Thermodynamic properties of isomeric iso-butoxybenzoic acids: Experimental and theoretical study

Article abstract of DOI:10.1016/j.tca.2015.07.011

Standard (p° = 0.1 MPa) molar enthalpies of formation at the temperature T = 298.15 K of the 2-, 3-, and 4-iso-butoxybenzoic acids were measured using the combustion calorimetry. Standard molar enthalpies of vaporization and sublimation were derived from

Full text of DOI:10.1016/j.tca.2015.07.011

Accepted Manuscript
Title: Thermodynamic properties of isomeric
iso-butoxybenzoic acids: experimental and theoretical study
´
Author: Michał Jakubczyk Andrzej Sporzynski Vladimir N.
Emel’yanenko Mikhail A. Varfolomeev Sergey P. Verevkin
PII:
S0040-6031(15)00301-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.tca.2015.07.011
TCA 77292
To appear in:
Thermochimica Acta
Received date:
Revised date:
Accepted date:
10-3-2015
16-7-2015
21-7-2015
´
Please cite this article as: M. Jakubczyk, A. Sporzynski, V.N. Emel’yanenko,
M.A. Varfolomeev, S.P. Verevkin, Thermodynamic properties of isomeric iso-
butoxybenzoic acids: experimental and theoretical study, Thermochimica Acta (2015),
http://dx.doi.org/10.1016/j.tca.2015.07.011
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Highlights (for review)
Highlights
Vapor pressures of butoxy benzoic acid derivatives were measured
Vaporization, sublimation and fusion enthalpies were derived
Molar enthalpies of formation were measured by calorimetry
Thermochemical data tested for consistency using additivity rules and computations
Simple additivity method suggested for prediction thermochemical properties
Page 1 of 25

*Manuscript
Thermochimica Acta, 2015
Thermodynamic properties of isomeric iso-butoxybenzoic acids:
experimental and theoretical study
Michał Jakubczyk,a Andrzej Sporzyński,a Vladimir N. Emel’yanenko,b Mikhail A.
Varfolomeev,b Sergey P. Verevkinb,c,*
aFaculty of Chemistry, Warsaw University of Technology, 00-664 Warszawa, Poland
bDepartment of Physical Chemistry, Kazan Federal University, 420008 Kazan, Russia
cDepartment of Physical Chemistry and Department „Science and Technology of Life, Light
and Matter“, University of Rostock, D-18059, Rostock, Germany
Standard (p° = 0.1 MPa) molar enthalpies of formation at the temperature T = 298.15 K of the
2-, 3-, and 4-iso-butoxybenzoic acids were measured using the combustion calorimetry.
Standard molar enthalpies of vaporization and sublimation were derived from the vapor
pressure temperature dependencies measured by the transpiration method. Molar enthalpies of
the solid state phase transitions were measured by the DSC. Thermodynamic data on alkoxy
substituted benzoic acids available in the literature were collected and combined with own
experimental results. This data set on alkoxybenzoic acids was evaluated by using quantum-
chemical and group-additivity methods.
Keywords: benzoic acid derivatives, vapor pressure measurements, enthalpy of sublimation,
enthalpy of vaporization, enthalpy of fusion, enthalpy of formation, quantum-chemical
calculations, group-additivity.
* Corresponding author. Tel.: +381 4986508; fax +381-498-6502.
E-mail address: sergey.verevkin@uni-rostock.de (S.P. Verevkin)
Page 2 of 25

1. Introduction
Alkoxybenzoic acids have become an object of recent increasing interest due to their
new applications. They are used as flavoring substances and preservatives of fragrance in
cosmetic formulations [1]. As some of them are natural products, they are good candidates for
pharmaceutical co-crystals [1]. Several alkoxybenzoic acids and their derivatives reveal
biological activity as inhibitors of phosphodiesterase [2]. The majority of the papers on
alkoxybenzoic acids deals with n-alkoxy derivatives. It is due to their liquid crystal properties.
Esters of these compounds with triethanolamine were found to be discotic mesogens [3]. It is
noteworthy that these compounds easily form mixed supramolecular liquid crystals induced
by hydrogen bonding which have a much wider mesophase range than their individual
components [4-5]. For all the isomers of methoxybenzoic acid molecular and crystal
structures have been determined by X-ray diffraction method [2,6]. The investigated
compounds form dimeric molecules with a moderately strong intermolecular hydrogen bonds.
Several polymorphs have been observed due to the flexibility of the molecule [1].
Thermochemistry of the methoxy substituted benzoic acids has been evaluated recently [7].
The molar enthalpy of formation and sublimation on benzoic acids with the n-alkoxy-
substituents were also reported [8,9]. The lack of the data for branched alkoxy-substituted
benzoic acids was the reason to investigate the iso-butoxy benzoic acids (see Figure 1) in the
present paper.
Fig. 1. Compounds studied in this work: 2-iso-butoxybenzoic acid, 3-iso-butoxybenzoic acid
and 4-iso-butoxybenzoic acid.
The new results together with the thermochemical data on alkoxybenzoic acid
available in the literature (see Table 1) have been checked for internal consistency using the
quantum chemical and the group-additivity methods.
Page 3 of 25

2.
Materials and methods
2.1.
Materials
Samples of iso-butoxybenzoic acids were synthesized from the commercially available
bromo-iso-butoxybenzenes according to a general procedure given on Fig. 2. Necessary
details are given below for each isomer as follows.
Fig. 2. General procedure for synthesis of iso-butoxybenzoic acids.
4-iso-butoxybenzoic acid. A solution of 4-bromophenyl-iso-butyl ether (40.0 g, 0.175 mol) in
diethyl ether was added dropwise to a solution of n-butyllithium (0.183 mol) in dry diethyl
ether (200 ml) and dry tetrahydrofuran (THF) (20 ml) at -70oC. The temperature did not
exceed –65°C. After completion of the addition the mixture was stirred for one hour at –70°C.
The mixture was carefully poured on 140 g of dry ice in diethyl ether (100 ml). After
completion of this step, 200 ml of water was added. The mixture was acidified to pH=6 with 3
M aq. HCl. The aqueous phase was extracted with ether (100 ml). The combined organic
layers were dried over sodium sulfate and the solvent was removed under reduced pressure.
Recrystallization from hexane yielded with 4-iso-butoxybenzoic acid (15.03 g, 44.2%) as a
1
white powder with the melting point of 139-141°C (lit. 142°C [10]. H NMR (400 MHz,
acetone-d6): 7.00 (d, 2H), 7.96 (d, 2H), 2.04 ( m, CH), 3.84 (d, CH2), 1.20 (d, CH3).
2- and 3-iso-butoxybenzoic acid were synthesized in similar manner.
2-iso-butoxybenzoic acid. Colorless liquid. Crude product was distilled under reduced
1
pressure at 160°C/20 Torr (lit. 135-136°C/1 Torr [11]. Yield 55%, H NMR (400 MHz,
CDCl3): 7.01 (dd, 1H), 7.06 (ddd, 1H), 7.50 (ddd, 1H), 8.11 (ddd, 1H), 2.18 ( m, CH), 3.98
(d, CH2), 1.50 (d, CH3).
Page 4 of 25

