J. Chem. Thermodynamics 1999, 31, 1429–1441 Article No. jcht.1999.0507 Available online at http://www.idealibrary.com on
Enthalpies of combustion, vapour pressures, and enthalpies of sublimation of three methoxy-nitrobenzoic acids. Vapour pressures and enthalpies of sublimation of the three nitrobenzoic acids Manuel A. V. Ribeiro da Silva,a M. Agostinha R. Matos, Manuel J. S. Monte, Dorothea M. Hillesheim, Miguel C. P. O. Marques, and Nuno F. T. G. Vieira Centro de Investigac¸a˜ o em Qu´ımica, Department of Chemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal
The standard ( p o = 0.1 MPa) molar enthalpies of formation for crystalline 3-methoxy-2nitro-, 4-methoxy-3-nitro-, and 3-methoxy-4-nitrobenzoic acid were derived from the standard molar enthalpies of combustion, in oxygen, at the temperature 298.15 K, measured by static bomb combustion calorimetry; the Knudsen mass-loss effusion technique was used to measure the vapour pressures of these crystals as functions of temperature, and the standard molar enthalpies of sublimation, at T = 298.15 K, were derived by the Clausius– Clapeyron equation. The standard molar enthalpies of sublimation, at T = 298.15 K, for the three nitrobenzoic acids were also determined using the Knudsen mass-loss effusion technique. g
Acid
o (cr)/(kJ · mol−1 ) −1c Hm
o /(kJ · mol−1 ) 1cr Hm
3-methoxy-2-nitrobenzoic acid 4-methoxy-3-nitrobenzoic acid 3-methoxy-4-nitrobenzoic acid 2-nitrobenzoic acid 3-nitrobenzoic acid 4-nitrobenzoic acid
3597.1 ± 2.1 3571.0 ± 3.7 3565.7 ± 6.4 — — —
141.9 ± 1.3 131.2 ± 0.8 131.0 ± 1.1 118.7 ± 0.5 110.0 ± 0.4 119.2 ± 0.6
The standard molar enthalpies of formation in the gaseous state are shown to fit a group c 1999 Academic Press additivity scheme. KEYWORDS: enthalpies of combustion; enthalpies of formation; enthalpies of sublimation; vapour pressures; nitrobenzoic acids; methoxynitrobenzoic acids
a To whom correspondence should be addressed (E-mail:
[email protected]).
0021–9614/99/111429 + 13
$30.00/0
c 1999 Academic Press
1430
M. A. V. Ribeiro da Silva et al.
1. Introduction The present work is part of a broader research programme on the thermochemistry of benzene derivatives,(1–7) in which our research group has been interested in the last 15 years, aiming to study the enthalpic effects of the introduction of different substituents into benzenic ring type compounds. The main purpose of this study is not only to fill the gap of available thermochemical data for benzenic substituted compounds, and so to provide some reliable data to be used for the estimation of the thermochemical properties of compounds with similar structures, but also to contribute to the study of the influence of polar and steric effects of substituents on the thermochemical stability of molecules. This paper reports the standard ( p o = 0.1 MPa) molar enthalpies of combustion, in oxygen, at the temperature 298.15 K, measured by static bomb combustion calorimetry, for the crystalline isomers 3-methoxy-2-nitro-, 4-methoxy-3-nitro-, and 3-methoxy-4nitrobenzoic acid. The Knudsen mass-loss effusion technique was used to measure the vapour pressures as functions of temperature for these compounds and for the three nitrobenzoic acid isomers. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation, at the mean temperature of the experimental range, were derived and corrected to T = 298.15 K. The only thermochemical data available in the literature for these compounds are the enthalpies of combustion of the three nitrobenzoic acids, first measured by Verkade(8) in 1926 and later by Lebedeva et al.(9) in 1971, from which the literature values of the standard molar enthalpies of formation of the three nitrobenzoic acids, in the condensed phase, have been derived. There is no thermochemical data for the crystal–gaseous phase transition, neither for any of the different isomers of the methoxynitrobenzoic acid.
