Reactivity of lignin subunits: the influence of dehydrogenation and formation of dimeric structures

Lignin is one of the most abundant natural materials around the world, accounting for about a quarter of the woody tissue. In general, it is well known that these highly branched aromatic bio-polymers are formed from the polymerization of p-coumaryl, coniferyl, and sinapyl alcohols; however, the connection between these structures are still not known in detail. In this work, we have employed electronic structure calculations to investigate local reactivities and details regarding the connectivity between the basic structures of lignin (unmodified mono and dilignols as well as dehydrogenated monolignols). Condensed-to-atoms Fukui indexes, local softness and hard and soft acids and bases principle were employed in the analyses. The results allow identifying reactive sites on the lignin subunits and access details on the synthesis and degradation of this bio-material. In particular, it is possible to identify a strong influence of the dehydrogenation and monomer dimerization on the monolignols reactivities, which activate the O-C4 and C5 positions. Graphical Abstract The local reactivities of lignin subunits were evaluated via DFT calculations.