Solvent and structural effects on the kinetics of the reactions of 2-substituted cyclohex-1-enylcarboxylic and 2-substituted benzoic acids with diazodiphenylmethane

Article abstract of DOI:10.1002/kin.20281

The rate constants for the reaction of 2-methyl-cyclohex-1-enylcarboxylic, 2-phenylcyclohex-1-enylcarboxylic, and 2-methylbenzoic and 2-phenylbenzoic acids with diazodiphenyl-methane were determined in 14 various solvents at 30°C. 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 quantitative relationship between the molecular structure and the chemical reactivity has been discussed, as well as the effect of geometry on the reactivity of the examined molecules. The geometric data of all the examined compounds corresponding to the energy minima in solvent, simulated as dielectric continuum, obtained using semiempirical MNDO-PM3 energy calculations.

Full text of DOI:10.1002/kin.20281

Solvent and Structural
Effects on the Kinetics
of the Reactions of
2-Substituted Cyclohex-
1-enylcarboxylic and
2-Substituted Benzoic Acids
with Diazodiphenylmethane
1
1
2
´
´
´
´
ˇ
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 29 June 2006; revised 19 February 2007, 21 May 2007, 11 July 2007; accepted 12 July 2007
DOI 10.1002/kin.20281
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The rate constants for the reaction of 2-methyl-cyclohex-1-enylcarboxylic,
2-phenylcyclohex-1-enylcarboxylic, and 2-methylbenzoic and 2-phenylbenzoic acids with
diazodiphenyl-methane were determined in 14 various solvents at 30◦C. To explain the ki-
netic 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 ki-
netic 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 quantitative relationship between the molecular structure and the chemical reactivity has
been discussed, as well as the effect of geometry on the reactivity of the examined molecules.
The geometric data of all the examined compounds corresponding to the energy minima in
solvent, simulated as dielectric continuum, obtained using semiempirical MNDO-PM3 energy
calculations. ꢀ 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 664–671, 2007
C
INTRODUCTION
Correspondence to: Jasmina Nikolic´; e-mail: jasmina@tmf.
bg.ac.yu.
Contract grant sponsor: Ministry of Science and Environmental
Protection, Republic of Serbia.
Contract grant numbers: 142063 and 142010.
2007 Wiley Periodicals, Inc.
Previous studies [1–5] of the kinetics of the reac-
tion of carboxylic acids with diazodiphenylmethane
(DDM) in various solvents have revealed the impor-
tant role of the nonspecific and specific solvent effects
c
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SOLVENT AND STRUCTURAL EFFECTS ON KINETICS OF CARBOXYLIC AND BENZOIC ACIDS
665
on the reactivity. The examined reaction may vary in
rate, depending on the reactants and conditions, but
it usually follows the second-order kinetics [1,2]. The
mechanism of this reaction has been thoroughly stud-
ied [6–9], and a spectrophotometric method and ki-
netic approaches have been developed [10,11]. It has
been shown that the reactivity of carboxylic acids is
influenced by the preferential solvation of the reac-
tants and/or the transition state, through the nonspe-
cific and specific solvent–solute interactions. Further-
more, it has been established that the multiple linear
regression analysis may well be used to separate and
quantify the impact of such solvent–solute interactions
on reactivity [3].
This paper extends our work on the reactivity of
α,β-unsaturated carboxylic acids in their reaction with
DDM in various solvents [12–15]. It is a very important
class of organic compounds, because they occur widely
in nature and because these molecules have powerful
biological properties and are the targets of the variety
of synthetic approaches [16].
In general, the presence of an α,β-double bond in
the ring of a cyclohex-1-enylcarboxylic acid increases
the acid reactivity, due to the inductive effect of the
unsaturated α-carbon atom. At the same time, the me-
someric effect of an α,β-double bond on the carboxylic
group decreases the reactivity of the acid, as in the
ground state the resonance interaction between the
double bond and the carboxylic group stabilizes the
acid; whereas in the case of the anion the resonance
stabilization is mainly within the carboxylate ion itself,
and the effect of a conjugate double bond is less sig-
nificant [17] (Fig. 1). Our previous investigations [18]
showed that the rate constant for the reaction of DDM
with α,β-unsaturated acids in ethanol is higher than for
the corresponding saturated compounds due to the ef-
fect of the polar and mesomeric factors, which oppose
one another, indicating the predominance of the former
one. The values of the rate constants for the correspond-
ing cycloalkenylacetic acids are still higher, which can
be interpreted as the evidence of the influence of the
mesomeric effect in the case of cycloalkenylcarboxylic
acids [18]. In our previous work [12], we examined the
reactivity of 2-substitued cyclohex-1-enylcabroxylic
acids with DDM in various alcohols. The results have
shown that the linear free energy relationships are ap-
plicable to the kinetic data for 2-subsituted cyclohex-1-
enyl-carboxylic system with the substituents of moder-
ate steric bulkiness. Comparisons were made with the
ortho-substitued benzoic acid system, under the same
experimental conditions. The results have shown that
there are slight differences in the composition of the
electronic effects depending on the type of the dou-
ble bond through which the effects are transmitted.
