Structure-reactivity correlation in the aminolysis of 4-fluorophenyl acetate in aqueous medium

Article abstract of DOI:10.1002/kin.10065

The reaction of the title substrate with a series of amines of varying pK_a, viz. ammonia, ethanolamine, glycine, 1,2-diaminopropane, 1,3-diaminopropane, n-butylamine, piperidine, hydrazine, imidazole, and hydroxylamine is subjected to a kinetic study in aqueous medium, 25°C, ionic strength 0.1 M (KCl). Pseudo-first-order rate coefficients (k_obs) are found throughout under amine excess at various pH values for each amine. For amines, excluding hydrazine, ammonia, and hydroxylamine the reaction follows clean second-order kinetics and the plots of (k_obs - k_H) against free amine concentration are linear at constant pH. The macroscopic nucleophilic substitution rate coefficients (k_N) are obtained as the slopes of these plots and found to be pH independent. For hydrazine, ammonia, and hydroxylamine, a rate dependence on more than first power of the amine is observed, accordingly, the rate constants for the assisted paths have been disseminated for these amines besides k_N. The Broensted-type plot (logk_N against amine pK_a) is linear with a slope value of β = 1.02. The magnitude of the slope value is consistent with a stepwise mechanism through a zwitterionic tetrahedral addition intermediate whose breakdown to products is rate-determining (k₂ step). A remarkable reactivity difference is observed among the diamines, the reason for which is discussed in detail.

Full text of DOI:10.1002/kin.10065

Structure-Reactivity
Correlation in the
Aminolysis of
4
-Fluorophenyl Acetate
in Aqueous Medium
1
2
2
D. RAJARATHNAM, T. JEYAKUMAR, P. ANANTHAKRISHNA NADAR
1Department of Chemical and Environmental Engineering, The National University of Singapore,
Engineering Drive 4, Singapore 117576
4
2
Department of Chemistry, Annamalai University, Tamil Nadu, India 608002
Received 5 November 2001; accepted 26 February 2002
DOI 10.1002/kin.10065
ABSTRACT: The reaction of the title substrate with a series of amines of varying pKa, viz. ammo-
nia, ethanolamine, glycine, 1,2-diaminopropane, 1,3-diaminopropane, n-butylamine, piperi-
dine, hydrazine, imidazole, and hydroxylamine is subjected to a kinetic study in aqueous
�
medium, 25 C, ionic strength 0.1 M (KCl). Pseudo-first-order rate coefficients (kobs) are found
throughout under amine excess at various pH values for each amine. For amines, excluding
hydrazine, ammonia, and hydroxylamine the reaction follows clean second-order kinetics and
the plots of (kobs � kH) against free amine concentration are linear at constant pH. The macro-
scopic nucleophilic substitution rate coefficients (kN) are obtained as the slopes of these plots
and found to be pH independent. For hydrazine, ammonia, and hydroxylamine, a rate depen-
dence on more than first power of the amine is observed, accordingly, the rate constants for the
assisted paths have been disseminated for these amines besides kN. The Br o¨ nsted-type plot
(
logkN against amine pKa) is linear with a slope value of � = 1.02. The magnitude of the slope
value is consistent with a stepwise mechanism through a zwitterionic tetrahedral addition in-
termediate whose breakdown to products is rate-determining (k step). A remarkable reactivity
2
difference is observed among the diamines, the reason for which is discussed in detail. �C 2002
Wiley Periodicals, Inc. Int J Chem Kinet 34: 366–373, 2002
INTRODUCTION
type plots, in which a zwitterionic tetrahedral interme-
�
diate (T ) in the reaction path has always been postu-
�
The aminolysis reactions of phenyl acetate in aqueous
mediumhavebeenstudiedextensively. Themechanism
of these reactions has been well established through
structure-reactivity correlations, especially Br o¨ nsted-
lated [1,2]. The stability of T and the rate-determining
step have been found to be dependent on the nature
and basicity of the amine moiety as well as the phe-
noxy group. For simple nucleophilic attack of amine
(with no significant second-order term on amine), the
reaction can be depicted as shown in Scheme 1, where,
Correspondence to: D. Rajarathnam; e-mail: chedr@nus.edu.sg.
�
either the formation of T is the rate-determining step
�
c 2002 Wiley Periodicals, Inc.