3-iso-butoxybenzoic acid. Yield 28.3%, melting point: 79-80°C, 1H NMR (400 MHz, CDCl3):
7.11 (ddd, 1H), 7.37 (dd, 1H), 7.62 (dd, 1H), 7.70 (ddd, 1H), 2.11 ( m, CH), 3.78 (d, CH2),
1.04 (d, CH3).
The products were re-crystalized or distilled under reduced pressure. No impurities
(greater than 0.001 mass fractions) could be detected in samples used for the before the
thermochemical measurements. The degree of purity was determined using the GC and the
DSC methods. Provenance and purity of the compound prepared for thermochemical studies
in this work are given in Table S1.
2.2 Combustion calorimetry
Standard molar enthalpies of combustion of iso-butoxybenzoic acids were measured
with an isoperibolic calorimeter with a static bomb and a stirred water bath. The detailed
procedure has been described previously [12]. The liquid sample of the ortho-isomer was
placed (under an inert atmosphere in a glove-box) in a polythene ampoule (Fa. NeoLab,
Heidelberg, Germany). The neck of the ampoule was compressed with a special tweezers and
was sealed outside the glove-box by heating of the neck in a close proximity to a glowing
wire. Then, the container was placed in the crucible and was burned in oxygen at a pressure
3.04 MPa. The solid samples were pressed into pellets of mass  700 mg, and were burned in
oxygen with a mass of 1.00 g of water added to the bomb. The combustion products were
examined for carbon monoxide (Dräger tube) and unburned carbon but neither was detected.
The energy equivalent of the calorimeter calor = 14816.00.7 JK-1 was determined with a
standard reference sample of benzoic acid (sample SRM 39j, NIST). For the reduction of the
data to standard conditions, conventional procedures [13] were used. Auxiliary data are
collected in Table S2. Correction for nitric acid formation was based on titration with 0.1
mol·dm-3 NaOH (aq). The residual water content 261.3 ppm in the liquid sample was
determined by Karl Fischer titration before starting experiments and appropriate corrections
have been made for combustion results. No discernable amount of moisture was detected in
the solid samples. Primary results from combustion experiments are summarized in Tables
S3-S5.
2.3. Transpiration method. Vapor pressure measurements
Vapor pressures of iso-butoxybenzoic acids were measured using the transpiration
method [14,15]. About 0.5 g of a solid sample was dissolved in acetonitrile and mixed with
small glass beads in a glass beaker. Under vigorous stirring with spatula and gentle heating
the solvent was removed from beaker producing glass beads covered uniformly with the
Page 5 of 25

sample. These covered glass beads were placed in a thermostatted U-shaped saturator. A well-
defined nitrogen stream was passed through the saturator at a constant temperature (± 0.1 K),
and the transported material was collected in a cold trap. The amount of condensed sample
was determined by weighing with microbalances of 0.0001 g resolution. Material collected in
the cold trap was systematically analyzed by the GC, but decomposition products were not
detected.
The absolute vapor pressure pi at each temperature Ti was calculated from the amount
of the product collected within a definite period. Assuming validity of the Dalton`s law
applied to the nitrogen stream saturated with the substance i, values of pi were calculated with
Eq. (1):
pi = mi·R·Ta / V·Mi ;
V= VN2 + Vi;
(VN2 » Vi)
(1)
where R = 8.314462 J·K-1·mol-1; mi is the mass of the transported compound, Mi is the molar
mass of the compound, and Vi; its volume contribution to the gaseous phase. VN2 is the volume
of the carrier gas and Ta is the temperature of the soap bubble meter used for measurement of
the gas flow. The volume of the carrier gas VN2 was determined from the flow rate and the
time measurement. Experimental results are given in Tables 1 and 2. Accuracy of vapor
pressures measured by the transpiration method is governed mostly by the reproducibility of
the GC analysis as well as by the volume VN2 determination [15]. The standard uncertainties
(u) of the measured vapor pressures have been calculated to be:
u(p/Pa) = 0.025 +0.025(p/Pa) for p > 5 to 1000 Pa
u(p/Pa) = 0.005 +0.025(p/Pa) for p < 5 Pa
To validate system operation, the vapor pressure of benzoic acid was determined at p = (1 to
500) Pa. Experimental vapor pressure of benzoic acid obtained using the transpiration agreed
with the results reported in the literature [16] to within (0.5 to 2.5) %. The system operation
was also successfully tested (see Fig. S1) with measurements on the 2-methoxybenzoic acid,
where reliable vapour pressures data are available from Knudsen effusion technique [17]. For
validation of our uncertainty estimations we measured vapor pressures for series of n-alkanols
[18] and aliphatic esters [14] where reliable data at p = (0.1 to 1000) Pa from different
methods were available. It has turned out that vapor pressures of n-alkanols and esters derived
from the transpiration method were comparable with the available high-precision data
generally within (1 to 3) %. Experimental results from transpiration methods are collected in
Table 2.
2.4. DSC. Enthalpies of the solid phase transitions measurements
Page 6 of 25

