Adventures in Thermochemistry

1. Adventures in Thermochemistry James S. Chickos* Department of Chemistry and Biochemistry University of Missouri-St. Louis MO 63121 E-mail: jsc@umsl.edu 7 Busch Stadium STL

2. Applications of the Correlation-Gas Chromatographic Method Objectives: To go where no one else has gone • Evaluation of the vaporization enthalpies of large molecules 2. Application of Correlation-Gas Chromatography to a Tautomeric Mixture - Acetylacetone

3. Acetylacetone • Acetylacetone has been studied for over a hundred years. It exists as a mixture of two tautomers. • The enol form predominates at equilibrium and has been used to prepare a variety of metal complexes many of them here in Porto. • These metal complexes are reasonably volatile and have been used in the study of metal-oxygen bond strengths. Hence its heat of formation of the enol form in the gas phase is an important thermodynamic property.

4. K 0.186 0.814 • 2,4-pentanedione and (z)-4-hydroxy-3-penten-2-one, coexist with the latter predominating at equilibrium. • The enthalpy of formation of acetylacetone in the liquid phase and the enthalpy of vaporization of acetylacetone have been measured several times.1-11 • The enthalpy of formation of the equilibrium mixture of the pure liquid, (-425.5±1.0)kJ·mol-1 reported by Hacking and Pilcher has been accepted by the thermochemical community. Hacking, J.M.; Pilcher, G. J. Chem. Thermodyn. 1979, 11, 1015-1017.

5. Vaporization Enthalpy at T = 298.15 K ∆Hk/e = +0.67 kJ mol-1 C5H8O2(gas, 93.3%enol) C5H8O2(gas, 100%enol) ∆lgHm(298.15K) = (41.8 ± 0.2) kJmol-1 measured calorimetrically ∆lgHm(298.15K) = (43.2 ± 0.2) kJ mol-1 C5H8O2(liquid, 81.4%enol) C5H8O2(liquid, 100%enol) ∆Hk/e = -2.1 kJ mol-1 • A trace of concentrated sulfuric acid was used to rapidly equilibrate the diketo and enol forms. Since the enol is more volatile, it was assumed that tautomerization of the diketo form to the enol contributed –2.1 kJ mol-1.. .It was also assumed that the composition in the gas phase was the equilibrium concentration. ∆lgHm(298.15K) = (41.8 ± 0.2) -2.1 + 0.67 = (43.2 ± 0.2) kJmol-1 Irving, R.J.; Wadso, I. Acta Chem.Scand. 1970, 24, 589-592

6. Table. Summary of all enthalpy differences between 2,4-pentanedione and (Z)-4-hydroxy-3-penten-2-one in the liquid and gas phase available to Hacking and Pilcher, and Irving and Wadso. Enthalpy differences measured by the temperature dependence of the equilibrium constant.

7. 0.186 0.814 Enthalpy of formation: fHºm(l, 298.15 K) = (–425.51.0) kJ.mol–1,1 Vaporization enthalpy (mixture) lgHºm(298.15 K)mix = (41.80.1) kJ.mol–1.2 keto/enolHºm (l, 298.15 K) = (–11.30.4) kJ.mol–1. 3 keto/enolHºm (g, 298.15 K) = (–10.30.8) kJ.mol–1.4 1Hacking and Pilcher, J. Chem. Thermodyn. 1979, 11, 1015-1017. 2 Irving, R.J.; Wadsö, I. Acta Chem. Scand. 1970, 24, 589-592. 3 Reeves, L. Can. J. Chem. 1957, 35, 1351-1365. 4 Powling, J.; Bernstein, H.J. J. Am. Chem. Soc. 1951, 73, 4353-4356.

8. gas, 100% diketo (–374.4  1.3) diketo/enolHm(g)=(–10.0  0.8) gas,100% enol (–384.4  1.3) fHm(298.15K) / kJ mol–1 lgHm(298.15K) = (43.2  0.2) liquid,100% diketo (–416.3  1.1) liquid, 81.4% enol 18.6% diketo (-425.5  1.0) diketo/enolHm(l)=(–11.3  0.4) (–427.6  1.1) liquid,100% enol 0 0.814 1 x(enol) The thermochemical scheme to calculate the enthalpy of formation of (Z)-4-hydroxy-3-pentene-2-one and 2,4-pentanedione scheme used by Hacking and Pilcher in 1979

9. The enthalpy difference of the two tautomers in the gas phase was measured by infrared spectroscopy in 1951 Gas Phase FT-IR spectrum of 2,4-pentanedione, Aldrich Chemical Co.

