Kinetic Study of the Reversible Formation of Cyclic Hemiacetals from 2-(Hydroxymethyl)benzaldehyde and 2-(β-Hydroxyethyl)benzaldehyde

Article abstract of DOI:10.1021/jo00318a015

The title aldehydes 1 and 3 exist in equilibrium with cyclic hemiacetal forms 2 and 4, respectively, with equilibrium constants in water of 6.7 (1<*>2) and 20 (3<*>4).These equilibria are displaced toward the hemiacetals in strongly basic solutions because of their ionization.Acidity constants have been determined to be 12.29 (2) and 12.38 (4).The kinetics of the equilibration have been studied in the pH range 1-8 in carboxylic acid and alkylphosphonic acid buffers.The base forms (RCOO^- and RPO3^2-) produce a common Bronsted line, while the acid forms produce two distinct Bronsted relations, with that for RHPO3^- being displaced ab out one logarithmic unit above that for RCOOH.It is concluded that the former are not acting as bifunctional catalysts but owe their enhanced reactivity to an electrostatic effect.It is, moreover, suggested that the acid-catalyzed equilibrations are proceeding by way of two competing mechanisms, one involving as an intermediate the protonated hemiacetal and the other the protonated benzaldehyde.Several acetals related to 2 and 4 have also been prepared, and the kinetics and mechanism of their hydrolyses are discussed.