Transesterification kinetics investigation of r-substituted phenyl benzoates with 4-methoxyphenol in the presence of K2CO3 in DMF

Article abstract of DOI:10.1002/kin.20822

Transesterification of R-substituted phenyl benzoates 1-5 with 4-methoxyphenol 6 was kinetically investigated in the presence of K ₂CO₃ in dimethylformamide (DMF) at various temperatures. The Hammett plots for the reactions of the 1-

Full text of DOI:10.1002/kin.20822

Transesterification Kinetics
Investigation of
R-Substituted Phenyl
Benzoates with
4-Methoxyphenol in the
Presence of K2CO3 in DMF
IRINA A. OS’KINA
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences, Novosibirsk
630090, Russian Federation
Received 7 May 2013; revised 31 July 2013; accepted 3 September 2013
DOI 10.1002/kin.20822
Published online 5 October 2013 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Transesterification of R-substituted phenyl benzoates 1–5 with 4-methoxyphenol
6 was kinetically investigated in the presence of K2CO3 in dimethylformamide (DMF) at var-
ious temperatures. The Hammett plots for the reactions of the 1–5 demonstrate good linear
correlations with σ0 constants. Low magnitude of ρLG values indicate that the leaving group
departure occurs after the rate-determining step. The Brønsted coefficient values for the reac-
tions (−0.2, −0.16, −0.13 at 15, 24, 36◦C, respectively) demonstrate the weak effect of leaving
group substituent on the reactivity of R-substituted phenyl benzoates 1–5 for the reactions
with 4-methoxyphenol 6 in the presence of K2CO3 in DMF. The leaving group substituent ef-
fect on free energy (ꢀG=), enthalpy (ꢀH=), and entropy (ꢀS=) of activation was examined.
It was shown that the activation parameters obtained depend weakly on the leaving group
substituent effect. The reactionꢁCis entropy controlled in case the leaving group substituent
becomes electron withdrawing. 2013 Wiley Periodicals, Inc. Int J Chem Kinet 46: 24–30, 2014
fect in a nonleaving group on the reactivity of 4-
nitrophenyl benzoates with 4-chlorophenol and with
benzenethiol in the presence of potassium carbonate in
DMF [23–26]. It is shown that the reaction proceeds
through a two-step addition–elimination mechanism,
and electron-donating groups in the benzoyl fragment
reduce the reactivity, whereas electron-withdrawing
substituents increase it [23–26]. In spite of leaving
group departure occurs after the rate-determining step
INTRODUCTION
Acyl transfer reactions have been intensively stud-
ied due to their great importance both in biological
processes and in synthetic applications [1–22]. Re-
cently, we have kinetically studied the substituent ef-
Correspondence to: I. A. Os’kina; e-mail: oi@nioch.nsc.ru.
C
ꢀ
2013 Wiley Periodicals, Inc.

