ORIGINAL PAPER Evaluation of the chemical reactivity in lignin precursors using the Fukui function Carmen Martinez & José L. Rivera & Rafael Herrera & José L. Rico & Nelly Flores & José G. Rutiaga & Pablo López Received: 25 July 2007 / Accepted: 7 November 2007 / Published online: 7 December 2007 # Springer-Verlag 2007 Abstract The hydroxycinnamyl alcohols: p-coumarol, con- iferol and sinapol are considered the basic units and precursors of lignins models. In this work, the specific reactivity of these molecules was studied. We investigate their intrinsic chemical reactivity in terms of the Fukui function, applying the principle of hard and soft acids and bases (HSAB) in the framework of the density functional theory (DFT). Comparisons of their nucleophilic, electrophil- ic and free radical reactivity show their most probably sites to form linkages among them. It is found that the most reactive sites, for reactions involving free radicals, are the carbons at the β-position in the p-coumarol and sinapol molecules, whilst the regions around the carbon-oxygen bond of the phenoxyl group are the most reactive in coniferol. Keywords DFT . Fukui function . Intrinsic Reactivity . Lignin . Monolignols Introduction Lignin is an aromatic heteropolymer that is mainly present in the walls of secondary thickened wood cells; it derives from simple units but contains significant complexity [1]. Typically only three hydroxycinnamyl alcohols (p-coumarol, coniferol and sinapol) are considered in the lignins forma- tion. Those alcohols are named monolignols and are shown in Fig. 1. Lignin polymerization is initiated by an oxidative enzymatic dehydrogenation of monolignols [2], which form five different resonance structures shown in Fig. 2. This leads to a large number of probable coupling reactions, yielding a disordered polymer. The macromolecular structure of lignins could be initiated by different combinations of interatomic linkages between two monolignols, but their relative abundance has not been established [3]. The different lignin linkages are not clearly determined, because the extraction process used to isolate the lignin modifies most of the original bonds. Furthermore, inside wood fibers the coupling reactions between monolignol units are influenced by the environ- ment of the cell wall, leading to a racemic polymer. Lignin is often called the “cementing agent” of woody tissues [4], thus the characterization of its bonds is the first step to understand its cementing behavior and to develop new synthetic lignins. Studies about chemical modification of lignin has been developed to improve polymer-lignin compatibility [5], however the reactivity at atomistic level was not included. Synthesis and characterization of syn- thetic lignin has been developed to study structural aspects in the polymerization [6], again, information of the intrinsic reactivity of lignin precursors was not reported. Relative electron densities proportionate an insight for the evaluation of the sites involved in coupling reactions. Previous results using quantum mechanical calculations have shown that phenoxy radicals have the highest π- electron density at the phenolic oxygen atom [7]. The high electron density on the oxygen atom promotes the forma- tion of aryl ether linkages such as the β-O-4′ linkages, J Mol Model (2008) 14:77–81 DOI 10.1007/s00894-007-0253-0 C. Martinez : N. Flores : J. G. Rutiaga : P. López (*) Wood Technology and Engineering Department, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán 58000, México e-mail: plopez@umich.mx J. L. Rivera : J. L. Rico Chemical Engineering Department, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán 58000, México R. Herrera Chemical and Biology Research Institute, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán 58000, México