Enthalpies of combustion, heat capacities, and enthalpies of vaporization of 1-ethylimidazole and 1-ethylpyrazole

Article abstract of DOI:10.1006/jcht.1998.0436

The standard (p° = 0.1 MPa) molar enthalpies of formation for liquid 1-ethylimidazole and 1-ethylpyrazole were derived from the standard molar enthalpies of combustion Δ_cH°_m, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The molar heat capacities of both liquids were measured in the temperature range (280 to 365 ) K by differential scanning calorimetry. The standard molar enthalpies of vaporization Δ^g₁H°_m, at the temperature T = 298.15 K were measured by Calvet microcalorimetry. The derived standard molar enthalpies of formation in the gaseous state are compared. -Δ_cH°_m(1)/(kJ·mol^-1) Δ^g₁H°_m/(kJ·mol^-1) 1-Ethylimidazole 3155.7 + 4.7 66.0 + 3.9 1-Ethylpyrazole 3190.2 + 2.1 53.3 + 2.4

Full text of DOI:10.1006/jcht.1998.0436

J. Chem. Thermodynamics 1999, 31, 129᎐138
Article No. jcht.1998.0436
Available online at http:rrwww.idealibrary.com on
Enthalpies of combustion, heat capacities,
and enthalpies of vaporization of
1-ethylimidazole and 1-ethylpyrazole
Manuel A. V. Ribeiro da Silva,a Maria das Dores M. C. Ribeiro
da Silva, M. Agostinha R. Matos,
Centro de In¨estiga¸cao em Quımica, Department of Chemistry, Faculty of Science,
˜
´
Uni¨ersity of Porto, Rua do Campo Alegre, 687, P-4150 Porto, Portugal
Pilar Jimenez, M. Victoria Roux,
Instituto de Quımica Fısica ‘‘Rocasolano’’, C.S.I.C., Serrano 119, 28006 Madrid,
´
´
Spain
Jose Elguero,
´
Instituto de Quımica Medica, C.S.I.C., Juan de la Cier¨a, 3, 28006 Madrid, Spain
´
´
Rosa Claramunt, Pilar Cabildo,
Departamento de Quımica Organica y Biologıa, Facultad de Ciencias, UNED,
´
´
´
28040 Madrid, Spain
and Ana Sanchez-Migallon
´
Facultad de Quımica, Uni¨ersidad de Castilla- La Mancha, 13071 Ciudad Real,
´
Spain
Ž
.
The standard pЊ s 0.1 MPa molar enthalpies of formation for liquid 1-ethylimidazole and
1-ethylpyrazole were derived from the standard molar enthalpies of combustion ⌬c HTm , in
oxygen, at T s 298.15 K, measured by static bomb combustion calorimetry. The molar heat
Ž
.
capacities of both liquids were measured in the temperature range 280 to 365 K by
differential scanning calorimetry. The standard molar enthalpies of vaporization ⌬gl HTm , at
the temperature T s 298.15 K were measured by Calvet microcalorimetry. The derived
standard molar enthalpies of formation in the gaseous state are compared.
T
y⌬c HTm l r kJ mol
⌬l Hmr kJ mol
y1
g
y1
.
.
Ž . Ž
.
Ž
.
1-Ethylimidazole
1-Ethylpyrazole
3155.7 " 4.7
3190.2 " 2.1
66.0 " 3.9
53.3 " 2.4
᭧
1999 Academic Press
KEYWORDS: enthalpies of combustion; enthalpies of formation; enthalpies of vaporization;
1-ethylimidazole; 1-ethylpyrazole; heat capacities
a
Ž
.
To whom correspondence should be adressed E-mail: risilva@fc.up.pt .
0021᎐9614r99r010129 q 10 $30.00r0
᭧ 1999 Academic Press