TRANSESTERIFICATION KINETICS of R-SUBSTITUTED PHENYL BENZOATES
25
K2CO3
OH
Ph
Ph
O
R
+
O
OCH3
DMF
15–36oC
O
H3CO
O
6
7
1–5
R = 4-CH3 (1), H (2), 4-Cl (3), 4-CN (4), 4-NO2 (5)
Scheme 1 Reaction of R-substituted phenyl benzoates 1–5 with 4-methoxyphenol 6 in the presence of K2CO3 in DMF.
(RDS), one can suggest the leaving group substituent
can have an effect on the reactivity and activation
parameters of the reaction. We have now extended
our study to the reaction of R-substituted phenyl ben-
zoates 1–5 with 4-methoxyphenol 6 in the presence of
K2CO3 in DMF at the temperature range 288–309 K
(Scheme 1) to investigate the effect of the leaving group
substituent on kinetic parameters of the reaction: rate
constant k, free energy of activation ꢀG=, enthalpy of
activation ꢀH=, and entropy of activation ꢀS=.
thermostated flask under argon was charged with
potassium carbonate (fraction d ≤ 0.5 mm) and 4-
methoxyphenol 6 in a ratio K2CO3: 4-methoxyphenol
≥1.4:1 in 2 mL DMF. The reaction mixture was stirred
(n ≥ 750 min−1) at constant temperature (15, 24, or
◦
36 C) for 30 min. Then thermostated solution of R-
substituted phenyl benzoates 1–5 in 2 mL DMF was
added into the reaction flask. The reaction mixture
was thermostated during different times (no more than
60 min), and an aliquot of a reaction solution was
sampled. The reactions were followed by monitoring
the appearance of 4-methoxyphenyl benzoate 7 using
gas-liquid chromatography (GLC) analysis. All kinetic
runs were carried out in triplicate, and the error was
≤6% for k (Table I). The second-order rate constants
(k) were calculated from
EXPERIMENTAL
Materials
Phenyl benzoates 1–5, 7 (Fig. 1) were prepared as de-
scribed [27–30]. Their purity was checked by means
of their melting point and spectral data. Commercial
4-methoxyphenol 6 of chemically pure grade was pu-
rified by standard methods. Commercial DMF of ana-
k = {2.303/[τ(a − b)]}log{[b(a − x)]/[a(b − x)]}
where a and b are initial reagent concentrations, M; x is
4-methoxyphenyl benzoate 7 concentration, M; and τ
is reaction time, s.
˚
lytical grade was dried over 4 A molecular sieves and
distilled over calcium hydride under reduced pressure.
Analysis
Procedures
Samples were analyzed by GLC using an LKhM-72
gas chromatograph equipped with a thermal conduc-
tivity detector (4000 × 4 mm column packed with 15%
SKTFT-803 on Chromaton-W, carrier gas helium, lin-
ear oven temperature ranging from 70 to 270◦C at the
The procedure was analogous to that reported in [25].
All the kinetic studies were performed under second-
order conditions in which the reagent concentrations
were varied over the range (2–6) × 10−2 M. The
O
Ph
Cl
Ph
O
CH3 Ph
O
O
O
O
1
2
3
O
Ph
O
OCH3
Ph
CN
Ph
O
NO2
O
O
O
7
4
5
Figure 1 Structures of phenyl benzoates used in the study.
International Journal of Chemical Kinetics DOI 10.1002/kin.20822

