Equilibrium constants for the mercuric acetate-catalyzed reaction,

cis-CH₃ OCH=CHOCH₃ D trans-CH₃ OCH=CHOCH₃,

have been determined in the liquid and vapor phase over the temperature range 30° - 175°C by gas chromatography analysis. A novel synthesis of the cis- and trans-isomers of 1,2-dimethoxyethylene is discussed. From a screening of 29 catalysts, mercuric acetate, mercuric benzoate, and mercuric salicylate were found to be favorable isomerization catalysts. For the liquid phase reaction,

ln K = -779.8/T + 0.6748

with ΔH° = +1.549 ± 0.019 Kcal/mole
and ΔS° = +1.34 ± 0.05 e.u./mole

Similarly, for the vapor phase reaction,

ln K = -727.2/T + 0.8335

with ΔH° = +1.445 ± 0.054 Kcal/mole
and ΔS° = +1.66 ± 0.15 e.u./mole

A linear regression analysis, employing a least-squares curve fit and an analysis of variance, was performed on the data. An independent experimental error analysis (for uncertainties in measurement, reproducibility, etc.) was performed using partial differentials and was consistent with the above findings.

cis-1,2-Dimethoxyethylene was more stable relative to its trans-isomer in both the liquid and vapor phase. The greater amount of cis-isomer present at equilibrium (the cis-effect) is discussed in terms of dipole-dipole interactions with a resultant increase in the C=C bond strength for the cis-isomer due to electron density shifts. The dimethoxyethylenes are correlated with previous results found in the literature for related systems (e.g. dihaloethylenes, etc.). Additional arguments are also advanced.