ADLER Phenol Oxidation

By means of Fremy salt oxidation, Adler and Lundquist (5) estimated that about half of the phenolic units contain free S-positions in M.W.L. Mikawa (J/), by Mannich reaction, obtained evidence that about 25-40% of the phenolic units in his sulfate lignin preparation contained no substituent in 5-position. In this investigation we also conclude that at least one-third of the phenolic units in our kraft lignin preparation has free 5-positions. 

Uncondensed phenolic units in lignin have been estimated by a color reaction based on oxidation with potassium nitrosodisulfonate (Fremy s salt) (Adler and Lundquist 1961) as illustrated below ... 

To eliminate interfering side reactions caused by units having carbonyl and hydroxyl groups at the a-position, the lignin is reduced with borohydride and alkylated with methanolic hydrochloric acid prior to oxidation to convert these groups to o-quinones. Following a procedure based on the above reaction, spruce milled-wood lignin was found to contain 15-18% of uncondensed phenolic units (Adler and Lundquist 1961). 

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... 

A method developed by Adler et al. (1958) is based on the oxidation of simple phenolic guaiacyl (Adler and Hernestam 1955) or syringyl (Chang et al. 1975) compounds with aqueous sodium periodate solution to ortho-quinone structures in which process nearly lmol of methanol per mol of phenolic hydroxyl group is released [Eq. (1)]. 

Although the periodate oxidation and aminolysis methods provide quantitative data on the frequency with which of phenolic hydroxyl groups occur in lignin, they do not reveal the structural environment in which they occur. Such information may be obtained by other means, e.g., UV spectroscopic or chemical degradation methods (Adler 1977). 

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... 

Phenol itself is only oxidised slowly by periodate (Adler and Magnusson ) but many substituted phenols are oxidised rapidly. Sklarz has reviewed the extensive work done on the stoichiometry of the oxidations of the latter. Few kinetic studies have been carried out. Kaiser and Weidman found second-order kinetics (first with respect to each reactant) for the oxidation of quinol and its monomethyl ether at 15° and 25 °C, i.e. 

E Adler, R Magnusson, B Berggren, and H Thomelius. Periodate Oxidation of Phenols... 

Little is known about the exchange of O18 between quinones and water. This is partly due to the difficulty of handling these compounds and to the ease with which they decompose. In connection with a tracer study on the periodate oxidation of phenol ethers, Adler et al. (1962) found that... 

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