A linear solvation energy relationship study for the reactivity of 2-(4-substituted phenyl)-cyclohe-1-enecarboxylic, 2-(4-substituted phenyl)-benzoic, and 2-(4-substituted phenyl)-acrylic acids with diazodiphenylmethane in various solvents

Article abstract of DOI:10.1002/kin.20497

The reactivities of 2-(4-substituted phenyl)-cyclohex-1-enecarboxylic acids, 2-(4-substituted phenyl)-benzoic acids, and 2-(4-substituted phenyl)-acrylic acids with diazodiphenylmethane in various solvents were investigated. To explain the kinetic results through solvent effects, the second-order rate constants of the examined acids were correlated using the Kamlet-Taft solvatochromic equation. The correlations of the kinetic data were carried out by means of multiple linear regression analysis, and the solvent effects on the reaction rates were analyzed in terms of initial and transition state contributions. The signs of the equation coefficients support the proposed reaction mechanism. The solvation models for all investigated carboxylic acids are suggested. The quantitative relationship between the molecular structure and the chemical reactivity is discussed, as well as the effect of geometry on the reactivity of the examined molecules.

Full text of DOI:10.1002/kin.20497

A Linear Solvation Energy
Relationship Study for the
Reactivity of
2-(4-Substituted Phenyl)-
cyclohex-1-enecarboxylic,
2-(4-Substituted
Phenyl)-benzoic, and
2-(4-Substituted
Phenyl)-acrylic Acids with
Diazodiphenylmethane in
Various Solvents
1
1
2
´
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J. B. NIKOLIC, G. S. USCUMLIC, I. O. JURANIC
1Department of Organic Chemistry, Faculty of Technology and Metallurgy, University of Belgrade,
Karnegijeva 4, P.O. Box. 3505, 11120 Belgrade, Serbia
2Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, P.O. Box. 158, 11001 Belgrade, Serbia
Received 13 November 2009; revised 15 January 2010, 1 February 2010; accepted 1 February 2010
DOI 10.1002/kin.20497
Published online in Wiley InterScience (www.interscience.wiley.com).
Correspondence to: J. B. Nikolic´; e-mail: jasmina@tmf.bg.ac.rs.
Contract grant sponsor: Ministry of Science and Environmental
Protection, Republic of Serbia.
Contract grant numbers: 142063 and 142010.
2010 Wiley Periodicals, Inc.
c
ꢀ

SOLVATION ENERGY RELATIONSHIP OF CARBOXYLIC ACIDS WITH DIAZODIPHENYLMETHANE
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ABSTRACT: The reactivities of 2-(4-substituted phenyl)-cyclohex-1-enecarboxylic acids, 2-
(4-substituted phenyl)-benzoic acids, and 2-(4-substituted phenyl)-acrylic acids with dia-
zodiphenylmethane in various solvents were investigated. To explain the kinetic results through
solvent effects, the second-order rate constants of the examined acids were correlated using the
Kamlet–Taft solvatochromic equation. The correlations of the kinetic data were carried out by
means of multiple linear regression analysis, and the solvent effects on the reaction rates were
analyzed in terms of initial and transition state contributions. The signs of the equation coef-
ficients support the proposed reaction mechanism. The solvation models for all investigated
carboxylic acids are suggested. The quantitative relationship between the molecular structure
and the chemical reactivity is discussed, as well as the effect of geometry on the reactivity of
the examined molecules. ꢀ 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 430–439, 2010
C
INTRODUCTION
cept developed by Kamlet and Taft [7] (Eq. (1)). The
correlation equations obtained by a stepwise regres-
sion for all the examined acids showed that the total
solvatochromic equation can be used in its complete
form.
The present paper demonstrates how the lin-
ear solvation energy relationship method can be
used to explain and present multiple interacting ef-
fects of the solvents on the reactivity of 2-(4-
substituted phenyl)-cyclohex-1-enecarboxylic acids,
2-(4-substituted phenyl)-benzoic acids, and 2-(4-
substituted phenyl)-acrylic acids (Fig. 1) in their reac-
tion with DDM and the quantitative estimation of the
solvent effects on the structure–reactivity relationship.
