Nonlinear Hammett plots in pyridinolysis of 2,4-dinitrophenyl X-substituted benzoates: Change in RDS versus resonance contribution

Article abstract of DOI:10.1039/c0ob00031k

Second-order rate constants (k_OH-) have been measured for nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzoates (1a-j) with Z-substituted pyridines in 80 mol% H₂O/20 mol% DMSO at 25.0 ± 0.1 °C. The Hammett plots for the reactions of 1a-j with pyridines consist of two intersecting straight lines, i.e., a large ρ value for the reactions of substrates (1a-c) possessing an electron-donating group (EDG) in the benzoyl moiety and a small one for substrates (1e-j) bearing an electron-withdrawing group (EWG). The nonlinear Hammett plots have been attributed to stabilization of the ground state of substrates 1a-c through resonance interactions between the electron-donating substituent and the carbonyl functionality, since the corresponding Yukawa-Tsuno plots exhibit excellent linear correlations with large r values. It has been shown that substrates 1e-j are not unusually more reactive than would be expected from the Hammett substituent constants, but rather, substrates 1a-c exhibit lower reactivity than would be predicted. The Bronsted-type plots for pyridinolysis of 1a-j are linear with β_nuc = 0.74-0.98, indicating that the reaction proceeds through a stepwise mechanism in which the second step is the RDS. It has been concluded that the electronic nature of the substituent X in the benzoyl moiety does not influence the RDS, but the degree of bond formation (or the effective charge on the nucleophilic site) in the transition state becomes more significant as the substituent X changes from a strong EDG to a strong EWG.

Full text of DOI:10.1039/c0ob00031k

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Nonlinear Hammett plots in pyridinolysis of 2,4-dinitrophenyl X-substituted
benzoates: change in RDS versus resonance contribution†‡
Ik-Hwan Um,* Li-Ra Im, Eun-Hee Kim and Ji Hye Shin
Received 22nd April 2010, Accepted 28th May 2010
First published as an Advance Article on the web 28th June 2010
DOI: 10.1039/c0ob00031k
Second-order rate constants (kOH–) have been measured for nucleophilic substitution reactions of
2,4-dinitrophenyl X-substituted benzoates (1a–j) with Z-substituted pyridines in 80 mol%
H2O/20 mol% DMSO at 25.0 0.1 ◦C. The Hammett plots for the reactions of 1a–j with pyridines
consist of two intersecting straight lines, i.e., a large r value for the reactions of substrates (1a–c)
possessing an electron-donating group (EDG) in the benzoyl moiety and a small one for substrates
(1e–j) bearing an electron-withdrawing group (EWG). The nonlinear Hammett plots have been
attributed to stabilization of the ground state of substrates 1a–c through resonance interactions
between the electron-donating substituent and the carbonyl functionality, since the corresponding
Yukawa–Tsuno plots exhibit excellent linear correlations with large r values. It has been shown that
substrates 1e–j are not unusually more reactive than would be expected from the Hammett substituent
constants, but rather, substrates 1a–c exhibit lower reactivity than would be predicted. The
Brønsted-type plots for pyridinolysis of 1a–j are linear with bnuc = 0.74–0.98, indicating that the
reaction proceeds through a stepwise mechanism in which the second step is the RDS. It has been
concluded that the electronic nature of the substituent X in the benzoyl moiety does not influence the
RDS, but the degree of bond formation (or the effective charge on the nucleophilic site) in the transition
state becomes more significant as the substituent X changes from a strong EDG to a strong EWG.