The thermal behavior of solid samples of 3- and 4-iso-butoxybenzoic acid was studied
with a Metller Toledo 822. The instrument was standardized using indium metal with a mass
fraction of 0.9999. The samples were hermetically sealed in 50 L pans supplied by Perkin
Elmer. The thermal behavior of the specimen was investigated during heating the sample with
a rate of 10 K·min-1. The DSC measurements were repeated in triplicate and values agreed
within the experimental uncertainties u( cr Hm ) = 0.2 kJ·mol-1 for the enthalpy of fusion and
l
u(T) = 0.5 K for the melting temperature. For the sample of 3-iso-butoxybenzoic acid only
phase transition solid-liquid at 353.5K was observed. For the sample of 4-iso-butoxybenzoic
acid also the solid-solid phase transition at 412.5 K was observed. The DSC results for
benzoic acid derivatives are reported in Table 3. Uncertainties in the temperature adjustment
of fusion enthalpies from Tfus to the reference temperature were assumed to amount to 30 % of
the total adjustment [19].
2.5. Computational details
Quantum chemical calculations of benzoic acid derivatives were performed with the Gaussian
09 series software [20]. Energies of molecules were calculated using the G3MP2 method [21].
General computational details within this approach were reported elsewhere [22]. Enthalpies,
H298, of each compound were calculated according to standard thermodynamic procedures
[23].
Table 1
Thermochemical data at T = 298.15 K (p° = 0.1 MPa) for alkoxybenzoic acids (in kJ·mol-1) a
cHm
f Hm (cr)
gcr Hm
gl Hm
f Hm (liq)
f Hm (g)
Compounds St.
2-iso-butoxy- liq
3-iso-butoxy- cr
4-iso-butoxy- cr
-
-
-5715.11.6
-5678.42.6
-5672.81.7
95.60.6
99.20.9
-614.32.2 -518.72.3
-629.53.2 c -530.33.3
-651.03.0
-656.62.2
120.71.4 b
119.00.6 104.12.1 b -641.73.0 c -537.62.3
4-etoxy-
4-butoxy-
4-octyloxy-
cr
cr
cr
-
-
-
95.5±2.8 b
108.8±2.6 b
126.7±3.8 b
-605.51.8 [9] 121.81.1 b
-647.12.5 [9] 123.21.0
-767.93.9 [9] 159.62.6 b
-579.23.2 c -483.72.1
-632.73.5 c -523.92.7
-745.04.8 c -608.34.7
aUncertainties correspond to expanded uncertainties of the mean (0.95 confidence level);
bFrom Table 3;
cCalculated from the experimental f Hm (cr)-values (this table) and the sum of the solid state
phase transitions with enthalpies   Hm values from Table 3.
3.
Results and Discussion
Page 7 of 25

3.1. Enthalpy of formation from the combustion calorimetry
Results of typical combustion experiments on 2-, 3-, and 4-iso-butoxybenzoic acids
are given in Tables S3-S5. The relative atomic masses used for the elements C, H, N and O
were calculated as the mean of the bounds of the interval of the standard atomic weights
recommended by the IUPAC commission in 2011 [24] for each of these elements. Values of
the standard specific energies of combustion  u° are given in Table S6. They have been used
c
to derive the standard molar enthalpies of combustion cHm and the standard molar
enthalpies of formation in the liquid state f Hm (l) or in the crystalline state f Hm (cr) (see
Table 1). Values of  u° and cHm refer to reaction:
c
C11H14O3 (l or cr) + 13 O2 (g) = 11 CO2 (g) + 7 H2O (l)
(2)
The standard molar enthalpies of formation, f Hm , of iso-butoxybenzoic acids were
calculated from the enthalpic balance according to Eq. (2) using standard molar enthalpies of
formation of H2O (l) and CO2 (g) recommended by CODATA [25]. Uncertainties related to
combustion experiments were calculated according to the guidelines presented by Hubbard et
al. [13] and by Olofsson [26]. The uncertainties of the standard molar energy of combustion
correspond to expanded uncertainties of the mean (0.95 confidence level). The uncertainty
assigned to the molar enthalpy of combustion and to the molar enthalpy of formation is twice
the overall standard deviation and includes the uncertainties from calibration, from the
combustion energies of the auxiliary materials, the uncertainties of the enthalpies of formation
of the reaction products H2O and CO2 [25], as well as the uncertainty due to the specific
intervals of atomic masses reported by IUPAC [24]. The standard molar enthalpies of
formation of all three isomers of the iso-butoxybenzoic acids (see Table 1) were measured for
the first time.
3.2. Vapor pressures of iso-butoxybenzoic acids
Samples of 2- and 3-iso-butoxybenzoic acids were measured above their melting
points. Vapor pressure for 4-iso-butoxybenzoic acid was measured over the crystalline
sample. Temperature dependences of vapor pressures pi measured for acids were fitted with
the following equation [14]:
Page 8 of 25



b
T

Rln pi  a   glCp,m ln




(3),
T
T0


gcr C
p,m ) is the difference of the
glCp,m
where a and b are adjustable parameters and
(or
molar heat capacities of the gaseous and the condensed phase respectively. T0 appearing in
Eq. (3) is an arbitrarily chosen reference temperature (which has been chosen to be T = 298.15
glCp,m
gcr C
or p,m in Eq. (3) were calculated
K) and R is the molar gas constant. Values of
(see Table S7) according to the procedure developed by Chickos and Acree [27] based on the
C
isobaric molar heat capacity p,m (l or cr, 298.15 K). The latter values were estimated by the
group-additivity procedure [28]. Experimental vapor pressures measured by the transpiration
method are given in Table 2.
Table 2
Absolute vapor pressures p, and standard vaporization/sublimation enthalpies, and
vaporization/sublimation entropies of iso-butoxybenzoic acids obtained by the transpiration
method.
gl Hm
kJmol-1 JK-1mol-1
glSm
T/
m/
V(N2)c /
Ta/
Flow/
p/
u(p)/
Pa
/
/
Ka
mgb
dm3
Kd
dm3·h-1
Pae
-1
2-iso-butoxybenzoic acid; gl Hm (298.15 K) = (95.60.6) kJ mol
.
381.0 126620.5 103.9
T/K






ln
ln(p/p°) =
R
R(T/K)
R
298.15


343.9
353.1
363.2
365.2
369.2
371.2
371.4
371.4
371.5
374.2
374.4
376.5
377.3
378.5
379.2
380.1
380.2
382.4
384.2
4.30
5.50
8.8
67.04
39.12
26.67
20.99
24.62
20.39
14.86
16.51
15.71
15.29
16.97
13.11
14.17
16.99
10.33
6.83
291.2
292.7
290.2
290.2
290.7
295.2
289.2
290.7
290.7
290.7
291.2
291.2
292.2
290.7
290.2
293.7
290.7
291.2
291.2
3.61
3.03
5.00
6.09
6.15
6.31
6.37
5.41
4.74
5.43
3.61
7.28
7.02
7.44
6.26
6.31
7.14
6.48
3.61
0.80
1.76
4.10
4.80
6.62
7.75
8.17
7.91
8.08
0.03
0.05
0.11
0.12
0.19
0.22
0.23
0.22
0.23
0.28
0.27
0.32
0.34
0.37
0.39
0.41
0.41
0.47
0.53
90.89
89.94
88.89
88.68
88.27
88.06
88.03
88.03
88.03
87.74
87.72
87.50
87.42
87.29
87.22
87.13
87.12
86.89
86.71
166.8
163.7
160.8
160.2
159.1
158.6
158.8
158.5
158.6
158.0
157.7
157.2
156.9
156.8
156.6
156.3
156.2
155.4
155.0
8.1
13.1
12.5
9.8
10.5
10.2
12.5
13.6
12.4
14.1
19.0
12.2
8.4
10.17
9.99
11.79
12.45
13.91
14.67
15.46
15.53
17.70
20.38
13.8
14.4
18.4
11.06
10.14
11.25
Page 9 of 25

