Reaction of Formaldehyde with Phenols: A Computational Chemistry Study

Phenolic resins are important adhesives used by the forest products industry. The phenolic compounds in these resins are derived primarily from petrochemical sources. Alternate sources of phenolic compounds include tannins, lignins, biomass pyrolysis products, and coal gasification products. Because of variations in their chemical structures, the reactivities of these phenolic compounds with formaldehyde vary in quite subtle ways. A method is needed for predicting the reactivity of phenolic compounds with formaldehyde in order to allow researchers to efficiently choose those compounds that might make the best candidates for new adhesive systems prior to conducting extensive laboratory trials. Computational chemistry has been used to study the relationship between the reactivity of a number of phenolic compounds with formaldehyde in an aqueous, alkaline system, and charges calculated for reactive sites on the aromatic ring of the phenolic compound. Atomic-charges for each phenolic compound were calculated by ab initio methods at the RHF/6-31 +G level of theory using the ChelpG method. Reaction rate constants were determined from measurements of the concentrations of the phenolic compounds and formaldehyde as functions of time. The reaction rate constants varied over a wide range (approx. 10 -2 to 10 4 L mol -1 hr. -1 ). An estimate of the reactivity per reactive site on the phenolic ring was determined by dividing the rate constant by the number of reactive sites. The charge per reactive site was estimated by summing the charges at all the reactive sites on the phenolic ring and dividing by the number of reactive sites. A strong correlation was observed between the reactivity per reactive site and the average charge per reactive site. Introduction In order to utilize phenolic adhesive systems more effectively and to develop new phenolic adhesives, it is important to understand the reactions of phenolic compounds with formaldehyde. To date, analytical studies on phenolic adhesives have concentrated mainly on kinetic studies (1-5). These studies involve not only the calculation of reaction rates but also complex processes for isolation and identification of intermediates, as well as reaction products. Recently, newer computational chemistry methods have been introduced that allow analysis of reaction mechanisms and prediction of the reactivities of chemical starting materials. Therefore, computational chemistry 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. This insight would also serve to decrease the time needed for development of new adhesive systems. Sprung (6) investigated the reactions of a series of methylphenols with formaldehyde. His kinetic measurements were based solely on the rate of disappearance of formaldehyde. As expected, differences in the reactivities of this series of phenolic compounds depended on subtle differences in chemical structure. Conner (7) demonstrated that the relative rates of these reactions could be correlated with electrostatic charges Session 2B: General • 147 at reactive positions in the phenolic-ring calculated using ab initio methods. Because of limitations on the analytical instruments in use at the time Sprung conducted his study, it was not clear whether either formaldehyde or the phenolic compounds were undergoing reactions other than those involved in hydroxymethylation. Moreover, Sprung's kinetic data were collected in non-aqueous systems rather than in the aqueous-based systems typically encountered in industrial applications of phenolic adhesives. Because of these limitations on the earlier work and the industrial significance of phenol-formaldehyde adhesives, we have employed aqueous-based systems to investigate the reactions of formaldehyde with a larger series of phenolic compounds (Table 1). These phenolic compounds included most of the phenols investigated by Sprung.