Contributions to the Reaction Kinetic Investigation of Acid-induced Aniline-Formaldehyde Condensation

The acid-catalyzed condensation of aniline with formaldehyde was investigated under conditions typical of industrial operation. Online-flow and conventional tube 1H and 13C NMR (DEPT) methods were evaluated and shown to provide richer information on the structures and concentrations of reaction intermediates than standard chromatographic analyses, while also enabling estimates of average molecular weight and conversion. Several major intermediates of previously unknown structure were detected, which limits the applicability of the widely used Wagner mechanism as a basis for simulation models. The kinetics of a key sub-reaction forming N-methylated products (Ploechl methylation) were studied using 2,4,6-trimethylaniline as a model substrate. In aqueous medium, the rate was first order in aniline, second order in formaldehyde, and zero order in hydrochloric acid, whereas in aniline medium it was first order in hydrochloric acid, second order in formaldehyde, and zero order in water. Rate constants and an activation energy are reported. A mechanistic interpretation is proposed in which unprotonated aniline reversibly forms a Schiff base with one formaldehyde molecule, followed by an acid-catalyzed reduction involving a second formaldehyde molecule. The Schiff base is identified as a stable intermediate and was isolated preparatively, and the reduction step is inferred to be rate-limiting in the studied pH range. The observed mono-/dimethylation product ratio is attributed to additional contributions from the Leuckart–Wallach reaction and catalytic methyl-group transfer.