AMINOLYSIS OF 4-FLUOROPHENYL ACETATE IN AQUEOUS MEDIUM
367
Scheme 1
�
(
k1 step) with �nuc = 0.1 � 0.3 or the breakdown of
tions under identical conditions to those of the kinetic
experiments [4,8].
�
T to the products is the rate-determining step (k2 step)
�
with �nuc = 0.8 � 1.0 as the basicity of the amine nu-
cleophile varies [3–16].
Kinetic Measurements
Though a great deal of information is available on
the aminolysis of phenyl acetates in aqueous medium
with various substituents [1,4,9–21] that of fluoro sub-
stituent is given less attention. In order to clarify
the mechanism of the aminolysis of fluoro-substituted
phenyl acetate and to assess the influence of amine ba-
sicity, in this work we investigate the kinetics of the
title reactions in aqueous medium with amines of vary-
ing basicity. Through the Br o¨ nsted-type plot and the
kinetic data a plausible mechanism is deduced.
These were carried out in aqueous medium, ionic
strength 0.1 M (KCl), using a JASCO UVIDEC 340
UV–VIS spectrophotometer equipped with a ther-
mostatted cell holder. The reactions were initiated by
adding a solution of 30 ␮l of the ester stock solution
(in dioxan) to 3 ml of thermally equilibrated (25.0 �
�
0.1 C) amine–amine hydrochloride buffer. The reac-
tions were followed by monitoring the release of the
fluorophenolate ion at regular intervals of time at
�
max = 272 nm. The initial ester concentration was
�
5
(5–6) � 10 M, and the amine concentration was kept
EXPERIMENTAL
Materials
at several times excess over the substrate. The pH val-
ues of the kinetic solutions were determined using ITL
digital pH meter both prior to and at the completion
of reactions. The glass electrode was calibrated using
standard procedures [23].
The 4-fluorophenyl acetate was prepared from corre-
sponding phenol by procedure from the literature [22].
Amines, with the exception of reagent grade imidazole
All the reactions in the present study fitted good
pseudo-first-orderkineticsuptogreaterthan90%ofthe
total reaction. Pseudo-first-order rate coefficients (kobs)
were found throughout by means of the Guggenheim
method employing least squares technique [24].
The correlation coefficients of the linear regressions
were always � 0.999. The experimental conditions of
aminolysis and the rate coefficients are given in Table I.
In order to evaluate the results, the alkaline hydrol-
ysis of the ester was also followed spectrophotometri-
cally using known concentrations of potassium hydrox-
(Fluka) and glycine (BDH), were redistilled from KOH
or recrystallized shortly before use. Ethanolamine
was simply redistilled. Ammonium chloride (Merck),
potassium chloride (Merck), and potassium hydroxide
(BDH) were reagent-grade and used as such. Water was
double-distilled.
The amines were employed as buffers to maintain
constant pH. The amine–amine hydrochloride buffers
were prepared shortly before the kinetic run by the
addition of calculated amounts of standardized hy-
drochloric acid or potassium hydroxide to the solutions
of known concentrations of amine-free base or amine
hydrochloride as the case may be. At any fixed pH
value, the concentration of free amine was evaluated
using Eq. (1).
�
ide at 25.0 � 0.1 C and ionic strength 0.1 M (KCl).
The second-order rate coefficient, khyd, (where, khyd =
�
� 1
� 1
M .
