Thermodynamic Analysis of Isomerization Equilibria of Chlorotoluenes and Dichlorobenzenes in a Biphasic Reaction Systems Containing Highly Acidic Chloroaluminate Melts

Article abstract of DOI:10.1021/acs.jpcb.6b10529

Thermodynamics and kinetics of the isomerization of chlorotoluenes and dichlorobenzene to the technically desired meta-isomers have been studied in the presence of highly acidic chloroaluminate melts with alkali metal and organic imidazolium cations. Enthalpies of four isomerization processes in reacting systems of chlorotoluenes and dichlorobenzene were obtained from temperature dependencies of the corresponding equilibrium constants in the liquid phase. Experimental reaction enthalpies, enthalpies of vaporization, and absolute vapor pressures of chlorotoluenes and dichlorobenzene have been used for the validation of quantum-chemical methods to predict thermodynamic functions of the four reactions under study successfully. Values of the standard Gibbs energies of formation, standard enthalpies and entropies of formation of chlorotoluenes and dichlorobenzenes in the liquid and in the gas phase have been derived. These values allow optimization of liquid-liquid biphasic manufacturing technologies for halogen-substituted benzenes.

Full text of DOI:10.1021/acs.jpcb.6b10529

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Article
Thermodynamic Analysis of Isomerisation Equilibria of
Chlorotoluenes and Dichlorobenzenes in a Biphasic Reaction
Systems Containing Highly Acidic Chloroaluminate Melts
Sergey P. Verevkin, Julia Messner, Vladimir N. Emel’yanenko,
Mikhail G Gantman, Peter S. Schulz, and Peter Wasserscheid
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b10529 • Publication Date (Web): 02 Dec 2016
Downloaded from http://pubs.acs.org on December 7, 2016
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J. Phys. Chem. B, 2016
Thermodynamic Analysis of Isomerisation Equilibria of Chlorotoluenes and
Dichlorobenzenes in a Biphasic Reaction Systems Containing Highly Acidic
Chloroaluminate Melts
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*
a
b
a,c
b
Sergey P. Verevkin, Julia Messner, Vladimir N. Emel´yanenko, Mikhail G. Gantman, Peter S.
b
b
Schulz, and Peter Wasserscheid
a
Department of Physical Chemistry and Department of „Science and Technology of Life, Light and
Matter“, University of Rostock, Dr-Lorenz-Weg 1, D-18059, Rostock, Germany
Lehrstuhl für Chemische Reaktionstechnik der Universität Erlangen Nürnberg, Egerlandstr. 3,
1058 Erlangen, Germany
Department of Physical Chemistry, Kazan Federal University, Kremlevskaya str. 18, 420008
Kazan, Russia
b
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c
ABSTRACT: Thermodynamics and kinetics of the isomerisation of chlorotoluenes and
dichlorobenzene to the technically desired metaꢀisomers have been studied in presence of highly
acidic chloroaluminate melts with alkali metal and organic imidazolium cations. Enthalpies of four
isomerisation processes in reacting systems of chlorotoluenes and dichlorobenzene were obtained
from temperature dependencies of the corresponding equilibrium constants in the liquid phase.
Experimental reaction enthalpies, enthalpies of vaporization, and absolute vapour pressures of
chlorotoluenes and dichlorobenzene have been used for the validation of quantumꢀchemical
methods to predict thermodynamic functions of the four reactions under study successfully. Values
of the standard Gibbs energies of formation, standard enthalpies and entropies of formation of
chlorotoluenes and dichlorobenzenes in the liquid and in the gas phase have been derived. These
values allow optimization of liquidꢀliquid biphasic manufacturing technologies for halogenꢀ
substituted benzenes.
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1. INTRODUCTION
The application of ionic liquids, both as reaction media and as catalyst opens new perspectives in
1
chemistry and chemical engineering. The use of ionic liquids in industrial processes, namely Lewis
acid catalyzed processes, allows to increase the yield of the desired product or to make the
separation and the purification of the product easier. Often these results can be achieved due to the
fact that the properties of ionic liquids can be varied in a wide range and also can be finely tuned. In
particular, the solubility of starting material and product in the ionic liquid and the ionic liquid´s
acidic properties can be adjusted according to the demands of the process under investigation. In the
present manuscript we describe the use of Lewisꢀacidic ionic liquids in the catalysis of two
industrially relevant processes: the isomerization of 4ꢀchlorotoluene (4ꢀCT) and the isomerization
of p-dichlorobenzene (see Figure 1).
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m-chlorotoluene
o-chlorotoluene
m-dichlorobenzene
o-dichlorobenzene
CH3
CH3
Cl
Cl
Cl
Cl
Cl
Cl
CH3
Cl
Cl
Cl
p-chlorotoluene
p-dichlorobenzene
Figure 1. Isomerization of the chloroꢀsubstituted benzenes studied in this work.
The main route to produce dichlorobenzene on an industrial scale is the direct chlorination of
benzene. This process is kinetically controlled and leads mainly to the formation of oꢀ and pꢀ
dichlorobenzene due to the directing effect of the first chloroꢀsubstituent. The methyl group of
toluene shows the same directing effect and the chlorination of toluene also leads to a mixture of
ortho- and para-isomers. Orthoꢀisomers have rather low importance for technology, the importance
2
of para-isomers is relatively higher , though namely the metaꢀisomers of dichlorobenzene and
chlorotoluene are broadly used as precursors for insecticides, herbicides, pharmaceuticals, and dyes.
However, currently for synthesis of these compounds only expensive and atom inefficient routes
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have been suggested (e.g. Sandmeyer reaction of mꢀchloraniline or the catalytic dechlorination of
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1
,2,4 dichlorobenzene ).