2. Experimental The 3-methoxy-2-nitro-, 4-methoxy-3-nitro-, and 3-methoxy-4-nitrobenzoic acid as well the three nitrobenzoic acid isomers were obtained commercially from Aldrich Chemical Co., and were then purified by vacuum sublimation. The purity of each sample was checked by melting temperatures and g.l.c. using two different columns, and no impurity could be detected. These purities were also confirmed by the mass of carbon dioxide recovered in the combustion experiments to that calculated from the mass of the sample; the averages, together with the standard deviation of the mean, were: 3-methoxy-2-nitrobenzoic acid (0.9996 ± 0.0007); 3-methoxy-4-nitrobenzoic acid (1.0004 ± 0.0011); 4-methoxy-3nitrobenzoic acid (0.9998 ± 0.0010). The combustion experiments were performed with a static bomb calorimeter; the apparatus and technique have been previously described.(1, 10) The energy equivalent of the calorimeter was determined from the combustion of benzoic acid (Bureau of Analysed Samples, Thermochemical Standard, BCS-CRM-190 n) having a massic energy of combustion under standard bomb conditions of −(26432.3±3.8) J · g−1 . The calibration results were corrected to give the energy equivalent ε(calor) corresponding to the average mass of water added to the calorimeter: 3119.6 g. From seven calibration experiments,
o for methoxynitrobenzoic acids 1f Hm
1431
ε(calor) = (16006.4 ± 1.4) J · K−1 , where the uncertainty quoted is the standard deviation of the mean. This value of ε(calor) was used for the combustion of 3-methoxy-4nitrobenzoic acid, and of 4-methoxy-3-nitrobenzoic acid. For the combustion experiments of the other isomer, 3-methoxy-2-nitrobenzoic acid, which was performed a year later, the value of ε(calor) = (16013.8 ± 1.2) J · K−1 was taken, since, in the meantime, the bomb went through some minor changes. For all experiments, samples in pellet form were ignited at T = (298.150 ± 0.001) K in oxygen at a pressure of 3.04 MPa, with a volume of 1 cm3 of water added to the bomb. 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 cottonthread fuse, empirical formula CH1.686 O0.843 , 1c u o = −16250 J · g−1 ,(11) this value has been previously confirmed in our laboratory. For the n-hexadecane used as an auxiliary combustion aid, 1c u o = −(47164.7 ± 2.7) J · g−1 from separate measurements. The corrections for nitric acid formation were based on −59.7 kJ · mol−1 , for the molar energy of formation of 0.1 mol · dm−3 HNO3 (aq) from N2 , O2 , and H2 O(l).(12) Corrections for carbon formation were based on 1c u o = −33 kJ · g−1(12) the massic energy of combustion of C(cr).(12) The amount of substance used in each experiment was determined from the total mass of carbon dioxide produced after allowance for that formed from the cotton thread fuse and hexadecane and the loss due to carbon formation. The density ρ = 1.1 g · cm−3 was assumed for the three isomers. An estimated pressure coefficient of specific energy (∂u/∂ p)T = −0.2 J · g−1 · MPa−1 at T = 298.15 K, a typical value for most organic solids, was assumed. For each compound, 1c u o was calculated by the procedure given by Hubbard et al.(11) The molar masses used for the elements were those recommended by the IUPAC Commission in 1995.(13) For the crystalline 3-methoxy-2-nitro-, 4-methoxy-3-nitro-, and 3-methoxy-4nitrobenzoic acids as well as for the three isomers of nitrobenzoic acid the vapour pressures as functions of temperature were measured using a mass-loss Knudsen-effusion apparatus. The detailed description of the apparatus, procedure and technique, and the results obtained with test substances (benzoic acid and ferrocene) have been reported.(14) This apparatus enables the simultaneous operation of three Knudsen cells, with three different holes. In each effusion experiment the mass loss 1m of the crystalline sample was measured by weighing the cell (containing the sample) to ±0.01 mg, before and after a convenient effusion time period t in a system evacuated to a pressure near 1 · 10−4 Pa. The cells were immersed in a thermostatically controlled silicone oil bath. At the temperature T of each effusion experiment, the vapour pressure p was calculated by p = (1m/A0 w0 t)(2πRT /M)1/2
(1)