The geometric data of the examined acids, correspond-
ing to the energy minima in the applied solvents, were
obtained using the semiempirical MO PM6 method.
Related to the study of the inßuence of the solvent
on the reactivity [1–6] of organic molecules, previous
work is extended toward the reactivity of α,β-unsatu-
rated carboxylic acids in their reaction with diazodi-
phenylmethane (DDM) in various protic and aprotic
solvents.
Kamlet et al. [7] established that the effect of a
solvent on the reaction rate should be given in terms of
the following properties: (i) the behavior of a solvent
as a dielectric, facilitating the separation of opposite
charges in the transition state, (ii) the ability of a solvent
to donate a proton in a solvent-to-solute hydrogen bond
and thus stabilize the carboxylate anion in the transition
state, (iii) the ability of a solvent to donate an electron
pair and therefore stabilize the initial carboxylic acid,
by way of a hydrogen bond between the carboxylic
proton and the solvent electron pair, and used Eq. (1)
to explain these properties:
MATERIALS AND METHODS
log k = Ao + sπ∗ + aα + bβ
(1)
2-(4-Substituted phenyl)-cyclohex-1-enecarboxylic
acids were prepared by the procedure for 2-
substituted-cyclohex-1-enecarboxylic acids [10] from
the corresponding 2-(4-substituted phenyl)-cyclo-
hexanone by the cyanohydrine reaction as reported
previously [11].
2-(4-Substituted phenyl)-benzoic acids were pre-
pared from the corresponding 4-substituted aniline ac-
cording to methods described in the literature [12].
2-(4-Substituted phenyl)-acrylic acids were pre-
pared by saponiÞcation of the cis-ethyl cinnamates,
synthesized by the hydrogenation of the appropriate
ethyl-phenylpropionate over the Lindlar catalyst [13]
as described previously [14].
The chemical structure and the purity of the ob-
tained acids were conÞrmed by melting points, as well
as 1H NMR, FTIR, and UV spectroscopy.
Diazodiphenylmethane was prepared by the method
described by Smith and Howard [15], and stock
solutions were stored in a refrigerator and diluted
The parameter π∗ is an appropriate measure of the Þrst
property, whereas the second and the third properties
are governed by the effects of the solvent acidity and
basicity, quantitatively expressed by the parameters α
and β, respectively. The linear dependence (LSER) on
the solvent parameters is used to correlate and predict a
wide variety of solvent effects, as well as to provide an
analysis in the terms of knowledge and the theoretical
concepts of the molecular structural effects [7].
To the best of our knowledge, the inßuence of
aprotic solvents on the reactivity of carboxylic acids
with DDM by the Kamlet–Taft treatment has not been
systematically presented before, except for benzoic
acid [7].
In recent papers [5,6,8,9], we examined the effects
of a set of 11 aprotic and 3 protic solvents on the reac-
tion of various carboxylic acids with DDM by means
of the linear solvation energy relationship (LSER) con-
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NIKOLIC, USCUMLIC, AND JURANIC
COOH
H
H
COOH
COOH
X
X
X
1
2
3
Figure 1 The structure of the investigated 2-(4-substituted phenyl)-cyclohex-1-enecarboxylic acids (1), 2-(4-substituted
phenyl)-benzoic acids (2), and 2-(4-substituted phenyl)-acrylic acids (3) (X = H, OCH3, CH3, Cl, Br).
before use. Solvents were puriÞed as described in
previous papers [16,17]. All the solvents used in ki-
netic studies were analytical grade. Rate constants for
the reaction of examined acids with DDM were de-
termined as reported previously, by the spectroscopic
method of Roberts and his co-workers [18], using a Shi-
matzu UV-1700 spectrophotometer. Absorbance mea-
surements were performed at 525 nm with 1-cm cells
at 30 0.05◦C. The second-order rate constants for
all acids were obtained by dividing the pseudo-Þrst-
order rate constants by the acid concentration (the con-
centration of acid was 0.06 mol dm−3 and of DDM
0.006 mol dm−3). Three to Þve rate determinations
were made on each acid in every case, and in particular
the second-order rate constants agreed within 3% of the
mean values. The correlation analysis was preformed
using Origin and Microsoft Excel computer software.
