Generalized Chemical Equilibrium Constant of Formaldehyde Oligomerization

Formaldehyde reacts with solvents that contain hydroxyl groups (R-OH) in oligomerization reactions to oxymethylene oligomers (R-(OCH₂)_n-OH). The chemical equilibria of these reactions have been studied in the literature for water, for the mono-alcohols methanol, ethanol, and 1-butanol, as well as for the diols ethylene glycol and 1,4-butynediol. In the present work, the collective data were analyzed. It was found that the prolongation of the oxymethylene chains by the addition of formaldehyde can be described very well with a generalized chemical equilibrium constant Kx,n≥2R-OH, which is independent of the substructure (R) of the solvent. This holds for the oligomerization reactions leading to R-(OCH₂)_n-OH with n ≥ 2. The chemical equilibrium constant Kx,1R-OH of the reaction of formaldehyde with the solvent R-OH depends on the solvent, but simple trends are observed. The hypotheses of the existence of a generalized chemical equilibrium constant Kx,n≥2R-OH was tested for the reactions of formaldehyde with ethanol and 1-propanol, for which neither Kx,1R-OH nor Kx,nR-OH was previously available. The corresponding equilibria were studied by ¹³C NMR spectroscopy and the equilibrium constants were determined. A novel method was developed and used in these studies to obtain data on Kx,1R-OH by NMR spectroscopy, which is difficult because of the low amount of molecular formaldehyde. It was found that the generalized equilibrium constant is even valid for the acid-catalyzed formation of poly(oxymethylene) dimethyl ethers (OME). 2020 American Chemical Society.