Condensation reactions of phenolic resins IV: self-condensation of 2,4-dihydroxymethylphenol and 2,4,6-trihydroxymethylphenol (2)

Abstract

Kinetics of the self-condensation of 2,4dihydroxymethylphenol (2,4-DHMP) and 2,4,6trihydroxymethylphenol (THMP) were investigated to elucidate the mechanisms of the condensation of hydroxymethylphenols (HMPs). Rate equations were derived on the assumptions of the formation of quinone methide intermediates as unimolecular reactions and the occurrence of bimolecular reactions between undissociated HMPs, between undissociated HMP and dissociated HMP, and between dissociated HMPs. Rate constants were determined numerically by comparing the calculated reaction rates with observed ones. The results of analyses are as follows: (1) Both unimolecular and bimolecular reactions occur as the rate-determining steps during the self-condensation of 2,4-DHMP and THMP with low concentrations. (2) Nothing but bimolecular reactions occur as the rate-determining steps during the self-condensation of THMP with high concentrations. (3) Differences in the activation energy and the reaction rate due to the unimolecular process between 2,4-DHMP and THMP are small. (4) Rates of bimolecular reactions of THMP are about five times as large as those of 2,4-DHMP. ( 5) The values of the rate constants and the activation energy for the bimolecular reactions of THMP of low concentrations differ from those of THMP of high concentrations, indicating the difference in reaction mechanisms.

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

“…The kinetics and mechanisms of the individual reactions in the PF system are apparently the key issues and also the foundation for understanding the chemistry of PF. However, by surveying the literature from the middle of the last century to recent years, discrepant results for the kinetics and mechanisms of the base-catalyzed condensation reactions between hydroxymethylphenols (HMPs) have been reported [9,10,11,12,13,14,15,16,17,18,19]. In some earlier studies, the condensations of HMPs were reported to be first-order [9,10,11], pseudo-first-order or second-order [12,13].…”

“…In 2001, Higuchi et al confirmed that the self-condensation of 2-hydroxymethylphenol (2-HMP) is kinetically of genuine first-order and the QM hypothesis seems to hold in such a reaction [ 15 ]. However, in their continuous studies on 4-hydroxymethylphenol (4-HMP) [ 16 ], 2,4-dihydroxymethylphenol (2,4-DHMP) [ 17 ] and 2,4,6-trihydroxymethylphenol (2,4,6-THMP) [ 18 ], the condensations appeared to be of fractional order, ranging from 1.1~2.0. Further, the reaction order varied with the change of the NaOH/HMP molar ratio and with the change of the HMP concentration.…”

Condensation reactions of phenolic resins. VI. Dependence of the molecular association of 2,4,6‐trihydroxymethylphenol on the concentration in an aqueous alkaline medium

“…Considering these results together with previous kinetic works, 9,10 we assume a reaction pathway for the self-condensation of THMP (Fig. 5).…”

“…The behavior of monomeric HMPs as model compounds is easier to study. In previous works, the kinetics of the self-condensations of 2-hydroxymethylphenol, 7 4-hydroxymethylphenol, 8 2,4-dihydroxymethylphenol, and 2,4,6-trihydroxymethylphenol (THMP) 9,10 were investigated. At THMP concentrations greater than 1.5 mol/ L, the reaction order was 2.0, whereas at concentrations below 1.0 mol/L, the reaction order changed to values between 1.2 and 1.6.…”

A 13C-NMR Study on the 1,3-Dimethylolurea-Phenol Co-Condensation Reaction: A Model for Amino-Phenolic Co-Condensed Resin Synthesis

“…Pathway (II) demonstrates the mechanism that forms the intermediates D , E and F for the base-catalyzed phenol-formaldehyde reaction. Among them, the quinine methide is the widely accepted intermediate that is involved in the condensations [40,41,42]. Based on these intermediates, the mechanisms for the UF and PF condensation reactions were proposed in Figure 3.…”