Ethylenic Structures in Lignin

Small amounts of ethylenic groups are present in in situ lignin mainly as components of unattached cinnamaldehyde and cinnamyl alcohol end groups (I and 11, respectively, Table 7.3.1) (Lai and Sarkanen 1971). Additionally, the presence of carbonyl-conjugated ethylenic groups as elements of p-quinonoid (Pew and Connors 1971) and o-quinonoid (Harkin 1966) ring systems has been proposed. 

Treatment of lignin with acid as is the situation, for example, in acidolysis (Chap. 6.1) gives rise to stilbene and phenylcoumarone structures (Adler et al. 1966). Stilbenes similar to those identified in acid sulfite pulping liquors have been detected in the steam hydrolyzate of aspen wood (Bardet et al. 1985), and a coniferyl aldehyde structure was formed when a yS-aryl ether-type lignin model compound was heated with slightly acidified water (Kratzl et al. 1959). 

Springer Series in Wood Science Methods in Lignin Chemistry (Edited by S.Y. Lin and C.W. Dence) 

Oxidative treatment of lignin typically results in the conversion of phenolic nuclei to o- and p-quinonoid structures or in the rupture of phenolic rings with concomitant formation of enoic and dienoic acids (Chang and Allan 1971). Quinonoid structures, particularly the ortho type, are prone to undergo condensation reactions in aqueous acidic or alkaline media, forming complex products in which some ethylenic groups may be retained although in an altered structural environment. 

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