Some preliminary investigations of the phenolformaldehyde reaction

Abstract

AbstractThe reaction between phenol and formaldehyde, takes place us a series of step‐wise reactions which are catalysed by both acids and bases and which proceed under neutral conditions at sufficiently high temperatures. Two types of reaction take place, namely the reaction of formaldehyde with phenol or a phenolic molecule, to form phenolic alcohols containing reacting methylol groups, and the subsequent condensation of these phenolic alcohols with phenol or a phenolic molecule to form larger molecules with elimination of water. Two studies of the initial portion of the reaction between phenol and formaldehyde in aqueous media are described, namely the Kinetics and the Thermochemistry.Kinetic studies show that a linear relationship exists between antilog (—pH) and the initial rate at which the formaldehyde reacts using HCl as catalyst between pH values of 1·3 and 3 at 100° and 80°. Catalysis by different acids at the same pH value gives rise to similar rates of reaction especially when pH and rate of reaction are measured at the same temperature. This points to catalysis by the hydrogen ion. The second order rate law is best obeyed by all the curves relating to formaldehyde reacted and time at various pH values, at 100° and 80° using HCl as catalyst. A decrease in temperature to 30° causes the rate law to approach first order. A theoretical enquiry shows that a decrease in order of reaction could be explained by the formation of dialcohols, which formation of dialcohols seems more probable at low temperature. In alkaline media, dialcohols also seem to be produced more readily, us indicated by the fact that a first order rate law is obeyed until about 45% of the formaldehyde has reacted.Thermochemical studies show that the heat evolved during the phenolformaldehyde reaction may also be regarded as derived from the reaction of formaldehyde with a phenolic molecule, and the subsequent elimination of water by condensation. The heats evolved per mole of formaldehyde reacted and water evolved are 4·1 and 16·9 kilocalories respectively. These values are confirmed by combining heats of combustion and heats of mixing data.