Kinetic study on aminolysis of 2-chloro-4-nitrophenyl x-substituted-benzoates in acetonitrile and in H2O Containing 20mol% DMSO: Effects of medium and substituent X on reactivity and reaction mechanism

Article abstract of DOI:10.1246/bcsj.20140048

Second-order rate constants for reactions of 2-chloro-4-nitrophenyl X-substituted-benzoates 1a-1j with a series of cyclic secondary amines in MeCN have been measured. Comparison of the kinetic results with those reported previously for the corresponding reactions carried out in H₂O containing 20 mol % DMSO has revealed that amines are less reactive in MeCN. The Bronsted-type plot for the aminolysis of 2-chloro-4-nitrophenyl benzoate (1f) in MeCN is linear with β_nuc= 0.64, which is in contrast to the curved Bronsted-type plot reported for the reaction performed in the H₂O-DMSO misxture. The Hammett plot for the reactions of 1a-1j with piperidine consists of two intersecting straight lines while the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρ_X= 1.22 and r = 0.60, indicating that the nonlinear Hammett plot is not due to a change in rate-determining step but is caused by stabilization of substrates possessing an electron-donating substituent through resonance interactions between the substituent and the carbonyl functionality. It has been concluded that medium change from the H₂O-DMSO mixture to MeCN forces the aminolysis of 1a-1j to proceed through a concerted mechanism by destabilizing the zwitterionic tetrahedral intermediate (T^±).

Full text of DOI:10.1246/bcsj.20140048

922
Bull. Chem. Soc. Jpn. Vol. 87, No. 8, 922-927 (2014)
© 2014 The Chemical Society of Japan
Kinetic Study on Aminolysis of 2-Chloro-4-nitrophenyl
X-Substituted-Benzoates in Acetonitrile and in H2O Containing
20 mol % DMSO: Effects of Medium and Substituent X
on Reactivity and Reaction Mechanism
Ha-Ram Kim,2 Jung Hwan Yoon,3 Min-Young Kim,1 and Ik-Hwan Um*1
1Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea
2Department of Science Education, Ewha Womans University, Seoul 120-750, Korea
3Young-il High School, Seoul 157-836, Korea
Received February 20, 2014; E-mail: ihum@ewha.ac.kr
Second-order rate constants for reactions of 2-chloro-4-nitrophenyl X-substituted-benzoates 1a-1j with a series of
cyclic secondary amines in MeCN have been measured. Comparison of the kinetic results with those reported previously
for the corresponding reactions carried out in H2O containing 20 mol % DMSO has revealed that amines are less reactive
in MeCN. The Brønsted-type plot for the aminolysis of 2-chloro-4-nitrophenyl benzoate (1f) in MeCN is linear with
¢
nuc = 0.64, which is in contrast to the curved Brønsted-type plot reported for the reaction performed in the H2O-DMSO
misxture. The Hammett plot for the reactions of 1a-1j with piperidine consists of two intersecting straight lines while the
Yukawa-Tsuno plot exhibits an excellent linear correlation with µX = 1.22 and r = 0.60, indicating that the nonlinear
Hammett plot is not due to a change in rate-determining step but is caused by stabilization of substrates possessing an
electron-donating substituent through resonance interactions between the substituent and the carbonyl functionality. It has
been concluded that medium change from the H2O-DMSO mixture to MeCN forces the aminolysis of 1a-1j to proceed
through a concerted mechanism by destabilizing the zwitterionic tetrahedral intermediate (T«).
Aminolysis of esters with amines has intensively been inves-
mediates (i.e., a zwitterionic tetrahedral intermediate T« and its
deprotonated form T ) depending on reaction conditions (e.g.,
the nature of electrophilic center, basicity of the incoming
amine and leaving group, reaction medium).2-5 Reactions of
4-nitrophenyl diphenylphosphinate with a series of cyclic
secondary amines have been reported to proceed through a
¹
tigated due to their importance not only in organic syntheses
but also in biological processes such as enzyme action and
peptide synthesis.1-10 As shown in Scheme 1, aminolysis of
esters has been reported to proceed through a concerted
mechanism or via a stepwise pathway with one or two inter-
kc
O
O
O
k1
H
k2
HN
R C O Ar
R C N
O Ar
R C O Ar
NH
k-1
PH
T
fast
k3[NH]
H
H
O
O
O Ar
R C O Ar
N
R C N
P
T
Scheme 1.