130
M. A. V. Ribeiro da Silva et al
FIGURE 1. Molecular structure of a, 1-ethylimidazole, and b, 1-ethylpyrazole.
1. Introduction
The thermochemical study of heterocyclic molecules has interested our research
centres for the last few years.Ž1 ᎐ 21. Some attention has been given to the study of
azoles, and their alkyl and aryl substituted carbon derivatives.Ž1 ᎐ 4. In this work we
begin the thermochemical study of N-substituted derivatives of imidazole and
pyrazole.
Ž
.
This paper reports the standard pЊ s 0.1 MPa molar enthalpies of combustion,
in oxygen, at the temperature 298.15 K, measured by static bomb combustion
Ž
.
calorimetry, for the liquid 1-ethylimidazole and 1-ethylpyrazole figure 1 .
The standard molar enthalpies of vaporization were determined by Calvet
microcalorimetry. Heat capacities were determined by using d.s.c. The derived
values for the standard molar enthalpies of formation for the compounds in the
gaseous state are analysed.
These compounds were selected as part of a multistep approach to the study of
the effect of N-substituents on the properties of aromatic compounds. This study
was undertaken to gain information on two related questions: how substituent
effects are transmitted through the nitrogen, and how the aromaticity of imidazole
and pyrazole rings is affected by N-substituents.Ž22. Concerning the first point,
knowledge about transmission of substituent effects is restricted to C-substituents
Ž22 ᎐ 24.
Ž
.
benzene and other aromatic rings including C-substituted heterocycles .
Regarding the second point, the only available information concerns the parent
compounds, imidazole and pyrazole themselves.Ž25᎐ 30. Concerning the geometry of
these compounds, we have previously shown that angular deformations which
Ž
.
Ž
.
affect the endocyclic angles in pyrazoles N1 and in benzenes C ipso are linearly
related.Ž31.
2. Experimental
The 1-ethylimidazoleŽ32. was prepared by mixing imidazole 10 mmol , potassium
Ž
.
Ž
.
Ž
.
tert-butoxide 11 mmol , and tetra-n-butylammonium bromide TBAB, 0.5 mmol ,

Thermochemistry of 1-ethylimidazole and 1-ethylpyrazole
131
Ž
.
and submerging in an ultrasonic cleaning bath 50 W, 200 MHz for 15 min. Ethyl
Ž
.
iodide 10 mmol was added at T s 273 K and the reaction mixture was subjected
to ultrasound for 3 h. The mixture was extracted with dichloromethane two
portions of 15 cm . After removal of the solvent and tert-butanol, the product was
Ž
3
.
distilled twice over sodium in a nitrogen atmosphere in a Vigreux column, giving a
yield of 80 per cent of a product with boiling temperature Tb s 378 K at p s
3
Ž33.
5
.
.
19 10 Pa. Literature values
are Tb s 481 K at p s 1.01 10 Pa.
The 1-ethylpyrazoleŽ32. was prepared by reacting pyrazole 10 mmol , finely
Ž
.
Ž
.
Ž
.
ground potassium hydroxide 20 mmol , and TBAB 0.5 mmol in an ultrasonic bath
Ž
.
for 15 min. Ethyl iodide 10 mmol was added at T s 273 K and the reaction
mixture was subjected to ultrasound for 24 h. The mixture was extracted with
dichloromethane two portions of 15 cm , and the product was distilled twice over
sodium in a nitrogen atmosphere in a Vigreux column. The yield was 85 per cent,
3
Ž
.
3
.
with a boiling temperature Tb s 343 K at p s 19 10 Pa. Literature values are
4
Ž33.
5
Ž34,35.
.
.
Ž
.
Tb s 409 to 410 K at p s 9.95 10 Pa, and Tb s 409 K at p s 1.01 10 Pa.
Purities were checked by C, H, N elemental analysis.
Both 1-ethylimidazole and 1-ethylpyrazole are hygroscopic liquids. Although the
handling of samples was performed under nitrogen, the average ratios of the mass
of carbon dioxide recovered after combustion to that calculated from the mass of
Ž
.
sample were: for 1-ethylimidazole 0.9980 " 0.0006 , and for 1-ethylpyrazole
Ž
.
0.9955 " 0.0007 , where the uncertainties are the standard deviation of the mean.
The combustion experiments were performed with a static bomb calorimeter,
and the apparatus and technique have been described.Ž36. Benzoic acid Bureau of
Ž
.
Analysed Samples, Thermochemical Standard, BCS-CRM-190 p was used for
calibration of the bomb. Its massic energy of combustion is y 26431.8 " 3.7 J g
under certificate conditions. The calibration results were corrected to give the
y1
.
Ž
.
Ž
.
energy equivalent ␧ calor corresponding to the average mass of water added
Ž
.
to the calorimeter, 3119.6 g. From eight calibration experiments, ␧ calor s
15911.2 " 1.5 J Ky1, where the uncertainty quoted is the standard deviation of
.
.
Ž
Ž
.
the mean. For all experiments, ignition was made at T s 298.150 " 0.001 K.
Combustion experiments were performed in oxygen at p s 3.04 MPa, with 1 cm3 of
water added to the bomb. The liquid 1-ethylimidazole and 1-ethylpyrazole were
Ž
.
burnt in sealed polyester bags Melinex by using the technique described by
Skinner and Snelson,Ž37. who determined the specific energy of combustion of dry
Melinex as ⌬c uЊ s y 22902 " 5 J gy1. That value was confirmed in the Porto
.
.
Ž
laboratory. The mass of Melinex used in each experiment was corrected for the
Ž
.
mass fraction of water 0.0032 , and the mass of carbon dioxide produced from it
was calculated by using the factor previously reported.Ž37. The electrical energy for
ignition was determined from the change in potential difference across a capacitor
when discharged through the platinum ignition wire. For the cotton-thread fuse,
y1 Ž38.
.
empirical formula CH 1.686O0.843, ⌬c uЊ s y16250 J g
.
The corrections for nitric
y1Ž39.
.
acid formation were based on y59.7 kJ mol
for the molar energy of formation
y3
.
Ž
.
Ž .
of 0.1 mol dm HNO3 aq from N2 , O2 , and H 2O l .
The amount of substance used in each experiment was determined from the
total mass of carbon dioxide produced after allowance was made for that formed

