it can be assumed that, in the alkylation step of the oxidative degradation procedure, a partial hydrolysis of such structures may occur, thereby creating new phenolic end groups that may affect the analysis 

Native, as well as technical lignins, whether present in pulp (wood) or in the form of isolated samples, usually contain trace amounts of catechol structures These can be identified and quantified if the alkylation step in the oxidative degradation procedure is performed with diethyl sulfate since, in this case, various types of catechol units in the form of 1,2-diethoxybenzene derivatives are distinguished from guaiacyl and synngyl structures (Gellerstedt and Lindfors 

Erickson M, Miksche GE (1974c) Charaktensierung der Lignine von Pteridophyten durch oxidativen Abbau Holzforschung 28 157-159 

Erickson M, Miksche GE (1974d) On the occurrence of lignin or polyphenols in some mosses and liverworts Phytochemistry 13 2295-2299 

Erickson M, Larsson S, Miksche GE (1973a) Gaschromatografische Analyse von 

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Adler E (1977) Lignin chemistry - past, present, and future Wood Sci Technol 11 169-218 Adler E, Hernestam S (1955) Estimation of phenolic hydroxyl groups in lignin I Periodate oxidation of guaiacol compounds Acta Chem Scand 9 319-334 Adler E, Hernestam S, Wallden I (1958) Estimation of phenolic hydroxyl groups in lignin Sven Papperstidn 61 64-647... 

The phenolic hydroxyl and etherified benzyl alcohol group can be selectively oxidized by periodate [385] and 2,3-dichloro-5,6-dicyano-l, 4-benzoquinone (DDQ) [386], respectively. The periodate oxidation of guaiacyl and syringyl nuclei yields 0-quinones plus methanol and has been used to estimate the phenolic hydroxyl group content of lignin. DDQ oxidation has been applied to estimate the content of lignin-carbohydrate linkages [28]. 

RC Francis, Y-Z Lai, CW Dence, TC Alexander. Estimating the Concentration of Phenolic Hydroxyl Groups in Wood Pulps. Tappi J 74(9) 219-224, 1991. 

Chatterjee observed stepwise neutralization of the hydroxyl groups in straight-chain polynuclear phenolic compounds, containing up to nine phenolic nuclei, and concludes that conductometric titration, in a suitable nonpolar solvent, provides an excellent method for the quantitative estimation of phenolic OH groups in such compounds. 

Kulai A. Kini I am surprised at the absence of phenolic hydroxyl in kerogen. What method was used to estimate phenolic groups ... 

Cyanuric fluoride has been used to modify tyrosine residues, substituting the phenolic hydroxyl group. A maximum of 3 residues in RNase was found to react at pH 10.9 and 25° (148a). However, some mystery surrounds this number, as with other estimates of accessibility, since alkaline-denatured material where all tyrosine residues are available still showed the reaction of only 3 residues with cyanuric fluoride. However, similar observations have been made on iodination in 8 Af urea (11 )- At pH 9.3, Takenaka et al. (149) found that only 2 residues reacted and that 115 was not one of them. Two more reacted after alkali denaturation. Two were resistant under all conditions tested. No enzymic activity data were reported. 

The oxidation of substituted phenols illustrates the importance of including speciation. Dissociation of the phenolic hydroxyl group results in an equilibrium mixture of the parent compound and its dissociated form, the phenoxide (or phenolate) anion. The undissociated phenol and the phenoxide anion react as independent species with very different rate constants, designated kArOU and kAr0. For the oxidation of 4-nitrophenol (pKa = 7.2) by C102, 1(1,0,11 = 1.4 x 10 1 M 1 s, andkArCr = 4.0 x 103 M 1 s 1 (42). Estimates of the pH-corrected second-order rate constant, kM, can be made using... 

Figure 6. Comparison of yields of oxygen in pyrolytic water with estimated content of oxygen in phenolic hydroxyl groups in raw coal. (-)...

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