kobs/[OH ]), for the ester is 2.46 � 0.07 s
Product Studies
[
N]T Ka
The product studies were carried out according to
the procedure already reported [25], based on the
method of Katz et al. [26]. The aminolysis of the es-
ter was allowed to go to completion and the acetamide
formed was determined by conversion to acetylhydrox-
amic acid – ferric ion complex. The absorbance of
the complex was determined spectrophotometrically at
�max = 540 nm and the concentration of acetamide was
[N]F =
(1)
Ka + aH+
where, [N]T is the total amine concentration, Ka is
the dissociation constant of the conjugate acid of the
amine, and aH is the hydrogen ion activity. The pKa
of the conjugate acid of amines were determined from
measurements of the pH of partially neutralized solu-
+

3
68
RAJARATHNAM, JEYAKUMAR, AND ANANTHAKRISHNA NADAR
Table I Summary of Experimental Conditions and Rate Coefficients for the Aminolysis of 4-Fluorophenyl Acetatea
[N]F (10 2 M)b
�
kobsd � kH (10� 3
s
� 1
)
k2 (10
0
� 2 � 1
s
� 1
M )
Amine
pH
Ammonia
8.73
12.44–22.39
16.44–29.59
21.85–39.33
28.15–50.67
7.74–17.02
10.11–22.25
13.42–29.53
16.34–35.95
10.41–15.41
14.95–22.13
20.72–30.67
26.15–38.70
13.93–27.85
16.96–33.93
23.04–46.08
26.57–53.14
0.321–0.964
0.494–1.48
0.727–2.18
0.990–2.97
0.154–1.08
0.238–1.66
0.359–2.51
0.524–3.67
0.144–1.01
0.243–1.70
0.359–2.51
0.472–3.30
4.49–6.25
1.12–2.69
1.34–3.23
1.61–4.32
1.83–4.70
1.59–3.31
1.83–4.31
2.29–5.20
2.79–6.19
2.39–4.61
3.49–6.15
3.79–7.55
4.96–9.42
3.27–6.80
3.94–8.02
4.53–9.97
5.54–12.00
2.33–6.47
3.07–8.30
3.62–11.64
6.43–18.08
0.491–3.33
0.810–5.32
1.15–7.69
1.62–11.58
1.28–7.86
2.04–13.24
2.84–19.35
3.78–26.01
2.49–4.62
5.51–10.06
8.58–15.63
14.50–29.25
1.06–3.17
1.23–3.77
1.60–4.61
1.82–4.89
1.75–8.92
2.09–10.80
2.78–15.25
3.45–18.72
0.90–1.20
0.82–1.10
0.74–1.10
8
9
9
.90
.10
.32
0.65–0.93
Ethanolamine
Glycine
9.30
1.97 � 0.02
1.90 � 0.09
1.73 � 0.07
1.72 � 0.06
2.65 � 0.04
2.56 � 0.06
2.13 � 0.09
2.15 � 0.03
2.42 � 0.05
2.33 � 0.05
2.13 � 0.07
2.17 � 0.02
70.8 � 0.5
59.3 � 0.8
52.1 � 0.6
63.2 � 0.8
30.2 � 0.2
34.1 � 0.4
25.9 � 0.4
31.6 � 0.3
82.2 � 0.4
80.8 � 0.6
78.8 � 0.4
79.7 � 0.9
5.54–7.40
9
9
.51
.81
1
0.15
9.13
9.34
9.56
9.75
1
,2-Diaminopropane
,3-Diaminopropane
9.75
.90
9
1
1
0.20
0.40
9.90
0.15
0.42
0.71
9.52
1
1
1
1
n-Butylamine
Piperidine
9
9
.73
.94
1
0.15
10.10
0.35
0.55
0.70
6.90
1
1
1
Hydrazine
7
7
7
.20
.50
.90
8.29–11.55
14.42–20.08
26.07–36.31
7.62–22.85
8.80–26.41
9.80–29.41
10.45–31.36
7.93–23.80
9.07–27.20
10.15–30.46
11.04–33.12
6.65–8.71
5.95–7.78
5.56–8.06
1.40 � 0.04
1.40 � 0.02
1.61 � 0.03
1.65 � 0.05
2.21–3.75
Imidazole
7.21
7.41
7.62
7.80
Hydroxylamine
6.10
6
6
6
.30
.55
.87
2.30–3.97
2.74–5.01
3.12–5.65
a
�
In aqueous solution at 25.0 � 0.1 C, ionic strength 0.1 M (KCl). At least five runs were carried out at each pH. The errors shown are
standard deviations.
b
Concentration of free amine.
determined from a calibration plot obtained through the
standard solutions of acetamide, which showed in all
the cases at least 90% of the formation of amide product
under the experimental conditions.