26
OS’KINA
Table I Summary of Second-Order Rate Constants k and Leaving Group Substituent Effect ρLG for the Reactions of
R-Substituted Phenyl Benzoates 1–5 with 4-Methoxyphenol 6 in the Presence of K2CO3 in DMF
k × 102 (dm3 mol−1 s−1
)
R-Substituted Phenyl Benzoates (R)
pKa
15◦C
24◦C
36◦C
1 (4-CH3)
2 (H)
19.7
18.8
17.6
14.2
11.9
(8.3 0.2) × 10−2
(1.6 0.1) × 10−1
(2.5 0.1) × 10−1
1.6 0.1
(2.2 0.1) × 10−1
(3.5 0.1) × 10−1
(5.2 0.1) × 10−1
2.2 0.1
(4.5 0.2) × 10−1
(7.3 0.4) × 10−1
(8.9 0.3) × 10−1
3.0 0.1
3 (4-Cl)
4 (4-CN)
5 (4-NO2)
3.3 0.2
1.07
1.6
4.0 0.1
0.84
1.3
5.0 0.2
0.69
1.2
ρLG (σ− b
)
ρLG (σ0)b
apK values in DMF are taken from [33].
bValues of σ− and σ0 are taken from Hansch et al. [34].
rate of 10 deg/min); the components were quantitated
by the absolute calibration technique using calibration
plots and identified by adding authentic samples, inter-
nal standard–hexamethyl benzene.
RESULTS AND DISCUSSION
Table I summarizes the second-order rate constants,
which were measured for the reaction of R-substituted
phenyl benzoates 1–5 with 4-methoxyphenol 6 in
the presence of K2CO3 in DMF at the temperatures
288–309 K. Table I shows that the second-order rate
constant k for the reaction increases as the electron-
withdrawing ability of substituent R in the leaving
group increases, i.e., as a substituent R in the leav-
ing group changes from 4-CH3 to 4-NO2. The effect
of substituent R in the leaving group on the reactivity
of R-substituted phenyl benzoates 1–5 is illustrated in
Figs. 2 and 3. The Hammett plots exhibit good lin-
Figure 2 Hammett correlations with σ◦ constants for
the reactions of R-substituted phenyl benzoates 1–5 with
4-methoxyphenol 6 in the presence of K2CO3 in DMF:
log k15 = 1.6σ0 – 2.84 (r 0.992, s 0.1, n 5); log k24
=
1.3σ0 – 2.48 (r 0.991, s 0.08, n 5); log k36 = 1.1σ0 – 2.19
(r 0.988, s 0.08, n 5).
ear correlations both with the σ0 constants: log k15
=
1.6σ0 – 2.84 (r 0.992, s 0.1, n 5); log k24 = 1.3σ0 –
2.48 (r 0.991, s 0.08, n 5); log k36 = 1.1σ0 – 2.19
(r 0.988, s 0.08, n 5), and with the σ− constants:
occur [31]. There is a slight difference in the correla-
tion coefficients of the obtained linear Hammett plots
with the σ0 and σ− constants. The difference cannot
clearly indicate that the leaving group departure oc-
curs either at the RDS or after RDS. On the other
hand, the obtained ρLG values are lower than ρ 3.43
at 24◦C when the substituent changes from an electron
donor to electron withdrawing in the nonleaving group
for the reaction of 4-nitrophenyl benzoates with 4-
chlorophenol in the presence of potassium carbonate in
DMF [23]. The large value of ρ = 3.43 for the reaction
of 4-nitrophenyl benzoates with 4-chlorophenol in the
presence of potassium carbonate in DMF is caused by
the electron-withdrawing substituents in the nonleav-
ing group effect that accelerates the rate-determining
stage of nucleophilic attack. At the same time, the
log k15 = 1.07σ− – 2.84 (r 0.998, s 0.05, n 5); log k24
=
0.84σ− – 2.48 (r 0.998, s 0.03, n 5) at 24◦C, log k 36
=
0.69σ− – 2.19 (r 0.997, s 0.04, n 5) (Table I, Figs. 2 and
3). If the leaving group departure is involved in the
RDS, a partial negative charge would develop on the
oxygen atom of the leaving phenyloxides at the tran-
sition state of the RDS. Such a negative charge can be
delocalized on the substituent R in the leaving group
through resonance; the Hammett plots for variations of
the substituent R in the leaving group should exhibit
a good Hammett correlation with the σ− constants.
A good Hammett correlation with the σ0 constants
indicates that direct resonance interactions of the leav-
ing group substituent R and the reaction center do not
International Journal of Chemical Kinetics DOI 10.1002/kin.20822