Starting from the assumption on similarity of the polar
interactions of the substituents with the reaction center
and on proximity effects, these differences were as-
cribed to the different polarizability of the examined
double bonds.
In the present work, the second-order rate con-
stants have been measured for the reaction of 2-methyl-
cyclohex-1-enylcarboxylic, 2-phenylcyclohex-1-enyl-
carboxylic, 2-methylbenzoic, and 2-phenylbenzoic
acids with DDM in 14 various solvents at 30◦C. Our
objective was to investigate the influence of the sol-
vents on the ortho-effect of the substituents in the
systems subjected to the secondary steric effects of
ortho-substituents. The solvent effects on the reaction
rate constants were interpreted by means of the linear
solvation energy relationships concept, developed by
Kamlet and Taft [19], expressed in the following form:
logk = A0 + sπ∗ + aα + bβ
(1)
where π∗, α, and β are solvatochromic parameters;
s, a, and b are the complementary solute-dependent
coefficients of the solvent parameters; and A0 is the
regression value of the examined solute property in the
reference solvent, cyclohexane.
In Eq. (1), π∗ is an index of solvent dipolar-
ity/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
Figure 1 The mesomeric effect of an α,β-double bond on
the carboxylic group in the ground state and in the anion.
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NIKOLIC, USCUMLIC, AND JURANIC
666
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 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
quantitative relationship between the molecular struc-
ture and the chemical reactivity has been discussed,
as well as the effect of geometry on the reactivity of
the examined molecules. The geometric data of all
the examined compounds corresponding to the energy
minima in solvent were obtained using semiempirical
MNDO-PM3 energy calculations.
in solvent, were obtained using semiempirical MNDO-
PM3 energy calculations [23,24].
RESULTS AND DISCUSSION
The second-order rate constants for the reac-
tion of 2-methylcyclohex-1-enylcarboxylic, 2-phenyl
cyclohex-1-enylcarboxylic, 2-methylbenzoic, and 2-
phenylbenzoic acids with DDM in 14 various solvents
at 30◦C, together with the previously determined [15]
rate constants for cyclohex-1-enylcarboxylic and ben-
zoic acids, are given in Tables I and II. Comparison
of the rate constants in protic and aprotic solvents in-
dicates that the examined reaction is slower in aprotic
solvents, which are in accordance with the supposed
reaction mechanism [6–9]. The mechanism of this re-
action in both protic and aprotic solvents was found to
involve the same rate-determining step: proton transfer
from the carboxylic acid to DDM, forming a diphenyl-
methanediazonium carboxylate ion-pair, which rapidly
reacts to give esters, or ethers in the case of hydroxylic
solvents.
MATERIALS AND METHODS
Cyclohex-1-enylcarboxylic, 2-methylcyclohex-1-enyl-
carboxylic, and 2-phenyl-clohex-1-enylcarboxylic
acids were prepared by the method of Wheeler and
Lerner [20], from the corresponding cyclohexanone
cyanohydrine, which was dehydrated to cyanocyclo-
hexene. Nitrile was hydrolyzed with phosphoric acid
to the corresponding cyclohex-1-enylcarboxylic acid.
Benzoic, 2-methylbenzoic, and 2-phenylbenzoic acids
were commercial products (Fluka, Germany).
Ph2CN2 + RCOOH → Ph2CHN+2 −O2CR
Our previous investigations of the reactivity of α,β-
unsaturated carboxylic acids with DDM in various
solvents [12–15] established that the effect of a sol-
vent 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 sol-
vent 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 car-
boxylic acid, by way of a hydrogen bond between the
carboxylic proton and the solvent electron pair. The
parameter π∗ is an appropriate measure of the first
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 chemical structure and the purity of the ob-
tained compounds were confirmed by melting or boil-
ing points, 1HNMR, FTIR, and UV spectra.
