Theoretical Confirmation of the Quinone Methide Hypothesis for the Condensation Reactions in Phenol-Formaldehyde Resin Synthesis

Abstract

Abstract:The mechanisms for the base-catalyzed condensation reactions in phenol-formaldehyde resin synthesis were investigated by using the density functional theory method. The structures of the intermediates and transition states, as well as the potential energy barriers of the involved reactions, were obtained. The hypothesis of quinine methide (QM) formation was theoretically confirmed. Two mechanisms were identified for QM formation, namely E1cb (elimination unimolecular conjugate base) and water-aided intra-molecular water elimination. The latter is energetically more favorable and is proposed for the first time in this work. Based on the QM mechanism, the condensation should be a unimolecular reaction because the following condensation between an ionized species (dissociated phenol or hydroxymethylphenol) with QM is much faster. The previously proposed S N 2 condensation mechanism was found to be not competitive over the QM mechanism due to a much higher energy barrier. The condensation reaction between neutral phenol or hydroxymethylphenol and QM was also found to be possible. The energy barrier of this reaction is close to or higher than that of QM formation. Therefore, the overall condensation reaction may appear to be bimolecular if such a reaction is incorporated. The theoretical calculations in this work rationalized the discrepant results reported in previous kinetics studies well.

Mechanism of Base-Catalyzed Resorcinol-Formaldehyde and Phenol-Resorcinol-Formaldehyde Condensation Reactions: A Theoretical Study

“…Obviously, the hydrogen bonding effect suppresses the leaving of an OH ion and the relative bonding strength is the key factor that determines the relative reactivity for HMRAs to form quinonemethide intermediates. In our calculations for PF reactions [18], such effect was also found. In addition, the quinonemethides, 2-QM, 4-QM, and 6-QM are different in stability.…”

“…Slower hydroxymethylation is not definitely in accordance with slower condensation reactivity. Theoretical calculations on PF reactions revealed that quinonemethide formation is the rate-determining step in the overall condensation reaction, and formation of para quinonemethide is faster than the formation of ortho quinonemethide [18]. This explained the experimental results that ortho - ortho -methylene linkage was always observed to be minor in comparison with ortho - para - and para - para -methylene linkages.…”

“…Figure 1 shows two mechanisms for base-catalyzed formation of quinonemethide which was derived from that identified for phenol-formaldehyde reactions [18]. Pathway (A) demonstrates the E1cb (unimolecular elimination of conjugate base) mechanism.…”

“…By employing a quantum chemistry method, we recently studied the base-catalyzed phenol-formaldehyde reactions and the quinonemethide (QM) was confirmed to be the key intermediate that initiates condensations [18]. RF condensation reactions may share the same mechanism.…”

“…But, 2-QMA and 4-QMA are anions which may exhibit different reactivity. According to the results for PF reactions [18], formation of quinonemethide is the rate-determining step of the overall condensation reaction. Thus, it can be speculated that formations of RF quinonemethides encounter lower energy barriers.…”