Published on the web August 15, 2014; doi:10.1246/bcsj.20140048

H.-R. Kim et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 8 (2014)
923
Cl
Cl
O
C O
O
C N
H
k
N
+
NO2 +
HN
O
NO2
X
X
1a-1j
X = 3,5-(NO2)2(1a), 4-Cl-3-NO2(1b), 3-NO2(1c), 4-CN(1d), 3-Cl(1e),
H(1f), 3-CH3(1g), 4-CH3(1h), 4-OCH3(1i), 4-N(CH3)2(1j).
HN
HN
Z ; R = H or CH3 ; Z = CH2, NH, NCH2CH2OH, O.
=
R
Scheme 2.
concerted mechanism on the basis of a linear Brønsted-type
plot with ¢nuc = 0.4 « 0.1.6 In contrast, a curved Brønsted-type
plot reported for aminolysis of esters possessing a weakly basic
leaving group (e.g., 2,4-dinitrophenoxide) has been taken as
evidence for a change in rate-determining step (RDS) of a
stepwise reaction.2-5,7-10
Table 1. Summary of Second-Order Rate Constants (kN) for
the Reactions of 2-Chloro-4-nitrophenyl Benzoate (1f)
with Cyclic Secondary Amines in MeCN and in H2O
Containing 20 mol % DMSO at 25.0 « 0.1 °C
MeCN
kN
H2Ob)
kN
/M¹1 s
Amines
a)
It is now firmly understood that RDS of aminolysis of esters
pKa
pKa
¹1
¹1
/M¹1 s
o
changes at pKa , which was defined as the pKa at the center of
the Brønsted curvature, from breakdown of T« to its formation
as the incoming amine becomes more basic than the leaving
group by 4 to 5 pKa units.2 However, the effect of nonleaving-
Piperidine
3-Methylpiperidine
Piperazine
18.8 19.0
18.6 14.0
18.5 12.8
11.02 30.0
10.8 25.3
9.85 12.3
1-(2-Hydroxyethyl)piperazine 17.6
2.53
0.767
®
9.38 2.97
8.65 1.81
5.95 0.0124
o
group substituents on the pKa is controversial.7-10 In nucle-
Morpholine
16.6
®
ophilic substitution reactions of X-substituted-phenyl 3,4-
dinitrophenyl carbonates with quinuclidine, Gresser and Jencks
have reported that the pKao increases as the substituent X in the
nonleaving group becomes a stronger electron-withdrawing
group (EWG).7 Similar results have been reported by Castro
et al. for pyridinolysis of diaryl carbonates and aminolysis of S-
2,4-dinitrophenyl X-substituted-benzoates.8 Thus, it has been
Piperazinium ion
a) The pKa values in MeCN were taken from Refs. 10a and 11.
b) The pKa and kN values in H2O containing 20 mol % DMSO
were taken from Ref. 12.
Table 2. Summary of Second-Order Rate Constants (kN) for
the Reactions of 2-Chloro-4-nitrophenyl X-Substituted-
Benzoates 1a-1j with Piperidine in MeCN at 25.0 « 0.1 °C
concluded that an EWG in the nonleaving group of esters
o
increases pKa by decreasing the k2/k ratio.7,8 In contrast,
¹1
¹1
we have shown that the k2/k ratio is independent of the
X
kN/M¹1 s
¹1
electronic nature of the substituent X for aminolysis of 2,4-
dinitrophenyl X-substituted-benzoates,9a 2,4-dinitrophenyl X-
substituted-benzenesulfonates9b and related reactions of various
esters.10
Our study has now been extended to the reactions of 2-
chloro-4-nitrophenyl X-substituted-benzoates 1a-1j with a
series of cyclic secondary amines in MeCN to obtain further
information on the effect of substituent X on the reaction
mechanism (Scheme 2). The kinetic results have been com-
pared with those reported previously for the corresponding
reactions carried out in H2O containing 20 mol % DMSO to
investigate the effect of medium on the reactivity and reaction
mechanism.
