Thermodynamics Research Center / ThermoML | Journal of Chemical and Engineering Data

ABSTRACT

The enthalpies of formation of pure liquid and gas-phase ethyl 3-oxobutanoate and ethyl Z-3-hydroxy- 2-butenoate are examined in the light of some recent NMR studies on the enthalpy differences between gas-phase enthalpies of the two tautomers. Correlation-gas chromatography experiments are used to evaluate the vaporization enthalpies of the pure tautomers. Values of (61.6 ( 2.2) and (54.7 ( 1.3) kJamol-1 at T ) 298.15 K are measured for pure ethyl 3-oxobutanoate and ethyl Z-3-hydroxy-2-butenoate, respectively. These values can be contrasted to a value of (54.2 ( 1.0) kJamol-1 when the vaporization enthalpy is measured as a mixture of tautomers. The difference is attributed to an endothermic enthalpy of mixing term of (7.1 ( 2.1) kJamol-1, which destabilizes the mixture relative to the pure components. Using the existing liquid-phase enthalpy differences measured by NMR, the endothermic enthalpy of mixing term and the condensed phase enthalpy of formation of the equilibrium mixture, new enthalpies of formation for ethyl 3-oxobutanoate and ethyl Z-3-hydroxy-2-butenoate in both the gas, centsfHdeg m(g) ) (-578.2 ( 3.3), (-593.0 ( 2.7) kJamol-1, respectively, and liquid phases, centsfHdeg m(l) ) (-639.8 ( 2.4), (-647.7 ( 2.4) kJamol-1, respectively, are calculated. A comparison of the difference in gas phase enthalpy of formation of the two tautomers reproduces the enthalpy difference measured previously by gas-phase NMR studies.