The goodness of Þt was discussed using correlation co-
efÞcient (R), standard deviation (SD), and the Fisher’s
value (F).
second-order rate constants for the same acids in hy-
droxylic solvents [3,4] were correlated using the total
solvatochromic equation (1), where π∗, α, and β are
solvatochromic parameters; s, a, and b are the solva-
tochromic coefÞcients; and Ao is the regression value
of the examined solute property in the reference sol-
vent, cyclohexane.
In Eq. (1), π∗ is an index of solvent dipolarity/
polarizability, which is a measure of the ability of a
solvent to stabilize a charge, or a dipole by its own
dielectric effect. The π∗ scale was selected to run from
0.00 for cyclohexanone to 1.00 for dimethyl sulfoxide.
The α parameter represents the scale of solvent hy-
drogen bond donor (HBD) acidity and has a range from
0.00 for non-HBD solvents (e.g., n-hexane, cyclohex-
ane) to 1.00 for methanol. It describes the ability of a
solvent to donate a proton or accept an electron pair
in a solvent-to-solute hydrogen bond. The β parameter
represents the scale of solvent hydrogen bond acceptor
(HBA) basicity, in other words the ability of a sol-
vent to donate an electron pair, or accept a proton in
a solvent-to-solute hydrogen bond. The β scale runs
from 0.00 for non-HBA solvents (e.g., n/hexane) to
about 1.00 for hexamethylphosphoric acid triamide.
The obtained second-order rate constants for the
examined acids in 11 aprotic solvents, together with
the previously determined rate constants for the same
acids in the hydroxylic solvents, are given in Tables I–
III. Comparison of the values of the reaction constants
in protic and aprotic solvents indicates that the ex-
amined reaction is slower in aprotic than in protic sol-
vents, which is in agreement with the supposed reaction
mechanism [20–22].
The reported conformations and the corresponding
heats of formation of the examined molecules were
obtained by the semiempirical MO PM6 method, using
MOPAC 2007 program package [19]. The following
keywords were used for optimization: EF GNORM =
0.100 MMOK GEO-OK PM6 EPS = xx PRECISE,
where xx stands for the appropriate dielectric constant
of a solvent (ethanol in this case).
RESULTS AND DISCUSSION
The second-order rate constants for the reaction
of 2-(4-substituted phenyl)-cyclohex-1-enecarboxylic
acids, 2-(4-substituted phenyl)-benzoic acids, and 2-
(4-substituted phenyl)-acrylic acids with DDM in 11
aprotic solvents at 30◦C were determined. To explain
the kinetic results through solvent effects, the second-
order rate constants of the examined acids in apro-
tic solvents together with the previously determined
Solvent–Reactivity Relationship
To explain the obtained kinetic results through solvent
dipolarity/polarizability and basicity or acidity, the rate
constants of the examined acids were correlated with
the solvent properties using the total solvatochromic
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Table I Second-Order Rate Constants (dm3 mol−1 min−1) for the Reaction of 2-(4-Substituted
Phenyl)-cyclohex-1-enecarboxylic Acids with Diazodiphenylmethane at 30◦C in a Set of Various Solvents
