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.