However, the effect of substituents in the nonleaving group
on mechanism remains controversial. In nucleophilic substitution
Nucleophilic substitution reactions of esters with amines have
reactions of diaryl carbonates with quinuclidines in H2O, Gresser
Introduction
intensively been investigated due to their importance in biological
processes as well as synthetic applications.1–12 Aminolysis of
carboxylic esters has been reported to proceed through a stepwise
and Jencks have concluded that the departure of the amine from
T (the k–1 process) is favored over that of the leaving group (the
k2 process), as the substituent in the nonleaving group changes
mechanism as shown in Scheme 1 on the basis of curved Brønsted-
from an electron-donating group (EDG) to a strong electron-
withdrawing group (EWG).10 A similar conclusion has been drawn
in pyridinolysis of 2,4-dinitrophenyl X-substituted benzoates,11
aminolysis of S-2,4-dinitrophenyl X-substituted thiobenzoates,12
pyridinolysis of aryl dithiobenzoates and related esters,8f ,g and
in theoretical calculations on phenolysis of aryl acetates.8h Ac-
cordingly, it has been concluded that the k2/k–1 ratio decreases
as the substituent X in the nonleaving group changes from
an EDG to an EWG.10–12 In contrast, we have proposed that
the k2/k–1 ratio is independent of the electronic nature of the
substituent in the nonleaving-group. This is because expulsion
of the nucleofuges (i.e., the k2 and k–1 processes) would be
retarded by an EWG in the nonleaving group but accelerated by
an EDG, since both nucleofuges depart with the electron pair
originally bonded to the remainder of the zwitterionic tetrahedral
intermediate T . In fact, we have shown that the k2/k–1 ratio is
little influenced by the nature of substituents X in the nonleaving
group for nucleophilic substitution reactions of O-4-nitrophenyl
X-substituted thionobenzoates with a series of pyridines.6d
type plots often observed for reactions of esters possessing a
good leaving group (e.g., 2,4-dinitrophenoxide).1–12 It is now firmly
understood that RDS is governed by the basicity of the incoming
amine and the leaving group, i.e., rate-determining step changes
from breakdown of a zwitterionic tetrahedral intermediate T (i.e.,
the k2 step) to its formation (i.e., the k1 step) as the amine becomes
more basic than the leaving group by 4 to 5 pKa units or the leaving
group becomes less basic than the amine.
Scheme 1 Aminolysis of carboxylic esters.
Department of Chemistry and Nano Science, Ewha Womans University,
Seoul, 120-750, Korea. E-mail: ihum@ewha.ac.kr; Fax: 82-2-3277-2844
† This paper is dedicated with respect and affection to the late Professor
Chi Sun Hahn, an inspiring teacher and mentor.
‡ Electronic supplementary information (ESI) available: Brønsted-type
plots for reactions of 1a–j with pyridines (Figures S1–S10). Kinetic
conditions and results for reactions of 1a–j with pyridines (Table S1–S30).
See DOI: 10.1039/c0ob00031k
Pyridinolysis of 2,4-dinitrophenyl benzoate has recently been
reported to proceed through a stepwise mechanism with a change
in rate-determining step on the basis of a curved Brønsted-
type plot, i.e., RDS changes from the k2 step to the k1 process
as the incoming pyridine becomes more basic than the leaving
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2,4-dinitrophenoxide ion by 5.4 pKa units.6f The microscopic
rate constants determined have also supported the proposed
mechanism. Our study has been extended to reactions of
2,4-dinitrophenyl X-substituted benzoates (1a–j) with three Z-
substituted pyridines (Scheme 2) to get further information on the
mechanism and transition-state (TS) structures. We wish to report
that the electronic nature of the substituent X does not influence
RDS but the degree of bond formation between the nucleophile
and electrophile (or the effective charge on the nucleophilic site)
in the TS becomes more significant as the substituent X changes
from a strong EDG to a strong EWG.
EDG to a strong EWG, e.g., the kOH– value for reactions of 1a–
j with 4-aminopyridine increases from 0.499 M-1s-1 to 32.0 and
2340 M-1s-1 as the substituent X in the benzoyl moiety changes
from 4-NMe2 to H and 3,5-(NO2)2, respectively. A similar result is
shown for the corresponding reactions with 3,4-dimethylpyridine
and unsubstituted pyridine, indicating that the reactivity of 1a–j
is highly dependent on the electronic nature of the substituent X
in the benzoyl moiety.
The effect of substituent X on reactivity is illustrated in Fig. 1
for the reactions of 1a–j with the three pyridines. One can see
that each Hammett plot consists of two intersecting straight lines.
The slope of the Hammett plot for substrates possessing EDGs
is steeper than that for substrates bearing EWGs, e.g., r = 1.68
when sX ≤ 0, while r = 0.91 when sX ≥ 0 for the reactions with
unsubstituted pyridine.
Scheme 2 Pyridinolysis of 2,4-dinitrophenyl X-substituted benzoates.
Results and discussion
All reactions in this study obeyed pseudo-first-order kinetics.
Pseudo-first-order rate constants (kobsd) were determined from
the equation ln (A• - At) = -kobsdt + C. The plots of kobsd
vs. pyridine concentration were linear and passed through the
origin, indicating that contribution of H2O and/or OH- ion from
hydrolysis of pyridines to kobsd is negligible. Thus, the rate law is
given by eqn (1), in which [S] and [Pyr] represent the concentration
of the substrate and pyridine, respectively. The second-order rate
constants (kOH–) were determined from the slope of the linear plots
of kobsd vs. pyridine concentration, and are summarized in Table 1.