385.8
388.1
390.3
393.0
395.2
398.1
17.1
21.2
12.1
13.9
12.0
27.9
9.59
10.18
4.94
4.75
3.36
6.56
293.2
294.2
292.2
290.7
293.2
295.2
7.02
8.04
6.31
7.31
6.31
7.87
22.37
26.22
30.64
36.39
44.78
53.73
0.58
0.68
0.79
0.93
1.14
1.37
86.54
86.30
86.07
85.79
85.56
154.5
153.8
153.3
152.5
152.4
151.6
85.26
-1
3-iso-butoxybenzoic acid; gl Hm (298.15 K) = (99.20.9) kJ mol
.
389.0 130218.5 103.9
T/K






ln
ln(p/p°) =
R
R(T/K)
R
298.15


354.8
356.2
357.4
359.5
361.2
363.0
364.2
365.7
368.3
370.9
372.7
373.7
375.3
378.2
380.1
382.7
16.1
21.3
13.5
12.4
22.6
22.3
20.2
16.0
14.6
12.5
15.8
14.5
17.3
17.5
21.5
16.9
125.5
148.2
84.13
65.76
101.20
87.84
71.10
49.62
36.81
26.12
28.14
23.94
25.09
20.06
20.91
13.48
294.2
294.2
294.2
294.2
294.2
294.2
294.2
294.2
290.2
293.2
294.2
294.2
295.2
294.2
295.2
294.2
3.18
5.00
5.77
5.08
4.00
5.06
3.99
6.49
6.92
7.43
8.28
7.85
8.01
5.60
4.50
1.62
1.81
2.02
2.38
2.81
3.20
3.58
4.06
4.93
6.01
7.07
7.63
8.71
0.05
0.05
0.06
0.06
0.08
0.08
0.09
0.11
0.13
0.18
0.20
0.22
0.24
0.30
0.35
0.42
93.36
93.21
93.09
92.87
92.69
92.50
92.38
92.22
91.95
91.69
91.50
91.40
91.23
90.92
90.73
90.46
.
171.4
171.0
170.6
169.8
169.5
168.7
168.5
168.1
167.2
166.4
166.1
165.8
165.4
164.6
164.3
163.6
10.99
13.00
15.79
8.34
-1
4-iso-butoxybenzoic acid (series I); cr Hm (298.15 K) = (119.00.6) kJ mol
g
371.5 132562.3 45.6
T/K






ln
ln(p/p°) =
R
R(T/K)
R
298.15


368.2 f
373.1 f
378.6 f
379.3 f
381.0 f
388.5 f
390.5 f
393.6 f
393.6 f
393.7 f
393.8 f
395.2 f
396.9 f
397.4 f
398.6 f
399.0 f
399.9 f
402.6 f
403.2 f
13.0
15.0
26.1
20.1
16.5
13.9
12.5
9.9
137.9
94.52
96.29
68.79
49.10
19.62
15.25
9.04
294.2
293.7
294.7
294.7
293.7
293.7
294.7
294.7
294.7
294.2
293.7
291.7
294.7
291.7
292.7
291.7
293.7
293.7
294.7
3.99
4.09
7.60
5.44
4.09
5.77
7.50
7.33
7.25
6.00
8.30
5.76
7.60
5.59
7.18
1.37
8.30
8.55
7.36
1.19
1.99
3.42
3.69
4.22
0.03
0.05
0.09
0.10
0.11
0.25
0.28
0.37
0.37
0.38
0.37
0.43
0.52
0.53
0.56
0.61
0.63
0.83
0.85
117.89
117.67
117.41
117.38
117.31
116.96
116.87
116.73
116.73
116.73
116.72
116.66
116.58
116.56
116.50
116.49
116.44
116.32
116.29
225.9
225.4
224.6
224.6
224.2
223.5
223.0
222.7
222.8
222.9
222.5
222.6
222.8
222.6
222.0
222.5
222.0
222.1
221.8
8.90
10.34
13.81
13.88
14.20
13.84
16.17
19.61
20.32
21.26
23.36
24.13
32.18
32.83
8.9
8.09
20.3
16.3
13.3
13.0
9.1
13.2
2.3
23.9
31.0
8.3
18.00
14.80
10.27
8.36
5.59
7.78
1.23
12.45
12.11
3.19
Page 10 of 25

4-iso-butoxybenzoic acid (series II);
342.3 g
343.5 g
346.4 g
348.5 g
349.2 g
353.4 g
353.6 g
356.9 g
359.2 g
360.4 g
361.6 g
363.4 g
363.5 g
365.4 g
365.9 g
367.8 g
368.7 g
368.9 g
370.7 g
373.3 g
0.76
0.66
0.95
0.27
1.09
0.73
0.40
0.59
0.72
0.55
1.05
0.67
0.82
0.44
0.95
1.32
0.82
0.91
0.98
1.09
133.98
103.13
107.88
23.14
88.48
35.55
19.66
19.09
19.21
12.49
19.81
10.58
13.09
5.66
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
290.2
4.09
4.50
4.21
5.24
4.14
3.99
4.14
4.09
4.30
4.21
4.34
3.94
4.20
4.14
2.48
4.17
4.14
4.42
4.05
4.14
0.07
0.08
0.11
0.14
0.15
0.26
0.25
0.38
0.46
0.55
0.66
0.78
0.78
0.96
1.02
1.22
1.34
1.40
1.62
2.16
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.02
0.02
0.02
0.02
0.03
0.03
0.04
0.04
0.04
0.05
0.06
118.95
118.89
118.76
118.66
118.63
118.44
118.43
118.28
118.18
118.12
118.07
117.99
117.98
117.89
117.87
117.78
117.74
117.73
117.65
229.7
229.4
228.8
228.7
228.5
228.1
227.8
227.7
226.9
227.0
227.3
227.0
226.8
226.6
226.6
226.2
226.1
226.2
225.7
225.6
11.63
13.42
7.59
8.10
7.48
6.28
117.53
-1
2-methoxybenzoic acid; cr Hm (298.15 K) = (106.30.9) kJ mol
g
.
345.4 1116657.1 34.7
T/K