Diazodiphenylmethane was prepared by the method
of Smith et al. [21], and stock solutions were stored in
a refrigerator and diluted before use. Solvents were
purified as described in previous papers [7,22]. All the
solvents used in the kinetic studies were of analytical
grade. Rateconstants for thereactionof examinedacids
with DDM were determined as reported previously, by
the spectroscopic method of Roberts and his coworkers
[10], using a Shimatzu UV-1700 spectrophotometer.
Absorbance measurements 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-first-order rate constants by the acid concen-
tration (the concentration of acid was 0.06 mol dm−3
and of DDM 0.006 mol dm−3). Three to five rate de-
terminations were made on each acid in every case,
and the particular second-order rate constants agreed
within 3% of the mean. The correlation analysis was
carried out using Origin and Microsoft Excel computer
software. The goodness of fit was discussed using cor-
relation coefficient (R), standard deviation (SD), and
the Fisher’s value (F).
Solvent–Reactivity Relationship
To explain the obtained kinetic results through sol-
vent dipolarity/polarizability and basicity or acidity,
the rate constants of the examined acids were corre-
lated with the solvent properties using the total solva-
tochromic equation (1). The solvent parameters, which
were determined by Kamlet et al. [25], are given in
Table III. The correlations obtained from the data given
The geometries of all the molecular species exam-
ined in this work, corresponding to the energy minima
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667
Table I The Second-Order Rate Constants (dm3 mol−1 min−1) for the Reaction of Cyclohex-1-enylcarboxylic,
2-Methylcyclohex-1-enylcarboxylic, and 2-Phenylcyclohex-1-enylcarboxylic Acids with Diazodiphenylmethane at 30◦C
in a Set of Various Solvents
k(dm3 mol−1 min−1
)
Cyclohex-1-
enylcarboxylic Acid
2-Methylcyclohex-1-
enylcarboxylic Acid
2-Phenylcyclohex-1-
enylcarboxylic Acid
Solvent
1
2
3
4
5
6
7
8
Methyl acetate
Cyclohexanone
Diethylketone
Carbontetrachloride
Ethyl acetate
Cyclopentanone
Dioxane
Acetonitrile
Acetone
Methanol
0.032
0.020
0.053
0.329
0.025
0.025
0.065
0.318
0.048
0.818
0.417
1.962
0.008
0.019
0.093
0.044
0.064
0.359
0.058
0.053
0.077
0.420
0.106
0.762
0.264
1.631
0.013
0.027
0.068
0.038
0.133
0.873
0.054
0.049
0.142
0.839
0.103
2.790
1.279
6.367
0.014
0.037
9
10
11
12
13
14
Ethanol
Ethylene glycol
DMSO
Tetrahydrofurane
by Marcus [26] did not give satisfactory results. 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.
2-Methylcyclohex-1-enylcarboxylic acid
logk = (−0.49 0.10) + (0.52 0.16)π∗
+ (1.66 0.07)α − (2.35 0.17)β
(R = 0.989, s = 0.09, F = 162, n = 14)
The correlation results obtained are as follows:
Cyclohex-1-enylcarboxylic acid
2-Phenylcyclohex-1-enylcarboxylic acid
logk = (−0.58 0.12) + (0.38 0.20)π∗
+ (2.07 0.09)α − (2.48 0.21)β
logk = (−0.14 0.08) + (0.35 0.22)π∗
+ (2.34 0.10)α − (2.70 0.24)β
(R = 0.990, s = 0.11, F = 168, n = 14)
(R = 0.991, s = 0.13, F = 175, n = 14)
Table II The Second-Order Rate Constants (dm3 mol−1 min−1) for the Reaction of Benzoic, 2-Methylbenzoic and
2-Phenylbenzoic Acids with Diazodiphenylmethane at 30◦C in a Set of Various Solvents
k(dm3 mol−1 min−1
)
Solvent
Benzoic Acid
2-Methylbenzoic Acid
2-Phenylbenzoic Acid
1
2
3
4
5
6
7
8
Methyl acetate
Cyclohexanone
Diethylketone
Carbontetrachloride
Ethyl acetate
Cyclopentanone
Dioxane
Acetonitrile
Acetone
Methanol
0.260
0.220
0.265
0.638
0.180
0.293
0.058
3.730
0.350
2.470
0.995
4.020
0.141
0.105
0.124
0.129
0.157
0.389
0.094
0.145
0.035
1.590
0.152
1.860
0.933
2.590
0.079
0.060
0.316
0.246
0.268
1.010
0.236
0.338
0.110
5.500
0.400
11.61
5.000
15.37
0.162
0.147
9
10
11
12
13
14
Ethanol
Ethylene glycol
DMSO
Tetrahydrofurane
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NIKOLIC, USCUMLIC, AND JURANIC
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Table III Solvent Parameters [24]
Table IV The Percentage Contributions of
Kamlet–Taft’s Solvatochromic Parameters to the