1a
1b
1c
1d
1e
1f
1g
1h
1i
3,5-(NO2)2
4-Cl-3-NO2
3-NO2
4-CN
3-Cl
H
3-CH3
4-CH3
4-OCH3
4-N(CH3)2
992
267
194
123
72.4
19.0
16.2
10.6
4.78
0.717
1j
uncertainty in the kN values is estimated to be less than «3%
from replicate runs. The kN values calculated in this way
are summarized in Table 1 for the reactions of 2-chloro-4-
nitrophenyl benzoate (1f) with a series of cyclic secondary
amines and in Table 2 for those of 2-chloro-4-nitrophenyl X-
substituted-benzoates 1a-1j with piperidine.
Effect of Medium on Reactivity and Reaction Mecha-
nism. As shown in Table 1, the kN value for the reactions
carried out in MeCN decreases as the amine basicity decreases,
e.g., kN decreases from 19.0 to 2.53 and 0.767 M¹1 s as the
Results and Discussion
All of the reactions in this study obeyed pseudo-first-order
kinetics. Pseudo-first-order rate constants (kobsd) were calcu-
lated from the equation, ln(A¨ ¹ At) = ¹kobsdt + C. The plots
of kobsd vs. [amine] were linear, indicating that general base
catalysis by a second amine molecule is absent. The second-
order rate constants (kN) were calculated from the slope of the
linear plots of kobsd vs. [amine]. The correlation coefficient
for the linear regression was always higher than 0.9995. The
¹1
pKa value of the conjugate acid of the amine decreases from

924
Bull. Chem. Soc. Jpn. Vol. 87, No. 8 (2014)
Aminolysis of 2-Chloro-4-nitrophenyl X-Substituted-Benzoates
3
2
1
2
(a)
(b)
2
1
3
1
1
5
2
4
1
3
0
4
5
0
-1
pKao = 10.3
6
-1
-2
βnuc = 0.64 0.04
R2 = 0.995
β1 = 0.25
β2 = 0.85
-2
-3
17
18
19
6
8
10
12
pKa + log (p/q)
pKa + log (p/q)
Figure 1. Brønsted-type plots for the reactions of 2-chloro-4-nitrophenyl benzoate (1f) with cyclic secondary amines in MeCN (a)
and in H2O containing 20 mol % DMSO (b) at 25.0 « 0.1 °C. The identity of points is given in Table 1.
18.8 to 17.6 and 16.6, in turn. A similar result is demonstrated
for the corresponding reactions carried out in H2O containing
20 mol % DMSO. However, the amines are less reactive in the
aprotic solvent than in the aqueous medium although they are
ca. 8 pKa units more basic in the former solvent. This indicates
that the reactivity of amines is not governed solely by the
basicity of amines.
It is well known that the effect of medium on reactivity is
strongly dependent on the nature of reactants. It has been
reported that a change in medium from H2O to a dipolar aprotic
solvent increases the reactivity of anionic nucleophiles due to
strong repulsion between the anionic species and the negative
dipole end of dipolar solvent.13 On the contrary, modification
of the reaction medium from H2O to a dipolar aprotic solvent
has often resulted in a decrease in rate for reactions between
neutral reactants which proceed through a partially charged
transition state (TS) or via a zwitterionic intermediate T«.13
Thus, one can suggest that destabilization of the partially
charged TS (or T«) in MeCN is a plausible reason for the
kinetic results that the amines are less reactive in MeCN than
in H2O.