TRANSESTERIFICATION KINETICS of R-SUBSTITUTED PHENYL BENZOATES
27
Figure 3 Hammett correlations with σ− constants for the
Figure 4 Brønsted-type plots for the reactions of R-
substituted phenyl benzoates 1–5 with 4-methoxyphenol 6
reactions of R-substituted phenyl benzoates 1–5 with 4-
methoxyphenol 6 in the presence of K2CO3 in DMF: log k =
1.07σ− – 2.84 (r 0.998, s 0.05, n 5; ◦) at 15◦C; log k =
0.84σ− – 2.48 (r 0.998, s 0.03, n 5; ꢀ) at 24◦C, log k =
0.69σ− – 2.19 (r 0.997, s 0.04, n 5; • at 36◦C). The identity
of the points is given in Table I.
in the presence of K2CO3 in DMF: log k = −0.20 pKDMF
1.0 (r 0.995, s 0.08, n 5; ◦) at 15◦C, log k = −0.16 pKDMF
0.6 (r 0.996, s 0.05, n 5; ꢀ) at 24◦C, log k = −0.13 pKDMF
+
+
+
0.33 (r 0.994, s 0.05, n 5; •) at 36◦C. The identity of the points
is given in Table I.
electron-withdrawing substituents in the nonleaving
group retard the departure of the leaving group. As
a consequence, a decrease in the ρ value can occur.
However, the substituent in the nonleaving group of R-
substituted phenyl benzoates 1–5 is absent. Therefore,
the leaving group substituent effect cannot be compen-
sated by the opposing effect of the substituent in the
nonleaving group. Thus, one might suggest that the
weak effect of the leaving group substituent on the re-
action rate constants for the reaction of R-substituted
phenyl benzoates 1–5 with 4-methoxyphenol 6 in the
presence of K2CO3 in DMF is responsible for the small
ρLG value. Jencks has shown that the nature of RDS in-
fluences the value of ρ, and he has reported the decrease
in the ρ value to a change in the RDS from formation of
the addition intermediate to its breakdown [32]. Thus,
one can suppose that the small ρLG value implies that
the leaving group departure occurs after the RDS. The
ρLG values obtained at various temperatures decrease
as the temperature rises. This fact suggests that the
sensitivity of the reaction rate constants to the leaving
group substituent effect decreases as the temperature
increases.
The leaving group substituent effect on R-substituted
phenyl benzoates 1–5 reactivity can also be estimated
from the magnitude of βLG values. Figure 4 shows
Brønsted-type plots for the reactions of R-substituted
phenyl benzoates 1–5 with 4-methoxyphenol 6 in the
presence of K2CO3 in DMF. The βLG values for the
reaction of R-substituted phenyl benzoates 1–5 with 4-
methoxyphenol 6 at various temperatures are low and
range from −0.20 at 15◦C to −0.13 at 36◦C. Small
βLG values have been observed for a reaction with an
addition intermediate formation in the RDS [35–37].
Considerably larger βLG values have been obtained for
the reactions in which the leaving group departure is
RDS [38]. Therefore, for the reactions of R-substituted
phenyl benzoates 1–5 with 4-methoxyphenol 6 in the
presence of K2CO3 in DMF, the small βLG values
show that the leaving group departure proceeds after
the RDS.
The activation parameters ꢀH= (enthalpy of acti-
vation), ꢀS= (entropy of activation), and ꢀG= (free
energy of activation) obtained by using the Eyring
equation (log k/T vs. 1/T) [39] are summarized in
Table II. We found positive ꢀH= values and highly
negative ꢀS= values. The range of variation of ꢀG=
remained typical of transesterification [25,26]. The re-
actions with compounds having the electron-donating
leaving group substituent are characterized by large
enthalpies of activation and negative entropies of
activation. The values of activation parameter be-
come smaller as the leaving group substituent changes
Table I shows that the reactivity of R-substituted
phenyl benzoates 1–5 increases when the leaving group
basicity reduces as the substituent R in the leaving
group changes from 4-CH3 to 4-NO2. The leaving
group basicity has been estimated as the pK of the con-
jugate acid (Table I). At the same time, the pK of the
conjugate acid decreases, the leaving group stability in-
creases, and the leaving group departure occurs easily.
International Journal of Chemical Kinetics DOI 10.1002/kin.20822