k (dm3 mol−1 min−1
)
Solvent
H
CH3
OCH3
Cl
Br
1. Methyl acetate
2. Cyclohexanone
3. Diethyl ketone
4. Carbon tetrachloride
5. Ethyl acetate
6. Cyclopentanone
7. Dioxane
8. Acetonitrile
9. Acetone
10. Methanol
11. Ethanol
0.0681
0.0382
0.133
0.873
0.0542
0.0491
0.142
0.839
0.103
2.79
0.0522
0.0281
0.100
0.735
0.0411
0.0373
0.104
0.703
0.0811
2.32
0.0442
0.0242
0.0851
0.665
0.0351
0.0322
0.0873
0.635
0.0711
2.032
0.924
4.89
0.00911
0.0232
0.0981
0.0561
0.194
1.10
0.0782
0.0712
0.216
1.06
0.141
3.44
1.71
7.97
0.0191
0.0551
0.0982
0.0563
0.195
1.11
0.078
0.072
0.217
1.07
0.142
3.81
1.87
7.99
0.0192
0.0552
1.28
6.37
0.0141
0.0371
1.07
5.39
0.0113
0.0272
12. Ethylene glycol
13. Dimethyl sulfoxide
14. Tetrahydrofuran
Table II Second-Order Rate Constants (dm3 mol−1 min−1) for the Reaction of 2-(4-Substituted Phenyl)-benzoic
Acids with Diazodiphenylmethane at 30◦C in a Set of Various Solvents
k (dm3 mol−1 min−1
)
Solvent
H
CH3
OCH3
Cl
Br
1. Methyl acetate
2. Cyclohexanone
3. Diethyl ketone
4. Carbon tetrachloride
5. Ethyl acetate
6. Cyclopentanone
7. Dioxane
8. Acetonitrile
9. Acetone
10. Methanol
11. Ethanol
0.316
0.246
0.268
1.01
0.236
0.338
0.110
5.50
0.269
0.207
0.226
0.909
0.200
0.285
0.091
4.95
0.245
0.188
0.206
0.856
0.183
0.259
0.0822
4.65
0.322
9.56
3.99
13.1
0.127
0.111
0.394
0.310
0.336
1.163
0.295
0.425
0.149
6.346
0.482
13.5
5.870
17.6
0.200
0.186
0.395
0.311
0.337
1.16
0.295
0.426
0.142
6.36
0.400
0.348
0.483
11.6
5.00
15.4
0.162
0.147
10.7
4.26
13.9
0.139
0.123
13.9
6.12
17.6
0.200
0.187
12. Ethylene glycol
13. Dimethyl sulfoxide
14. Tetrahydrofuran
equation (1). The solvent parameters [23] are given in
Table IV. The correlation of the kinetic data was carried
out by means of the multiple linear regression analysis.
It was found that the rate constants in the applied set of
14 solvents show satisfactory correlation with π∗, α,
and β solvent parameters together in the same equation.
The obtained correlation results are given in Table V.
From the results presented in Table V, the gen-
eral conclusion can be made that the solvent effects
inßuence the carboxylic acid–DDM reaction by two
opposite contributions. The opposite signs of the elec-
trophilic and the nucleophilic parameters are, as ex-
pected, in accordance with the described mechanism
of the reaction. The positive signs of the s and a co-
efÞcients prove that the classical solvation and HBD
effects increase the reaction rate, supporting the for-
mation of the transition state, and the negative sign
of the b coefÞcient points out that HBA effects de-
crease the reaction rate and stabilize the state before
the reaction begins. The degree of success of the above
correlations is shown in Fig. 2 by means of a plot of
log k calculated vs. log k obtained experimentally for
2-(4-substituted phenyl)-cyclohex-1-enecarboxylic, 2-
(4-substituted phenyl)-benzoic, and 2-(4-substituted
phenyl)-acrylic acids in different solvents. From the
values of regression coefÞcients, the contribution of
each parameter to reactivity, on a percentage basis,
was calculated and is listed in Table VI.