in which A0 is the area of the effusion hole, w0 is the respective Clausing factor, w0 = {1 + (3l/8r)}−1 , where l is the thickness of the effusion hole and r is the radius, M is the molar mass of the effusing vapour and R is the gas constant. The effusion time period of each experiment, depending on the temperature and on the compound, was usually between 3 h, for vapour pressures near 1 Pa (the upper limit of the measured vapour pressures) and
1432
M. A. V. Ribeiro da Silva et al. TABLE 1. Typical combustion results for 3-methoxy-2-nitrobenzoic acid, 4-methoxy3-nitrobenzoic acid, and 3-methoxy-4-nitrobenzoic acid, at T = 298.15 K
m(CO2 , total)/g
3-methoxy-2-nitro-
4-methoxy-3-nitro-
3-methoxy-4-nitro-
benzoic acid
benzoic acid
benzoic acid
1.85046
2.02211
1.39658
m 0 (cpd)/g
0.57989
0.78339
0.41716
m 00 (fuse)/g
0.00246
0.00378
0.00244
m 000 (hexadecane)
0.26076
0.19841
0.20827
ε(cal)/(J · K−1 )
16013.8
16006.4
16006.4
16.60
16.75
15.99 1.08945
εf /(J · K−1 ) 1Tad / K
1.43298
1.47733
1m(H2 O)/g
0.0
0.1
0.1
−1U (IBP)/J
22971.86
23672.78
17457.25
1U (hexadecane)/J
12293.88
9357.75
9822.89
1U (HNO3 )/J
31.04
40.27
20.44
1U (ign)/J
0.62
0.68
1.20
1U6 /J
13.66
16.77
9.79
0.0
0.0
0.0
1U (carb)/J 1U (fuse)/J −1c u o /(J · g−1 )
39.95
61.39
39.63
18625.69
18120.27
18127.58
6 h for vapour pressures near 0.1 Pa (the lower limit). The thickness of the effusion holes, l, was 0.049 mm and their areas and Clausing factors were: hole 1, A0 = 0.596 mm2 , w0 = 0.959; hole 2, A0 = 0.754 mm2 , w0 = 0.964; hole 3: A0 = 0.862 mm2 , w0 = 0.966. The integrated form of the Clausius–Clapeyron equation, ln( p/Pa) = a − b(K/T ), g where b = 1cr Hmo (hT i)/R, was used to derive the standard molar enthalpies of sublimation g at the mean temperature of the experimental temperature range, 1cr Hmo (hT i).
3. Results Results for a typical combustion experiment on each compound are given in table 1, where 1m(H2 O) is the deviation of the mass of water added to the calorimeter from the mass (3119.6 g) assigned for ε(calor), and 1U6 is the correction to the standard state; the remaining quantities are as previously described.(11) As samples were ignited at T = (298.150 ± 0.001) K: 1U (IBP) = −{ε(calor) + 1m(H2 O)c p (H2 O, l) + ε f }1Tad + 1Uign . uo
(2)
The individual values of −1c for all the combustion experiments, together with the mean value and standard deviation for each compound, are given in table 2. Table 3 lists the derived standard molar energies and enthalpies of combustion, 1c Umo (cr) and
o for methoxynitrobenzoic acids 1f Hm
1433
TABLE 2. Individual values of the standard massic energies of combustion −1c u o for 3-methoxy-2-nitrobenzoic acid, 4-methoxy-3-nitrobenzoic acid, and 3-methoxy-4-nitrobenzoic acid, at T = 298.15 K 3-methoxy-2-nitro-
4-methoxy-3-nitro-
3-methoxy-4-nitro-
benzoic acid
benzoic acid
benzoic acid
−1c u o /(J · g−1 ) 18271.46
18126.88
18114.94
18266.16
18169.11
18112.65
18253.22
18103.64
18151.53
18265.69
18120.27
18059.92
18240.16
18134.97
18047.95
18271.04
18146.35
18127.58
18101.27 h−1c u o i/(J · g−1 ) 18261.3 ± 5.0
18128.9 ± 9.0
18102.4 ± 16.4
TABLE 3. Derived standard ( p o = 0.1 MPa) molar values in the crystalline state at T = 298.15 K Acid
o (cr) −1c Um kJ · mol−1
o (cr) −1c Hm kJ · mol−1
o (cr) −1f Hm kJ · mol−1
3-methoxy-2-nitro-
3600.2 ± 2.1
3597.1 ± 2.1
551.4 ± 2.3
3574.1 ± 3.7
3571.0 ± 3.7
577.5 ± 3.8
3568.8 ± 6.4
3565.7 ± 6.4
582.8 ± 6.5
benzoic acid 4-methoxy-3-nitrobenzoic acid 3-methoxy-4-nitrobenzoic acid
1c Hmo (cr), and the standard molar enthalpies of formation 1f Hmo (cr) for the crystalline methoxynitrobenzoic acids studied at T = 298.15 K. In accordance with normal thermochemical practice, the uncertainties assigned to the standard molar enthalpies of combustion and formation are, in each case, twice the overall standard deviation of the mean and include the uncertainties in calibration(15) and in the values of the auxiliary quantities used. To derive 1f Hmo (cr) from 1c Hmo (cr) the standard molar enthalpies of formation, at T = 298.15 K, for H2 O(l), −(285.830 ± 0.042) kJ · mol−1 ,(16) and for CO2 (g), −(393.51 ± 0.13) kJ · mol−1 ,(16) were used. The Clausius–Clapeyron equation was used to derive the standard molar enthalpies of g sublimation at the mean temperature of the experimental temperature range, 1cr Hmo (hT i).