ln
ln(p/p°) =
R
R(T/K)
R
298.15


342.4
346.2
350.3
348.2
352.2
354.3
356.2
358.9
361.4
363.2
366.2
367.9
371.5
9.5
157.0
113.3
125.3
53.78
147.2
76.00
80.40
62.62
39.46
58.90
14.83
37.90
13.10
294.7
295.9
295.9
295.9
295.9
295.9
295.9
292.2
295.9
295.9
291.7
290.7
295.9
4.18
3.97
4.17
3.97
6.07
6.00
6.00
4.04
6.07
6.00
5.23
2.93
5.50
0.98
1.52
2.21
1.81
2.72
3.47
4.19
5.36
6.80
8.13
0.03
0.04
0.06
0.05
0.07
0.09
0.11
0.14
0.20
0.23
0.29
0.34
0.46
104.78
104.65
104.50
104.58
104.44
104.36
104.30
104.20
104.12
104.06
103.95
103.89
210.1
210.0
209.2
209.6
209.2
209.2
209.0
208.5
208.3
208.2
207.8
207.8
207.3
10.6
17.1
6.0
24.7
16.3
20.8
21.0
16.6
29.6
9.9
10.64
12.62
17.28
30.1
14.0
103.77
-1
4-butoxybenzoic acid; cr Hm (298.15 K) = (123.21.0) kJ mol
g
.
378.3 137149.4 46.8
T/K






ln
ln(p/p°) =
R
R(T/K)
R
298.15


377.8
383.4
391.0
396.2
399.3
401.3
12.0
20.5
11.6
13.3
10.4
13.4
92.71
90.28
25.01
17.21
10.80
11.02
295.9
293.7
295.2
294.2
294.5
295.2
5.13
5.09
5.09
5.09
5.10
5.09
1.64
2.85
5.86
9.73
12.14
15.36
0.05
0.08
0.15
0.27
0.33
0.41
119.47
119.21
118.85
118.61
118.46
118.37
224.7
224.0
223.0
222.6
221.7
222.0
Page 11 of 25

406.4
411.4
415.4
417.3
417.3
18.2
19.7
12.9
9.8
9.741
6.751
3.316
2.126
2.891
295.9
294.3
295.7
295.9
5.13
5.13
5.10
5.10
5.10
23.66
36.76
49.23
58.38
58.26
0.62
0.94
1.26
1.48
1.48
118.13
117.90
117.71
117.62
117.62
221.3
220.9
220.1
220.0
220.0
13.3
295.9
a
b
c
Saturation temperature (u(T) = 0.1 K). Mass of transferred sample condensed at T = 293 K. Volume of
nitrogen (u(V) = 0.005 dm3) used to transfer m (u(m) = 0.0001 g) of the sample. Ta is the temperature of the
d
soap bubble meter used for measurement of the gas flow. e Vapor pressure at temperature T, calculated from the
f
m and the residual vapor pressure at T = 293 K calculated by an iteration procedure. Series I: the mass of the
transported sample was determined by weighing of the cold trap. g Series II: the mass of the transported sample
was determined by GC.
3.2. Vaporization/sublimation enthalpies of iso-butoxybenoic acids
Vaporization enthalpies at temperature T were derived from the temperature
dependence of vapor pressures using Eq. (4):
g
g
l Hm (T)  b lCp,m T
(4)
Vaporization entropies at temperature T were also derived from the temperature dependence
of vapor pressures using Eq. (5):
glSm (T)  gl Hm /T  Rln(pi / p )
(5)
Experimental absolute vapor pressures measured by the transpiration method, coefficients a
and b of Eq. (3), as well as values of gl Hm (T) and glSm (T) are given in Table 2. Eqs. 3-5 are
also valid for vapor pressure measurements over the solid sample giving the standard molar
enthalpy of sublimation gcr Hm (T) and standard molar sublimation entropy gcr Sm (T).
Procedure for calculation of the combined uncertainties of the vaporization/sublimation
enthalpies was described elsewhere [15, 29]. It includes uncertainties from the transpiration
experimental conditions, uncertainties in vapor pressure, and uncertainties in the temperature
adjustment to T = 298.15 K. The standard molar enthalpies of sublimation or vaporization of
all three isomers of iso-butoxybenzoic acids (see Tables 1 and 2) were measured for the first
time.
In order to extend the temperature range of the 4-iso-butoxybenzoic acid transpiration
study possibly close to the reference temperature we have performed two independent sets of
experiments with this compound. The first set of transpiration experiments was performed
between 368.2 and 403.2 K as it was described for all three isomers by weighing of the
transported material collected in the cold trap accompanied with the GC analytic control of
purity. From our experiences, about 8-20 mg of the collected material are essential for reliable
measurements. Due to very low volatility of 4-iso-butoxybenzoic acid, experiments below
Page 12 of 25

360 K become unreasonable time consuming in order to collect enough of sample for the
weighing. In contrast, for the GC determination of the collected in the trap sample, already
amount of 0.5-1 mg is usually sufficient. Thus, application of the GC for measurements of the
mass m (see Eq. 1) has allowed significantly extend the range towards lower temperatures of
saturation. The second set of experiments with 4-iso-butoxybenzoic acid was performed in the
range 342.3-373.3 K by using the GC analysis (external standard hexadecane) for
determination of m in the cold trap. Experimental vapor pressures measured in the both sets
are given in Table 2. Results from both series were consistent within 1-3% and they were
treated together in order to develop coefficients of Eq. (3) (see Table 2) and the sublimation
enthalpies (see Tables 1 and 2).
No values of enthalpies of vaporization/sublimation of the iso-butoxybenzoic acids are
available in the literature for comparison. For this reason, it is important to consider
consistency of our new data for iso-butoxybenzoic acids in the context of the available in the
literature data for isomeric methoxybenzoic acids [7], as well as with the recent systematic
study of 4-n-alkoxybenzoic acids [9]. In the latter paper the sublimation enthalpies were
measured by the Knudsen effusion method and adjusted to the reference temperature 298.15
K by using the Domalski and Hearing procedure [30]. In order to derive the consistent data
set, the primary experimental vapor pressures reported in ref. [9] were treated in this work
similar to our own results with Eqs. (3) and (4) and by using the Chickos and Acree procedure
glC
[27] with the
p,m -values from Table S7. Enthalpies of sublimation of alkoxybenzoic acids
derived in this way are listed in Tables 1 and 3.
3.3. Solid state phase transitions of 3- and 4-iso-butoxybenoic acids
It has turned out that volatility of the 3-iso-butoxybenzoic acid in the temperature
range below the melting point of 353.5 K was too low in order to perform transpiration
measurements in extended temperature range with a reasonable accuracy. In such cases we
used to perform transpiration experiments above the melting point. The enthalpy of
vaporization derived from vapor pressures temperature dependences according to Eqs. (3) and
l
(4), combined with the DSC measured enthalpy of fusion, cr Hm , provides the desired
enthalpy of sublimation for the compound of interest. Enthalpies of fusion measured by the
DSC are referenced to the melting temperature. In order to adjust these values to 298.15 K,
the procedure established by Chickos and Acree [27] was applied. However, as reported by
Fonseca et al. [9], the solid state phase transitions with enthalpies   Hm are often observed
Page 13 of 25