The percentage contribution of solvatochromic pa-
rameters, for the reaction of examined acids with
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Table III Second-Order Rate Constants (dm3 mol−1 min−1) for the Reaction of 2-(4-Substituted Phenyl)-acrylic Acids
with Diazodiphenylmethane at 30◦C in a Set of Various Solvents
k (dm3 mol−1 min−1
)
Solvent
H
CH3
OCH3
Cl
Br
1. Methyl acetate
2. Cyclohexanone
3. Diethyl ketone
4. Carbon tetrachloride
5. Ethyl acetate
6. Cyclopentanone
7. Dioxane
8. Acetonitrile
9. Acetone
10. Methanol
11. Ethanol
0.244
0.151
0.424
2.00
0.201
0.186
0.447
1.94
0.178
0.108
0.307
1.64
0.146
0.134
0.312
1.58
0.149
0.089
0.255
1.46
0.122
0.111
0.253
1.40
0.372
0.236
0.656
2.63
0.309
0.289
0.727
2.56
0.374
0.237
0.659
2.64
0.311
0.290
0.732
2.57
0.343
5.28
2.58
0.263
4.62
2.26
0.226
3.86
1.83
0.491
7.44
3.76
0.493
7.87
4.11
12. Ethylene glycol
13. Dimethyl sulfoxide
14. Tetrahydrofuran
10.3
0.0662
0.147
8.52
0.0491
0.105
7.63
0.0412
0.086
13.4
0.0991
0.232
13.4
0.0993
0.233
DDM, show that the most of the solvatochromism
is due to solvent basicity and acidity rather than
to the solvent dipolarity/polarizability. Considering
these results, the solvation models of the reac-
tants and the transition states, separately for 2-(4-
substituted phenyl)-cyclohex-1-enecarboxylic acids,
2-(4-substituted phenyl)-benzoic acids, and 2-(4-
substituted phenyl)-acrylic acids can be represented as
Generally, the presence of a substituent at the C-4
position on the benzene ring in all types of examined
acids has a secondary inßuence on the reaction with
DDM and do not seem to cause steric hindrance be-
tween the reactants and the solvent. The principal in-
ßuences on the reaction rate are apparently the solvent
properties and the general form of the carboxylic acid
molecule.
2-(4-substituted phenyl)-cyclohex-1-enecarboxylic acids:
Reactants
HBA solvation (51%)
⇒
Transition state
HBD and solvation by
nonspeciÞc interactions (49%)
⇒
⇒
⇒
Products
Products
Products
2-(4-substituted phenyl)-benzoic acids:
Reactants
HBA solvation (38%)
⇒
Transition state
HBD and solvation by
nonspeciÞc interactions (62%)
2-(4-substituted phenyl)-acrylic acids:
Reactants
HBA solvation (50%)
⇒
Transition state
HBD and solvation by
nonspeciÞc interactions (50%)
The suggested solvation models indicate that
the 2-phenylcyclohex-1-enecarboxylic and the 2-
phenylacrylic acid systems are more sensitive to the
HBA solvent interactions than the 2-phenylbenzoic
acid system (Table VI) and less sensitive to the
HBD solvent ability. The same results were ob-
tained for a comparative LSER study of the reac-
tivity of 2-substituted cyclohex-1-enecarboxylic and
2-substituted benzoic acids [8] and 2-substituted
cyclohex-1-eneacetic and 2-substituted phenylacetic
acids [9] presented in previous papers.
Structure–Reactivity Relationship
Taking into account the results presented in this work,
it can be concluded that the solvation differences of
the examined acids in their reaction with DDM derive
from the structural differences between the cyclohex-
1-enylcarboxylic, benzoic, and the open-chain acrylic
acids. Such a conclusion can be drawn from the
minimal energy molecular conformations. The geo-
metric layouts of the unsubstituted representatives of
all carboxylic acid systems examined in this work,
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SOLVATION ENERGY RELATIONSHIP OF CARBOXYLIC ACIDS WITH DIAZODIPHENYLMETHANE