Reactivity
Solvent
π∗
α
β
1
2
3
4
5
6
7
8
Methyl acetate
Cyclohexanone
Diethylketone
Carbontetrachloride
Ethyl acetate
Cyclopentanone
Dioxane
Acetonitrile
Acetone
Methanol
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
Compound
Pπ∗ (%) Pα (%) Pβ (%)
1
2
3
Cyclohex-1-
8
11
7
42
37
43
50
52
50
enylcarboxylic acid
2-Methylcyclohex-1
-enylcarboxylic acid
2-Phenylcyclohex-1
-enylcarboxylic acid
Benzoic acid
9
4
5
6
28
24
20
31
37
42
41
39
38
10
11
12
13
14
2-Methylbenzoic acid
2-Phenylbenzoic acid
Ethanol
Ethylene glycol
DMSO
Tetrahydrofurane
reactivity, on a percentage basis, was calculated and
is listed in Table IV. From these results it can be no-
ticed that the nonspecific interactions (π∗) are less pro-
nounced than the specific (α,β). However, the specific
interactions have more influence on the cyclohexenyl
systems (87%–93%) than on benzoic systems (72%–
80%). It probably means the carboxyl group of the
cyclohexenyl acids is more susceptible to the proton–
donor and proton–acceptor solvent effects than the car-
boxyl group of benzoic acids.
Benzoic acid
logk = (−0.64 0.28) + (1.34 0.47)π∗
+ (1.51 0.22)α − (1.98 0.49)β
(R = 0.915, s = 0.26, F = 17, n = 14)
The π∗ term is needed to account for the observed
rate enhancing effect of the behaviour of a solvent as
a dielectric (all the applied solvents have significant
π∗ values, Table III). In this work, the π∗ term is sta-
tistically insignificant in equations for 2-subistituted
cyclohex-1-enylcarboxylic acids; however, for benzoic
acids, it has an influence of 20%–28%, which is too
high to neglect it. The removal of π∗ parameter de-
creases the reliability of the Kamlet–Taft model and
makes it incomplete, because it can cause the reversal
of arithmetic signs in front of the parameters and the
drop of the correlation coefficient (R) to about 0.80.
Therefore, the authors suggest that π∗ should remain
included in the mentioned equations.
2-Methylbenzoic acid
logk = (−0.83 0.27) + (1.05 0.44)π∗
+ (1.64 0.20)α − (1.75 0.46)β
(R = 0.932, s = 0.25, F = 22, n = 14)
2-Phenylbenzoic acid
logk = (−0.34 0.26) + (0.99 0.41)π∗
+ (2.11 0.19)α − (1.90 0.44)β
(R = 0.961, s = 0.24, F = 40, n = 14).
To get a complete view of the solvent interactions
with the molecules of the examined carboxylic acids,
the solvent effects are expressed quantitatively, for both
acid systems and referring separately to the ground and
the transition state.
From all the above equations, it can be concluded
that the solvent effects influence the carboxylic acid–
DDM reaction by two reverse effects. The opposite
signs of the electrophilic and the nucleophilic param-
eters are in accordance with the described mechanism
of the reaction. The positive signs of the s and a co-
efficients prove that the classical solvation and HBD
effects dominate the transition state and increase the
reaction rate, and the negative sign of the b coeffi-
cient points out that HBA effects stabilize the initial
state before the reaction starts and are responsible for a
decrease in the reaction rate. From the values of regres-
sion coefficients, the contribution of each parameter to
2-Substituted cyclohex-1-enylcarboxylic acid
system:
Reactants
⇒
Transition state ⇒ Products
HBD and solvation
by the nonspecific
interactions
HBA solvation
(∼51%)
(∼49%)
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SOLVENT AND STRUCTURAL EFFECTS ON KINETICS OF CARBOXYLIC AND BENZOIC ACIDS
669
2-Substituted benzoic acid system:
Reactants
⇒
Transition state ⇒ Products
HBD and solvation
by the nonspecific
interactions
HBA solvation
(∼39%)
(∼61%)
The suggested solvation models indicate that the
2-substituted cyclohex-1-enylcarboxylic acid system is
more sensitive to the HBA solvent interactions than the
2-substituted benzoic acid system (Table IV) and less
sensitive to the HBD solvent ability. However, the more
general conclusion that comes from these results is that
the substituents at the C-2 position in both carboxylic
acid types have very weak influence on the solvation
effects during the reaction with DDM.