for the alkoxybenzoic acids. For such compounds we calculated a total sum of solid state
T29fu8s H
l
transition enthalpies,
m , as a sum of the enthalpy of fusion cr Hm and   Hm . We

adjusted T
H
values to 298.15 K (see Table 3) according to recommendation by Chickos
fus
298
m
and Acree [31] in order derive values of gcr Hm (298.15 K) or gl Hm (298.15 K) of
alkoxybenzoic acids listed in Table 3.
Table 3
Compilation of experimental data on enthalpies of phase transitions (in kJmol-1).
d
g
gcr Hm
lHm

c
T
H
fus
a

a,b
T
H
298
m
fus
Compounds
Tfus
298
m
at 298.15 K
5
1
2
3
4
6
3-iso-butoxybenzoic acid 353.5e
4-iso-butoxybenzoic acid 415.3
25.2±0.2
18.9±0.2
21.5±1.1
120.71.4i 99.20.9 f
14.9±2.0 f 119.0±0.6 g 104.1±2.1
4-ethoxybenzoic acid
4-butoxybenzoic acid
4-octyloxybenzoic acid
471.8e [9] 35.1±0.2[9] 26.3±2.6 121.8±2.1 h 95.5±2.8
433.7[9] 22.3±0.2[9] 14.4±2.4 123.2±1.0 g,j 108.8±2.6
421.0[9] 32.2±0.2[9] 22.9±2.8 159.6±2.6 h 126.7±3.8
a
The uncertainty for melting and solid-solid phase transition temperatures u(T) = 0.5 K. The
l
uncertainty for enthalpies of fusion and enthalpies of solid-solid phase transition u(cr Hm ) =
0.2 kJ·mol-1.
b
l
Sum of the enthalpy of fusion cr Hm and the sum of enthalpies of the solid state phase
transitions   Hm (uncertainties of  Hm and cr Hm were not reported in ref. [9] and they
l
were assessed in this work).
c Adjusted to the reference temperature according to ref. [31].
d Difference between column 5 and column 4 in this table.
e No solid state phase transition prior to the melting was observed.
f Sum of the enthalpy of fusion cr Hm = 18.9±0.2 kJmol-1 and the enthalpy of the solid state
l
phase transitions  Hm = 2.8±0.2 kJmol-1 at 412.5±0.5 K was adjusted to the reference
temperature according to ref. [31].
g From Table 2.
h
Primary experimental vapor pressures reported in ref. [9] were treated similar to our own
glCp,m
results with eqs. 3 and 4 and by using the
values from Table S7.
i Calculated as sum of column 4 and 6 from this table.
j
For comparison, experimental value gcr Hm (298.15 K) = 127.8±1.0 kJmol-1 was reported in

= 14.4±2.4 kJmol-1 from column 4, the value lHm (298.15 K) =
g
T
H
fus
ref. [9]. With the
298
m
113.4±2.6 kJmol-1 was calculated.
3.4. Experimental gas and liquid phase standard molar enthalpies of formation
Page 14 of 25

Values of vaporization, sublimation, and solid phase transition enthalpies of
alkoxybenzoic acids (Tables 1-3) can now be used together with the combustion calorimetry
data, to yield the liquid phase, f Hm (liq), and the gaseous standard enthalpies of formation,
f Hm (g) at 298.15 K. The resulting values are given in Table 1. These values can now be
compared to the results from group-additivity procedure and those from the high-level
quantum-chemical calculations.
4. Quantum chemical calculations
The modern high-level quantum-chemical (QC) calculations provide a valuable tool to
test experimental and theoretical results for the mutual consistency [32-33]. We have used the
composite method G3MP2 for calculation of the gaseous enthalpies of formation of
alkoxybenzoic acids for comparison with the experimental data.
4.1. Theoretical gas phase standard molar enthalpies of formation of alkoxybenzoic acids
The optimized structures obtained for each of the iso-butoxybenzoic acids are
illustrated in Fig. 3.
Fig. 3 Optimized structures of 2-, 3- and 4- iso-butoxybenzoic acids obtained with the
G3MP2 method.
Page 15 of 25

The enthalpies H298 calculated for the alkoxybenzoic acid were converted to enthalpies
of formation f Hm (g, 298.15 K) using the conventional ring-conserved homodesmic
reactions:
(6)
where R1 and R2 are alkoxy and carboxy substituents. Using enthalpies of reaction (6)
participants calculated for benzoic acid derivatives by G3MP2 (see Table S8) together with
the experimental enthalpies of formation, f Hmo (g), for benzene, benzoic acid, and
alkoxybenzenes (see Tables S8 and S9) the enthalpies of formation of all three iso-
butoxybenzoic acids, as well as for available from ref. [9] alkoxybenzoic acids were
calculated (Table 4, column 3).
As it can be seen in Table 4, there is an overall good agreement (except for the ortho-
substituted species) between theoretical and experimental data, particularly if the common
uncertainties of about 2-4 kJ·mol-1 associated with the G3MP2 gas-phase enthalpies of
formation are taken into account. This good agreement further reinforces the reliability of the
experimental and theoretical results presented in this work.
Table 4
Comparison of the experimental and calculated by G3MP2 and GA gas-phase enthalpies of
formation of alkoxybenzoic acids at T = 298.15 K (in kJ·mol-1)
c
f Hm (g)a
f Hm (g)
f Hm (g)
b
Compounds
1
exp
2
G3MP2
3
add
5
-
-
-
4
6
-
-
2-methoxy-benzoic acid -431.9±1.2 [7] -437.8 5.9
3-methoxy-benzoic acid -444.9±1.2 [7] -446.6 1.7
4-methoxy-benzoic acid -449.5±1.4 [7] -452.3 2.8
-
-0.7
-0.7
-2.0
2.1
5.5
3.1
2-iso-butoxybenzoic acid
3-iso-butoxybenzoic acid
4-iso-butoxybenzoic acid
4-etoxybenzoic acid
-524.5 5.3 -518.0
-532.6 2.3 -531.0
-538.0 0.4 -535.6
-518.72.3
-530.33.3
-537.62.3
-483.72.1
-523.92.7
-608.34.7
-485.8
-529.4
-611.4
-483.3 -0.4
-527.1 3.2
-609.4 1.1
4-butoxybenzoic acid
4-octyloxybenzoic acid
a From Table 1.
b Difference between column 2 and 3 in this table
c Difference between column 2 and 5 in this table
Page 16 of 25

