Enhanced reactivity in the ammonolysis of phenyl thiolacetates in aqueous medium

Article abstract of DOI:10.1002/kin.10014

The ammonolysis of several substituted phenyl thiolacetates is kinetically studied in aqueous medium, 18°C, ionic strength 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ_max = 260-410 nm), under excess free ammonia, pseud

Full text of DOI:10.1002/kin.10014

Enhanced Reactivity
in the Ammonolysis
of Phenyl Thiolacetates
in Aqueous Medium
D. RAJARATHNAM,1 J. BABU,2 P. ANANTHAKRISHNA NADAR2
1Department of Chemical and Environmental Engineering, The National University of Singapore, 4 Engineering Drive 4,
Singapore 117576
2Department of Chemistry, Annamalai University, Tamil Nadu 608002, India
Received 13 April 2001; accepted 9 July 2001
ABSTRACT: The ammonolysis of several substituted phenyl thiolacetates is kinetically studied
in aqueous medium, 18 C, ionic strength 0.1 M (KCl). By following the leaving groups spec-
trophotometrically ( max = 260–410 nm), under excess free ammonia, pseudo-first-order rate
coefficients (kobs) are obtained. The plots of (kobs kH) against free ammonia concentration are
linear at constant pH. The macroscopic nucleophilic substitution rate coefficients (kN) are ob-
tained as the slopes of these plots and found to be pH-independent for all the thiolesters. The
¨
Bronsted-type plot (log kN against pKa of leaving groups) and the Hammett plot (log kN against
values of substituents) obtained for the title reactions of thiolesters are linear with slope val-
ues of
= 0.34 and = 0.74 respectively. From the magnitude of these values, experimental
lg
data, the kinetic law, and the analysis of products, it is deduced that the ammonolysis of thi-
olesters proceeds through a simple bimolecular nucleophilic substitution pathway with a zwit-
terionic tetrahedral addition intermediate (T ), whereby its formation is rate-determining (k1
step). Comparison of this reaction of thiolesters with a similar reaction of analogue oxyesters
shows a mechanistic difference. Further, for thiolesters there is a rate enhancement with larger
kN values. The change in mechanism and enhanced reactivity observed by substitution of the
C
oxygen atom by sulphur atom on the phenyl moiety are discussed in detail. 2001 John Wiley
& Sons, Inc. Int J Chem Kinet 34: 18–26, 2002
INTRODUCTION
medium, a general mechanism as shown in Scheme 1
has been proposed, which involves the formation of
a zwitterionic tetrahedral intermediate (T ) whose
pathway for decomposition to products depends on the
presence of proton acceptor agents and on the basic-
ity of the leaving group. In this mechanism the rate-
determining step is first-order in amine concentration
for nucleofuges from more reactive esters and leads
to the formation of T (k1 step). As the leaving abil-
ity decreases there is a change towards a rate-limiting
The nucleophilic substitution reactions of aryl esters by
various amines have been important subjects of numer-
ous kinetic studies. Extensive works have been carried
outfortheaminolysisofarylacetates[1–28]inaqueous
medium. For the aminolysis of aryl esters in aqueous
Correspondence to: D. Rajarathnam; e-mail: chedr@nus.edu.sg.
2001 John Wiley & Sons, Inc.
DOI 10.1002/kin.10014
c

AMMONOLYSIS OF PHENYL THIOLACETATES IN AQUEOUS MEDIUM
19
Scheme 1
step which is second-order in free amine concentration,
EXPERIMENTAL
Materials
wherein a proton abstraction from T occurs leading
to an anionic tetrahedral intermediate T (k3 step) with
subsequent breakdown to products.
All the 3- and 4-substituted phenyl thiolacetates
were prepared from corresponding thiophenols respec-
tively by procedures from the literature [36]. Ammo-
nium chloride (Merck), potassium chloride (Merck),
and potassium hydroxide (BDH) were reagent-grade
and used as such. Water was double-distilled. The
ammonia–ammonium chloride buffers were prepared
shortlybeforeusebytheadditionofcalculatedamounts
of standardized potassium hydroxide solution to aque-
ous solutions of known concentrations of ammonium
chloride. At any fixed pH value, the concentration of
free ammonia was evaluated using Eq. (1).
