The Addition Mechanisms Of Phenol Toward Formaldehyde:a Theoretical Investigation

Phenolic Resins (PF) are polycondensation products of phenols and aldehydes formed on the catalyst, in particular phenol and formaldehyde. As the first commercial synthetic resin, phenolic resins have been extensively studied for nearly one century. However, conventional phenolic materials from either novolac or resole type resins suffer from the common shortcomings of brittleness and release of volatiles. These shortcomings limit the applications of PF in various fields. To improve the chemical structure of PF resins and decrease the formaldehyde emission, researches in the field of PF resin have never stopped from its first introduction to nowadays. The general theory system for PF resin about reactivity of o-, m-, p-position on benzene ring was built in the last century, but this system was based on the classic organic concepts and experiences. Many details about the reaction mechanisms and potential energy surfaces (PESs) remain unclear. Especially, with the presence of catalyst, the reaction pathways must be different from non-catalytic situation. The simple terms of "addition" and "condensation" are not enough to illustrate the mechanisms of PF resin formation. Some experiments were carried out on the kinetics of some initial P-F reactions and some possible mechanisms were proposed. But up to date, these mechanisms have never been examined by any reliable theoretical calculations.Computational chemistry based on reliable quantum theories can be an alternative choice to study the reaction mechanisms. By using theoretical methods, each single reaction can be investigated on computer. And then, theoretically speaking, the competitive relationships of all the possible reactions can be understood. However, after searching of the literatures, we found that the applications of theoretical methods in the field of resins are rather limited. As a part of the project to understand the mechanisms of PF formation, herein, we present an exploring theoretical study on some initial phenol-formaldehyde (PF) reactions catalyzed by acid and alkali, respectively. The theoretical calculation results might be used to predict the reactivities of phenolic compounds with formaldehyde and thereby provide new insight into the reaction mechanisms. Such information would be useful in developing strategies for formulation and cure of phenolic adhesives. The results involving addition of formaldehyde to phenol both in acid/alkali solution are reported in this paper.The reaction mechanisms of phenol with formaldehyde in first and second addition at ortho-and para-position in acid solution were theoretically investigated at PW91/DNP level with solvent effects included. The reaction of phenol with the protonated methanediol firstly forms an adduct intermediate IM1, via a SN2mechanism with a water molecule as leaving group. From the adduct intermediate, there are two reaction channels involving a proton transfer to form the addition products. One is that a proton directly transfers via a four-member ring transition state with a notable energy barrier (Four-member mechanism). Another mechanism involving a water molecule as catalyst to mediate the proton transfer (WCP mechanism), is a barrierless process, indicating that the formation of the adduct intermediate, the first reaction step, is rate-limiting. The reaction products are free hydroxymethyl phenols and/or hydroxybenzy carbocation (HOC6H4CH2+) which plays an important role in the following formation of methylene and methylene ether linkages. The second addition reactions between formaldehyde and hydroxymethyl phenol at all possible reaction sites of the phenol ring in acid solution were also investigated and discussed.The reaction mechanisms of phenol with formaldehyde in first addition at ortho-and para-position in alkali solution were theoretically investigated at PW91/DNP level with solvent effects included. The reaction of phenol anion with the methanediol firstly forms an adduct intermediate IM2, via a SN2mechanism with a hydroxyl anion as leaving group. From the adduct intermediate, there are also two reaction channels involving a proton transfer to form the addition products. One is that a hydrogen ion of the intermidiate was abstracted by a hydroxyl anion and form mono-methylol phenolate. Another mechanism was involving a water molecule as catalyst to mediate the hydrogen atom transfer (WCP). The reaction products are free hydroxymethyl phenolate and/or o-/p-Quinone methides (o-/p-QMs), which plays an important role in the following formation of methylene and methylene ether linkages. The reactions of phenol anion with the free formaldehyde also involve two reaction channels of a proton transfer to form the addition products. Under alkaline conditions the reaction of phenol anion with free formaldehyde is more favorable than that with methanediol.