However, it is well established that the metaꢀisomers are the thermodynamically most stable
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,6
compounds in their respective isomeric mixtures. As a consequence, a promising route to produce
the metaꢀisomer efficiently is the isomerisation of pꢀdichlorobenzene or p-chlorotoluene. This
highly selective reaction can be carried out under typical Friedel-Crafts conditions with water
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,6
promoted aluminium chloride. However, these classical catalysts have an important drawback:
the product isolation procedure demands for the hydrolysis of the reaction system, which leads to
the complete destruction of the catalyst. This makes the reuse of the catalyst impossible and the
isomerisation protocol not sustainable and not economic. A very interesting alternative was
presented by Messner et al. who demonstrated that liquidꢀ liquid biphasic reaction systems
involving alkali chloroaluminate melts ꢀ and in particular lithium chloroaluminate melts – form
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highly active and recyclable catalyst phases for the dichlorobenzene isomerisation.
In focus of this work is the design of novel Lewisꢀacidic catalytic systems for chlorobenzene
isomerization processes. The target is to tune the miscibility of the ionic liquid with the isomers in a
way that the isomers are sufficiently soluble in the ionic liquid to allow an isomerisation reaction.
At the same time, a miscibility gap between catalyst phase and isomers is required to allow for a
liquidꢀliquid biphasic process that ensures easy product separation and catalyst recycling. Basic
requirement is also a very pronounced Lewis acidity of the applied ionic liquid to promote
isomerisation to the desired, thermodynamic equilibrium mixture of isomers.
It is well established, that catalytic reactions in alkylꢀbenzenes and chloroꢀbenzenes (see
Figure S1 in ESI) are usually limited by the equilibrium of isomerization and disproportionation
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reactions.
However, relative rates of these chemical reactions are significantly different and
they are crucially dependent on the temperature of experiment. For example, as shown by Olah and
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Meyer, starting from mixtures with different amount of orthoꢀ, meta-, and paraꢀisomers, the
distribution of isomers approximates equilibrium only after 50 h at 373 K. Therefore, reactions
given in Figure 1, have to be considered both under kinetic as well as under thermodynamic control.
For the development and the optimisation of novel biphasic reaction systems, the equilibrium
distribution of orthoꢀ, meta, and paraꢀisomers have to be exactly known at any temperature in order
to assess and compare activity of catalytic systems. Unfortunately, only few experimental studies
3
dealing with chlorotoluene and dichlorobenzene isomerisation in presence of AlCl ꢀbased catalysts
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are available.
Information on isomers distribution available from the literature is not only
fragmentary and contradictory, but it also suffers from an ambiguity whether the experimental time
at a certain temperature was enough to reach equilibrium or not.
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The purpose of this work is threefold: First, we wish to resolve the contradiction existing
between available experimental equilibrium constants for para ↔ meta ↔ ortho interconversions
with help of own experimental data. The reliable temperature dependence of equilibrium constants
in the liquid phase is able to provide thermodynamic characteristics for isomerisation reactions: a
o
m
o
m
standard molar reaction enthalpy, ꢀrH , and a standard molar reaction entropy, ꢀrS . The
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consistency of the latter values derived from equilibrium studies (Second Law Method) can be
validated with help of the Hess´s Law by comparison of the standard molar enthalpies of formation,
o
o
ꢀf
, or the standard molar entropies, , of the reaction participants (Second Law Method). This
H S
m m
part of the study aims at providing reliable equilibrium constants and ꢀ as a consequence ꢀ the
quantitative distribution of orthoꢀ, meta-, and paraꢀisomers at any temperature. This knowledge is
indispensable for the assessment of the catalytic activity of the novel biphasic reaction systems
within this work.
However, this knowledge is also of great value to further develop quantumꢀchemical
methods towards their use in optimizing industrial processes. In recent years some of us have
intensively applied highꢀlevel quantumꢀchemical methods for the calculation of thermodynamic
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properties and charcteristics of different, industrially relevant reactions.
With reliable
experimental equilibrium constants for the para ↔ meta ↔ ortho interconversions at hand, it was
reasonable to apply quantum chemistry for an “in silico” assessment of these equilibrium constants.
Finally, a further aim of the current study is the development of comprehensive tables with
thermodynamic data for isomeric chlorotoluenes and dichlorobenzenes. These tables are required
for the design and optimisation of industrial processes and for implementation of further novel
biphasic reaction systems.
2
. Experimental procedures
Isomerization of 4ꢀchlorotoluene (4ꢀCT) or 1,4ꢀdichlorobenzene with acidic chloroaluminate
melts has been studied in a glass flask equipped with a mechanical stirrer and a reflux condenser at
ambient pressures, as well as under pressure in a glass autoclave. The detailed description of the
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experiment has been given elsewhere. In short: an acidic mixture of a chloride salt and AlCl
3
3 3
(n(AlCl ) > n(chloride salt); χAlCl = 0.67) was heated above its melting point under gentle stirring
within 5 minutes and the required amount of the starting isomer was added. The resulting biphasic
reaction system was stirred at 500 rpm for 24 h. At certain time intervals, about 1 ml of sample was
taken from the flask and dissolved in 5 ml dichloromethane and extracted with water. Afterwards,
the organic phase was separated and analysed by gas chromatography. Sampling was continued
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until no further change of the compositions was observed indicating that the chemical equilibrium
was established.