1434
M. A. V. Ribeiro da Silva et al. TABLE 4. Knudsen-effusion results for the studied complexes. The equilibrium vapour pressures are denoted by p, and the deviations of experimental results from those given by the Clausius–Clapeyron equations are denoted by 100 · 1 ln( p/Pa) T /K
100 · 1 ln( p/Pa)
p/Pa hole 1
hole 2
hole 3
hole 1
hole 2
hole 3
3-methoxy-2-nitrobenzoic acid 398.17
0.1222
0.1275
0.1315
−0.03
2.08
1.64
399.15
0.1342
0.1359
0.1396
−0.79
−1.69
−2.49
400.22
0.1516
0.1516
0.1598
0.39
−1.79
402.25
0.1855
0.1900
0.1953
−0.14
0.03
404.16
0.2307
0.2329
0.2405
2.36
1.05
0.94
406.15
0.2709
0.2850
0.2938
−1.49
1.28
1.07
408.25
0.3336
0.3450
0.3593
−1.48
−0.46
0.42
409.15
0.3738
0.3816
0.3970
1.04
0.75
1.55
410.15
0.4086
0.4126
0.4201
0.15
−1.25
−2.57
387.10
0.2493
0.2469
0.2512
0.20
−0.31
−0.41
389.08
0.3059
0.3014
0.3063
0.65
−0.28
−0.66
391.30
0.3733
0.3766
0.3858
−1.63
−0.10
0.15
393.25
0.4624
0.4582
0.4718
0.49
0.31
0.92
395.10
0.5440
0.5468
0.5594
−1.38
−0.05
−0.23
397.15
0.6856
0.6770
0.6972
1.89
1.51
1.84
399.05
0.8165
0.8029
0.8192
1.10
0.40
−0.34
401.15
0.9732
0.9613
0.9917
−1.31
−1.47
−1.27
0.03 −0.6
4-methoxy-3-nitrobenzoic acid
3-methoxy-4-nitrobenzoic acid 388.15
0.2359
0.2352
0.2333
3.32
2.04
1.45
390.20
0.2753
0.2777
0.2747
−1.78
−1.87
−2.72
392.15
0.3371
0.3469
0.3485
−0.88
1.05
1.77
394.05
0.4140
0.4166
0.4099
1.00
0.71
−0.63
396.05
0.4826
0.4883
0.5080
−3.13
−2.84
1.42
398.15
0.5981
0.6063
0.6010
−1.89
−1.39
−1.95
400.15
0.7434
0.7439
0.7358
0.79
0.02
−0.73
402.15
0.9139
0.9187
0.9071
2.57
2.28
1.38
The experimental results obtained from each cell for each compound, together with the residuals of the Clausius–Clapeyron equation obtained from least-squares adjustment, are presented in table 4. Table 5 presents, for each compound and each hole used, the detailed parameters of the
o for methoxynitrobenzoic acids 1f Hm
1435
TABLE 4—continued T /K
100 · 1 ln( p/Pa)
p/Pa hole 1
hole 2
hole 3
hole 1
hole 2
hole 3
346.19
0.1046
0.1024
0.1050
349.21
0.1486
0.1475
0.1505
−0.15
0.54
−0.16
−0.52
−1.22
352.16
0.2069
0.2052
0.2065
−1.24
−0.25
−0.88
0.65
355.65
0.3003
0.2968
0.3057
1.30
1.01
0.42
356.67
0.3405
0.3324
0.3418
−0.02
0.92
0.55
359.17
0.4487
0.4396
0.4499
−0.49
0.08
0.32
362.16
0.6063
0.6015
0.6167
1.36
0.66
0.89
366.18
0.9484
0.9331
0.9642
−1.23
−1.10
−1.43
2-nitrobenzoic acid
3-nitrobenzoic acid 347.16
0.2145
0.2137
0.2164
−0.87
−0.68
−1.01
349.12
0.2629
0.2629
0.2665
−0.36
−0.50
−1.11
351.16
0.3197
0.3215
0.3204
1.63
0.94
1.91
353.16
0.4033
0.3987
0.3994
−0.74
0.30
0.60
354.21
0.4430
0.4449
0.4469
0.70
0.17
0.13
355.19
0.4887
0.4929
0.4934
0.94
0.00
0.23
357.14
0.6102
0.5954
0.5973
−1.36
1.03
0.90
359.17
0.7333
0.7435
0.7460
0.70
−0.72
−1.01
361.16
0.9055
0.9045
0.9045
−0.62
−0.54
−0.63
−0.36
−0.12
0.00 0.00
4-nitrobenzoic acid 367.15
0.2294
0.2243
0.2289
369.17
0.2783
0.2786
0.2807
0.89
−1.26
371.17
0.3468
0.3394
0.3460
−0.85
−0.78
0.00
373.17
0.4229
0.4083
0.4179
−0.65
0.74
0.00
375.15
0.5081
0.5010
0.5055
0.60
−0.15
0.01
377.17
0.6146
0.6113
0.6190
1.43
−0.23
0.01
379.31
0.7723
0.7579
0.7827
−0.70
−1.06
−0.02
381.21
0.9237
0.9028
0.9234
−0.38
−0.37
0.00
Clausius–Clapeyron equation, the mean temperature hT i of the experiments, the values for the global treatment of all the ( p, T ) points obtained for each compound and the g standard molar enthalpy of sublimation at T = 298.15 K, 1cr Hmo (298.15 K). For each substance, the calculated enthalpies of sublimation obtained from each individual hole are in agreement within experimental error. The uncertainties associated with the calculated values of the enthalpies of sublimation were obtained from the standard deviation of the slope of the linear regression. The mean of these values is also similar to the value derived