product of khyd, the second-order rate coefficient for the
alkaline hydrolysis of esters and the concentrations
of hydroxide ion present in the system for aminol-
�
ysis, calculated from aOH and the activity coeffi-
cient of hydroxide ion [27]. For amines ethanolamine,
glycine, 1,2-diaminopropane, 1,3-diaminopropane, n-
butylamine, piperidine, and imidazole k2 values are
RESULTS AND DISCUSSION
0
The kobs values for the aminolysis of the ester were con-
almost constant within a given pH and are indepen-
dent of free amine concentration (Table I). The plots
of log(kobs � kH) against log[N]F for these amines are
verted to the apparent second-order rate coefficients,
0
0
k2 , where k2 = (kobs � kH)/[N] , where kH is the
F

AMINOLYSIS OF 4-FLUOROPHENYL ACETATE IN AQUEOUS MEDIUM
369
where Ar represents 4-fluorophenyl, kobs is the pseudo-
first-order rate coefficient, k0 is the coefficient of as-
sisted paths comprising general acid, general base, and
specific base assisted attack of amines, kN is the macro-
scopic nucleophilic substitution rate coefficient, FN is
the free-amine fraction and [N]T is the concentration of
total amine (free-amine plus protonated form). Since,
FN = [N] /[N] , Eq. (3) becomes,
F
T
(kobs � kH) = k0 + kN[N]F
(4)
The values of k0 and kN were obtained as the inter-
cept and slope, respectively, of linear plots of (kobs �
k ) against [N] at constant pH. For the reactions by
Figure 1 Plot of log(kobs � kH) against log [N]F for the
H
F
aminolysis of 4-fluorophenyl acetate by ethanolamine at pH
these amines, the value of k was negligible compared
0
9
.30.
to the aminolysis term in Eq. (4) and kN values obtained
were pH-independent. These plots are exemplified for
the case of ethanolamine at various pH values in Fig. 2.
The mean kN values are given in Table II.
linear with unit slope. A representative plot of
ethanolamine at pH 9.30 is shown in Fig. 1. Thus, the
aminolysis by these amines depends only upon the first
power of free amine concentration and the reaction pro-
ceeds by simple nucleophilic attack on ester.
The reactions for these amines are governed by the
kinetic law under the experimental conditions given by
Eqs. (2) and (3),
0
An increase in k2 values is observed for hydrazine,
ammonia, and hydroxylamine by varying the concen-
tration of free amine within a given pH (Table I). The
plots of log(kobs � kH) against log[N]F for this group of
amines give slope value more than 1, for which a repre-
sentativeplotisshowninFig. 3byhydrazineatpH6.90.
For these amines which show dependence on second-
molecule of amine, Eq. (5) governs the reaction:
�
d[ArO ]
=
(kobs � kH)[Ester]
(2)
(3)
+
dt
(
kobs � kH)/[N] = kN + (kgb + kga aH /Ka)[N]
F
F
(
kobs � kH) = k0 + kN FN[N]T
(5)
Figure 2 Plots of (kobs � kH) against [N]F for the aminolysis of 4-fluorophenyl acetate by ethanolamine at various pH values.

3
70
RAJARATHNAM, JEYAKUMAR, AND ANANTHAKRISHNA NADAR
Table II Macroscopic Nucleophilic Substitution Rate Coefficients for the Aminolysis of 4-Fluorophenyl Acetatea
kN (10 2
�
s
� 1
� 1
M
)
No.
Amine
Ammoniab
Ethanolamine
Glycine
1,2-Diaminopropane
1,3-Diaminopropane
n-Butylamine
Piperidine
Hydrazineb
Imidazole
Hydroxylamineb
pKa
1
2
3
4
5
6
7
8
9
9.21 � 0.02
9.50 � 0.03
9.71 � 0.05
9.95 � 0.07
10.50 � 0.09
10.60 � 0.08
11.21 � 0.09
7.96 � 0.04
6.97 � 0.05
5.81 � 0.03
0.41 � 0.01
1.88 � 0.05
3.83 � 0.09
2.39 � 0.07
58.8 � 0.2
31.4 � 0.5
77.4 � 0.8
1.01 � 0.02
1.46 � 0.02
1.62 � 0.03
1
0
a
b
�
Values of both pK
For these amines k
a
and k
N
in aqueous solution, at 25.0 � 0.1 C, ionic strength 0.1 M (KCl). The errors shown are standard deviations.
