28 THE PLANT CELL WALL 



When lignins are subjected to pyrolysis, eugenol, /soeugenol, 

 //-propylguaiacol, o- and /7-cresol, guaiacol and catechol are in- 

 cluded among the representative products. 



Hydrogenolysis and other reductive processes tend to yield 

 phenylpropanes with various side chain oxygen functions. 



Alkali fusion yields acetic, oxalic, and protocatechuic (3,4-di- 

 hydrobenzoic) acids. 



From methylated lignin treated with alkali and subsequently 

 permanganate, veratric (3, 4-dimethoxy benzoic) acid, isohemipinic 

 (3,4-dimethoxy-5-carboxybenzoic) acid, and gallic (3,4,5-trihydro- 

 xybenzoic) acid are obtained. 



One of the well studied degradative procedures is alkaline 

 nitrobenzene oxidation. This treatment yields vanillin (3-methoxy- 

 4-hydroxybenzaldehyde) from sprucewood. The corresponding 

 acid, vanillic acid, and its 5-carboxylic derivatives are produced 

 by oxidation of the aldehyde. Other aldehydes formed during 

 nitrobenzene oxidation are /7-hydroxybenzaldehyde and syringal- 

 dehyde (3,5-dimethoxy-4-hydroxybenzaldehyde). The latter occurs 

 only in angiosperm lignins, not in conifers. 



Mercaptolysis with thioglycolic acid and sulfonation of lignins 

 yields solubilized acidic products containing, respectively the 

 — S— CH 2 — COOH and — SO3H moieties. On mercaptolysis, 

 each mole of thioglycolate taken up is accompanied by the appear- 

 ance of a phenolic hydroxyl. Sulfonation, although more complex 

 includes a similar phenomenon. 



Other oxidation products of lignins include acidic benzene 

 derivatives with 1,2,4,5-tetracarboxylic, pentacarboxylic, or hexa- 

 carboxylic (mellitic) acid patterns. 



The aldehydes formed on nitrobenzene oxidation vary consider- 

 ably in amount from species to species. Hydroxybenzaldehyde 

 ranges from less than 1 per cent to as much as 10 per cent of the 

 total lignin and vanillin from 17-25 per cent. The syringaldehyde 

 content of hardwoods ranges from 2-13 per cent. These aldehydes 

 may comprise in toto as much as 40 per cent of the overall weight 

 of lignin samples, or as little as 20 per cent. 



The information carried by this mass of analytical data is 

 truly impressive. It is without doubt a key to many of the problems 



