The study of the reactivity of derivatives of hydroxycinnamic alcohol as model compounds of lignin

Abstract

In the framework of method of the B3LYP hybrid density functional and the restricted Hartree-Fock method in the basis of 6-311 (d, p), quantum-chemical calculations of model compounds of lignin, i.e. derivatives of p-hydroxycinnamic alcohol were carried out. The structures and reactivity of coumaric, coniferyl and synapol alcohols were studied. The structural units of lignin contain hydroxyl groups which can be in the plane of benzene ring but can be turned up to 90° with respect to this plane. In the case of methoxy groups which are also present in coniferol and synapol alcohols, the methyl group is turned to 90° with respect to the plane of ring, as the most favorable conformation. Moreover, these compounds contain π,π-conjugations of the aromatic ring with an aliphatic fragment of molecule that affects the geometric characteristics of the molecule. For coumaric and coniferyl alcohols, the Сар–Сα bond length is 1.47 Ǻ. A slight deformation of valence angles for coumaric and synapol alcohols equal to 118.94° and 117.72° respectively instead of 120° at sp2 hybridization indicates the availability of conjugation. The use of a charge as a descriptor of attack selectivity of nucleophilic and electrophilic particles allows us to analized of the reactivity of these acids are given. It is found that the electronic structure of lignin is determined primarily by the charge distribution in its structural phenylpropane unit. In the molecules of all model compounds of lignin, the center for nucleophilic attack is the carbon of aromatic ring (E-ring) with a hydroxyl group, and in the molecule of synapol alcohol, this center is also the carbon of the aromatic ring (E-ring) with a methoxy group. In all three compounds, a center with an increased electron density appears on the Сβ carbon atom.