In spite of the considerable amount of mechanistic
details available for the aminolysis of oxyesters, less is
knownfortheirthiolanalogues. Somereportsofsimilar
reactions on the latter in aqueous medium include the
aminolyses of phenyl and 4-nitrophenyl thiolacetates
[29], and 2,4-dinitrophenyl and 2,4,6-trinitrophenyl
thiolacetates [30], imidazolysis of phenyl thiolacetates
[31], aminolyses of phenyl dithioacetate [32] and 4-
nitrophenyl dithioacetate [33]. There have been some
studies on the aminolyses of thionoesters too [34,35],
in aqueous medium.
In order to gain more information on the nucle-
ophilic reactions of thiolesters and to investigate the
influence of the nature of the leaving thiol anions on the
reaction mechanism, we carry out in this work a sys-
tematic kinetic study on the ammonolysis of several
substituted phenyl thiolacetates in aqueous medium.
The results are interpreted through structure–reactivity
correlations, based on which a probable mechanism is
proposed. Further, it is of interest to compare the results
of thiolesters (1) to that of oxyesters (2) under present
reactionconditions. Hence, theearlierwell-studiedam-
monolysis of phenyl acetates [7] are repeated under
identical reaction conditions and the results obtained
showed a remarkable change in mechanism between
the two series of esters.
[N]T Ka+
[NH3]F =
(1)
Ka + aH
where [N]T = [NH4+] + [NH3]F, Ka = dissociation
constant of NH4+, and aH+ = hydrogen ion activ-
ity. The pKa of ammonium ion (9.68) under iden-
tical conditions to that of the kinetic measurements
was determined from measurement of the pH by half-
neutralization method [10].
Kinetic Measurements
These were carried out in aqueous solution, 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 so-
lution (in dioxan) to 3 ml of thermally equilibrated
(18.0 0.1 C) ammonia–ammonium chloride buffer.

20
RAJARATHNAM, BABU, AND ANANTHAKRISHNA NADAR
The reactions were followed by monitoring the re-
lease of the corresponding thiophenolate ions at regular
intervals of time. The wavelengths of maximum ab-
were found throughout by means of the Guggenheim
method [38] employing least squares technique. The
correlation coefficients of the linear regressions were
always higher than 0.999. The experimental conditions
of ammonolysis and the rate coefficients for phenyl
thiolacetates are given in Table I.
In order to evaluate the results, the alkaline hydro-
lysis of phenyl thiolacetates was also followed spectro-
photometrically using known concentrations of potas-
sium hydroxide at 18.0 0.1 C and ionic strength
0.1 M (KCl). The second-order rate coefficients,
khyd (where khyd = kobs/[OH ]), for the substrate with
various substituents are H = 0.521 0.001; 3-CH3 =
0.441 0.003; 4-CH3 = 0.411 0.003; 3-Br = 1.02
0.01; 4-Br = 0.832 0.003; 3-Cl = 1.01 0.01; 4-Cl
= 0.737 0.004; 4-NO2 = 3.30 0.01.
sorption,
(nm), for the substituted thiophenolate
max
ions are as follows: H, 260; 3-CH3, 263; 4-CH3, 264;
3-Br, 267; 4-Br, 271; 3-Cl, 267; 4-Cl, 271; 4-NO2, 410.
Theinitialesterconcentrationwas(5–6) 10 5 M, and
that of free ammonia concentration was kept at several
times excess over the substrate. The pH values of the
kinetic solutions were determined using an ITL digital
pH meter both prior to and at the completion of reac-
tions. The glass electrode was calibrated using standard
procedures [37].