Thus, one can suggest that the medium change from H2O
containing 20 mol % DMSO to MeCN forces the reaction to
proceed through a concerted mechanism by decreasing the
stability of T« in MeCN. This idea is also consistent with the
report that MeCN is a poor solvent for ionic species.13
Effect of Substituent X on Reactivity and Reaction
Mechanism. To investigate the effect of nonleaving-group
substituents on the reaction mechanism, the reactions of 2-
chloro-4-nitrophenyl X-substituted-benzoates 1a-1j with pipe-
ridine have been carried out in MeCN. As shown in Table 2,
the kN value for the reactions of 1a-1j decreases as the
substituent X changes from a strong EWG to a strong EDG,
¹1
e.g., it decreases from 992 to 19.0 and 0.717 M¹1 s as the
substituent X changes from 3,5-(NO2)2 to H and 4-N(CH3)2, in
turn.
The effect of the substituent X on the reactivity of 1a-1j is
illustrated in Figure 2. It is noted that the Hammett plot con-
sists of two intersecting straight lines. Nonlinear Hammett plots
have traditionally been interpreted as a change in the reaction
mechanism or RDS depending on the shape of curvature.1c,15
Upward curvature often reported for nucleophilic substitution
reactions of benzylic systems, where EDGs and EWGs are
located on the downward and upward portions of the plot,
respectively, has been attributed to a change in the reaction
mechanism from an SN1 mechanism to an SN2 pathway.15 In
contrast, downward curvature has been ascribed to a change
in RDS on changing the substituents from EDGs to EWGs.15
In fact, Jencks concluded that the reactions of semicarbazide
with a series of X-substituted-benzaldehydes in weakly acidic
solution proceed through a stepwise mechanism with a change
in RDS on the basis of a curved Hammett plot (i.e., from a large
µX to a small one).1c
To investigate the effect of medium on the reaction mecha-
nism, Brønsted-type plots have been constructed for the reac-
tions of 1f carried out in MeCN and in H2O containing 20 mol %
DMSO. As shown in Figure 1a, the Brønsted-type plot for the
reactions in MeCN exhibits an excellent linear correlation with
¢
nuc = 0.64 « 0.04 when the kN and pKa values are corrected
statistically using p and q (i.e., p = 2 while q = 1 except q = 2
for piperazine).14 This is in contrast to the curved Brønsted-type
plot shown in Figure 1b for the corresponding reactions carried
out in H2O containing 20 mol % DMSO. The reactions in the
aqueous medium has been reported to proceed through a
stepwise mechanism, in which the RDS changes from break-
down of T« to its formation as the amine basicity increases, on
the basis of the curved Brønsted-type plot.12 A linear Brønsted-
type plot with ¢nuc = 0.5 « 0.1 is typical for reactions reported
previously to proceed through a concerted mechanism.2-5,7-10
Thus, one might attribute the nonlinear Hammett plot shown
in Figure 2 to a change in RDS, i.e., from formation of T« to its
breakdown to the products as the substituent X changes from
EDGs to EWGs. This idea appears to be reasonable since an
EDG in the benzoyl moiety would retard nucleophilic attack

H.-R. Kim et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 8 (2014)
925
4
O
O
Me
Me
Me
Me
N
C OAr
N
C OAr
1a
3
2
IV
V
1b
Chart 2.
1c
1d
1e
1a
3
2
1f
1g
1b
1
1c
1d
1h
1i
1e
0
1f
1j
1g
1h
1
1i
-1
-1
0
1
σX
1j
0
ρX = 1.22 0.03
r = 0.60
Figure 2. Hammett plot for the reactions of 2-chloro-4-
nitrophenyl X-substituted-benzoates 1a-1j with piperidine
in MeCN at 25.0 « 0.1 °C. The identity of points is given
in Table 2.
R2 = 0.998
-1
-1
0
1
2
σXo + r (σX − σX )
+
o
O
O
O
OR'
R C OR'
R C OR'
R C
Figure 3. Yukawa-Tsuno plot for the reactions of 2-chloro-
4-nitrophenyl X-substituted-benzoates 1a-1j with piper-
idine in MeCN at 25.0 « 0.1 °C. The identity of points is
given in Table 2.