Fig. 1 Hammett plots for the reactions of 2,4-dinitrophenyl X-substituted
benzoates (1a–j) with 4-aminopyridine (᭺), 3,4-dimethylpyridine (᭹), and
pyridine (ꢀ) in 20 mol% DMSO at 25.0 0.1 ◦C. The identity of points is
given in Table 1.
-
Rate = kobsd[S], where kobsd = kOH [Pyr]
(1)
Nonlinear Hammett plots have traditionally been interpreted as
a change in reaction mechanism or RDS depending on the shape
of curvature.13,14 Upward curvature often found for nucleophilic
substitution reactions of benzylic systems has been attributed
to a change in mechanism from SN1 to SN2, e.g., substrates
possessing an EDG proceed through an SN1 mechanism with a
Effect of substituent X on reactivity and RDS
As shown in Table 1, the second-order rate constant (kOH–)
increases significantly as the substituent X changes from a strong
Table 1 Summary of second-order rate constants (kOH–) for reactions of 2,4-dinitrophenyl X-substituted benzoates (1a–j) with 4-aminopyridine, 3,4-
dimethylpyridine, and pyridine in 20 mol% DMSO at 25.0 0.1 ◦
C
k
OH–/M-1s-1
X
4-Aminopyridine
3,4-Dimethylpyridine
Pyridine
1a
1b
1c
1d
1e
1f
4-NMe2
4-MeO
4-Me
H
4-Cl
3-Cl
4-CN
4-NO2
4-Cl-3-NO2
3,5-(NO2)2
(4.99 0.08) ¥ 10-1
5.97 0.04
15.8 0.1
32.0 1.6
62.0 1.4
(2.27 0.05) ¥ 10-3
(1.77 0.03) ¥ 10-2
(4.53 0.07) ¥ 10-2
(7.25 0.09) ¥ 10-2
(1.24 0.02) ¥ 10-1
(2.61 0.05) ¥ 10-1
(5.31 0.08) ¥ 10-1
(7.13 0.08) ¥ 10-1
(7.43 0.08) ¥ 10-1
2.28 0.03
(3.47 0.02) ¥ 10-4
(2.31 0.10) ¥ 10-3
(4.96 0.08)¥ 10-3
(8.61 0.01)¥ 10-3
(1.14 0.01) ¥ 10-2
(2.48 0.004)¥ 10-2
(4.85 0.07) ¥ 10-2
(6.14 0.40) ¥ 10-2
(5.34 0.07) ¥ 10-2
(1.64 0.06) ¥ 10-1
142
321
446
520
4
3
3
5
1g
1h
1i
1j
2340 20
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negative r value, while those bearing an EWG proceed through
an SN2 pathway with a positive r value.13,14 In contrast, downward
curvature has been ascribed to a change in RDS upon changing
substituents from EDGs to EWGs.13,14 Such a downward Hammett
plot has been reported for reactions of a series of X-substituted
benzaldehydes with ammonia, i.e., benzaldehydes possessing an
EDG result in a positive r value while those with an EWG yield a
negative r value.13a A similar downward curvature has been found
for the corresponding reactions with semicarbazide in a weakly
acidic medium (pH = 3.9), i.e., the r for the Hammett plot changes
from a large positive value to a small one as the substituent changes
from EDGs to EWGs.13b
Therefore, one might attribute the nonlinear Hammett plots
shown in Fig. 1 to a change in RDS, i.e., from formation of T
to its breakdown to yield the products as the substituent in the
benzoyl moiety changes from EDGs to EWGs. This idea appears
to be reasonable, since one can expect that an EDG in the benzoyl
moiety would retard nucleophilic attack (i.e., a decrease in k1) but
would accelerate departure of the negatively charged leaving group
(i.e., an increase in k2). In contrast, an EWG would increase k1 but
would decrease k2. Accordingly, one can expect a large r value
when the k1 step is the RDS but a small one when the k2 process
is the RDS due to the opposite substituent effect. In fact, Fig. 1
shows that the r value decreases from 1.68 for the reactions of
pyridine with substrates 1a–d to 0.91 for those with 1d–j. Thus,
the nonlinear Hammett plots might be taken as evidence for a
change in RDS upon changing the substituent X in the benzoyl
moiety.