N
values are determined using Eq. (5).
and 7.9 � 10�
2
s
� 1 � 2
M
respectively. These results are
where, kgb, kga, and Ka are the rate constants for gen-
eral base assisted attack of amine, the general acid as-
sisted attack of amine, and the dissociation constant
consistent to that observed earlier for the aminolysis
of phenyl acetate by these amines in aqueous medium
[3,8,17,18].
of aminium ion respectively. The values of kN and
+
(
kgb + kgaaH /Ka) are obtained as the intercepts and
The kN values thus obtained and the pKa val-
ues were statistically corrected [28–30] before plot-
ting the Br o¨ nsted-type equation. The plot is linear for
the nucleophiles ammonia, ethanolamine, glycine, 1,3-
diaminopropane, n-butylamine, and piperidine with a
slope value of � = 1.02 (r = 0.995). The plot is shown
in Fig. 6. The numbers given in the plot are according
to the order of amines given in Table II. Expectedly, hy-
droxylamine and hydrazine deviate from the plot owing
to the “␣-effect” [13,31] and imidazole because of its
reactivity towards the ester as a tertiary amine [3]. The
deviation of 1,2-diaminopropane from the plot and its
lesser reactivity are discussed in the later part.
slopes from the plots of (kobs � kH)/[N] against [N]F
F
for each pH of the medium. Further plot of these slope
+
values against aH /Ka should give kgb and kga as the
intercept and slope, respectively. For ammonia and hy-
droxylamine the slope values of the plots of Eq. (5) are
almost constant over the pH range employed and hence
taken as kgb. The kN and kgb values thus derived for am-
�
2
� 1
monia and hydroxylamine are 0.41 � 0.01 � 10
s
�
1
� 2 � 1
� 1
M
M
, 1.62 � 0.03 � 10
s
and 2.07 � 0.07 �
�
2
� 1
� 2
� 2 � 1
� 2
10
s
M
, 3.71 � 0.03 � 10
s
M , respec-
tively. For hydrazine the slope values of the plots of
Eq. (5) are not constant and decrease with the pH of
the medium (Fig. 4). Thus, a further plot of these slope
The value of the slope is consistent with a stepwise
mechanism, where a T is found on the reaction path-
+
�
values against aH /Ka is drawn for hydrazine as shown
in Fig. 5, and the derived kN, kgb, and kga values are
way and its breakdown to products is rate-determining.
Larger Br o¨ nsted slope values (�nuc = 0.8–1.0) have
been found in the aminolysis of phenyl acetates [14],
�
2
� 1
� 1
� 2 � 1
� 2
M ,
1.01 � 0.02 � 10
s
M
, 13.27 � 10
s
2
2
-nitrophenyl acetate [15], 4-nitrophenyl acetate [12],
,4-dinitrophenyl acetate [9], and other reactive car-
bonyl compounds [32–38]. In all these studies a step-
wise mechanism has been proposed with the break-
�
down of T to products being the rate-determining
step. Contrarily, smaller Br o¨ nsted slope values (�nuc =
0
.1–0.3) have been found in the aminolyses of these
�
esters when the formation of T is rate determining
[19,39,40].
According to the results and rate law obtained, the
analysis of products and the Br o¨ nsted-type plot found
for the reactions under study, the mechanism of these
reactions can be assumed to adopt Scheme I with k2
step being rate-limiting. Since, for the present reactions
k� 1 � k2, it follows that kN = K1k2 through the rate
Figure 3 Plot of log (kobs � kH) against log [N]F for the
aminolysis of 4-fluorophenyl acetate by hydrazine at pH 6.90.

AMINOLYSIS OF 4-FLUOROPHENYL ACETATE IN AQUEOUS MEDIUM
371
Figure 4 Plots of (kobs � kH)/[N] against [N]F for the aminolysis of 4-fluorophenyl acetate by hydrazine at various pH values.
F
Structure I
expression kN = k1k2/(k� 1 + k2), for the formation of
�
T assumed under steady-state conditions. Where K1
is the equilibrium constant for the first step.
The transition state for the aminolysis of this ester
may exist as shown in Structure I. Analogue transition
Figure 5 Plot of slope values (from Fig. 4) against [N]F for
the aminolysis of 4-fluorophenyl acetate by hydrazine.
state and a similar rate equation with a preequilibrium
step has been proposed in the pyridinolysis of 2,4-
dinitrophenyl 4-nitrobenzoate (�nuc = 0.9) [34] and, in
the aminolysis of 4-acetyl-1-naphthyl acetate (�nuc =
0
.74) and of 6-acetyl-2-naphthyl acetate (�nuc = 0.94)
[41] in aqueous medium.