1436
M. A. V. Ribeiro da Silva et al.
TABLE 5. Experimental results for the studied compounds where a and b are from g o the Clausius–Clapeyron equation: ln( p/Pa) = a − b · (K/T ), and b = (1cr Hm hT i/R); −1 −1 R = 8.31451 J · K · mol hT i K
g
o (hT i) 1cr Hm kJ · mol−1
Hole number
a
b
Hole 1
39.16 ± 0.41
16429 ± 168
136.6 ± 1.4
Hole 2
39.26 ± 0.47
16462 ± 191
136.9 ± 1.6
Hole 3
39.16 ± 0.54
16407 ± 220
Global results
39.20 ± 0.37
16433 ± 151
g
o (T = 298.15 K) 1cr Hm kJ · mol−1
3-methoxy-2-nitrobenzoic acid
136.4 ± 1.8 404.18
136.6 ± 1.3
141.9 ± 1.3
4-methoxy-3-nitrobenzoic acid Hole 1
37.93 ± 0.43
15220 ± 168
126.5 ± 1.4
Hole 2
37.75 ± 0.27
15151 ± 108
126.0 ± 0.9
Hole 3
38.07 ± 0.32
15271 ± 126
Global results
37.93 ± 0.22
15220 ± 88
Hole 1
37.64 ± 0.75
15184 ± 298
126.3 ± 2.5
Hole 2
37.60 ± 0.62
15165 ± 244
126.1 ± 2.0
Hole 3
37.56 ± 0.57
15148 ± 227
125.9 ± 1.9
Global results
37.60 ± 0.35
15166 ± 137
127.0 ± 1.0 394.12
126.5 ± 0.8
131.2 ± 0.8
3-methoxy-4-nitrobenzoic acid
395.15
126.1 ± 1.1
131.0 ± 1.1
2-nitrobenzoic acid Hole 1
37.92 ± 0.20
13909 ± 70
115.6 ± 0.6
Hole 2
37.89 ± 0.20
13905 ± 72
115.6 ± 0.6
Hole 3
38.12 ± 0.19
13977 ± 69
Global results
37.98 ± 0.16
13930 ± 57
116.2 ± 0.6 356.19
115.8 ± 0.5
118.7 ± 0.5
3-nitrobenzoic acid Hole 1
35.67 ± 0.30
12920 ± 105
107.4 ± 0.9
Hole 2
35.71 ± 0.12
12934 ± 69
107.5 ± 0.6
Hole 3
35.45 ± 0.30
12843 ± 107
Global results
35.61 ± 0.15
12899 ± 52
106.8 ± 0.9 354.16
107.2 ± 0.4
110.0 ± 0.4
4-nitrobenzoic acid Hole 1
36.30 ± 0.26
13868 ± 99
115.3 ± 0.8
Hole 2
36.21 ± 0.20
13840 ± 73
115.1 ± 0.6
Hole 3
36.46 ± 0.29
13928 ± 110
Global results
36.32 ± 0.20
13879 ± 74
115.8 ± 0.9 374.18
115.4 ± 0.6
119.2 ± 0.6
from the overall treatment of the global results. The plots of ln p = f(1/T ) for the global results of each compound are presented in figure 1. The sublimation enthalpies at T = 298.15 K were derived from the values calculated at
o for methoxynitrobenzoic acids 1f Hm
–0.2
–0.8
ln ( p /Pa)
–0.6
–1.1
–1.0 –1.4 –1.4 –1.7
–1.8
–2.0
–2.2 a –2.6 2.70
ln ( p /Pa)
1437
2.75
2.80
2.85
d –2.3 2.90 2.43
–0.2
–0.1
–0.6
–0.4
2.45
2.47
2.49
2.51
–0.7 –1.0 –1.0 –1.4
–1.3 b
–1.8 2.76 2.78
2.80 2.82
2.84
2.88
2.54
2.56
2.58
2.6
–0.4
–0.4 –0.7
–0.7
–1.0
–1.0
–1.3
–1.3 c
–1.6 2.61
2.52
–0.1
–0.1
ln ( p /Pa)
2.86
e –1.6 2.48 2.50
f 2.65
2.69 1000 K/T
–1.6 2.73 2.48
2.50
2.52
2.54
2.56
2.5
1000 K/T
FIGURE 1. Plot of ln( p/Pa) against 1/T for the studied compounds. 2, hole 1; O, hole 2; +, hole 3. a, 2-nitrobenzoic acid; b, 3-nitrobenzoic acid; c, 4-nitrobenzoic acid; d, 3-methoxy-2-nitrobenzoic acid; e, 4-methoxy-3-nitrobenzoic acid; f, 3-methoxy-4-nitrobenzoic acid.