However, we propose that the nonlinear Hammett plots in Fig. 1
are not due to a change in RDS. This is because the RDS of a
stepwise mechanism should be determined by the k2/k–1 ratio (i.e.,
RDS = k1 when k2/k–1 > 1 or RDS = k2 when k2/k–1 < 1) but
not by the magnitude of k1 and k2. Furthermore, k1 and k2 values
cannot be compared directly, since the former is a second-order
rate constant while the latter is a first-order rate constant.
Fig. 2 Yukawa–Tsuno plots for reactions of 2,4-dinitrophenyl X-substi-
tuted benzoates (1a–j) with 4-aminopyridine (᭺), 3,4-dimethylpyridine
(᭹), and pyridine (ꢀ) in 20 mol% DMSO at 25.0 0.1 ◦C. The identity of
points is given in Table 1.
Resonance structures as modeled by I and II are possible for
substrates possessing an EDG in the benzoyl moiety (e.g., 1a–
c). Accordingly, one can suggest that stabilization of the ground
state (GS) through resonance interactions is responsible for the
negative deviation exhibited by substrates 1a–c from the Hammett
plots shown in Fig. 1. This argument can be further supported by
the fact that the substrate possessing a stronger EDG (e.g., 1a)
deviates more significantly from the Hammett plot. Thus, one can
conclude that the nonlinear Hammett plots shown in Fig. 1 are
not due to a change in RDS and deduction of reaction mechanism
based just on a nonlinear Hammett plot can be misleading.
Origin of nonlinear Hammett plots
To examine the above argument that the nonlinear Hammett
plots shown in Fig. 1 are not due to a change in RDS, Yukawa–
Tsuno plots have been constructed. The Yukawa–Tsuno equation,
eqn (2), has originally been derived to account for solvolysis of
benzylic systems in which a positive charge develops in the TS.15,16
We have recently shown that Yukawa–Tsuno plots are highly
effective to elucidate ambiguities in the mechanism for nucleophilic
substitution reactions of aryl diphenylphosphinates and related
esters.17
GS destabilization vs. GS stabilization
Table 1 shows that substrates possessing an EWG in the benzoyl
moiety are more reactive than those bearing an EDG. Neuvonen
et al. have investigated the origin of enhanced reactivities of esters
possessing an EWG in the leaving group of aryl acetates or in
the nonleaving group of alkyl benzoates.18 They measured 13C
=
NMR shifts of the carbonyl carbon and frequencies of the C O
stretching vibration in various Y-substituted phenyl X-substituted
benzoates and acetates, and found that an EWG in the leaving- or
nonleaving-group causes upfield 13C NMR shifts of the carbonyl
log (kX/kH) = r[sX + r(sX - sX◦)]
(2)
o
+
As shown in Fig. 2, the Yukawa–Tsuno plots exhibit excellent
linear correlations with r = 0.92–1.31 and r = 0.79–0.92, indicating
that the reactions proceed through a common mechanism without
changing the RDS. The r value in eqn (2) represents the resonance
demand of the reaction center or the extent of resonance contri-
18
=
carbon and higher frequencies of the C O stretching vibration.
Their PM3 calculations have also shown that an EWG increases
the electron density of the carbonyl carbon, implying that an
EWG in the leaving- or nonleaving-group does not increase the
electrophilicity of the carbonyl carbon.18 Thus, the enhanced
reactivity of esters possessing an EWG has been attributed to
destabilization of the GS of substrates, since an EWG in the
leaving group or in the nonleaving benzoyl moiety would inhibit
the resonance interactions III ↔ IV ↔ V. 18
bution, while the term (sX - sX◦) is the resonance substituent
+
constant that measures the capacity for p-delocalization of the p-
electron donor substituent.15,16 The r values of 0.79–0.92 found in
the current study suggest that resonance interaction is relatively
significant.
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been suggested to change from the k2 step to the k1 process as the
pKa of the conjugate acids of pyridines becomes larger than 9.5
on the basis of the magnitude of the bnuc values, i.e., bnuc = 0.90
when pKa < 9.5 and bnuc = 0.40 when pKa > 9.5.6f
It is noted that the pKa of the conjugate acids of the pyridines
employed in this study is smaller than 9.5. Furthermore, bnuc
values of 0.74–0.98 obtained in this study are typical for reactions
reported previously to proceed through a stepwise mechanism
in which breakdown of T is the RDS.1–12 Therefore, one can
suggest that the pyridinolysis of 1a–j proceeds through a stepwise
mechanism with breakdown of T to the products being the RDS
regardless of the nature of substituent X. This is consistent with
the preceding argument that the RDS is not influenced by the
nature of substituent X in the benzoyl moiety.