132
M. A. V. Ribeiro da Silva et al
from the cotton thread fuse and Melinex. The densities at T s 298.15 K for both
1-ethylimidazole and 1-ethylpyrazole were measured by using an Anton Paar DMA
y3
.
Ž
02D densimeter, as 0.959 g cm
and 1.08 g cmy3. An estimated pressure
.
y1
y1
.
.
.
coefficient of massic energy, Ѩ urѨ p T s y0.2 J g MPa
at T s 298.15 K, a
typical value for most organic compounds, was assumed. For each compound, the
massic energy of combustion ⌬c uЊ was calculated by the procedure given by
Hubbard et al.Ž40.
Ž
.
A differential scanning calorimeter Perkin-Elmer DSC-2C , connected to a
Model 3600 Data Station and equipped with an Intracooler-2-unit, was used in this
research. Its temperature and power scales were calibrated by measuring the
melting temperatures of the usual high-purity reference materials: n-octadecane,
n-octadecanoic acid, benzoic acid, and indium. The power scale was calibrated by
Ž41.
Ž
.
using high-purity indium x ) 0.99999 as reference material.
Heat capacities were calculated by means of the heat-capacity programme.
Synthetic sapphire and benzoic acid were used as standard materialsŽ42. for checking
all the procedures. The complete temperature range for determination of the heat
capacities was divided into three intervals of approximately 40 K, overlapping by
15 K from one interval to another. The heat capacity values were the average
of four experiments performed for each interval of temperature. Fresh samples of
y1
.
mass 8 mg were scanned using a heating rate of 0.17 K s
and a sensitivity of
0.008 W full scale. All the samples were handled under a nitrogen atmosphere. The
.
.
estimated error of the molar heat capacities was between 0.01 Cp, m and 0.02 Cp, m
.
The enthalpies of vaporization of 1-ethylimidazole and 1-ethylpyrazole were
measured by using a method similar to that used for sublimation of solids, the
‘‘vacuum sublimation’’ drop microcalorimetric method,Ž43. which was previously
tested in the Porto laboratory.Ž9. Samples of about 7 to 11 mg of each liquid
compound, contained in a thin glass capillary tube sealed at one end, were
dropped, at room temperature, into the hot reaction vessel in a high-temperature
Ž
.
Ž
.
Ž
Calvet microcalorimeter held at T s 371 K 1-ethylimidazole and T s 373 K 1-
.
ethylpyrazole , and then removed from the hot zone by vacuum vaporization. The
observed enthalpies of vaporization were corrected to T s 298.15 K by using the
T
T
Ž44.
Ž .
value of H298.15 K Cp, m g dT estimated by the Benson group method,
with values
from Stull et al.Ž45. The microcalorimeter was calibrated in situ for these
measurements by using the reported enthalpy of vaporization of undecane.Ž46. The
molar masses used for the elements were those recommended by IUPAC in
1993.Ž47.
The geometries of imidazole and pyrazole, as well as those of their 1-ethyl
Ž
.
derivatives, were optimized at the B3LYPr6-31G d,p level, which was shown to
yield quite reliable geometries for this kind of system.Ž48. For the 1-ethyl derivatives,
Ž
.
we have initially considered two alternative conformers, EI1 1-ethylimidazole 1 ,
Ž
.
Ž
.
Ž
.
EI2 1-ethylimidazole 2 , EP1 1-ethylpyrazole 1 , and EP2 1-ethylpyrazole 2 ,
which differ in the relative positions of the methyl group of the ethyl substituent
Ž
.
with respect to the azole ring see figure 2 .
In all cases, both conformers evolve to yield a sole equilibrium conformation, EI
and EP, respectively, in which the methyl group is located in a plane almost