Figure 6 Br o¨ nsted-type plot (statistically corrected) ob-
tained in the aminolysis of 4-fluorophenyl acetate in water,
�
2
5 C, ionic strength 0.1 M KCl.
Structure II

3
72
RAJARATHNAM, JEYAKUMAR, AND ANANTHAKRISHNA NADAR
7
8
9
. Bruice, T. C.; Willis, R. G. J Am Chem Soc 1965, 87,
31.
. Bruice, T. C.; Donzel, A.; Huffman, R. W.; Butler, A. R.
J Am Chem Soc 1967, 89, 2106.
5
. Castro, E. A.; Freudenberg, M. J Org Chem 1980, 45,
9
06.
0. Castro, E. A.; Borquez, M. T.; Parada, P. M. J Org Chem
986, 51, 5072.
1. Castro, E. A.; Ureta, C. J Org Chem 1990, 55, 1676.
2. Bond, P. M.; Castro, E. A.; Moodie, R. B. J Chem Soc,
Perkin Trans 2 1976, 68.
1
1
Structure III
Structure IV
1
1
Among the diamines, the reactivity of 1,3-
diaminopropane is higher than 1,2-diaminopropane.
Enhanced reactivity was observed in the reactions of
diamines with phenyl acetate [7] and acetylimidazole
13. Edwards, J. O.; Pearson, R. G. J Am Chem Soc 1962,
84, 16.
1
1
1
4. Johnson, S. L. Adv Phys Org Chem 1967, 5, 237.
5. Arcelli, A.; Concilio, C. J Org Chem 1996, 61, 1682.
6. Castro, E. A.; Ibanez, F.; Lagos, S.; Schick, M.; Santos,
J. G. J Org Chem 1992, 57, 2691.
[42] when compared to simple primary amines, which
has been attributed to the existence of intramolecular
general base catalysis. In the present case, the reaction
of 1,3-diaminopropane proceeds in a similar manner
with a transition state as shown as Structure II, similar
to the one already proposed (7).
1
1
7. Bruice, T. C.; Mayahi, M. F. J Am Chem Soc 1960, 82,
3
067.
8. Bruice, T. C.; Benkovic, S. J. J Am Chem Soc 1964, 86,
18.
4
However, no such comparable reactivity is observed
for 1,2-diaminopropane and also it shows a negative
deviation from the plot owing to its decreased reac-
tivity. It has been proposed that for 1,2-disubstituted
ethane, XCH2–CH2X or XCH2–CH2Y (X, Y = polar
substituents), molecules prefer “gauche” conforma-
tion in solution, wherein the structure has the maxi-
mum number of gauche interactions between adjacent
electron pairs or polar bonds, and is termed as “gauche
effect” [43–50]. This effect was ascribed to nuclear
electron attractive forces between the groups or un-
shared pair. The diaminopropane can be considered as
disubstituted ethane and the “gauche” conformations
for 1,2-diaminopropane and 1,3-diaminopropane are
shown as Structures III and IV, respectively, where for
the former “gauche effect” can be expected to oper-
ate between two adjacent amino groups thereby reduc-
ing its nucleophilicity compared to latter for which the
amino groups are apart and also the assistance by one
amine moiety to other is more probable regardless of
the conformation of the C C bond.
19. Satterthwait, A. C.; Jencks, W. P. J Am Chem Soc 1974,
96, 7018.
20. Deady, L. W.; Finlayson, W. L. Aust J Chem 1980, 33,
2
441.
2
2
2
1. Kovach, I. M.; Belz, M.; Larson, M.; Rousy, S.;
Schowen, R. L. J Am Chem Soc 1985, 107, 7360.
2. Baliah, V.; Ganapathy, K. Trans Faraday Soc 1955, 77,
2
771.
3. Franson, M. A. H. (Ed.). Standards Methods for the Ex-
amination of Water and Wastewater, 16th ed.; American
Public Health Association: Baltimore, 1985; Sec. 423.
4. Frost, A. A.; Pearson, R. G. Kinetics and Mechanism,
2nd ed.; Wiley: New York, 1961; Ch. 3.
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