However, we propose that the GS stabilization is more responsi-
ble than the GS destabilization suggested previously by Neuvonen
et al. for the reactivity pattern found in the current reactions
on the basis of the following reasons. If destabilization of the
GS through decreased resonance interactions (e.g., III, IV and
V) were responsible for the high reactivity shown by substrates
possessing an EWG, these substrates should have resulted in
positive deviation from the Hammett plot. A careful examination
of the Hammett plots in Fig. 1 reveals that substrates possessing
an EWG (i.e., 1e–j) do not exhibit positive deviation from the
Hammett plots. On the contrary, substrates bearing an EDG (i.e.,
1a–c) result in negative deviation from the linear Hammett plot
composed of substrates 1d–j. This is consistent with the preceding
argument that 1a–c exhibit lower reactivity than would be expected
from the Hammett substituent constant due to stabilization of the
GS through the resonance interaction as modeled by I and II.
A similar conclusion has been drawn for solvolysis of methyl
chloroformate and acetyl chloride in aqueous acetone. The former
was reported to be 9 ¥ 103 times less reactive than the latter.19
Kevill et al. have concluded that stabilization of the GS through
the resonance interaction VI ↔ VII, analogous to that suggested
in the current system (i.e., I ↔ II), is responsible for the
decreased reactivity of methyl chloroformate, since such resonance
interactions are not possible for acetyl chloride.19
To investigate the effect of the substituent X on the TS structure,
the bnuc values are correlated with sX constants. As shown in Fig. 3,
bnuc increases linearly as the substituent X changes from a strong
EDG to a strong EWG. It has generally been understood that
the magnitude of bnuc values represents a relative degree of bond
formation between the nucleophile and electrophile (or an effective
charge developed on the nucleophilic site) in the TS.13 Thus, one
can suggest that the degree of bond formation (or the effective
charge) in the TS becomes more significant as the substituent X in
the benzoyl moiety changes from a strong EDG to a strong EWG
since bnuc increases linearly with increasing electron withdrawing
ability of the substituent X.
Effect of substituent X on TS structure
To investigate the effect of substituent X on the TS structure,
Brønsted-type plots for the reactions of 1a–j with three pyridines
have been constructed. The plots shown in Figures S1–S10 in
the ESI† exhibit excellent linear correlation with bnuc = 0.74–0.98
(see also Table 2).20 One can get useful information on reaction
mechanisms from the magnitude of bnuc values as well as the shape
of Brønsted-type plots.13 In fact, the biphasic Brønsted-type plot
obtained from reactions of 2,4-dinitrophenyl benzoate (1d) with
a series of pyridines has been taken as evidence for a stepwise
mechanism.6f Besides, the RDS for the pyridinolysis of 1d has
Fig. 3 Plot of bnuc versus s constants for the reactions of 1a–j with
pyridines in 80 mol% H2O/20 mol% DMSO at 25.0 0.1 ◦C. The identity
of points is given in Table 2.
Table 2 Summary of bnuc values for the reactions of 2,4-dinitrophenyl
X-substituted benzoates (1a–j) with pyridines in 80 mol% H2O/20 mol%
DMSO at 25.0 0.1 ◦
C
X
bnuc
Conclusions
1a
1b
1c
1d
1e
1f
4-NMe2
4-MeO
4-Me
H
4-Cl
3-Cl
4-CN
4-NO2
4-Cl-3-NO2
3,5-(NO2)2
0.74 0.05
0.81 0.01
0.83 0.02
0.85 0.01
0.88 0.02
0.89 0.02
0.90 0.02
0.91 0.01
0.94 0.04
0.98 0.03
The current study has allowed us to conclude the following: (1)
the Hammett plots obtained from reactions of 1a–j with pyridines
consist of two intersecting straight lines. (2) The nonlinear
Hammett plots are not due to a change in RDS but are caused
by stabilization of the GS of substrates possessing an EDG (e.g.,
1a–c) throughthe resonance interactions I ↔ II, since the Yukawa–
Tsuno plots exhibit excellent linear correlations with r = 0.92–1.31
and r = 0.79–0.92. (3) Substrates possessing an EWG (e.g., 1e–j)
1g
1h
1i
1j
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are not unusually more reactive than would be predicted from
their Hammett substituent constants, but rather, substrates 1a–c
exhibit lower reactivity than would be expected due to stabilization
of the GS through resonance interactions. (4) The pyridinolysis
of 1a–j proceeds through a stepwise mechanism with the k2 step
being the RDS. (5) The RDS is not influenced by the nature of
the substituent X. However, the degree of bond formation (or the
effective charge on the nucleophilic site) in the TS becomes more
significant as the substituent X changes from a strong EDG to a
stronger EWG.