The disagreement between the theoretical and the experimental values observed for
ortho-substituted benzenes have already been observed in our previous study of the enthalpy
of formation of 2-methoxybenzoic acid [7], as well as for 1,2-difluorobenzene [34]. In the
current work we also observe the disagreement for 2-iso-butoxybenzoic acid. A systematic
study of a series of 1,2-di-substituted benzenes with the composite methods will help to reveal
the reason in the future.
4.2 Pairwise interactions of substituents on the benzene ring
Pairwise interactions of substituents in the benzene ring belong to the basics of organic
chemistry. Energetics of interplay of electron-donating and electron-accepting substituents in
the ortho-, meta-, and para-position in the ring determine the mechanism of a chemical
reaction. Having established a set of G3MP2 enthalpies H298 for iso-butoxybenzoic acids (see
Table S8) we are able now to calculate effects of mutual interactions of carboxyl-group with
the iso-butoxy-group on the benzene ring in the ortho-, meta-, and para-position (see Table
5). Similar to our recent results for the isomeric methoxybenzoic acids [7], only strong
destabilization was observed for the ortho isomer. Mutual interactions in the meta position
seems to be negligible within a few kJmol-1. According to the G3MP2 calculations the para-
iso-butoxybenzoic acid is slightly stabilised about 5 kJmol-1. It make oneself conspicuous,
that G3MP2 derived pairwise interactions of methoxy- and iso-butoxy-substituents with the
carboxyl-group are very similar. Also resulting interaction in methoxybenzoic calculated with
the more sophisticated and time consuming G4-method are indistinguishable from those from
G3MP2 (see Table 5). This fact can be considered as the manifestation of the internal
consistency of the composite methods used in this work, except for the systematic deviations
discussed for ortho-isomers above. The experimental ortho interactions of the methoxy- and
iso-butoxy substituent with the carboxyl (see Table 5 column 4) are remarkably similar,
proving also the consistency of the experimental data sets. Experimental pairwise interactions
in the meta- and para-position of the alkoxy substituent are not large and hardly
distinguishable within 3-4 kJmol-1 in agreement with the theoretical calculations.
Table 5
Pairwise interactions of substituents on the benzene ring for di-substituted benzenes expressed
as the enthalpy of reaction r Hm (at 298.15 K, in kJ·mol-1). a
r Hm (G3MP2) r Hm (G4) r Hm (exp)
Compounds
Page 17 of 25

2-iso-butoxybenzoic acid
3-iso-butoxybenzoic acid
4-iso-butoxybenzoic acid
2-methoxy-benzoic acid
3-methoxy-benzoic acid
4-methoxy-benzoic acid
8.6
0.5
-4.9
9.9
1.1
-4.6
-
-
-
16.9±3.8
-5.3±4.3
2.0±3.7
15.8±2.9 [7]
2.8±2.9 [7]
-1.8±3.0 [7]
9.8 [7]
1.2 [7]
-4.7 [7]
a Reaction enthalpy r Hm is ascribed to the reverse reaction (6).
4.3. Validation of thermochemical properties of alkoxybenzoic acids using the group-
contribution method
In spite of the good agreement of the theoretical and experimental values for 3- and 4-
iso-butoxybenzoic acids, quantum-chemical methods failed to proove our experimental data
for the ortho-isomer. Following, another additional method have to be applied for validation
of the experimental thermochemical results measured in this work. Good agreement of the
experimental and theoretical gas-phase enthalpies of formation of the iso-butoxybenzoic acids
shown in the previous chapter does not completely exclude a possible inconsistency in the
vaporization enthalpies and in the liquid phase enthalpies, f Hm (liq), which are constituted
parts of the f Hm (g).
Group-additivity methods (GA) are admittedly belong to complementary tools to
assess consistency of experimental results [35,36]. One of the best flags to possible
experimental errors is a large discrepancy between experimental and estimated values –
especially if other, closely related compounds show no such discrepancy [36]. It is well
established that enthalpies of vaporization, enthalpies of formation in the liquid and in the gas
state obey the group additivity rules [37]. Application of the group additivity methods to
sublimation enthalpies and solid state enthalpies of formation is possible [38] but less
successful, because both the latter thermochemical properties include significant amount of
the fusion enthalpy. For this reason, in this work we systematically derived the vaporization
enthalpies and the liquid phase enthalpies of alkoxybenzoic acids from the experimental data
in order to apply the GA procedure.
In order to assess consistency of the f Hm (g, 298.15 K), f Hm (liq, 298.15 K) as well
as vaporization enthalpies gl Hm (298.15 K) of alkoxybenzoic acids, we applied in this work a
simple group-additivity method based on the thermochemical properties of benzene and an
appropriate substituted benzenes [15,34]. Experimental thermodynamic properties of benzene
are well established and accurate [39]. Thus it reasonable to take this molecule as the starting
Page 18 of 25

point and exchange the H-atoms with the different substituents. Indeed, the difference
between f Hm (298.15 K) of methoxybenzene [33] and benzene [39] provides the increment
H(H→OCH3) = -153.6 kJmol-1 for substitution of an H atom by the methoxy-group in the
benzene ring. Likewise, applying the same procedure for the benzoic acid and benzene yields
the increment H(H→COOH) = -378.4 kJmol-1 [7]. Introduction of the methoxy and
carboxyl substituents into the benzene ring produces three additional increments [7]: (ortho
CH3O - COOH) = 15.8 kJmol-1, (meta CH3O - COOH) = 2.8 kJmol-1, and (para CH3O -
COOH) = -1.8 kJmol-1, which are taking into account the pairwise interactions of substituents
on the benzene ring (see Table 6). These pairwise interactions were calculated (e.g. for
vaporization enthalpy) as the difference between the experimental vaporization enthalpy of an
appropriate methoxybenzoic acid [7] and the sum: gl Hm (298.15 K, benzene) +
H(H→OCH3) + H(H→COOH). The quantities of contributions from pairwise
interactions to the vaporization enthalpy (see Tables 5 and 6), are not large (except for ortho-
destabilization) but they are significant for prediction vaporization enthalpies of poly-
substituted benzenes.
In our recent study we measured, collected, and evaluated available thermochemical
data on methoxybenzoic acids [7]. We have reconcile data from numerous experimental
studies and recommended a data set of the benchmark quality for the ortho-, meta, and para-
isomers. Analysis of the pairwise interactions of alkoxy substituents on the benzene ring with
the carboxyl (see Table 5) have revealed that values for interaction of methoxy or iso-butoxy
group with the carboxyl in their ortho, meta- and para-position were very similar. This
similarity has allowed to propagate more reliable experimental pairwise interactions derived
from the methoxybenzoic acids for estimation thermochemical properties of another parent
alkoxybenzoic acids.
Table 6
Parameters for the calculation of enthalpies of vaporization and enthalpies of formation of
substituted benzenes at 298.15 K (in kJmol-1)a
f Hm (g)
gl Hm
f Hm (liq)
Parameters
benzene [39]
H(H→CH3O)
H(H→COOH)
82.9
-153.6
-378.4
33.9
12.5
41.9
49.0
-166.1
-420.4
ortho (CH3O - COOH)
meta (CH3O - COOH)
para (CH3O - COOH)
15.8
2.8
-1.8
-2.2
2.2
3.4
19.3
2.1
-2.7
Page 19 of 25