Thermochemistry of 1-ethylimidazole and 1-ethylpyrazole
133
Ž
.
Ž
.
FIGURE 2. Conformers of 1-ethylimidazole ET1 and ET2 , and of 1-ethylpyrazole EP1 and EP2 .
Ž
.
perpendicular to the plane of the azole ring figure 2 . The harmonic vibrational
frequencies were evaluated at the same level of theory. This allows us both to
confirm that the stationary points found correspond to local minima of the
corresponding potential energy surfaces, and to evaluate the zero-point energy
Ž49.
Ž
.
ZPE corrections, which were scaled by the empirical factor 0.98.
To have reliable energetics, the final total energies of these systems were
Ž
.
obtained in single-point calculations at the B3LYPr6-311 q G 3df,2p level, which
usually yields thermodynamic properties, such as proton affinities, or enthalpies of
formation, in fairly good agreement with those obtained from the G2-theory, which
is prohibitively expensive for systems of this size.
3. Results
Results for a typical combustion experiment of each compound are given in table 1,
Ž
.
where ⌬m H 2O is the deviation of the mass of water added to the calorimeter
from 3119.6 g, and ⌬U⌺ is the correction to the standard state. The remaining
TABLE 1. Results of typical combustion experiments for 1-ethylimidazole and 1-ethylpyrazole at
T s 298.15 K
1-Ethylimidazole
1-Ethylpyrazole
Ž
.
m CO2 rg
1.76867
0.69893
0.07121
0.00353
1.55103
16.20
1.47980
0.59327
0.05045
0.00385
1.31586
15.89
Ž
.
mЈ cpd rg
Ž
Ž
.
mЉ Melinex rg
.
mٞ fuse rg
⌬TadrKy1
.
Ž
.
␧fr J K
Ž
.
.
⌬m H 2O rg
y0.2
0.3
20958.37
56.73
1.10
11.98
1155.49
62.52
33158.01
a
Ž
y⌬U IBP rJ
24701.92
68.66
Ž
Ž
.
⌬U HNO3 rJ
.
⌬U ign. rJ
0.66
14.57
1630.89
57.33
32807.96
⌬U⌺rJ
Ž
Ž
.
⌬U Melinex rJ
.
Ž
⌬U fuse rJ
y1
.
.
y⌬c uЊr J g
a
Ž
.
Ž
.
⌬U IBP already includes the ⌬U ign. .

