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
Applications of the Correlation-Gas Chromatographic Method
Objectives: To go where no one else has gone
1)
Evaluation of the vaporization enthalpies of large molecules
2. Application of Correlation-Gas Chromatography to a
Tautomeric Mixture - Acetylacetone
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.
H
O
O
O
O
C
CH 3
CH 2
0.186
CH 3
K
CH 3
CH
CH 3
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.
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
C5H8O2(liquid, 81.4%enol)
∆lgHm(298.15K) = (43.2 ± 0.2) kJ mol-1
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
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.
m)liq
Hdiketo/enol(T
–1
kJ mol
–11.9±0.8
Tm/K Hdiketo/enol(Tm)gas
–1
kJ mol
Tm/K
Method
Year
–18.0
388
UV
1977
–7.5±1.5
373
Photoelectron
Spectroscopy
1974
NMR
1966
306
–16.3
1959
–11.3±0.4
NMR
1957
–7.8
273
Bromination
1952
–10.0±0.8
386
IR
1951
H
CH3
O
O
O
O
C
C
C
C
CH2
CH3
C
CH3
CH3
H
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
1
Hacking 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.
gas, 100%
diketo
(–374.4  1.3)
diketo/enolHm(g)=(–10.0  0.8)
gas,100% enol
(–384.4  1.3)
fHm(298.15
K) / kJ mol–1
lgHm(298.15K) = (43.2  0.2)
liquid,100%
diketo
(–416.3  1.1)
diketo/enolHm(l)=(–11.3  0.4)
liquid,
81.4% enol
18.6% diketo
(-425.5  1.0)
(–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-3pentene-2-one and 2,4-pentanedione scheme used by Hacking and Pilcher in 1979
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.
The enthalpy difference of the two tautomers in the gas phase was remeasured by gas phase 1H NMR spectroscopy in 1985.
5.3 ppm enol vinyl 1H
3.3 ppm keto methylene
1H
Folkendt, M.M.J.et.al.
Phys. Chem. 1985, 89,
3347-3352
1.9 ppm enol methyl 1H
2.0 ppm keto methyl 1H
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.
m)liq
Hdiketo/enol(T
–1
kJ mol
Tm/K
–11.7
303
Hdiketo/enol(T–1m)gas
kJ mol
Tm/K
Method
Year
NMR
1996
-
–17.0
422
Photoelectron
Spectroscopy
1987
–11.8
394.5
–19.5
409
NMR
1985
–11.7±1.3
311
NMR
1982
–11.9±0.8
–18.0
388
UV
1977
–7.5±1.5
373
Photoelectron
Spectroscopy
1974
NMR
1966
306
–16.3
1959
–11.3
NMR
1957
–7.8
273
Bromination
1952
–10.0±0.8
386
IR
1951
The gas phase and condensed phase enthalpies are different, suggesting significant tautomer
interaction in the condensed phase
H
O
O
O
O
C
CH 3
CH 2
CH 3
CH 3
CH
CH 3
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)
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
Intensity / arbitrary units
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
0
50
100
150
200
Time / s
Gas Chromatograph of acetylacetone
250
300
Standards for 2,4-pentanedione
2,3-butanedione
1,4-benzoquinone
2,5-hexanedione
2,2,4,4-tetramethylcyclobutanedione
∆lgHm (298 K)/ kJ mol-1(lit.)
39.0 ±0.6
53.4
57.5
54.2 ±0.3
Standards for (Z) 4-hydroxy-3-penten-2-one
3-hydroxy-2-butanone
ethyl-2-hydroxypropanoate
4-hydroxy-4-methyl-2-pentanone
ethyl-3-hydroxybutanoate
o-hydroxyacetophenone
p-hydroxyacetophenone
methyl-o-hydroxybenzoate
methyl p-hydroxybenzoate
ethyl 3-hydroxyhexanoate
ethyl o-hydroxybenzoate
∆lgHm (298 K)/ kJ mol-1(lit)
48.7 ±0.4
52.5 ±3.0
52.3 ±1.4
55.9 ±0.6
59.6 ±0.6
82.5
62.0 ±1.8
83.1
61.9 ±0.6
66.5
Table. Enthalpy of transfer and vaporization enthalpy obtained for (Z)-4-hydroxy-3penten-2-one.
Compound
∆slngHm(387 K) ∆lgHm(298.15 K) ∆lgHm(298.15 K)
/kJ mol-1
/kJ mol-1(lit) /kJ mol-1(calcd)
27.92
48.7
48.7
Slope
Intercept
-3358.8
10.092
(Z)-4-hydroxy-3-penten-2- -3703.9
one
ethyl 2-hydroxypropanoate -3942.7
10.520
30.79
10.977
32.78
52.5
52.3
4-hydroxy-4-methyl-2pentanone
ethyl 3-hydroxybutanoate
-3998.0
10.914
33.24
52.3
52.6
-4516.7
11.712
37.55
55.9
55.8
ethyl 3-hydroxyhexanoate
-5476.8
13.020
45.53
61.9
61.6
o-hydroxyacetophenone
-5213.3
12.053
43.34
59.6
60.0
3-hydroxybutanone
50.8±0.6
lgHm(298.15 K)/kJ mol–1 = (0.734±0.021) slngHm(359 K) + (28.21±0.32) r2 = 0.997
Table. Enthalpy of Transfer and Vaporization Enthalpies obtained for 2,4pentanedione
Compound
∆slngHm(328 K) ∆lgH (298.15 K) ∆lgHm(298.15K)
/kJ mol-1
/kJ mol-1(lit) /kJ mol-1(calcd)
Slope
Intercept
2,3-butanedione
-3153.8
1.493
26.22
2,4-pentanedione
-4305.8
12.034
35.80
2,2,4,4-tetramethylcyclobutanedione
-4603.4
12.285
38.27
54.2
54.3
benzoquinone
-4614.4
12.111
38.36
53.4
54.4
2,5-hexanedione
-4800.5
12.592
39.91
57.5
56.4
39.0
38.9
51.2±2.2
lgHm(298.15 K)/kJ mol–1 = (1.283±0.1) slngHm(328 K) + (5.21±1.1)
r2 = 0.989
(Z)-4-hydroxy-3-penten-2-one
H
O
O
O
O
C
CH 3
CH 2
CH 3
CH 3
CH
CH 3
∆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-enol tautomerism observed = ∆Hketo-enol tautomerism real +∆Hmix
∆Hketo-enol tautomerism real = (-11.3)-(+7.6±0.6) = -18.9±0.6 kJ mol-1
The enthalpies of formation of the tautomers of acetylacetone in the liquid phase
and in the gas phase
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
x(enol)
1
Table. Summary of Standard Molar Enthalpies at T = 298.15 K of the Two
Acetylacetone Tautomers
Compound
fHºm(l) / kJ mol–1
 lgHm / kJ mol–1
fHºm(g) / kJ mol–1
2,4-pentanedione
–410.1  1.2
[–416.3  1.1]
51.2  2.2
–358.9  2.5
[–374.4  1.3]
Z 4-hydroxy-3penten-2-one
–429.0  1.0
[–427.6  1.1]
50.8  0.6
[43.2  0.1]
–378.2  1.2
[–384.4  1.3]
∆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.
Graduate Students
Patamaporn Umnahanant
Dmitry Lipkind
Darrell Hasty
C. Plienrasri
Manuel Notario