Alkaline hydrolysis of lignin

Another product that is produced commercially from the alkaline hydrolysis of lignin is vanillin. It was found that the yield could be improved by the presence of oxygen during the alkaline hydrolysis reaction. A number of commercial ventures have been based on this procedure (8,9). 

Vanillin is obtained from sulfite waste liquor by further alkaline hydrolysis of lignin. It is the same substance that can be obtained from vanilla bean extract and is the common flavoring in foods and drinks. Natural and synthetic vanillin can be distinguished from each other by a slight difference in the amount of 13C in their structure, since one is biosynthetic in the bean and the other is isolated from a second natural product, wood, by hydrolysis of the lignin. 

Alkaline hydrolysis of lignin increases the number of reactive benzylic hydroxyl groups and may also be important in further depolymerizing the lignin once the oxidative-cleavage reaction has occurred. The formation of a p-electron-withdrawing -CHO substituent on aryl lignin units should increase the rate of hydrolysis of the ether bonds (26). Hydrolysis also forms p-phenylate ions, which then protect the benzaldehyde from further reaction via the Cannizzaro reaction, as mentioned earlier. 

Vanillin (Cgl O ) which is produced firom alkaline hydrolysis of lignin is also a trihalomethane precursor. Both Resorcinol and Vanillin have been investigated as model compoimds to further studies in understanding the role played by aquatic bacteria in the decomposition of complex naturally occurring humic acids (3). 

A most important aroma compound worldwide, vanillin, is obtained primarily by alkaline hydrolysis of lignin (sulfite waste of the wood pulp industry), which yields coniferyl alcohol. It is converted to vanillin by oxidative cleavage ... 

Formaldehyde liberation from y-methylol groups by alkaline cleavage of the p-y C-C bond has been observed, and the action of hot alkali on lignin to form vanillin by cleavage of the a-p C-C bonds is well known. The simultaneous formation of acetaldehyde in the latter case results from a reverse aldol condensation. Traces of guaiacol found after alkaline hydrolysis of wood may result from cleavage of the C-C bond between the a carbon and the ring. 

Alkaline oxidation of lignin is the commercial source of vanillin. This procedure converts a significant portion of the lignin to aromatic fragments that are, for the most part, aromatic aldehydes. The mechanism probably involves two steps, hydrolysis of the aryl ether linkages followed by side chain oxidation. 

WE Collier, TH Eisher, LL Ingram, AL Harris, TP Schultz. Alkaline Hydrolysis of Nonphenohc (3-0-4 Lignin Model Dimers Enrther Studies of the Substituent Effect on the Leaving Phenoxide. Holrforschung 50 420, 1996. 

SA Braddon, CW Dence. Structure and reactivity of chlorolignin. I. Alkaline hydrolysis of chlorine-substituted lignin model compounds. Tappi 51(6) 249-256, 1968. 

The quinone methide can also be generated in situ, at least in aqueous NaOH, directly from the peracetate, as hydrolysis of the phenolic acetate is faster than the benzylic acetate (see an example in Section 12.5.3). This method was used to demonstrate the addition of anthrahydroquinone (AHQ) and anthranol to (actual polymeric) lignin quinone methides in studies elucidating the anthraquinone (AQ)-catalyzed 8-0-4-aryl ether cleavage mechanisms in alkaline pulping.64-66... 

Certain derivatives of the lignin sulfonic acids can be determined directly in water. The nitroso derivatives, which are easily formed in solution, can be determined by differential pulse polarography [438]. Vanillin can be formed by alkaline hydrolysis [439] or alkaline nitrobenzene oxidation [440], extracted into an organic solvent and determined by gas chromatography. 

When radioactive lignin precursors are applied to resistant host plants infected with an avirulent pathogen, the autoradiographic localization of radioactivity in resistant reacting host cells may help to corroborate the participation of lignification in the resistance response. Thorough extraction of non-polymerized precursor with organic solvents and the removal of esterified phenolics by alkaline hydrolysis are important steps in these experiments (25,28,30,31). 

Alkaline hydrolysis, as compared to other methods of lignin degradation, has certain advantages. The catalytic reagents are inexpensive and available commercially, and require no elaborate method of preparation. 

This study has again demonstrated the effectiveness of alkaline hydrolysis as a prime method of lignin degradation to readily identifiable phenolic products. 

If, as a criterion of value to the study of lignin depolymerization by alkaline hydrolysis, the maximum yield of oxygen-bearing, phenylpropanoid derivatives is chosen, then the conditions of such a study have been optimized here at a treatment severity corresponding to a reaction temperature of 300°C for 1 hour. Under these conditions, 20% of the lignin is recovered as ether-solubles of which 55% is identifiable as monomeric derivatives. The rest of this material probably consists of dimeric-type compounds not identified by capillary gas chromatography. 

Under alkaline conditions isolating lignin degradation products which are essentially of a phenyl.ethyl rather than a phenylpropyl nature is structurally important and requires a lignin structure by which the 7-carbon may be removed as a result of a 0-7 carbon-carbon cleavage reaction, either by direct alkaline hydrolysis or alkali-catalyzed hydrogenolysis. 

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