28
OS’KINA
Table II Activation Parameters for the Reactions of R-Substituted Phenyl Benzoates 1–5 with 4-Methoxyphenol 6 in
the Presence of K2CO3 in DMF
R-Substituted Phenyl Benzoates (R) ꢀH= (kJ mol−1
)
ꢀS= (J mol−1 K−1
)
−T·ꢀS= (kJ mol−1 a
)
ꢀG= (kJ mol−1 a
)
1 (4-CH3)
2 (H)
3 (4-Cl)
4 (4-CN)
5 (4-NO2)
56.5 9.1
50.7 4.3
41.8 6.8
19.5 1.6
12.2 0.3
−106.9 0.2
−121.9 0.1
−148.9 0.1
−210.9 0.1
−230.8 0.1
31.7
36.2
44.2
62.6
68.5
88.2
86.9
86.0
82.1
80.7
aAt 297 K.
from an electron-donating substituent to an electron-
withdrawing substituent.
An increase in the electron-acceptor power of the
leaving group substituent leads to considerable reduc-
tion of the contribution of enthalpy, and an increase in
the contribution of entropy leads to the energy barrier
of the reaction of R-substituted phenyl benzoates 1–
5 with 4-methoxyphenol 6 in the presence of K2CO3
in DMF. The enthalpy component provides the main
contribution to the Gibbs energy of activation in the
reactions of benzoates 1, 2 with 4-methoxyphenol 6 in
the presence of K2CO3 in DMF. The entropy compo-
nent is dominant for the reactions of benzoates 3–5.
Electron-withdrawing substituents in the leaving
group accelerate the leaving group departure. The ac-
celeration of the leaving group departure results in
reduction of activation enthalpy. It should be noted
that the observed change in activation entropy with the
substituents is caused by the change in the solvation
and by the reorganization of the solvent [40–42]. The
difference in the ground state and the transition state
solvation rises in case the leaving group substituent
becomes electron withdrawing. Through the decrease
in activation entropy values, the reaction becomes en-
tropy controlled. The free energy of activation ꢀG=
is weakly sensitive to the leaving group substituent
effect. The little activation enthalpy and high free en-
ergy of activation indicate the formation of a strongly
solvated transition state. It is important to emphasize
that the negative entropy activation values point to the
addition–elimination mechanism.
Figure 5 ꢀH=, TꢀS=, and ꢀG= versus σ0 plots for the
reactions of R-substituted phenyl benzoates 1–5 with 4-
methoxyphenol 6 in the presence of K2CO3 in DMF: ꢀH=
−47.1 σ0 + 50 (r 0.996, s 1.9, n 5); TꢀS= = −39.3 σ0
=
–
36.8 (r 0.997, s 1.5, n 5) at 297 K; ꢀG= = −7.8 σ0 + 87.1
(r 0.992, s 0.5, n 5) at 297 K. The identity of the points is
given in Table II.
to the leaving group substituent effect for transesteri-
fication of R-substituted phenyl benzoates 1–5 with 4-
methoxyphenol 6 in the presence of K2CO3 in DMF is
weaker than the sensitivity of the activation parameters
to the nonleaving group substituent effect for transes-
terification of R-substituted 4-nitrophenyl benzoates.
A plot of ꢀH= against ꢀS= for the reac-
tion of R-substituted phenyl benzoates 1–5 with 4-
methoxyphenol 6 gave a good straight line of slope
356 3 K (Fig. 6), which is the value of compensation
temperature for the reaction. A similar value of com-
pensation temperature (Tcomp) for the reaction 356
14 K is obtained from a relation of ꢀG= against ꢀS=
The changes in activation parameters with the leav-
ing group substituent illustrate the dependence of
ꢀH=, ꢀG=, and TꢀS= activation parameters on the
σ0 substituent constants (Fig. 5).
The slopes (in absolute values) of the depen-
dences of activation parameters for transesterifica-
tion of R-substituted phenyl benzoates 1–5 with 4-
methoxyphenol 6 in the presence of K2CO3 in DMF
are less than that for the reaction of 4-nitrophenyl
benzoates with 4-chlorophenol and benzenethiol [26].
Probably, the sensitivity of the activation parameters
(ꢀG= = 0.1655
0.00641)ꢀH= + 78.8 (r 0.998,
s 0.25, n 5). The existence of the compensation re-
lationship is evident in favor of an identical reaction
mechanism [43,44].
The value of Tcomp 380
1 K for the re-
action R-substituted 4-nitrophenyl benzoates with
4-chlorophenol [23] was obtained. The reaction
International Journal of Chemical Kinetics DOI 10.1002/kin.20822

TRANSESTERIFICATION KINETICS of R-SUBSTITUTED PHENYL BENZOATES
29
parameters, ꢀH=, ꢀS, and ꢀG=, are obtained for the
reaction. In the case of the electron-withdrawing leav-
ing group substituent, the reaction becomes entropy
controlled.
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International Journal of Chemical Kinetics DOI 10.1002/kin.20822