Inorganic as well as organic chloroaluminate melts represent possible acidic catalysts for the
isomerization of dichlorobenzenes (DCB) and chlorotoluenes (CT). Organic chloride salts or alkali
metal chlorides form with AlCl
in the mixture determines the chemical properties of the resulting chloroaluminate melts. A
systematic study of the catalytic activity of different inorganic melts AlCl ꢀMCl (MCl = LiCl, LiClꢀ
NaCl, KClꢀNaCl) and of AlCl ꢀ[EMIM]Cl has been performed. The following chloroaluminate
melts were prepared according to a general procedure: AlCl
3
the above mentioned chloroaluminate melts. The nature of cation
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/LiCl/NaCl, AlCl
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/LiCl/NaCl,
2.0:1.0;
AlCl
AlCl
3
3
/LiCl/[N(CH
3
3
)
4
4
]Cl,
2:0.6:0.4;
AlCl
3
/[C
2
mim][Cl],
AlCl /[N(CH ]Cl,
3
3 4
)
/LiCl/[N(CH
)
]Cl, 2:0.5:0.5, and AlCl
3
/LiCl/[N(CH ) ]Cl, 2:0.4:0.6. The experimental details
3 4
are reported in Table 1.
3
. Results
3.1 Isomerization of 4-chlorotoluene with Lewis-acidic chloroalumiante melts (system I).
Equilibrium interconversions of orthoꢀ, meta, and paraꢀisomers of chlorotoluenes according to
reactions (1 and 2) were studied at 413 K. The disproportion reactions of chlorotoluenes have been
out of focus of this work. For the three isomers considered in this study only two equilibrium
constants are independent. For the discussion we selected the reaction 1 with the equilibrium
constant Kx(m/p) and the reaction 2 with the equilibrium constant Kx(o/p) (see Table 1, Fig. 3).
CH3
CH3
CH3
Cl
Reaction 1
Reaction 2
Cl
Cl
Figure 2. Equilibrium system I for the isomerization of chlorotoluenes.
Different inorganic and organic chloroaluminate melts were used as acidic catalysts. Among these,
inorganic melts and AlCl ꢀ[N(CH ) ]Cl formed a biphasic system upon mixing with the reagents,
3
3 4
3
while AlCl ꢀ[EMIM]Cl was completely miscible with the reaction mixture. At first, it was
important to find out, how fast the system I was able to reach the equilibrium. The interplay of
kinetic and thermodynamic control in the system I is evident from Figure 3 showing the
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3 3 2
isomerization of pꢀchlorotoluene with AlCl /AlCl *6H O as catalyst at 413 K. During the first hour
of isomerization the system I was completely under kinetic control and the concentration of metaꢀ
chlorotoluene in the reaction mixture even reached a local maximum of 58% before 0.5 h (see
Figure 3). However, in the course of the equilibration, the amount of this desired compound slightly
decreased to the value predicted by thermodynamics, due to the concurrent formation of orthoꢀ
chlorotoluene. After 4 hours the reaction mixture reached equilibrium and contained 47 mol. % of
metaꢀ, 17.5 mol. % of paraꢀ, and 35.5 mol % of orthoꢀchlorotoluene. These concentrations were
practically constant within the next 8 hours of equilibration.
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Figure 3. Isomerization of pꢀchlorotoluene at 413 K in the presence of AlCl /AlCl *6H O as a
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2
catalytic system.
Table 1. System I (p-chlorotoluene isomerization). Experimentally determined compositions
of equilibrium mixtures and K –values in the liquid phase. (T is temperature of investigation
in K; x is the mole fraction measured chromatographically)
x
a
i
Catalyst
T
x
p
x
m
x
o
K
x(m/p)1
K
x(o/p)2
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
3
3
3
3
3
3
3
3
3
3
/HCl, 4.25 h [6]
369 17.9 49.2 32.9 2.7
373 19.0 38.0 43.0 (2.0)
413 17.5 47.0 35.5 2.7
413 17.5 47.0 35.5 2.7
413 17.5 47.0 35.5 2.7
413 17.0 45.0 38.0 2.6
(1.8)
2.3
2.0
2.0
2.0
(2.3)
(0.01)
(0.01)
(1.5)
(1.1)
(0.6)
/AlCl
/AlCl
3
*H
*6H
2
O, 200 h [11]
O, 4h
3
2
/KCl/NaCl, 4h
/LiCl/NaCl, 4h
/LiCl/NaCl, 24h
/[EMIM]Cl, 4h
413 85.0 14.5 0.5
413 93.0 6.5 0.5
(0.2)
(0.1)
3 4
/[N(CH ) ]Cl, 4h
/LiCl/[N(CH
/LiCl/[N(CH
3
)
)
4
]Cl 2:0.6:0.4, 4h
]Cl, 2:0.5:0.5, 4h
413 20.0 51.0 29.0 2.6
413 22.0 55.0 23.0 2.5
413 26.0 59.0 15.0 2.6
3
4
AlCl /LiCl/[N(CH ) ]Cl, 2:0.4:0.6, 4h
3
3 4
a
Values in brackets are not at equilibrium either for reaction 1 or for reaction 2.
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Experimental data for the isomerization of pꢀchlorotoluene by using different catalytic
mixtures are presented in Table 1. Both ternary inorganic chloroaluminate melts lead to the
3 3 2
formation of mixture with the same content of isomers as AlCl /AlCl *6H O. It is important to
note, that the extension of reaction time up to 24 h in case of inorganic catalytic systems hardly
changed the products distribution. This indicates that after 4 hours at 413 K the composition of the
reaction mixture is close to the equilibrium.