All the reactions in the present study fitted good
pseudo-first-orderkineticsuptogreaterthan90%ofthe
total reaction. Pseudo-first-order rate coefficients (kobs
)
Table I Summary of Experimental Conditions and Rate Coefficients for the Ammonolysis of Substituted Phenyl
Thiolacetatesa
[NH3]F (10 2M)
kobs kH (10
s
)
k20 (10
s
M
)
kN (10
s
M
)
b
3
1
2
1
1
2
1
1
Substituents
H
pH
9.54
9.77
9.94
10.15
9.28
9.41
9.58
9.73
9.40
9.57
9.72
9.87
9.22
9.40
9.73
10.03
9.28
9.40
9.74
9.97
9.25
9.40
9.61
9.75
9.24
9.40
9.74
9.98
9.22
9.40
9.61
9.83
1.60–8.02
2.15–10.63
2.67–12.75
3.11–15.01
1.09–5.44
1.34–6.86
1.69–8.60
2.02–10.32
1.31–6.57
1.67–8.41
1.99–10.06
2.32–11.69
0.98–4.92
1.29–6.47
2.03–10.11
2.66–13.25
1.09–5.43
1.23–6.57
2.04–10.20
2.52–12.61
1.03–5.17
1.29–6.47
1.76–8.78
2.05–10.31
0.97–4.83
1.22–6.27
2.04–10.20
2.54–12.71
0.74–3.69
0.98–3.91
1.32–5.27
1.67–6.72
2.13–11.17
2.83–14.12
3.47–16.59
4.08–20.42
1.24–6.43
13.6 0.2
13.2 0.1
13.3 0.2
13.3 0.3
11.5 0.4
11.6 0.4
11.6 0.3
11.8 0.3
9.21 0.11
9.47 0.09
9.59 0.12
9.60 0.14
22.5 0.2
23.7 0.2
23.7 0.3
23.2 0.3
18.6 0.3
18.9 0.4
18.6 0.4
18.3 0.3
22.1 0.5
23.4 0.5
21.7 0.6
23.0 0.4
18.0 0.4
18.1 0.3
17.8 0.5
17.6 0.5
52.97 0.38
53.26 0.29
53.10 0.45
52.94 0.41
13.6 0.2
11.8 0.2
9.73 0.09
23.1 0.3
18.7 0.3
22.1 0.3
17.7 0.3
53.4 0.5
3-CH3
4-CH3
3-Br
1.58–8.09
2.00–10.20
2.41–12.29
1.21–6.36
1.57–8.16
1.90–9.73
2.22–11.33
2.18–11.31
3.14–15.06
4.91–23.75
6.19–30.96
2.04–10.17
2.39–12.36
3.75–19.06
4.58–23.50
2.26–11.63
3.15–14.23
3.84–18.71
4.71–23.88
1.73–8.40
2.18–10.88
3.66–18.26
4.48–22.37
3.91–19.53
5.18–21.02
6.98–28.44
8.91–35.77
4-Br
3-Cl
4-Cl
4-NO2
aIn aqueous solution at 18.0 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.
bConcentration of free ammonia.

AMMONOLYSIS OF PHENYL THIOLACETATES IN AQUEOUS MEDIUM
21
The values of k0 and kN were obtained as the intercept
and slope, respectively, of plots of (kobs kH) against
[NH3]F at constant pH, for at least four pH values. The
values of kN were found to be pH-independent over the
pH range employed and the mean kN values are given
in Table I. For all the reactions, the value of k0 was
negligible compared to the ammonolysis term (kN) in
Eq. (4). Thus, the ammonolysis of phenyl thiolacetates
follows a clean second-order kinetics through the sim-
ple nucleophilic attack, and the catalytic pathways have
no significance.
Product Studies
The product studies were carried out according to the
procedure already reported [26], based on the method
of Katz et al. [39], which showed in all the cases for
phenyl thiolacetates at least 90% of the formation of
amide product under the experimental conditions.
RESULTS AND DISCUSSION
The pseudo-first-order rate coefficients (kobs) observed
for the ammonolysis of phenyl thiolacetates are con-
vertedtotheapparentsecond-orderratecoefficients, k20 ,
where k20 = (kobs kH)/[NH3F, where kH is the product
of khyd], the second-order rate coefficient for the al-
kaline hydrolysis of phenyl thiolacetates, and the con-
centration of hydroxide ion present in the system for
ammonolysis, calculated from aOH and the activity
coefficient of hydroxide ion [40]. In the ammonolysis,
k20 values calculated are almost constant within a given
pH and are independent of free ammonia concentra-
tion. The values of k20 are given in Table I for various
pH values. The nature of k20 values, which are invari-
ant to various pH values, rules out the possibility of
general-acid or specific-base catalyzed pathways. Fur-
ther, the plots of log(kobs kH) against log [NH3]F for
all the phenyl thiolacetates are linear with unit slopes
which pass through the origin (plots not shown). Hence
the general-base catalyzed pathway has also no signif-
icance. These indicate that ammonolysis of phenyl thi-
olacetates depends only upon the first power of the free
ammonia concentration, and the assistance by a second
molecule of ammonia or by a specific base leading to
catalyzed pathways is absent.