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Table IV Solvent Parameters [23]
Table VI Percentage Contributions of Kamlet–Taft’s
Solvatochromic Parameters to the Reactivity
Solvent
π∗
α
β
Pπ∗
(%)
Pα
(%)
Pβ
(%)
1. Methyl acetate
2. Cyclohexanone
3. Diethyl ketone
4. Carbon tetrachloride
5. Ethyl acetate
6. Cyclopentanone
7. Dioxane
8. Acetonitrile
9. Acetone
10. Methanol
11. Ethanol
0.60
0.76
0.72
0.28
0.55
0.76
0.55
0.85
0.72
0.60
0.54
0.92
1.00
0.58
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.19
0.08
0.93
0.83
0.90
0.00
0.00
0.42
0.53
0.45
0.00
0.45
0.52
0.37
0.31
0.48
0.62
0.77
0.52
0.76
0.55
Acids
2-Phenylcyclohex-1-enecarboxylic
acid
2-(4-Methylphenyl)-cyclohex-1-
enecarboxylic acid
2-(4-Methoxyphenyl)-cyclohex-1-
enecarboxylic acid
2-(4-Chlorophenyl)-cyclohex-1-
enecarboxylic acid
6.50 43.50 50.00
7.00 47.50 45.50
6.70 42.90 50.40
6.00 43.60 50.40
5.00 44.60 50.40
2-(4-Bromophenyl)-cyclohex-1-
enecarboxylic acid
2-Phenylbenzoic acid
12. Ethylene glycol
13. Dimethyl sulfoxide
14. Tetrahydrofuran
20.00 42.00 38.00
20.00 42.00 38.00
2-(4-Methoxyphenyl)-benzoic acid 20.00 42.00 38.00
2-(4-Methylphenyl)-benzoic acid
2-(4-Chlorophenyl)-benzoic acid
2-(4-Bromophenyl)-benzoic acid
2-Phenylacrylic acid
2-(4-Methylphenyl)-acrylic acid
2-(4-Methoxyphenyl)-acrylic acid
2-(4-Chlorophenyl)-acrylic acid
2-(4-Bromophenyl)-acrylic acid
20.00 42.00 38.00
20.00 42.00 38.00
6.50 43.50 50.00
6.00 44.00 50.00
7.50 42.50 50.00
6.00 43.00 51.00
5.00 45.00 50.00
corresponding to the energy minima in solvent, were
obtained using the semiempirical MO PM6 method
and are shown in Figs. 3–5.
These geometrical conformations are taken into
consideration because they are the most stable forms
in which the examined molecules generally exist and
Table V Results of the Correlations of the Kinetic Data with Eq. (1)
Acid
Ao
sa
aa
ba
Rb
sc
Fd
ee
2-Phenylcyclohex-1-enecarboxylic
acid
2-(4-Methylphenyl)-cyclohex-1-
enecarboxylic acid
2-(4-Methoxyphenyl)-cyclohex-1-
enecarboxylic acid
2-(4-Chlorophenyl)-cyclohex-1-
enecarboxylic acid
−0.140 0.35 0.22 2.34 0.10 −2.70 0.24 0.991 0.13 175 14
−0.230 0.38 0.24 2.39 0.11 −2.30 0.21 0.989 0.14 152 14
−0.290 0.38 0.24 2.41 0.11 −2.83 0.26 0.988 0.14 136 14
−0.002 0.32 0.24 2.25 0.11 −2.59 0.26 0.988 0.14 136 14
2-(4-Bromophenyl)-cyclohex-1-
enecarboxylic acid
2-Phenylbenzoic acid
0.010
0.27 0.24 2.27 0.11 −2.55 0.26 0.988 0.14 138 14
−0.340 0.99 0.41 2.11 0.19 −1.90 0.44 0.961 0.24
−0.400 1.02 0.42 2.15 0.19 −1.96 0.45 0.960 0.24
40 14
40 14
40 14
41 14
41 14
2-(4-Methylphenyl)-benzoic acid
2-(4-Methoxyphenyl)-benzoic acid −0.440 1.04 0.43 2.17 0.20 −1.99 0.45 0.960 0.23
2-(4-Chlorophenyl)-benzoic acid
2-(4-Bromophenyl)-benzoic acid
2-Phenylacrylic acid
−0.260 0.97 0.40 2.07 0.19 −1.84 0.42 0.962 0.23
−0.260 0.95 0.40 2.08 0.19 −1.82 0.43 0.961 0.23
0.230
0.130
0.060
0.400
0.410
0.31 0.19 1.92 0.09 −2.25 0.20 0.990 0.11 163 14
0.30 0.19 2.01 0.09 −2.30 0.21 0.989 0.11 160 14
0.36 0.19 2.03 0.09 −2.38 0.21 0.990 0.11 167 14
0.32 0.24 2.25 0.11 −2.59 0.26 0.989 0.11 145 14
0.21 0.19 1.86 0.09 −2.07 0.21 0.988 0.11 139 14
2-(4-Methylphenyl)-acrylic acid
2-(4-Methoxyphenyl)-acrylic acid
2-(4-Chlorophenyl)-acrylic acid
2-(4-Bromophenyl)-acrylic acid
a Calculated solvatochromic coefÞcient.
b Correlation coefÞcient.
c Standard deviation of the estimate.
d Fisher’s test.
e Number of points used in the calculation.