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In contrast, the use of catalysts with organic chloroaluminate melts leads to a significant
decrease of the isomerization rates and interconversions of isomers occurs completely under kinetic
control far away from the equilibrium (see Table 1). It has turned out that the catalytic activity of
the systems AlCl
catalysts. However, the activity of the monophasic system AlCl
in comparison to the biphasic AlCl ꢀ[N(CH ) ]Cl system (which gave only 6.5 mol. % of metaꢀ
3
ꢀ[EMIM]Cl and AlCl
3
ꢀ[N(CH )
3 4
]Cl is much lower compared with the inorganic
3
ꢀ[EMIM]Cl is significantly higher
3
3 4
chlorotoluene, see Table 1). It is likely that diffusion limitation accounts for the decrease in the
reaction rate in the AlCl ꢀ[N(CH ]Cl system under the applied conditions.
According to the results listed in Table 1, catalysts based on AlCl /LiCl/[N(CH ) ]Cl
3
3 4
)
3
3 4
mixtures show significantly higher activity in the isomerization compared to organic
chloroaluminates. Interestingly, when using the system AlCl ꢀLiClꢀ[N(CH ]Cl (χAlCl = 0,67) in
3
3
)
4
3
catalysis, the activity depends on the amount of LiCl in the mixture. The higher the content of LiCl,
the higher the reactivity and the degree of conversion paraꢀchlorotoluene (see Figure S2).
An important characteristic of catalytic systems under study is the amount of orthoꢀ
chlorotoluene produced in the reaction mixture. The latter isomer is concurrently formed according
to reaction 2 (see Figure 2). As shown in Table 1, the ternary catalytic compositions based on
3 3 4
AlCl /LiCl/[N(CH ) ][Cl] allows to utilize the kinetic control over the reaction 2 (see Figure 2) and
as a consequence significantly higher concentrations (up to 69 % at 413 K, see Table 1) of the
desired metaꢀchlorotoluene were achieved in the reaction mixture far away from the equilibrium.
3.2 Isomerization of p-dichlorobenzene with Lewis-acidic chloroaluminate melts (System II)
The isomerization of paraꢀdichlorobenzene (Figure 4) is an atomꢀeffective way to synthesize metaꢀ
dichlorobenzene. Classical Friedel-Crafts catalysts are active in this reaction, but hydrolysis for
product isolation lead to complete catalyst loss and large amounts of aluminium oxide and
hydroxide waste (contaminated with aromatic compounds) that is difficult to dispose.
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Cl
Cl
Cl
Cl
Reaction 3
Reaction 4
Cl
Cl
Figure 4. Equilibrium system II for the isomerization of dichlorobenzenes.
Compared to system I, the isomerization of dichlorobenzenes (system II) requires harsher
conditions. We used liquidꢀliquid biphasic systems with (X(AlCl )= 0.67 with 20 molꢀ% AlCl in
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
3
respect to the organic compound and experimental studies have been performed at 443 K. The latter
was limited from one side by the boiling point of paraꢀdichlorobenzene (447 K) and from another
3
side by the melting point of the binary AlCl ꢀLiCl system (442 K), which is important for the
comparison of the activity of chloroaluminate melts. Ternary systems containing chloroaluminates
possess lower melting points compared to binary mixtures, which also enables to perform the
isomerization process at temperatures lower than 443 K with these systems.
Results of the isomerization of pꢀdichlorobenzene in the presence of both inorganic
chloroaluminates AlCl
organic chloroaluminates AlCl
collected experimental data on equilibrium compositions of dichlorobenzenes available from the
3
ꢀLiCl (2:1), AlCl
3 3
ꢀLiClꢀNaCl (2:0.4:0.6), AlCl ꢀKClꢀNaCl (2:0.4:0.6) and
3
ꢀ[EMIM]Cl (2:1) are given in Table 2. For comparison we also
5
, 12, 15
literature.
Table 2. System II (p-dichlorobenzene isomerization)
composition of equilibrium mixtures and K –values in the liquid phase. (T is temperature of
investigation in K; x is the mole fraction measured chromatographically)
i
.
Experimentally determined
x
a
Catalyst
T
x
p
x
m
x
o
K
x(m/p)
K
x(o/p)
0.28
ꢀ
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
3
3
3
3
3
3
3
3
3
3
[12]
[12]
[12]
[12]
*6H
393
433
433
453
473
513
443
443
443
443
33.6
25.7
30.0
30.6
32.6
37.0
34.0
49.0
37.0
90.0
55.1
45.2
54.0
57.2
59.5
42.0
60.0
49.0
58.0
5.0
11.3
29.1
16.0
12.2
7.9
21.0
6.0
2.0
4.0
5.0
1.6
1.8
1.8
1.9
1.8
(1.1)
1.8
(1.0)
(1.6)
ꢀ
ꢀ
0.39
(0.24)
0.56
(0.18)
(0.04)
(0.11)
ꢀ
2
O[5]
O[15]
*H
2
/LiCl/HCl, 4h
/LiCl/NaCl, 4h
/LiCl/NaCl/HCl, 4h
/[EMIM]Cl, 24h
AlCl
a
Values in brackets are not at equilibrium either for reaction 3 or for reaction 4.
The interplay of kinetic and thermodynamic control on system II with different catalysts is
presented in Figure 5. The highest activity was observed for the reference catalyst
(
3 3 2
AlCl /AlCl *6H O). In this case the concentration of metaꢀdichlorobenzene reached 21 % after 2
hours reaction time and further extension of the reaction time to 24 h showed no further change.
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Note, that the organic chloroluminate system AlCl
3
ꢀ[EMIM]Cl is not active in this very demanding
7
isomerization reaction (see Figure 5) which is in full agreement with earlier reports.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. pꢀDichlorobenzene isomerization – formation of metaꢀdichlorobenzene over time
applying different, acidic chloroaluminate melts at 443 K.