1438
M. A. V. Ribeiro da Silva et al.
TABLE 6. ( p, T ) values from the vapor-pressure equations for 3-methoxy-2-nitrobenzoic acid, 4methoxy-3-nitrobenzoic acid, 3-methoxy-4-nitrobenzoic acid, 2-nitrobenzoic acid, 3-nitrobenzoic acid, and 4-nitrobenzoic acid p/Pa
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
3-methoxy-2-nitrobenzoic acid
395.95 402.68
406.72
409.63 411.93 413.82
415.43
416.84
418.09
419.21
378.30 384.93
388.92
391.80 394.07 395.94
397.53
398.92
400.16
401.27
4-methoxy-3-nitrobenzoic acid 3-methoxy-4-nitro380.07 386.79
390.83
393.75 396.05 397.94
399.56
400.97
402.22
403.35
2-nitrobenzoic acid
benzoic acid
345.84 351.90
355.54
358.17 360.24 361.94
363.40
364.67
365.80
366.81
3-nitrobenzoic acid
340.23 346.57
350.39
353.15 355.32 357.11
358.64
359.98
361.16
362.23
4-nitrobenzoic acid
359.35 365.92
369.87
372.73 374.97 376.83
378.41
379.80
381.03
382.13
the mean temperature hT i of the experiments by the equation g
g
g
1cr Hmo (T = 298.15 K) = 1cr Hmo (hT /Ki) + 1cr C op,m (298.15 K − hT /Ki)
(3)
g
with 1cr C op,m = −50 J · K−1 · mol−1 estimated for all compounds, in accordance with similar estimations made by other authors,(17) which we have also used in previous papers for other organic compounds.(18–24) Table 6 lists the ( p, T ) values calculated from the equations derived for the six compounds, over the vapour pressure range 0.1 Pa to 1 Pa. The derived standard molar enthalpies of formation in the crystalline state, standard molar enthalpies of sublimation, and standard molar enthalpies of formation in the gaseous state, 1f Hmo (g) at T = 298.15 K, are summarised in table 7.
4. Discussion Cox(25)
proposed an empirical scheme to estimate the standard molar enthalpies of formation 1f Hmo (g) of di- and tri-substituted benzenes in the gaseous state to within ±10 kJ · mol−1 . This scheme is based in a constant increment in 1f Hmo (g) on substitution of a particular group into the benzene ring, independently of the position of the substitution. This assumption implies that the transferability of bond energy holds, and so this simple procedure would predict the same calculated values of 1f Hmo (g) for all members of a set of positional isomers. Due to the lack of enough reliable experimental data in 1978 this model considers only alkyl and polar substituents, so the suggested correction implies more than one single effect. In order to improve the estimation performance, the model was refined to take account of steric effects between adjacent substituted groups, with an additive correction of +4 kJ · mol−1 for every pair of ortho-substituents, except if both substituents are CH3 groups or one is CH3 and the other COOH, when no correction is
o for methoxynitrobenzoic acids 1f Hm
1439
TABLE 7. Derived standard ( p o = 0.1 MPa) molar values at T = 298.15 K g
Acid
o (cr) −1f Hm kJ · mol−1
o 1cr Hm kJ · mol−1
o (g) −1f Hm kJ · mol−1
3-methoxy-2-nitro-
551.4 ± 2.3
141.9 ± 1.3
409.5 ± 2.6
577.5 ± 3.8
131.2 ± 0.8
446.3 ± 3.9
582.8 ± 6.5
131.0 ± 1.1
451.8 ± 6.6
benzoic acid 4-methoxy-3-nitrobenzoic acid 3-methoxy-4-nitrobenzoic acid 2-nitrobenzoic acid
398.5 ± 0.7a
118.7 ± 0.5
279.8 ± 0.9
3-nitrobenzoic acid
414.0 ± 0.5a
110.0 ± 0.4
304.0 ± 0.6
4-nitrobenzoic acid
426.9 ± 0.9a
119.2 ± 0.6
307.7 ± 1.1
a Reference 9.
