Kinetics and mechanisms of the condensation reactions of phenolic resins II. Base-catalyzed self-condensation of 4-hydroxymethylphenol

Kinetics of the base-catalyzed self-condensation of 4-hydroxymethylphenol ( 4-HMP) was investigated for the purpose of clarifying the mechanisms of the phenolic resin curing. In order to determine the order of reaction, the de pendence of the initial rate of reaction on the initial concentration of th e reactant was closely investigated under a wide range of reaction conditio ns, It was found that the order of reaction was a-round 1.3. An analysis re vealed that unimolecular- and bimolecular reactions occur simultaneously as the rate-determining steps. This is different from the case of the self-co ndensation of 2-hydroxymethylphenol (2-HMP) where the order of reaction was found to be 1.0 that means only unimolecular reactions take place as the r ate-determining step. The rate of reaction due to the unimolecular process could be explained quantitatively by the rate equation based on a quinone m ethide intermediate hypothesis. The rate of reaction due to the bimolecular process could be explained quantitatively by the rate equation that assume d the following three types of reactions: reactions occurring (1) between t wo undissociated 4-HMP molecules, (2) between an undissociated 4-HMP molecu le and a dissociated 4-HMP molecule, and (3) between two dissociated 4-HMP molecules. As to the bimolecular reactions, the authors suggest that the S( N)2 type mechanism in which the negatively charged 2- or 4-position of the phenolate ion attacks the methylol carbon of the other molecule is insuffic ient to explain the kinetics.