134
M. A. V. Ribeiro da Silva et al
TABLE 2. Individual values of the massic energy
of combustion ⌬c uЊ of 1-ethylimidazole
and 1-ethylpyrazole at T s 298.15 K
y1
.
Ž
.
y⌬c uЊr J g
1-Ethylimidazole
1-Ethylpyrazole
32818.07
32767.97
32842.10
32807.96
32803.13
32795.90
32787.64
32782.19
33138.25
33127.03
33159.23
33191.76
33158.01
33183.70
y1
a
.
²
: Ž
.
y ⌬c uЊ r J g
32800.6 " 8.0
33159.7 " 10.2
a Mean value and standard deviation of the mean.
quantities are as previously described.Ž40. As samples were ignited at T s 298.15 K,
.
⌬U IBP s y ␧ calor q ⌬m H O
c
H O , l q ␧ ⌬T q ⌬U
.
1
Ž
.
Ž
.
Ž
.
Ž
.
Ž .
Ä
f 4
2
p
2
ad
ign
The individual results of all combustion experiments, together with the mean
value and its standard deviation, are given for each compound in table 2. Table 3
lists the derived standard molar energies and enthalpies of combustion, ⌬cUmT
l
Ž .
l
T Ž .
and ⌬c HTm l , respectively, and the standard molar enthalpies of formation ⌬f Hm
Ž .
for the two liquid compounds at T s 298.15 K. In accordance with normal
thermochemical practice, the uncertainties assigned to the standard molar
enthalpies of combustion are, in each case, twice the overall standard deviation
of the mean, and include the uncertainities in calibrationŽ50. and in the values
of auxiliary quantities. To derive ⌬f HTm
Ž .
l
from ⌬c HTm
Ž .
l the standard molar
Ž .
Ž .
Ž
enthalpies of formation of H 2O l and CO2 g at T s 298.15 K, y 285.830 "
y1Ž51.
y1 Ž51.
.
.
.
Ž
.
0.042 kJ mol
and y 393.51 " 0.13 kJ mol
,
respectively, were used.
The molar heat capacities for the liquid 1-ethylimidazole and 1-ethylpyrazole
were measured from T s 280 K to the temperature of vaporization; the
temperatures were corrected by a programme that includes the calibrations of
temperature and power scales of the d.s.c. apparatus, and values of Cp, m are given
in table 4.
TABLE 3. Derived standard pЊ s 0.1 MPa molar energies of combustion ⌬cUmT, standard molar
enthalpies of combustion ⌬c HTm , and standard molar enthalpies of formation ⌬f HTm for 1-ethylimidazole
and 1-ethylpyrazole at T s 298.15 K
Ž
.
y⌬cUmT l r kJ mol
y⌬c HTm l r kJ mol
⌬f HTm l r kJ mol
y1
y1
y1
.
.
.
Ž . Ž
.
Ž . Ž
.
Ž . Ž
.
1-Ethylimidazole
1-Ethylpyrazole
3153.2 " 1.7
3187.7 " 2.1
3155.7 " 1.7
3190.2 " 2.1
44.8 " 1.8
79.3 " 2.2

Thermochemistry of 1-ethylimidazole and 1-ethylpyrazole
135
TABLE 4. Experimental molar heat capacities Cp, m of 1-ethylimidazole and 1-ethylpyrazole in the
liquid state
y1
y1
y1
y1
y1 y1
.
mol
.
.
.
.
.
Ž
.
Ž
.
Ž
.
TrK
Cp, mr J K
mol
TrK
Cp, mr J K
mol
TrK
Cp, mr J K
1-ethylimidazole
280.1
285.2
290.2
295.3
303.3
305.4
180.99
183.96
183.53
186.16
187.48
188.20
310.4
315.4
320.4
325.4
330.4
335.4
188.45
190.66
192.63
194.99
198.17
202.03
340.4
345.4
350.3
355.3
359.2
365.2
200.53
202.66
203.21
205.84
207.48
211.80
1-ethylpyrazole
280.3
285.3
290.4
295.4
300.4
305.4
170.90
177.76
179.95
185.37
192.26
195.27
310.5
315.5
320.5
325.5
330.5
335.5
195.55
196.35
199.45
204.28
208.35
215.61
340.5
345.5
350.4
355.4
360.4
365.4
216.62
218.19
217.09
224.90
227.27
233.87
T
TABLE 5. Standard pЊ s 0.1 MPa molar enthalpies of vaporization ⌬gl, THm at various temperatures
Ž
.
T for 1-ethylimidazole and 1-ethylpyrazole, determined by microcalorimetry
g, THTm
H2T98.15 K CTp, m g dT
⌬gl HTm 298.15 K
No. of
expts
⌬
Ž .
Ž
.
l
TrK
y1
y1
y1
.
.
.
kJ mol
kJ mol
kJ mol
1-Ethylimidazole
1-Ethylpyrazole
7
5
371
373
75.6 " 3.9
63.2 " 2.4
9.6
9.9
66.0 " 3.9
53.3 " 2.4
Values of the enthalpies of vaporization ⌬gl HTm are given in table 5, with
uncertainties of twice the standard deviation of the mean.
The derived enthalpies of formation in both the condensed and gaseous phases
for 1-ethylimidazole and 1-ethylpyrazole are summarized in table 6.
4. Discussion
To discuss the enthalpies of formation of 1-ethylimidazole and 1-ethylpyrazole
Ž1.
Ž
.
table 6 we have first studied the imidazole᎐pyrazole pair. In a previous paper,
we have reported the following values for ⌬f HTm g : pyrazole 179.4 " 0.8 kJ mol
,
y1
.
Ž .
Ž
.
and imidazole 132.9 " 0.6 kJ moly1. Thus, of the two compounds of formula
.
Ž
.
T
Ž
.
TABLE 6. Derived standard pЊ s 0.1 MPa molar enthalpies of formation ⌬f Hm of 1-ethylimidazole
and 1-ethylpyrazole at T s 298.15 K
T
⌬f HTm l r kJ mol
⌬l Hmr kJ mol
⌬f HTm g r kJ mol
y1
g
y1
y1
.
.
.
Ž . Ž
.
Ž
.
Ž . Ž
.
1-Ethylimidazole
1-Ethylpyrazole
44.8 " 1.8
79.3 " 2.2
66.0 " 3.9
53.3 " 2.4
110.8 " 4.3
132.6 " 3.3