10. The enthalpy difference of the two tautomers in the gas phase was re-measured by gas phase 1H NMR spectroscopy in 1985. 5.3 ppm enol vinyl 1H 3.3 ppm keto methylene 1H 1.9 ppm enol methyl 1H 2.0 ppm keto methyl 1H Folkendt, M.M.J.et.al. Phys. Chem. 1985, 89, 3347-3352

11. Table. A summary of all the enthalpy differences measured between 2,4-pentanedione and (Z)-4-hydroxy-3-penten-2-one in the liquid and gas phase. Enthalpy differences measured by the temperature dependence of the equilibrium constant. The gas phase and condensed phase enthalpies are different, suggesting significant tautomer interaction in the condensed phase

12. If the pure enol form is mixed with the pure keto form at the equilibrium concentrations, ( 0.814 mol enol/ (0.186 mol)will ∆H = 0 ? Is ∆Hmix = 0 ? If ∆Hmix ≠ 0 the consequences on vaporization are • If the solution heats up when the pure diketo and enol are mixed at their equilibrium concentration, it will take more energy to vaporize the two liquids as a mixture at T = 298.15 K ; • If the solution cools down, it will take less heat to vaporize the two liquids as a mixture at T = 298.15 K. • Since Hdiketo/enol(liq) ≠ Hdiketo/enol(gas),we decided to measure lgHm(298.15K)

13. Correlation Gas Chromatography: an ideal method for determining the vaporization enthalpy of a pure material even though the material of interest may be present in the mixture provided all components can be separated The mass spectrum of the enol and diketo forms by gcms were also consistent with the literature. The two resolved peaks were observed as a function of temperature over a T = 30 K temperature range Gas Chromatograph of acetylacetone

14. Standards for 2,4-pentanedione ∆lgHm (298 K)/ kJ mol-1(lit.) 2,3-butanedione 39.0 ±0.6 1,4-benzoquinone 53.4 2,5-hexanedione 57.5 2,2,4,4-tetramethylcyclobutanedione 54.2 ±0.3 Standards for (Z) 4-hydroxy-3-penten-2-one ∆lgHm (298 K)/ kJ mol-1(lit) 3-hydroxy-2-butanone 48.7 ±0.4 ethyl-2-hydroxypropanoate 52.5 ±3.0 4-hydroxy-4-methyl-2-pentanone 52.3 ±1.4 ethyl-3-hydroxybutanoate 55.9 ±0.6 o-hydroxyacetophenone 59.6 ±0.6 p-hydroxyacetophenone 82.5 methyl-o-hydroxybenzoate 62.0 ±1.8 methyl p-hydroxybenzoate 83.1 ethyl 3-hydroxyhexanoate 61.9 ±0.6 ethyl o-hydroxybenzoate 66.5

15. Table. Enthalpy of transfer and vaporization enthalpy obtained for (Z)-4-hydroxy-3-penten-2-one. lgHm(298.15 K)/kJ mol–1 = (0.734±0.021) slngHm(359 K) + (28.21±0.32) r2 = 0.997

16. Table. Enthalpy of Transfer and Vaporization Enthalpies obtained for 2,4-pentanedione lgHm(298.15 K)/kJ mol–1 = (1.283±0.1) slngHm(328 K) + (5.21±1.1) r2 = 0.989

17. (Z)-4-hydroxy-3-penten-2-one ∆lgHm(298.15K)/kJ.mol-1(corr- gas chromatography)=(50.8±0.6) kJ mol-1 ∆lgHm(298.15K)/kJ mol-1(calc for pure material) = (43.2 0.2) kJ.mol–1 a a Measured as a mixture but calculated for the pure material ∆Hmix = (50.8±0.6) - (43.2 ±0.2) = 7.6±0.6 kJ.mol-1 ∆Hketo-enoltautomerism observed = ∆Hketo-enoltautomerism real +∆Hmix ∆Hketo-enoltautomerism real = (-11.3)-(+7.6±0.6) = -18.9±0.6 kJ mol-1

18. gas, diketo (–358.9±2.5) kJ mol-1 ∆diketo/enol Hm(g) = (-19.3±2.8) kJ mol-1 (-19.5)kJ mol-1 Folkendt,M. et al. gas, 100% enol (–378.2±1.2) kJ mol-1 ΔlgHm=(51.2±2.2) kJ mol-1 lgHm= (50.8±0.6) kJ mol-1 liquid, diketo (–410.1±1.2) kJ mol-1 ∆diketo/enol Hm(l)= -18.9 kJ mol-1 (–429.0±1.0)kJ mol-1 liquid, 100% enol 0 0.814 1 x(enol) The enthalpies of formation of the tautomers of acetylacetone in the liquid phase and in the gas phase

19. Table. Summary of Standard Molar Enthalpies at T = 298.15 K of the Two Acetylacetone Tautomers ∆fHm (T = 298.15 K, liquid, 81.4% enol and 18.6% diketo) = -425.5±1.0 kJ mol-1. values in the brackets are the previous accepted values. Temprado, M.; Roux, M. V.; Umnahanant, P.; Zhao, H.; Chickos, J. S. J. Phys. Chem. B.2005; 109, 12590-12595.

20. Graduate Students Patamaporn Umnahanant Dmitry Lipkind Darrell Hasty C. Plienrasri Manuel Notario