I
II
III
Chart 1.
(a decrease in k1 in Scheme 1) but would accelerate expulsion
of the leaving group (an increase in k2 in Scheme 1) or vice
versa. Nevertheless, we propose that the nonlinear Hammett
plot is not due to a change in RDS. Because RDS is not
governed by the magnitude of the k1 and k2 values but it should
stabilization of substrates bearing an EDG, because they can be
stabilized through the resonance interactions as illustrated by
resonance structures IV § V in Chart 2.
To examine the above argument, a Yukawa-Tsuno plot has
been constructed in Figure 3 for the reactions of 1a-1j with
piperidine. The r value in the Yukawa-Tsuno equation, eq 1,
represents the resonance demand of the reaction center or the
be determined by the k2/k ratio (e.g., RDS = the k1 step
¹1
when k2/k¹1 > 1 but RDS = the k2 step when k2/k¹1 < 1).
Origin of the Nonlinear Hammett Plot. Neuvonen et al.
have reported that a strong EWG in the leaving or nonleaving
group of aryl acetates or alkyl benzoates increases the reactivity
of the esters with upfield 13C NMR chemical shifts of the
carbonyl carbon and higher frequencies of the C=O stretching
vibration.16 Since a strong EWG in the leaving or nonleaving
group would inhibit the resonance interaction as illustrated by
resonance structures I § II § III in Chart 1, Neuvonen et al.
have attributed the increased reactivity of the esters bearing
a strong EWG to destabilization of their ground state (GS)
through decreased resonance interactions.16 In fact, Table 2
shows that the substrate possessing a stronger EWG in the
benzoyl moiety (e.g., 1a-1e) becomes more reactive.
However, a careful examination of the nonlinear Hammett
plot shown in Figure 2 reveals that the substrates possessing an
EDG in the benzoyl moiety (e.g., 1g-1j) deviate negatively
from the linear line composed of the substrates bearing an
EWG (e.g., 1a-1e). Furthermore, such deviation is more
significant for the substrate possessing a stronger EDG. Thus,
we propose that the nonlinear Hammett plot is caused by
+
extent of resonance contribution, while the term (·X ¹ ·Xo) is
the resonance substituent constant that measures the capacity
for π-delocalization of the π-electron donor substituent.17,18
Equation 1 was originally derived to account for the kinetic
results obtained from solvolysis of benzylic systems in which a
partial positive charge develops in TS.17 However, we have
shown that eq 1 is also highly effective in elucidation of ambi-
guities in the reaction mechanism for aminolysis of various
esters.9,10
o
o
logðkX=kHÞ ¼ µX½·X þ rð·Xþ ꢀ ·X Þꢁ
ð1Þ
As shown in Figure 3, the Yukawa-Tsuno plot exhibits an
excellent linear correlation with µX = 1.22 and r = 0.60. Such
a linear Yukawa-Tsuno plot clearly indicates that the nonlinear
Hammett plot is not due to a change in RDS but is caused by
the resonance stabilization as illustrated by resonance structures
IVand V. Thus, one can suggest that the electronic nature of the
substituent X does not affect the k2/k¹1 ratio and that deduction
of reaction mechanism based just on a linear or nonlinear
Hammett plot can be misleading.

926
Bull. Chem. Soc. Jpn. Vol. 87, No. 8 (2014)
Aminolysis of 2-Chloro-4-nitrophenyl X-Substituted-Benzoates
The µX value of 1.22 obtained in this study is much larger
than that for reactions reported to proceed through a stepwise
mechanism (e.g., µX = 0.41 and 0.60 for piperidinolysis of
4-nitrophenyl X-substituted-2-methylbenzoates19 and 4-pyridyl
X-substituted-benzoates10b in MeCN, respectively) but is
similar to that for reactions reported to proceed through a
forced concerted mechanism (e.g., µX = 1.30 and 1.28 for the
piperidinolysis of 2,4-dinitrophenyl X-substituted-benzoates10f
and 2-pyridyl X-substituted-benzoates10a in MeCN, respec-
tively). Thus, one can suggest that the current reactions also
proceed through a forced concerted mechanism, in which bond
formation between the incoming amine and electrophilic center
is much more advanced than expulsion of the leaving group
on the basis of the large µX value. This is consistent with the
proposal in the preceding section that the aminolysis of 1f
proceeds through a concerted mechanism on the basis of the
linear Brønsted-type plot with ¢nuc = 0.64.