TABLE 8. Experimental and estimated values for the standard molar enthalpy of formation at T = 298.15 K Acid
o (g)/kJ · mol−1 −1f Hm
1/kJ · mol−1
Experimental
Estimated
409.5 ± 2.6
449.0 ± 1.7
39.5 ± 3.1
446.3 ± 3.9
457.0 ± 1.7
10.7 ± 4.3
benzoic acid
451.8 ± 6.6
457.0 ± 1.7
5.2 ± 6.8
2-nitrobenzoic acid
279.8 ± 0.9
306.5 ± 1.3
26.7 ± 1.6
3-nitrobenzoic acid
304.0 ± 0.6
310.5 ± 1.3
6.5 ± 1.4
4-nitrobenzoic acid
307.7 ± 1.1
310.5 ± 1.3
2.8 ± 1.4
2-methoxybenzoic acid
433.8 ± 1.2a
441.9 ± 1.5
8.1 ± 1.9
3-methoxybenzoic acid
446.1 ± 1.3a
445.9 ± 1.5
−0.2 ± 2.0
4-methoxybenzoic acid
451.9 ± 1.4a
445.9 ± 1.5
−6.0 ± 2.1
3-methoxy-2-nitrobenzoic acid 4-methoxy-3-nitrobenzoic acid 3-methoxy-4-nitro-
a Reference 28. 1 = {1 H o (g) experimental − 1 H o (g) estimated } f m f m
needed; an additional correctional factor of +4 kJ · mol−1 must be used for every set of three substituents in positions 1, 2, 3. From the available values for the standard molar enthalpies of formation in the
1440
M. A. V. Ribeiro da Silva et al.
gaseous state at T = 298.15 K, for benzene (82.6 ± 0.7) kJ · mol−1 ,(26) benzoic acid −(295.4 ± 0.2) kJ · mol−1 ,(27) nitrobenzene (67.5 ± 0.5) kJ · mol−1 ,(26) and methoxybenzene −(67.9 ± 0.9) kJ · mol−1 ,(26) one can calculate the enthalpic increments for the entrance of the groups –COOH, –NO2 and –OCH3 in the benzene ring, respectively, as −(378.0 ± 0.7) kJ · mol−1 , −(15.1 ± 0.9) kJ · mol−1 , and −(150.5 ± 1.1) kJ · mol−1 . Table 8 resumes the experimental values of 1f Hmo (g) determined for the six compounds studied in this paper together with the values for the three isomers of methoxybenzoic acids,(28) and the values estimated by the Cox scheme.(28) The comparison of the experimental values with those estimated by the Cox scheme shows that, for these compounds, this scheme fulfils the claim that the calculated values are reliable to within ±10 kJ · mol−1 , except for the ortho-nitro substituted compounds. The reason for that discrepancy is the high steric hindrance between the adjacent nitro and carbonyl groups, as well as the fact that both groups are electron-withdrawing groups. The difference 1 between the experimental and estimated values of 1f Hmo (g) on all the benzoic acid compounds with nitro substituents is always positive, showing a destabilization due to the fact that both groups withdraw electrons from the benzene ring. This destabilization is considerable higher for the ortho-substitution due to the existence of an additional strong steric effect, being lower for the substitutions in the meta- and para-positions. The 1 values for the three isomers of methoxybenzoic acid show a positive value for the ortho isomer, due to steric hindrance, and negative values for the meta and para isomers, where this interaction is small or non-existent. Thanks are due to Junta Nacional de Investigac¸a˜ o Cient´ıfica e Tecnol´ogica (JNICT) for financial support to the Faculty of Science of the University of Porto (Project PBIC/C/QUI/2193/95) as well as to Centro de Investigac¸a˜ o em Qu´ımica of the University of Porto (research unit number 81, CIQ-UP./L.5). One of us (DH) thanks PRAXIS XXI for the award of a research grant (BD/9349/96) under Programa Ciˆencia. REFERENCES 1. Ribeiro da Silva, M. A. V.; Ribeiro da Silva, M. D. M. C.; Pilcher, G. J. Chem. Thermodynamics 1984, 16, 1149–1155. 