The activity of inorganic chloroaluminates was dependent on the nature of the cation. The
highest yield of metaꢀdichlorobenzene was reached in presence of the AlCl
after 24 h the yield of 24 % of metaꢀdichlorobenzene, was even higher, than the yield obtained with
the reference system (AlCl /AlCl *6H O). Other ternary systems showed lower catalytic activity as
compared to the binary AlCl ꢀLiCl. The system AlCl ꢀLiClꢀNaCl reached 12 mol % of metaꢀ
ꢀKClꢀNaCl reached only 5% after 24 h reaction time (see
3
ꢀLiCl melt. In this case,
3
3
2
3
3
dichlorobenzene, and the melt AlCl
3
Figure 5).
3 3 2
It is interesting to note, that despite, the reference system (AlCl /AlCl *6H O) showed the
highest activity in the first 2 hours the reaction stopped significantly far away from the expected
equilibrium ratios (see Table 2). This could be due to partial sublimation of aluminum chloride
under the harsh reaction conditions. Moreover, the formation of gaseous HCl during the
equilibration was observed. Apparently both factors reduce the catalytic activity of the reference
system already after 2 hours.
Compositions of isomers obtained in presence of different binary and ternary melts are
collected in Table 2. As can be seen from this table after 4 h of equilibration the catalytic systems
under study have been able to reach distribution of isomers close to the equilibrium with the good
yield of the desired metaꢀdichlorobenzene at the level of 49ꢀ60 mol%.
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4. Discussion
°
4.1
General Aspects. Standard molar enthalpies of chemical reactions, ꢀrH , can be
m
determined experimentally from the temperature dependence of chemical equilibrium constants (in
accordance with the Second Law of thermodynamics) or by means of calorimetric measurements of
the standard molar enthalpies of formation of all reaction participants (with the help of Hess´s Law
or in accordance with the First Law of thermodynamics).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The Second Law Method (Equilibrium Constant Measurements). For a general chemical reaction in
the liquid phase, the true thermodynamic equilibrium constant, K , is defined as the ratio of the
a
i
activities a of the products and the reagents under equilibrium conditions. From our previous
experiences, activity coefficients of isomers are very close to unity and for this reason we can use
for the current equilibrium study an assumption that K , = K , where the equilibrium constant, K , is
a
x
x
calculated from the experimental mole fractions of the reactions (1) – (4) participants as they given
in Tables 1 and 2. From the Van`t Hoff relation
°
m
2
d ln K
x
/dT = ꢀrH / RT
(1)
°
m
the slope of a plot (ln K
x
) vs 1/T gives the standard enthalpy of reaction ꢀrH if the temperature
dependence of the equilibrium constant K
x
is known. This procedure, involving study of the
changes of equilibrium constants with temperature, is referred to as Second Law Method. Hence,
°
m
reaction enthalpies, ꢀrH , for reactions (1) – (4) have been calculated from the temperature
x
dependence of the equilibrium ratios K (see Tables 1 and 2).
The First Law Method (Combustion Enthalpies Measurements). The standard molar enthalpies of
°
formation ꢀ H of organic compounds in the liquid or crystalline state are usually derived from
f
m
combustion or solution calorimetry. Enthalpies of formation of chloroꢀsubstituted benzenes
16ꢀ18
(relevant to the present work) have been evaluated just recently
and they are collected in Table
°
m
S1 (see Supporting Information). Thus, having the evaluated enthalpies of formation ꢀ H (liq) of
f
°
m
all participants of reactions (1) – (4) at hand, the reaction enthalpies, ꢀ H , can be independently
r
°
m
derived according to the Hess´s Law and used for an additional prove of the ꢀrH , obtained from
the temperature dependences of equilibrium constants of the reactions (1) – (4) according to the The
Second Law Method.
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4
.2
of the chemical equilibria studies of systems I and II are listed in Tables 1ꢀ2. The experimental
values of K for all reactions considered in this work are only slightly changing with the
temperature. Experimental values of K
Liquid Phase: Equilibrium Constants and Reaction Enthalpies. The experimental results
x
x
for reactions (1) ꢀ (4) were approximated as a linear
ꢀ1
function of temperature by the equation ln K
x
= a + b×(T/K) using the method of least squares.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The slopes of these lines when multiplied by the gas constant afford the standard enthalpies of these
°
m
reactions, ꢀrH . The intercept of these lines when multiplied by the gas constant afford standard
°
m
entropies of these reactions ꢀrS . Numerical results are presented in Table 3. The errors in the
thermodynamic functions from equilibrium studies are given by the standard deviations for the
meaningful level 0.05.
The reaction enthalpies of positional isomerization on the benzene ring usually reflect the
relative intensity of substituents interactions. As can be seen from Table 3 reaction enthalpies in the
system I are slightly exothermic. The enthalpy of reaction (2) is responsible for the sterically
demanding para ↔ortho interconversion, that is why the enthalpy of this reaction is obviously
larger than those for the reaction 1, where para ↔ meta isomerization do not exhibit any noticeable
3
interactions of CH and Clꢀsubstituents at the benzene ring. The reaction enthalpies in the system II
are slightly endothermic. Also, the enthalpy of reaction (4) accounting for the para ↔ortho
interconversion is noticeably larger than that for the reaction (3), the para ↔ meta isomerization,
where the less intense interaction of two Clꢀsubstituents on the benzene ring are similar to metaꢀ
interactions of CH and Clꢀsubstituents.
