The Mechanism of the Alkali catalysed Phenol-Formaldehyde reaction

Abstract

Kinetic studies on the 2,3,4,5-tetramethylphenol(prehnitenol)- and 2,6-xylenol-formaldehyde reactions indicate that the alkali catalysed reaction is not a simple bimolecular reaction. The rate of the 2,6-xylenol-formaldehyde reaction in the presence of excess alkali has been shown to be proportional to [phenoxide]^1.4 [formaldehyde]^1.4 [free alkali]^-0.4. The mechanism of the reaction has been interpreted as a reaction between the phenoxide ion and CH₂=O together with other simultaneous reactions. It is unlikely that the ⁺CH₂OH plays any part in the alkali catalysed reaction. Attempts have been made to interpret the results on the basis that a hemiformal rearranges to a phenol alcohol and that the ion ⁺CH₂OPh reacts with a phenoxide ion. In any case it is not possible to give a complete mechanism with certainty. The degree of formation of hemiformal is too small to be detected by hydrogen ion measurements. When the Cannizzaro reaction of formaldehyde is carried out in the presence of a phenol, the phenoxide Ions catalyse a condensation which 1s presumably an aldol condensation. This reaction having a long induction period and being autocatalytic does not assume importance In the early stages of the reaction. A compound, probably 2,2'-dihydroxy-3,3',4,4',5,5',6,6'-octamethyldiphenylmethane, has been isolated from the alkaline reaction between prehnitenol and formaldehyde.