2. Ribeiro da Silva, M. D. M. C.; Ribeiro da Silva, M. A. V.; Pilcher, G. J. Chem. Thermodynamics 1986, 18, 295–300. 3. Ribeiro da Silva, M. D. M. C.; Ribeiro da Silva, M. A. V.; Pilcher, G. J. Chem. Thermodynamics 1988, 20, 969–974. 4. Ribeiro da Silva, M. A. V.; Ribeiro da Silva, M. D. M. C.; Teixeira, J. A. S.; Bruce, J. M.; Guyan, P. M.; Pilcher, G. J. Chem. Thermodynamics 1989, 21, 265–274. 5. Ribeiro da Silva, M. A. V.; Reis, A. M. M. V.; Monte, M. J. S.; B´artolo, M. S. S. F.; Rodrigues, J. A. R. G. O. J. Chem. Thermodynamics 1992, 24, 653–659. 6. Ribeiro da Silva, M. A. V.; Ferr˜ao, M. L. C. C. H.; Jiye, F. J. Chem. Thermodynamics 1994, 26, 839–846. 7. Ribeiro da Silva, M. A. V.; Ribeiro da Silva, M. D. M. C.; Monteiro, M. F. B. M.; Gomes, M. L. A. C. N.; Chickos, J. S.; Smith, A. P.; Liebman, J. F. Structural Chemistry 1996, 7, 367–373. 8. Verkade, P. E. Verslag Akad. Wetenschappen Amsterdam 1926, 35, 492–504. 9. Lebedeva, N. D.; Ryadnenko, V. L.; Kuznetsova, I. N. Russ. Zh. Fiz. Khim. 1971, 45, 980–981; J. Phys. Chem. (English Translation) 1971, 45, 549.
o for methoxynitrobenzoic acids 1f Hm
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10. Ribeiro da Silva, M. A. V.; Ribeiro da Silva, M. D. M. C.; Pilcher, G. Rev. Port. Quim. 1984, 26, 163–172. 11. Hubbard, W. N.; Scott, D. W.; Waddington, G. Experimental Thermochemistry: Vol. 1. Rossini, F. D.: editor. Interscience: New York. 1956, Chap. 5. 12. J. Phys. Chem. Ref. Data 1982, Supplement no 2. 13. IUPAC. J. Phys. Chem. Ref. Data 1997, 26, 1239–1253. 14. Ribeiro da Silva, M. A. V.; Monte, M. J. S. Thermochimica Acta 1990, 171, 169–183. 15. Rossini, F. D. Experimental Thermochemistry: Vol. 1. Rossini, F. D.: editor. Interscience: New York. 1956, Chap. 14. 16. Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values for Thermodynamics. Hemisphere: New York. 1989. 17. Burkinshaw, P. M.; Mortimer, C. T. J. C. S. Dalton Trans. 1984, 75–77. 18. Ribeiro da Silva, M. A. V.; Monte, M. J. S.; Matos, M. A. R. J. Chem. Thermodynamics 1989, 21, 159–166. 19. Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Monte, M. J. S. J. Chem. Thermodynamics 1990, 22, 609–616. 20. Ribeiro da Silva, M. A. V.; Monte, M. J. S. J. Chem. Thermodynamics 1992, 24, 715–724. 21. Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Monte, M. J. S.; Alves, M. C. B.; Vieira, J. M. A. P. J. Chem. Thermodynamics 1993, 25, 579–590. 22. Ribeiro da Silva, M. A. V.; Monte, M. J. S. J. Chem. Thermodynamics 1992, 24, 1219–1228. 23. Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Amaral, L. M. P. F. J. Chem. Thermodynamics 1995, 27, 1187–1196. 24. Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Amaral, L. M. P. F. J. Chem. Thermodynamics 1997, 29, 1129–1136. 25. Cox, J. D. A Method for Estimating the Enthalpies of Formation of Benzene Derivatives in the Gas State. NPL Report CHEM 83. 1978. 26. Pedley, J. B. Thermochemical Data and Structures of Organic Compounds: Vol. 1. Thermodynamics Research Center: College Station, TX. 1994. 27. Pilcher, G. The Chemistry of Acid Derivatives: Vol. 2. Patai, S.: editor. John Wiley: New York. 1992, Chap. 2. 28. Colomina, M.; Jimenez, P.; Roux, M. V.; Turrion, C. J. Chem. Thermodynamics 1978, 10, 661– 665. (Received 13 January 1999; in final form 22 February 1999)
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