varied over the range (2-100) © 10¹3 M, while the substrate
concentration was ca. 4 © 10¹5 M. Pseudo-first-order rate con-
stants (kobsd) were calculated from the equation, ln(A¨ ¹ At) =
¹kobsdt + C. The plots of ln(A¨ ¹ At) vs. time were linear
over 90% of the total reaction. Usually, five different amine
concentrations were employed to obtain the second-order rate
constants (kN) from the slope of linear plots of kobsd vs. amine
concentrations.
Product Analysis.
2-Chloro-4-nitrophenoxide ion was
liberated quantitatively and identified as one of the products by
comparison of the UV-vis spectrum after completion of the
reaction with that of authentic sample under the same reaction
conditions.
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education (No. 2012-
R1A1B-3001637). JHY is also grateful for the Intensive
Science Program of Young-il High School.
Conclusion
The current study has allowed us to conclude the follow-
ing: (1) The linear Brønsted-type plot with ¢nuc = 0.64 for the
reactions of 1f suggests that the reactions proceed through a
concerted mechanism. (2) The Hammett plot for the reactions
of 1a-1j with piperidine consists of two intersecting straight
lines, while the Yukawa-Tsuno plot results in an excellent
linear correlation with µX = 1.22 and r = 0.60. (3) The non-
linear Hammett plot is not due to a change in RDS but is caused
by stabilization of the substrates through resonance interac-
Supporting Information
The kinetic results and conditions for the reactions of 2-
chlorophenyl X-substituted-benzoates with amines. This mate-
rial is available free of charge on J-STAGE.
References
1
a) M. I. Page, A. Williams, Organic and Bio-Organic
Mechanisms, Longman, Singapore, 1997, Chapt. 7. b) T. H.
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¹1
nature of the substituent X in the nonleaving group. (5)
Modification of the reaction medium from H2O containing
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proceed through a concerted mechanism by destabilizing T«.
2
a) E. A. Castro, Pure Appl. Chem. 2009, 81, 685. b) E. A.
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Experimental
Materials. 2-Chloro-4-nitrophenyl X-substituted-benzoates
1a-1j were readily prepared from the reaction of the respective
benzoyl chloride with 2-chloro-4-nitrophenol in anhydrous
ether in the presence of triethylamine as reported previous-
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3
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4
a) A. Llinás, M. I. Page, Org. Biomol. Chem. 2004, 2, 651.
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spectrophotometer for slow reactions (e.g., t1/2 ² 10 s) or a
stopped-flow spectrophotometer for fast reactions (e.g., t1/2
b) L. Perreux, A. Loupy, M. Delmotte, Tetrahedron 2003, 59,
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2002, 41, 2581.
<
10 s) equipped with a constant temperature circulating bath to
maintain the reaction mixture at 25.0 « 0.1 °C. The reactions
were followed by monitoring the appearance of 2-chloro-4-
nitrophenoxide ion. All of the reactions in this study were
carried out under pseudo-first-order conditions, in which the
concentration of the amine was kept in excess over that of the
substrate.
Typically, the reaction was initiated by adding 5 ¯L of a
0.02 M solution of the substrate in acetonitrile to a 10-mm
quartz UV cell containing 2.50 mL of the thermostated reaction
mixture made up of solvent and aliquot of the amine stock
solution. All solutions were transferred by gas-tight syringes.
Generally, the amine concentration in the reaction mixtures was
5
a) D. D. Sung, I. S. Koo, K. Yang, I. Lee, Chem. Phys. Lett.
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Sung, I. Lee, Org. Biomol. Chem. 2005, 3, 1240.
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H.-R. Kim et al.
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