С-(С)(H)3
С-(С)2(H)2
С-(С)3(H)
С-(С)4
-41.8
-23.1
-10.5
-3.2
6.33
4.52
1.24
-2.69
0.26
3.64
-0.69
-2.43
-48.5
-27.2
-10.4
1.9
(С-C)1-4
2.6
2.0
C-(C)(Н)2(О)
C-(C)2(Н)(О)
C-(C)3(О)
-34.9
-31.1
-25.0
-38.2
-29.4
-20.6
a
Group contributions for methoxybenzoic acids was taken from ref. [7]. Increments for alkyl chains and ethers
were taken from our previous study [37].
f H 
m (g,
In Table 6 we collected group-additivity values required for calculation of
f H 
m (liq, 298.15 K) of alkoxycarboxylic acids of
298.15 K), gl Hm (298.15 K), and
different structures. As an example let us consider GA estimation of thermochemical
properties of 4-ethoxybenzoic acid, e.g for vaporization enthalpy we need to sum
gl Hm (298.15 K, 4-ethoxybenzoic acid) = gl Hm (298.15 K, benzene) + H(H→OCH3) +
H(H→COOH) + (para CH3O - COOH) + C-(C)(Н)2(О)
The latter increment C-(C)(Н)2(О) takes into account extending of the methoxy group. More
sophisticated example with estimations for 4-octyloxybenzoic acid is given in Table S8.
Comparison of the experimental and GA calculated enthalpies of formation f Hm (g,
298.15 K) for alkoxybenzoic acids is given in Table 4, column 5. As can be seen from this
table, independent on the structure and branching estimated values agree with the experiment
within the boundaries of experimental uncertainties of 2-3 kJmol-1.
The agreement of the calculated and experimental vaporisation enthalpies, gl Hm
(298.15 K) was generally acceptable within the experimental errors of 1-3 kJmol-1 (see Table
7, column 4). However it should be mentioned that comparison of the experimental and
additive values for the 4-butoxybenzoic acid in Table 7 was performed with the vaporization
enthalpy derived from the experimental gcr Hm which was re-measured in this work (see
Tables 1-3). It has turned out that the value gl Hm (298.15 K) = 113.4±2.6 kJmol-1 derived
from the experimental value gcr Hm (298.15 K) = 127.8±1.0 kJmol-1 reported in ref. [9] (see
Table 3, footnote i), has shown the deviation from the additivity of 9.1 kJmol-1. We were not
able to suggest any reasonable explanation for that and decided to measure 4-butoxybenzoic
acid with the transpiration method. In spite of agreement shown for the absolute vapor
pressures (see Fig. S2), our new values gcr Hm (298.15 K) = 123.2±1.0 kJmol-1 and gl Hm
Page 20 of 25

(298.15 K) = 108.8±2.6 kJmol-1 fit significantly better to the consistency tests performed with
the quantum-chemical method (Table 4) and GA procedure (Table 7).
f H 
Agreement for the liquid phase enthalpies of formation
m (liq, 298.15 K) was less
sufficient (see Table 7, column 7), especially for the 4-octyloxybenzoic acid. However, the
disagreement for the latter compound is rather due to the numerous solid state transitions
f H 
m (liq, 298.15 K) (see Table 3), than

T
fus
collected in to the 298Hm -term and used to derive
in the deficiencies of the GA procedure.
Table 7
Comparison of the experimental and GA calculated data for alkoxybenzoic acids at T =
298.15 K (in kJ·mol-1).
a
gl Hm
exp
gl Hm
add
3
f H 
m (l) a
f H 
m (l)
b
c
Compounds
exp
5
add
6
1
2
4
7
96.5 -0.9
101.3 -2.1
102.5 1.6
95.3 0.2
104.7 4.1
124.0 2.7
-0.2
1.8
-5.6
-0.8
-1.9
-13.4
2-iso-butoxybenzoic acid
3-iso-butoxybenzoic acid
4-iso-butoxybenzoic acid
4-etoxybenzoic acid
4-butoxybenzoic acid
4-octyloxybenzoic acid
-614.1
-631.3
-636.1
-578.4
-630.8
-731.6
95.60.6
99.20.9
104.12.1
95.52.8
108.82.6
126.73.8
-614.32.2
-629.53.2
-641.73.0
-579.23.2
-632.73.5
-745.04.8
a From Table 1
b Difference between column 2 and column 3 in this table.
c Difference between column 5 and column 6 in this table.
5. Conclusions
We have applied different experimental thermochemical methods to measure highly
pure samples of the 2-,3-, and 4-iso-butoxybenzoic acids. Experimental data on gcr Hm
(298.15 K), lcr Hm (298.15 K), and gl Hm (298.15 K) were derived and together with
f H 
enthalpies of formation
enthalpies of formation
m (liq or cr, 298.15 K) were used to obtain the gas-phase
f H 
m (g, 298.15 K) for all three isomers. Mutual validation of the
experimental and theoretical gas phase enthalpies of formation was performed with the
Page 21 of 25

G3MP2 quantum-chemical method. Simple group-additivity procedures have been developed
for prediction of enthalpies of formation and enthalpies of vaporization of alkoxybenzoic
acids of different structures.
Acknowledgment
This work has been partly supported by the Russian Government Program of
Competitive Growth of Kazan Federal University.
Appendix A. Supplementary data
Supplementary data associated with this article: provenance and purity of the materials
(Table S1), formula, density, massic heat capacity and expansion coefficients of the materials
used in the present study (Table S2), experimental results for combustion experiments at T =
298.15 K (p° = 0.1 MPa) for isomeric iso-butoxybenzoic acids (Tables S3-S5), values of
specific energies of combustion as the results of all combustion experiments (Table S6),
compilation of data on molar heat capacities at 298.15 K of the benzoic acid derivatives
(Table S7), G3MP2 total energies at 0 K and enthalpies at 298.15 K of the molecules studied
in this work (Table S8), results of calculation of the standard enthalpies of formation for the
molecules studied in this work in the gaseous phase at 298.15 K (Table S9), experimental
vapor pressures of the 2-methoxybenzoic acid (Fig. S1) and experimental vapor pressures of
the 4-butoxybenzoic acid (Fig. S2) can be found, in the online version, at http://
Page 22 of 25

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