2 (a) F. M. Menger and J. H. Smith, J. Am. Chem. Soc., 1972, 94, 3824–
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1997, 179–183.
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H. F., J. Phys. Chem. A, 2008, 112, 6700–6707.
4 (a) I. H. Um, Y. M. Park, M. Fujio, M. Mishima and Y. Tsuno, J. Org.
Chem., 2007, 72, 4816–4821; (b) I. H. Um, J. Y. Lee, S. H. Ko and S. K.
Bae, J. Org. Chem., 2006, 71, 5800–5803; (c) I. H. Um, S. E. Jeon and
J. A. Seok, Chem.–Eur. J., 2006, 12, 1237–1243; (d) I. H. Um, J. Y. Lee,
H. W. Lee, Y. Nagano, M. Fujio and Y. Ysuno, J. Org. Chem., 2005,
70, 4980–4987; (e) I. H. Um, K. H. Kim, H. R. Prak, M. Fujio and Y.
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6 (a) I. H. Um, E. H. Kim and J. Y. Lee, J. Org. Chem., 2009, 74, 1212–
1217; (b) I. H. Um, S. J. Hwang, S. Yoon, S. E. Jeon and S. K. Bae,
J. Org. Chem., 2008, 73, 7671–7677; (c) I. H. Um, S. Yoon, H. R. Park
and H. J. Han, Org. Biomol. Chem., 2008, 6, 1618–1624; (d) I. H. Um,
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9197; (e) I. H. Um, E. Y. Kim, H. R. Park and S. E. Jeon, J. Org. Chem.,
2006, 71, 2302–2306; (f) I. H. Um, H. J. Han, M. H. Baek and S. Y.
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Experimental
Materials
Compounds 1a–j were readily prepared from the reaction of
X-substituted benzoyl chloride with 2,4-dinitrophenol in the pres-
ence of triethylamine in anhydrous ether as reported previously.2
Their purity was confirmed from melting point and spectral data
such as 1H NMR. Pyridines and other chemicals were of the
highest quality available. Doubly glass distilled water was further
boiled and cooled under nitrogen just before use. Due to low
solubility of 1a–j in pure H2O, 80 mol% H2O/20 mol% DMSO
was used as the reaction medium.
7 (a) I. H. Um, J. Y. Hong and J. A. Seok, J. Org. Chem., 2005, 70, 1438–
1444; (b) I. H. Um, S. M. Chun, O. M. Chae, M. Fujio and Y. Tsuno,
J. Org. Chem., 2004, 69, 3166–3172; (c) I. H. Um, J. Y. Hong, J. J. Kim,
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Kinetics
The kinetic study was performed using a UV-vis spectrophotome-
ter for slow reactions (t1/2 ≥ 10 s) or a stopped-flow spectropho-
tometer for fast reactions (t1/2 < 10 s) equipped with a constant
temperature circulating bath to keep the reaction temperature
at 25.0
0.1 ◦C. The reactions were followed by monitoring
the leaving 2,4-dinitrophenoxide at 410 nm. All reactions were
carried out under pseudo-first-order conditions in which the
pyridine concentration was at least 20 times greater than the
substrate concentration. The pyridine stock solution of ca. 0.2 M
was prepared by dissolving 2 equiv. of pyridine and 1 equiv. of
standardized HCl solution to keep the pH constant by making a
self-buffered solution.
Product analysis
2,4-Dinitrophenoxide was liberated quantitatively and identified
as one of the reaction products by comparison of the UV-vis
spectra after completing the reactions with those of authentic
samples under the same kinetic conditions.
Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2009-0075488). L. R. Im, E. H. Kim and J. H. Shin are grateful
for the BK 21 Scholarship.
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3806 | Org. Biomol. Chem., 2010, 8, 3801–3806
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