Investigation of the reaction of 1,3-dimethylurea with formaldehyde by quantitative on-line NMR spectroscopy: a model for the urea-formaldehyde system

Quantitative on-line NMR spectroscopy is applied to study equilibria and reaction kinetics of the reaction of formaldehyde with 1,3-dimethylurea. This reaction system serves as a model system for the much more complex but industrially relevant urea-formaldehyde system. The aim is to study individual reactions and intermediates. The 1,3-dimethylurea-formaldehyde system undergoes only four reactions and, unlike urea-formaldehyde, does not form polymers. The following reactions are studied in detail: (1) the hydroxymethylation, (2) the formation of hemiformals of the hydroxymethylated intermediate, and (3) two condensation reactions of which the first leads to methylene bridges, the other to ether bridges. NMR spectroscopic chemical shift data of the reacting species are provided for the H-1, C-13, and N-15 domains. Equilibrium data of reactions (1), (2), and (3) are determined by quantitative H-1 and C-13 NMR spectroscopy at molar ratios of formaldehyde to 1,3-dimethylurea between 1:2 and 16:1 at a pH value of 8.5. Reaction kinetic experiments using an NMR spectrometer coupled to a batch reactor led to a reaction kinetic model parametrized with true species concentrations. The model takes into account reactions (1), (2), and (3). It describes the reaction system well for molar ratios of 1:1, 2:1, and 4: 1, temperatures of 303 to 333 K, and pH values from 5.0 to 9.5. Dilution experiments with a micro mixer coupled to the NMR spectrometer are conducted to estimate the time to equilibrium of reaction (2) of which the time constant is significantly lower than those of reactions (1) and (3). Copyright (C) 2015 John Wiley & Sons, Ltd.