136
M. A. V. Ribeiro da Silva et al
Ž
.
TABLE 7. Total energies U, zero-point energies U T ª 0 , and relative energies ⌬U of imidazole,
pyrazole, 1-ethylimidazole, and 1-ethylpyrazole
d
⌬U
b
b
b
UarE h
UcrE h
U T ª 0 rE h
Ž
.
Compound
y1
.
kJ mol
Imidazole
Pyrazole
1-Ethylimidazole EI
y226.22309
y226.20703
y304.85577
y304.84243
y226.29940
y226.29870
y304.95039
y304.93602
0.07126
0.07140
0.12784
0.12783
0.0
44.8
0.0
Ž
.
.
Ž
1-Ethylpyrazole EP
33.7
b
c
a E h s 2625.50 kJ. B3LYPr6-31G d,p . B3LYPr6-311 q G 3df,2p rrB3LYPr6-31G d,p . d These
Ž
.
Ž
.
Ž
.
Ž
.
values include the corresponding corrections for U T ª 0 .
C3H 4 N2 , imidazole is more stable by 46.5 " 1.0 kJ moly1. The results of the
.
Ž
.
y1
.
theoretical calculation, after ZPE correction, reported in table 7, is 44.8 kJ mol
.
This excellent agreement encourages us to do the same kind of calculation for the
y1
.
1-ethylimidazole᎐1-ethylpyrazole pair. The theoretical result, 33.7 kJ mol
,
although showing a decrease in the difference in stability, is still far from the
experimental value, 21.8 " 5.4 kJ moly1, which is in line with that recently found
by Curtiss et al.Ž52. for a large number of molecules, for which the mean absolute
.
Ž
.
difference for calculated enthalpies of formation by B3LYP is 13 kJ moly1. Before
.
going further from the preliminary conclusion that the differences in aromaticity
between imidazoles and pyrazoles decrease from N᎐H to N-ethyl derivatives, it is
necessary to study other pairs of N-substituted azoles, both experimentally and
theoretically.
Ž
.
Thanks are due to Junta National de Investiga¸cao Cientıfica e Tecnologica JNICT ,
˜
´
´
Ž
.
Lisbon, Portugal, and Consejo Superior de Investigaciones Cientıficas CSIC ,
Madrid, Spain for a joint research project CSICrJNICT. Financial support from
DGICYT, Spain PB 96-0972 C02 01 is ackwnowledged. Thanks are given to
´
Ž
.
´˜
Ž
.
Professors Otilia Mo and Manuel Yanez Universidad Autonoma de Madrid for
´
´
the theoretical calculations of table 7.
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1. Jimenez, P.; Roux, M. V.; Turrion, C.; Gomis, F. J. Chem. Thermodynamics 1987, 19, 985᎐992.
´
2. Jimenez, P.; Roux, M. V.; Turrion, C. J. Chem. Thermodynamics 1989, 21, 759᎐764.
´
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Recei¨ed 18 May 1998; in final form 26 August 1998
O-728