Generalized Chemical Equilibrium Constant of Formaldehyde Oligomerization

Abstract

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).