3
°
m
°
m
Table 3. Thermodynamic functions ꢀrH and ꢀ S of the reactions (1-4) in the liquid phase
r
-
1
and temperature dependences ln K
x
=
a
+
b
×(T/K)
a
°
m
b
°
m
b
K
x
/298 K
3.0
K
x
/443 K
2.5
Τ
ꢀrH
ꢀr S
Reaction
a
b
kJ.molꢀ1
ꢀ1.2 1.0
J.mol K
5.0 3.5
ꢀ1
ꢀ1
K
R1
373ꢀ413
0.6
144
±
±
c
ꢀ
ꢀ
1.4
±
2.0
3.5
d
1.7
±
R2
R3
R4
373ꢀ413
393ꢀ453
393ꢀ513
ꢀ0.6
1.8
1.6
538
ꢀ4.5
±
2.0
3.5
0.4
1.8
3.5
1.0
1.8
3.5
1.0
ꢀ5.0
±
3.5
3.3
1.0
0.1
1.8
1.8
0.4
c
ꢀ3.9
ꢀ4.0
±
±
d
ꢀ535
4.5
±
15.2
13.7
±
1.0
2.2
c
4
ꢀ
.3
0.1
9.6
±
±
±
d
ꢀ1156
±
c
7.5
±
d
4.1
±
a
Temperature range of the equilibrium study;
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2
2
2
2
2
2
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4
4
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5
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5
5
5
5
5
6
b
°
m
°
m
The values of the enthalpies ꢀ H and entropies ꢀ S of reactions (1) to (4) were derived for the
r
r
average temperatures of the experimental intervals given in the column 2. It was assumed that the
enthalpies of the reaction hardly change on passing from the average temperature of the
experimental range to T = 298 K;
c
Calculated from experimental enthalpies of formation (see Table S1) of reaction participants;
d
Calculated with Eq. 2.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Analysis of equilibrium constants for reactions (1) – (4) reported in the literature (see Table
1
and 2) have revealed that the position of equilibrium was not always easy to approach starting
from the individual paraꢀ, meta-, or ortho-isomer. In many cases the true position of equilibrium
was not reached even after hundreds hours of equilibration. This fact has heavily aggravated the
analysis of the available experimental distribution of isomers. For this reason the independent
°
m
validation of reaction enthalpies, ꢀ H , obtained from temperature dependences of equilibrium
r
constants of reactions (1) – (4) according to the Second Law Method was virtually required.
Enthalpies of reactions (1) – (4) independently derived according to the Hess´s Law using the
°
m
liquidꢀphase enthalpies of formation, ꢀ H (liq), of chlorotoluenes and dichlorobenzenes from
f
Table S1 are in agreement to those from the Second Law Method within the boundaries of
experimental uncertainties (see Table 3). This validates the selection of equilibrium constants taken
for the thermodynamic analysis of the interconversions of paraꢀ, meta-, or ortho-isomers.
ꢀ1
Coefficients of temperature dependences ln K
x
= a + b×(T/K) given in Table 3 can be used for
calculation of the benchmark equilibrium constants K at any temperature, which are important as
x
indicators of the catalytic activity of the liquid–liquid biphasic reaction system.
4
.3
Gas Phase: Reaction Enthalpies and Equilibrium Constants from Quantum Chemical
9
, 10, 13, 14
Calculations. In the recent work by some of us,
we established a remarkable ability of
highꢀlevel quantumꢀchemical methods to predict gaseous thermodynamic functions of organic
compounds accurately. Admittedly, the chemical reactions of isomerization and transalkylation are
thermodynamically controlled, thus, the desired yields of goal products can be predicted by the
quantumꢀchemical calculations. Having established in this work the reliable experimental
equilibrium constants for the para ↔ meta ↔ ortho interconversions of chloroꢀsubstituted
benzenes, it was reasonable to test the ability of quantum chemistry for the “in silico” assessment of
these equilibrium constants. In order to validate this procedure, the following thermodynamic
functions were calculated for the gas phase reactions (1) – (4) using the G3MP2 method at 298 K:
o
o
o
m
the Gibbs energy ꢀ G , the reaction enthalpy, ꢀ H , and the reaction entropy, ꢀrS
. Calculated
m
r
m
r
values are compiled in Table 4.
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Results of the quantumꢀchemical calculations are referred to the gas phase. The standard molar
°
m
°
m
reaction enthalpies ꢀ H (g) in the gas phase are related to reaction enthalpies, ꢀ H (liq), in the
r
r
liquid state by the equation:
o
m
o
m
g
l
o
m,i
ꢁ H (liq) = ꢁ H (g) − νiꢀ H
(2),
r
r
∑i
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
g
l
o
where ꢀ H are the molar enthalpies of vaporization of the pure compounds i at the reference
m,i
temperature 298 K. The latter values for compounds involved in reactions (1) – (4) are well
16ꢀ18
established and they are available in the literature
(see Table S1 in Supporting Information).
°
m
Enthalpies of reactions ꢀ H (g) required for Eq. 2 were calculated (see Tables 4) directly from
r
H
298 enthalpies of the reaction participants using the G3MP2. The resulting enthalpies of reactions
°
m
(1) – (4), ꢀ H (liq) in the liquid phase are given for comparison with the experimental values in
r
°
m
Table 3. The calculated values of the ꢀ H (liq) for reactions (1) and (2) are in good agreement
r
with those derived from the chemical equilibrium studies. Agreement for reaction (3) and (4) is
acceptable within the combined boundaries of uncertainties. As a consequence, the energetics of
isomerization reactions in the liquid phase can be reliably assessed from quantumꢀchemical
calculations with help of empirical data for the vaporization enthalpies.
o
m
°
m
°
m
Table 4. G3MP2 calculated thermodynamic functions, ꢀ G ꢀ H and ꢀ S and
thermodynamic equilibrium constants of reactions (1) – (4) in the gas phase at 298 K.
r
r
r
a
b
o
°
m
°
m
KP
Kx
Kx
ꢀr
G
m
ꢀ H
ꢀ S
r
r
Reaction
ꢀ1
ꢀ1
ꢀ1 ꢀ1
.
