H-bond catalytic mechanism of aromatic electrophilic substitution between phenol and formaldehyde

摘要

Phenolic resins are widely used as moulding plastics, coatings, industrial bondings and ablative materials. An essential reaction step for preparing phenolic resins is aromatic electrophilic substitution. However, the current understanding of this reaction mechanism is kind of too general, making them not able to explain some phenomena such as highly exothermic characteristic of the early reaction stage between phenol and aldehyde under acid catalysis, and how attacking species form when the reaction begins. We studied the detailed catalytic mechanism by quantum mechanics. A new hydrogen-bond (H-bond) catalytic mechanism is suggested. It reveals that the essence of the acid catalytic mechanism is electrophile activation through hydrogen-bonding, and the substrate-catalyst complex formed through an H-bond is the attacking species. Besides, the study discloses that oxalate anion takes part in the reaction through bonding with arenium ion. The thermodynamic and kinetic characters based on this mechanism are quite different from the current theories. (C) 2020 Elsevier Ltd. All rights reserved.