Figure 3 The most stable conformation of cyclohex-1-
enylcarboxylic acid. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
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 and benzoic acids. Such a conclusion
can be drawn from the minimal energy molecular con-
formations. The geometric data of all the examined
acids corresponding to the energy minima in solvent
were obtained using the semiempirical MNDO-PM3
energy calculations (including COSMO facility in it)
and are shown in Figs. 2–7 and Table V.
Figure 4 The most stable conformation of 2-methyl-
cyclohex-1-enylcarboxylic acid. [Color figure can be viewed
in the online issue, which is available at www.interscience.
wiley.com.]
In the molecule of benzoic acid, the carboxylic
group is almost planar with the ring (Fig. 2), which
is a cause of the conjugation of the carbonyl group
of the carboxylic group and the benzene ring. In the
case of cyclohex-1-enylcarboxylic acid (Fig. 3), the
carboxylic group is 142◦ twisted out of the plane
of the double bond and of the opposite orienta-
Figure 5 The most stable conformation of 2-phenyl-
cyclohex-1-enylcarboxylic acid. [Color figure can be viewed
in the online issue, which is available at www.interscience.
wiley.com.]
tion comparing to benzoic acid. The double bond
of cyclohex-1-enylcarboxylic acid is much nearer to
the carboxylic group, which can have as a consequence
an interaction between the carboxylic proton and
the π-electrons of the double bond. This is hardly
possible for benzoic acid because the position of its
carboxylic group is quite different. The torsion an-
gles between the carboxylic group and the C-2 atom
Figure 2 The most stable conformation of benzoic acid.
[Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
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NIKOLIC, USCUMLIC, AND JURANIC
670
Table V Data Calculated Using Semiempirical MNDO-PM3 Method
Compound
Torsion Angle (◦)
Heat of Formation (kcal mol−1
)
1
2
3
4
5
6
Cyclohex-1-enylcarboxylic acid
2-Methylcyclohex-1-enylcarboxylic acid
2-Phenylcyclohex-1-enylcarboxylic acid
Benzoic acid
2-Methylbenzoic acid
2-Phenylbenzoic acid
142.0
−94.97
−103.1
−68.23
−66.27
−73.78
−38.86
−93.30
−94.10
−16.60
−87.50
92.50
Generally, the presence of the substituent affects
the orientation of the carboxylic group (torsion an-
gles), comparing to the unsubstituted molecules, but
the secondary steric effect is obviously of little or no
significance in these systems. The electronic effect of
substituents is dominant in both systems of the exam-
ined carboxylic acids in the reaction with DDM.
CONCLUSION
Figure 6 The most stable conformation of methylbenzoic
acid. [Color figure can be viewed in the online issue, which
is available at www.interscience.wiley.com.]
On the basis of the results presented in this work
and our previously reported results for more than 50
carboxylic acids, it can be concluded that the solva-
tochromic concept of Kamlet and Taft is applicable
to kinetic data for the reaction of different carboxylic
acids with DDM in various solvents. This means that
this model gives a correct interpretation of the solvat-
ing effects on the carboxylic group in various solvents.
The solvation models for 2-substituted cyclohex-1-
enylcarboxylic and 2-substituted benzoic acids are sug-
gested. The results show that the substituents at the C-2
position of the ring have rather a weak influence on
the solvation effects during the reaction of carboxylic
acids with DDM. For these reasons, we consider that
the results presented in this work may be used to quan-
titatively estimate and separate the overall solvent ef-
fects into initial and transition state contributions in the
reaction of DDM with carboxylic acids.
Figure
7 The most stable conformation of 2-2-
phenylbenzoic acid. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
are given in Table V for all the examined acids,
together with the corresponding conformation ener-
gies. The conformations with the minimal energy con-
tents for the other four examined acids are given in
Figs. 3–6.
The presence of the methyl group at ortho-position
causes the increase in the heat of formation of the
compound, which is in fact the energy content of the
most stable conformation. The presence of the phenyl
group at ortho-position of both benzoic and cyclohex-
1-enylcarboxylic acid significantly decreases the en-
ergy content of the compound, which points to the
high extent of conjugation between the double bond in
the cyclohexene ring and the phenyl group, or between
two phenyl groups, and therefore to the absence of the
ortho-effect for these phenyl-substituted acids.
The reactivity of the examined carboxylic acids in
the reaction with DDM is in agreement with the geo-
metric characteristics obtained using the semiempirical
MNDO-PM3 energy calculations.
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