.
.
theor
exp
kJ mol
ꢀ0.67
ꢀ1.72
ꢀ0.36
5.69
kJ mol
ꢀ1.07
ꢀ4.07
ꢀ0.38
5.26
J mol K
ꢀ1.36
R1
R2
R3
R4
1.31
1.99
1.15
0.10
1.33
1.86
1.07
0.13
3.0
3.3
1.0
0.1
ꢀ7.99
ꢀ0.09
ꢀ1.42
a
Calculated by Eq. 3;
Calculated by equation ln K
b
ꢀ1
x
= a + b×(T/K) with coefficients given in Table 3.
The thermodynamic equilibrium constant, KP , calculated for the gas phase reactions (1) –
(
4) is related to the thermodynamic constant K
x
of these reactions in the liquid phase by Eq (3):
P
paraꢀisomer,0
P
paraꢀisomer,0
K = K ⋅
or
K = K ⋅
(3),
x
P
x
P
Pmetaꢀisomer,0
P
orthoꢀisomer,0
where P are the saturated vapour pressures of the pure components. Saturated vapour pressures for
i,0
15ꢀ17
chlorobenzenes were available from the literature
(see Table 5 and Tables S2 and S3 in the
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1
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1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Supporting Information). Using the values of K for the gaseous reactions (1) – (4), calculated with
P
help of G3MP2 and presented in Table 4, the theoretical thermodynamic constants K
liquid phase were calculated (see Table 4). Theoretical ratios K for the reactions (1ꢀ4) (calculated
using Eq (3) given in Table 4) are compared there with the experimental values K at 298 K.
Theoretical values of K calculated at the G3MP2 level are in acceptable agreement with the
x
(theor) in the
x
x
x
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
experimental values. Thus, the procedure developed in this work could be practically applied for the
assessment of yields of isomerisation reactions similar to reactions (1ꢀ4) studied in this work.
Following such a procedure could be recommended in general for a broad range of reactions, such
as alkylation and interconversion of substituted aromatic compounds.
Table 5. Thermodynamic properties of halogen-substituted benzenes at
T
= 298.15 K
Tꢀrange
g
l
o
m
g
l
o
m
a
o
m
b
o
m
c
Compound
p
0
ꢀ H
ꢀ S
S
(g)
S
(liq)
exp.
K
exp.
Pa
exp.
exp.
G3MP2
est.
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
kJ
⋅mol
J
⋅K
⋅mol
J
⋅K
⋅mol
J⋅K ⋅mol
2
3
4
1
1
ꢀchloroꢀmethylbenzene (liq) 274.2ꢀ306.2 451 46.4±0.4 [15]
ꢀchloroꢀmethylbenzene (liq) 274.2ꢀ309.2 412 46.9±0.5[15]
ꢀchloroꢀmethylbenzene (liq) 274.3ꢀ309.2 421 46.5±0.5[15]
110.9±1.3
111.8±1.7
110.5±1.1
112.0±0.2
109.1±0.3
166.4±0.6
361.5
368.1
369.5
344.3
345.7
340.4
250.6
256.3
259.0
232.3
236.6
ꢀ
,2ꢀdichlorobenzene (liq)
,3ꢀdichlorobenzene (liq)
,4ꢀdichlorobenzene(cr)
263.2ꢀ373.2 188 48.8±0.5[16]
278.3ꢀ311.3 264 47.3±0.5[16]
273.5ꢀ303.5 245 66.0±0.3[16]
1
a
From treatment of vapour pressures temperature dependencies (see Supporting Information);
From Table 6;
b
c
o
m
g
l
o
m
Calculated from
S
(g) and entropy of vaporization ꢀ S (298 K) given in this table.
4.4. Standard Molar Thermodynamic Functions of Halogen-Substituted Benzenes. The basic
thermodynamic equation for the Gibbs free energy,
o
m
o
o
m
ꢀ G
=
ꢀ H ꢀ T× ꢀ S
(4),
f
f
m
f
o
m
o
m
where ꢀ H and ꢀ S are the enthalpy and the entropy of formation, is responsible for the
f
f
1
9
assessment of feasibility of any chemical process. Recently, some of us have shown that the gasꢀ
o
m
phase standard molar entropies,
S
(g, 298 K), of different organic molecules can be calculated
o
m
with an acceptable accuracy. In this work, we calculated
S
(g, 298 K)ꢀvalues for isomeric chloroꢀ
toluenes and dichlorobenzenes by the G3MP2 method, optimizations and frequency calculations
were performed at the HF/6ꢀ31G(d) level of theory. These values for dichlorobenzenes are in good
20
agreement with values derived from statistical thermodynamics (ST).