The Bro¨nsted-type plot, log kN against pKa of thio-
phenols [41] and the Hammett plot, log kN against
values of substituents [42], for phenyl thiolacetates
are linear with slopes
= 0.34 (r = 0.998) and
lg
= 0.74 (r = 0.983) respectively. The plots are shown
in Figs. 1 and 2. The numbers given in the plots are
according to the order of substituents given in Table
I. The linearity in these plots indicates that there is no
change in mechanism and all the thiolacetates follow
the same pathway irrespective of the basicity of leaving
groups.
The value of Bro¨nsted slope is similar to the cor-
responding ones found in the aminolyses of acetate
esters [3] and phenyl acetates [22,43], in the reaction
of piperidine with thiolacetates [30], and in the acyl
transfer between sulfur and oxygen nucleophiles [44]
in aqueous medium. In all these studies low
value,
lg
between 0.2 and 0.35, has been associated with a
stepwise mechanism where the formation of the tetra-
hedral intermediate is the rate-determining step.
Itisalsoinlinewiththoseobtainedintheaminolyses
of 4-nitrophenyl thiolacetate [29], of 2,4-dinitrophenyl
and 2,4,6-trinitrophenyl thiolacetates [30], of phenyl
The reactions subjected to the present investigation
for phenyl thiolacetates can be kinetically described by
Eqs. (2) and (3),
d[ArS ]
= (kobs kH) [Thiolester]
(2)
(3)
dt
(kobs kH) = k0 + kN FN[N]T
where ArS is the liberated thiophenoxide ion, kobs
is the pseudo-first-order rate coefficient, k0 is the co-
efficient of assisted paths comprising general-acid,
general-base, and specific-base assisted attack of
ammonia, kN is the macroscopic nucleophilic sub-
stitution rate coefficient, FN is the free ammonia
fraction, and [N]T is the concentration of ammonia–
ammonium chloride buffer ([NH4+] + [NH3]F). Since,
FN = [NH3]F/[N]T, Eq. (3) becomes
FIG. 1 Bro¨nsted-type plot obtained in the ammonolysis of
substituted phenyl thiolacetates in aqueous medium, 18 C,
ionic strength 0.1 M (KCl).
(kobs kH) = k0 + kN[NH3]F
(4)

22
RAJARATHNAM, BABU, AND ANANTHAKRISHNA NADAR
Structure 1
Similar reactions are repeated for substituted phenyl
acetates, prepared by procedures from the literature
[36], under identical reaction conditions to that of
phenyl thiolacetates. The essential kinetic data are
given in Table II. The results obtained are in agree-
ment with the earlier reported one for the ammonoly-
sis of phenyl acetates [7] in aqueous medium, with
poor leaving groups showing general base catalysis,
where an increase in k20 value is observed by vary-
ing the concentration of free ammonia within a given
pH and is independent over the range of pH values
employed, and good leaving groups showing the sim-
ple nucleophilic attack of ammonia, where k20 values
are constant over the concentration of free ammonia
employed.
Thus, the substituent effects dictate the reaction
pathway for the ammonolysis of oxyesters while for
thiolesters irrespective of the nature of the substituents,
the reaction proceeds through a common mechanism
with simple nucleophilic attack of ammonia. A com-
parative plot of thiolesters and oxyesters for kN values,
obtained through Eq. (4), is nonlinear (Fig. 3). The
deviation of the plot occurs for the parent substrate
and substrates with electron-donating substituents, for
which the mechanism differs for both series, where the
catalysis by a second molecule of ammonia is in effect
for oxyesters. Identical mechanism prevails for other
substituents in both series. However, in all cases, the
reactivity of thiolesters is higher than that of oxyesters
towards ammonia.
