Quantitative NMR spectroscopy of binary liquid mixtures (aldehyde + alcohol). Part II: (Propanal or butanal or heptanal) + (methanol or ethanol or 1-propanol)

•Formation of hemiacetal/poly(oxymethylene) hemiacetals in liquid binary mixtures.•Aldehyde (1-propanal or 1-butanal or 1-heptanal) and alcohol (methanol or ethanol or 1-propanol).•Quantitative 13C NMR spectroscopy at temperatures between (255 and 295) K.•High conversion rate to hemiacetals.•(1-Propanal + 1-propanol (50 + 50)) at 273 K: mole fraction of hemiacetal (polymers) ≈55% (≈6%).

The chemical reactions of aldehydes with alcohols to (hemiacetals and poly(oxymethylene) hemiacetals) have an essential influence on the thermodynamic properties and related phenomena like, for example, the vapor + liquid phase equilibrium of such liquid mixtures. This is well known in the literature for systems such as, for example, formaldehyde and methanol. Experimental information on the chemical reaction equilibria in mixtures with aldehydes other than formaldehyde and alcohols is extremely scarce. Therefore, in the first part of this series, quantitative NMR spectroscopy was used to investigate the chemical reaction equilibrium in binary liquid mixtures of acetaldehyde and an alcohol (methanol or ethanol or 1-propanol) at temperatures between (255 and 295) K. That work is here extended to three other aldehydes, viz. (1-propanal, 1-butanal and 1-heptanal). The results confirm the expectations from the first part of this series, i.e., that the majority of the constituents of the mixture is present as hemiacetal and the first two poly(oxymethylene) hemiacetals. For example, in an equimolar liquid mixture of {1-heptanal + methanol (or + ethanol or + 1-propanol)} at T = 273 K about 88% (or 81% for both other alcohols) of the aldehyde is bound to hemiacetal and the first two poly(oxymethylene) hemiacetals, i.e., the conversion rates are nearly the same as in the previous investigations with acetaldehyde instead of 1-heptanal. In the series investigated of combinations of aldehydes and alcohols, the particular aldehyde has only a small influence on the conversion rate. In the series of alcohols investigated only methanol has a somewhat larger influence whereas the results (speciation and conversion) for ethanol and 1-propanol are very similar. The NMR-spectroscopic results were also evaluated to determine the mole-fraction based chemical reaction equilibrium constants for the formation of the hemiacetals and the first two poly(oxymethylene) hemiacetals and the chemical reaction enthalpies.

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