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o
Table 6. Comparison of the
S
(g, 298 K)-values from quantum chemical calculations (G3MP)
m
-1
-1
and from statistical thermodynamics (ST) in J
⋅K
⋅
mol .
o
m
o
m
S
(g)
S
(g)
Compounds
ꢀClꢀtoluene
ꢀClꢀtoluene
ꢀClꢀtoluene
,2ꢀdiꢀClꢀbenzene
,3ꢀdiꢀClꢀbenzene
1,4ꢀdiꢀClꢀbenzene
G3MP2
361.5
ST
a
2
3
4
1
1
368.1
369.5
344.3
345.7
345.8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
341.5
343.5b
340.4
a
o
m
ꢀ1
ꢀ1
The value is calculated as the sum
S
(g, 298 K)G3MP2 = 346.6 J
⋅
K
⋅
mol and the contribution R×ln
σ
for the symmetry number (
σ
= 6);
b
o
m
ꢀ1
ꢀ1
The value is calculated from the experimental
S
(cr, 298 K) = 174.0 J⋅K ⋅mol [22] and the
g
cr
o
m
ꢀ1
ꢀ1
sublimation entropy ꢀ S = 166.4 J⋅K ⋅mol derived from the vapour pressure measurements [24]
between 273.5 K and 303.5 K (see Table S3).
o
m
Having such a good agreement the liquidꢀphase entropies
S
(liq, 298 K) for chloroꢀsubstituted
g
l
o
m
benzenes were calculated by substracting the molar entropy of vaporization ꢀ S (see Table 5)
o
m
from the entropy in the gas state
S
(g, 298 K) given in Table 6, column 2. The molar entropy of
g
l
o
m
vaporization ꢀ S ꢀvalues were derived (see Table 7) from the experimental vapour pressures
available from the literature (see Tables S2 and S3). The standard molar entropies of formation,
o
m
ꢀ S , in the gas and in the liquid phase were calculated on the basis of reaction (5):
f
a Cgraphite + (b/2) H (g) + (c/2) Cl (g) = C H Cl (g or liq or cr)
(5),
2
2
a
b
c
o
m
using the
S
ꢀvalues given in Table 7 and the values of entropy of formation for Cgraphite (5.74±0.13)
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
J⋅
K
⋅
mol , for H
2
(g) (130.52±0.02) J
⋅K
⋅
mol , and for Cl
2
(g) (223.081±0.010) J
⋅K
⋅mol
21
recommended by Chase.
o
m
Values of the standard Gibbs energies of formation, ꢀ G , were estimated by Eq. 4 from the
f
o
m
o
m
values of ꢀ H and ꢀ S (see Table 7). The standard molar thermodynamic functions in the liquid
f
f
and in the gas phase collected in Table 7 can be used for optimizing technologies for the
manufacturing of halogenꢀsubstituted benzenes as well as for design and implementation of novel
biphasic reaction systems.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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5
5
5
5
5
5
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Table 7. Experimental standard molar thermodynamic properties of chloro-substituted
benzenes at = 298.15 K.
T
Compound
o
o
a
o
b
o
c
o
d
State
ꢀ H
ꢀ S
ꢀ G
S
ꢀ1
C
ꢀ1
f
m
f
m
f
m
m
p
,
m
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
kJ·mol
J⋅K ⋅mol
ꢀ357.9
ꢀ247.0
ꢀ352.2
ꢀ240.4
ꢀ349.5
ꢀ239.0
ꢀ286.3
ꢀ174.3
ꢀ282.0
ꢀ172.9
ꢀ344.6
ꢀ288.8
ꢀ178.2
kJ·mol
J⋅K ⋅mol
250.6
361.5
256.3
368.1
259.0
369.5
232.3
344.3
236.6
345.7
J⋅K ⋅mol
2ꢀchloroꢀmethylbenzene
liq
gas
liq
gas
liq
gas
liq
gas
liq
gas
cr
ꢀ33.0 [15]
13.1 [15]
ꢀ30.5[15]
16.3[15]
ꢀ29.1[15]
73.7
86.8
74.5
88.0
75.1
88.4
67.9
83.4
63.6
78.3
60.4
61.0
75.6
178.1
121.2
171.7
116.5
176.6
120.1
171.3
111.9
170.6
112.3
143.8 [21]
179.4
112.3
3
4
1
1
1
ꢀchloroꢀmethylbenzene
ꢀchloroꢀmethylbenzene
,2ꢀdichlorobenzene
,3ꢀdichlorobenzene
,4ꢀdichlorobenzene
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
17.1[15]
ꢀ17.4±1.3[16]
31.4±1.4[16]
ꢀ20.5±1.0[16]
26.8±1.1[16]
ꢀ42.3±1.0[16]
ꢀ25.1±1.1[16]
22.5±1.2[16]
174.0 [21]
229.8
340.4
e
liq
gas
a
b
c
d
Calculated by Eq. 5;
Calculated by Eq. 4;
From Tables 5 and 6;
From Table S4;
e
o
ꢀ1
ꢀ1
The value is calculated from the experimental
S
(cr, 298 K) = 174.0 J
⋅
K
⋅
mol [22] and the
m
l
cr
o
m
ꢀ1
ꢀ1
entropy of fusion ꢀ S = 55.8 J⋅K ⋅mol [23].
5
. Conclusions
In conclusion, we have studied thermodynamics and kinetics of the isomerization of chlorotoluenes
and dichlorobenzenes. Our experiments using highly acidic ionic liquid and molten salt catalyst
systems provided temperatureꢀdepended equilibrium constants in the liquid phase from which thre
reaction enthalphies of four isomerisation processes were derived. We could demonstrate that
thermodynamic functions derived from quantumꢀchemical calculation matched the experimental
values within the boundaries of uncertainty. These will allow in the future to optimize isomerization
processes of aromatic compounds, for example in liquidꢀliquid biphasic reaction mode, by the help
of quantum chemical calculations.
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AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: sergey.verevkin@uniꢀrostock.de Phone: +49ꢀ381ꢀ498ꢀ6508.
Fax: +49ꢀ381ꢀ498ꢀ6502.
Notes
10
11
12
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14
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23
24
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50
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56
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60
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work has been supported by the German Science Foundation (DFG) in frame of the priority
program SPP 1708 “Material Synthesis Near Room Temperature”. This work has been partly
supported by the Russian Government Program of Competitive Growth of Kazan Federal
University and Russian Foundation for Basic Research No. 15ꢀ03ꢀ07475.
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