FIG. 2 Hammett plot obtained in the ammonolysis of sub-
stituted phenyl thiolacetates in aqueous medium, 18 C, ionic
strength 0.1 M (KCl).
dithioacetates [32], and of 4-nitrophenyl dithioacetate
[33]aswellasintheaminolysisofphenylthionoacetate
[35] in aqueous medium, where smaller Bro¨nsted slope
value ( nuc), between 0.1 and 0.2, has been found for
the rate-determining formation of T .
The slope value ( lg) of the Hammett plot is quite
small, which is again indicative of a stepwise mech-
anism with rate-limiting attack of the nucleophile
[10,19,27,45]. Contrarily, larger lg values are obtained
when rate-limiting expulsion of the leaving group is in-
volved [46–48]. Generally, for most of the reactions of
acyl compounds, a low magnitude in and values
have always been attributed to the rate-limiting attack
of the nucleophiles [2,15,49–57].
According to these arguments, the kinetic results,
and the product studies, the mechanism of the reac-
tions of phenyl thiolacetates under scrutiny can be de-
scribed completely by Scheme 2. Since, for the present
reactions k2
k 1, the kN values obtained correspond
to k1 through the rate expression kN = k1k2/(k 1 + k2),
assumed under steady-state conditions for the forma-
tion of tetrahedral addition intermediate.
The transition state for the ammonolysis of phenyl
thiolacetates may exist as shown in Structure 1. Ana-
logue transition state has been proposed for reac-
tive oxyesters with rate-determining formation of T
[26,27,54].
For the substrates of phenyl acetates, which show
general-base catalysis, Eq. (5) governs the ammono-
lysis:
(kobs kH)/[NH3]F = kN + kgb[NH3]F
(5)
Scheme 2

AMMONOLYSIS OF PHENYL THIOLACETATES IN AQUEOUS MEDIUM
23
Table II Summary of Experimental Conditions and Rate Coefficients for the Ammonolysis of Substituted Phenyl
Acetatesa
k20 (10
s
M
)
kN (10
s
M
)
b
2
3
1
2
1
1
2
1
1
Substituents
H
pH
[NH3]F (10 M)
kobs kH (10
s
)
9.32
9.45
9.56
9.72
9.99
10.10
10.45
10.87
9.99
10.10
10.45
10.91
9.84
18.95–33.41
20.38–36.69
23.72–42.70
24.19–43.19
37.96–64.25
41.34–68.91
49.09–81.46
60.06–74.89
37.96–64.25
41.34–68.91
49.09–81.50
47.81–76.39
7.08–28.35
10.72–31.94
5.63–28.21
7.11–28.22
13.98–27.97
11.20–28.01
13.49–36.85
9.02–32.39
7.08–28.35
5.40–26.80
5.63–28.98
7.11–28.22
9.44–22.02
10.12–23.35
11.20–28.01
8.47–30.82
0.089–2.30
1.33–3.35
1.50–3.62
1.52–3.80
1.75–4.39
1.81–4.56
2.39–5.49
2.66–5.86
3.32–6.89
5.12–7.00
1.78–4.27
1.96–4.58
2.41–5.47
2.61–5.23
1.91–8.65
2.99–9.20
1.63–8.10
2.19–8.49
2.02–4.50
1.54–4.50
1.98–6.15
1.53–6.17
1.56–7.74
1.38–7.39
1.46–8.33
2.08–8.87
1.28–3.68
1.36–3.74
1.49–4.38
1.22–4.90
2.28–5.95
3.29–8.34
0.792–1.08
0.746–1.04
0.738–1.03
0.748–1.06
0.630–0.854
0.643–0.850
0.676–0.846
0.852–0.935
0.469–0.665
0.474–0.665
0.491–0.671
0.546–0.685
2.86 0.03
2.90 0.04
2.97 0.05
3.05 0.04
1.56 0.01
1.49 0.02
1.58 0.03
1.80 0.04
2.61 0.03
2.63 0.03
2.73 0.04
3.02 0.04
1.52 0.02
1.48 0.03
1.44 0.03
1.49 0.03
25.8 0.1
1.43 0.08
1.19 0.05
0.930 0.07
3.28 0.05
1.86 0.02
2.99 0.04
1.79 0.07
25.9 0.2
3-CH3
4-CH3
3-Br
9.99
10.28
10.57
9.57
4-Br
9.86
10.20
10.59
9.84
3-Cl
9.99
10.28
10.57
9.40
9.55
9.86
10.20
8.71
8.86
4-Cl
4-NO2
24.8 0.2
8.97
9.10
1.67–4.06
2.29–5.33
4.20–10.27
6.34–17.22
25.2 0.1
28.5 0.9
aIn aqueous solution at 18.0 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.