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0
Table VII Data Calculated Using the Semiempirical
MO PM6 Method
Heat of
Formation
(kcal mol−1
)
logkexp = −0.0099 + 0.997logkcalc
r = 0.977, s = 0.16, n =210
Acid
Torsion Angle (◦)
2-Phenylcyclohex-1- (C2-C1-C7-O8) 134.8
enecarboxylic acid
−79.93
k
2-Phenylbenzoic acid (C2-C1-C7-O8) 132.5
2-Phenylacrylic acid (C2-C1-C3-O4) 136.9
−47.20
−57.14
−2
pected (Figs. 3–5). The degree of these interactions is
in agreement with the obtained kinetic data and solva-
tion models for all the examined acids.
−2
0
2
Additional evidence of the solvent effect on the
structure–reactivity relationship in the reaction of the
examined 2-phenyl-substituted carboxylic acids with
DDM was also obtained by the correlation of log k
values for the examined acids with the Hammett equa-
tion (2) [24]:
log kcalc
Figure 2 The plot of log k observed against log k cal-
culated from Eq. (1) for 2-(4-substituted phenyl)-cyclohex-
1-enecarboxylic, 2-(4-substituted phenyl)-benzoic, and 2-(4-
substituted phenyl)-acrylic acids in different solvents. [Color
Þgure can be viewed in the online issue, which is available
at www.interscience.wiley.com.]
log k = log k0 + ρσp
(2)
react; their torsion (C2–C1–C7–O8 or C2–C1–C3–O4)
and the heats of formation corresponding to them are
given in Table VII. Generally, the presence of the sub-
stituted phenylene group in the position 2 affects the
orientation of the carboxylic group (torsion angles).
The interactions of this type are the strongest in the
case of 2-phenylbenzoic acid, weaker in the case of 2-
phenylcyclohex-1-enecarboxylic acids, and in the case
of 2-phenylacrylic acids they should not even be ex-
where ρ is the reaction constant, reßecting the sen-
sitivity of log k to substituent effects. The substituent
constant σp [25] is a measure of the electronic effects of
a substituent. The results of the correlations are given
in Tables VIII–X.
On the basis of the values of the reaction constants,
it can be concluded that the resonance interactions
are the least disturbed in the case of 2-(4-substituted
phenyl)-acrylic acids, somewhat more in the case
Figure 3 The most stable conformation of 2-phenylcyclohex-1-enecarboxylic acid. [Color Þgure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
International Journal of Chemical Kinetics DOI 10.1002/kin

SOLVATION ENERGY RELATIONSHIP OF CARBOXYLIC ACIDS WITH DIAZODIPHENYLMETHANE
437
Figure 4 The most stable conformation of 2-phenylbenzoic acid. [Color Þgure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
Figure 5 The most stable conformation of 2-phenylacrylic acid. [Color Þgure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
of 2-(4-substituted phenyl)-cyclohex-1-enecarboxylic
acids, and the most in the case of 2-(4-substituted
phenyl)-benzoic acids.
These results are in agreement with the struc-
tural characteristics of the examined carboxylic acids
obtained by the semiempirical MO PM6 method
and the solvation models determined by the LSER
method.
CONCLUSION
The results of the presented investigations and the pre-
viously reported results [5,6,8,9] show that the solva-
tochromic concept of Kamlet and Taft (LSER) is appli-
cable to the kinetic data for the reaction of more than 50
different carboxylic acids with diazodiphenylmethane
in various solvents, meaning that this model gives the
International Journal of Chemical Kinetics DOI 10.1002/kin

´ ´ ´ ´
ˇ
NIKOLIC, USCUMLIC, AND JURANIC
438
Table VIII Hammett Reaction Constants for
Table X Hammett Reaction Constants for
2-(4-Substituted Phenyl)-cyclohex-1-enecarboxylic Acids
2-(4-Substituted Phenyl)-acrylic Acids
Solvent
ρa
rb
sc
dd
Solvent
ρa
rb
sc
dd
1. Methyl acetate
2. Cyclohexanone
3. Diethyl ketone
0.695 0.004 0.999 0.0020
0.745 0.006 0.999 0.0030
0.722 0.002 0.999 0.0010
5
5
5
5
5
5
5
5
5
5
5
5
5
5
1. Methyl acetate
2. Cyclohexanone
3. Diethyl ketone
0.801 0.002 0.999 0.0010
0.852 0.002 0.999 0.0009
0.826 0.002 0.999 0.0008
5
5
5
5
5
5
5
5
5
5
5
5
5
5
4. Carbon tetrachloride 0.441 0.001 0.999 0.0005
4. Carbon tetrachloride 0.516 0.001 0.999 0.0006
5. Ethyl acetate
6. Cyclopentanone
7. Dioxane
8. Acetonitrile
9. Acetone
10. Methanol
11. Ethanol
12. Ethylene glycol
0.699 0.001 0.999 0.0005
0.706 0.008 0.999 0.0030
0.795 0.002 0.999 0.0008
0.452 0.001 0.999 0.0050
0.603 0.003 0.999 0.0010
0.498 0.040 0.990 0.0180
0.571 0.040 0.992 0.0180
0.426 0.001 0.999 0.0004
5. Ethyl acetate
6. Cyclopentanone
7. Dioxane
8. Acetonitrile
9. Acetone
10. Methanol
11. Ethanol
12. Ethylene glycol
13. Dimethyl sulfoxide 0.767 0.002 0.999 0.0009
14. Tetrahydrofuran
0.815 0.003 0.999 0.0010
0.836 0.001 0.999 0.0006
0.923 0.003 0.999 0.0010
0.528 0.001 0.999 0.0007
0.679 0.002 0.999 0.0008
0.580 0.040 0.992 0.0190
0.643 0.060 0.987 0.0270
0.489 0.001 0.999 0.0006
13. Dimethyl sulfoxide 0.632 0.030 0.997 0.0120
0.764 0.006 0.999 0.0030
14. Tetrahydrofuran
0.866 0.002 0.999 0.0010
a Reaction constant.
a Reaction constant.
b Correlation coefÞcient.
b Correlation coefÞcient.
c Standard deviation of the estimate.
d Number of points used in the calculation.
c Standard deviation of the estimate.
d Number of points used in the calculation.
Table IX Hammett Reaction Constants for
2-(4-Substituted Phenyl)-benzoic Acids
acids, and 2-(4-substituted phenyl)-acrylic acids are
suggested. The results show that the presence of the
substituted phenylene group at the C-2 position affects
the orientation of the carboxylic group. The degree of
these interactions is in agreement with the obtained ki-
netic data, solvation models, and characteristics of the
examined carboxylic acid molecule.
Solvent
ρa
rb
sc
dd
1. Methyl acetate
2. Cyclohexanone
3. Diethyl ketone
0.415 0.001 0.999 0.0006
0.438 0.002 0.999 0.0007
0.429 0.002 0.999 0.0009
5
5
5
5
5
5
5
5
5
5
5
5
5
5
4. Carbon tetrachloride 0.268 0.001 0.999 0.0003
5. Ethyl acetate
6. Cyclopentanone
7. Dioxane
8. Acetonitrile
9. Acetone
10. Methanol
11. Ethanol
12. Ethylene glycol
0.418 0.002 0.999 0.0010
0.431 0.001 0.999 0.0006
0.503 0.020 0.997 0.0090
0.271 0.001 0.999 0.0003
0.353 0.001 0.999 0.0005
0.299 0.020 0.993 0.0090
0.360 0.020 0.996 0.0080
0.256 0.001 0.999 0.0003
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