bConcentration of free ammonia.
where kgb is the rate constant for general-base as-
sisted attack of ammonia. The values of kN and kgb
are obtained as the intercepts and slopes from the plots
of (kobs kH)/[NH3]F against [NH3]F. The kN and kgb
values, thus derived for phenyl acetate, 3-methy-
lphenyl acetate, and 4-methylphenyl acetate are 3.73
3
1
3
1
1
0.42 10
s
M
1, 3.33 0.37 10
s
M ,
3
1
1
3
2.22 0.29 10
s
M
and 1.74 0.31 10
1
3
1
s
M
s
2, 7.08 0.54 10
s
M
2, 6.23 0.29
3
1
2
10
M
respectively. The kN values obtained so
for these esters are used in the comparative plot from
where a linear plot is obtained (r = 0.971), as shown in
Fig. 4.
FIG. 3 Comparative plot of kN in the ammonolysis of
phenyl thiolacetates and phenyl acetates.
For transmission of substituent effects through a sul-
fur atom a “fall-off” factor of 1.63 can be assumed

24
RAJARATHNAM, BABU, AND ANANTHAKRISHNA NADAR
For the abnormal reactivity of sulfur-containing
compounds, the involvement of its vacant “d” orbital in
the reactions, is often predicted. The small value of
nuc
obtained for the attack of thiol anions on aldehydes has
been attributed to the transfer of electron density from
the carbonyl oxygen to a “d” orbital on the attacking
sulfur atom (4) in the transition state [44].
FIG. 4 Comparative plot of kN in the ammonolysis of
phenyl thiolacetates and phenyl acetates [for phenyl acetates
showing general-base catalysis, kN values obtained through
Eq. (5) are used].
A zero value of has been found for the addi-
tion of thiol anions to methylcobaloxime where the
“d” orbital participation was suggested [69]. The re-
versal order of dissociation constants for the mer-
captobenzoic acids in the series 4-CH3SC6H4COOH
(pKa = 5.80), 4-C2H5SC6H4COOH (pKa = 5.71), and
4-iC3H7SC6H4COOH (pKa = 5.66) has been at-
tributed to the hyperconjugation effect involving the
“d” orbitals of sulfur [70]. Similar effect can be ex-
pected to operate in the present reactions wherein the
sulfur atom accommodates the electron density from
the attacking nucleophile in its vacant “d” orbitals,
which subsequently lowers the basicity of PhS com-
pared to PhO making the former a better nucleofuge
from T than the latter. The canonical structure for this
assumption is shown as below:
[29] by comparing the
values for acid dissocia-
tion of ArCH2COOH ( = 0.49) and ArSCH2COOH
( = 0.30) in water [58]. Jencks observed that the equi-
librium transfer of acyl groups to substituted thiols
or thiol anions is less sensitive to polar substituents
[44]. However, he did not account this to a relative
poor transmission of electron-withdrawing effect of
the acyl group through sulfur compared to oxygen or
nitrogen, since the “fall-off” factors for transmission
of substituent effects through X in the ionization of
ArXCH2COOH in 50% ethanol are 0.51, 0.52, 0.38,
and 0.33–0.39 for S, O, NH, and CH2 respectively
[59,60]. He proposed that the significant contribution
of the resonance form 3c of thiol esters accounted for
the small sensitivity to substituents on sulfur.
The observed enhanced reactivity of thiolesters of
thepresentinvestigationisinagreementwiththeresults
in the acetate series, where the rate of PhS expulsion
is larger compared to PhO expulsion [34,49,61,62].
It is generalized further that the expulsion of thiol an-
ions favor over oxygen anion expulsion with a constant
substituent on the leaving group [63]. This is due to
the fact that the PhO is more electron-donating than
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