Dynamic Impact of RuCl3 Catalyst on the Oxidation of Cyclamate with Acidified Bromamine-T: Elucidation of Mechanistic Pathways and Kinetic Modeling

Article abstract of DOI:10.1007/s10562-017-2034-4

Abstract: Oxidation-Kinetics of cyclamate by bromamine-T (BAT) in HClO₄ medium catalyzed by RuCl₃ exhibits a first-order dependence on [BAT]₀, fractional-order on both [Cyclamate]₀ and [RuCl₃], and inverse fractional-order on [HClO₄]. RuCl₃ catalyst accelerated the reaction by a splendid 48 times. Plausible mechanism and related rate law have been deduced. Graphical Abstract: The Stoichiometry of the reaction is:[Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2034-4

Catal Lett (2017) 147:1392–1402
DOI 10.1007/s10562-017-2034-4
Dynamic Impact of RuCl Catalyst on the Oxidation of Cyclamate
3
with Acidified Bromamine-T: Elucidation of Mechanistic
Pathways and Kinetic Modeling
Ramya Radhakrishnan · Puttaswamy1
1
Received: 11 January 2017 / Accepted: 8 March 2017 / Published online: 7 April 2017
©
Springer Science+Business Media New York 2017
Abstract Oxidation-Kinetics of cyclamate by bro-
fractional-order on [HClO ]. RuCl catalyst accelerated the
4 3
mamine-T (BAT) in HClO medium catalyzed by RuCl
reaction by a splendid 48 times. Plausible mechanism and
related rate law have been deduced.
4
3
exhibits a first-order dependence on [BAT] , fractional-
0
order on both [Cyclamate] and [RuCl ], and inverse
0
3
Graphical Abstract The Stoichiometry of the reaction
is:
O
H+
NHSO H
3
TsNHBrNa
(BAT)
+
H O
+
+
NH SO H
2 3
+
2
(
cyclamate)
(cyclohexanone)
(sulfamic acid)
+
-
TsNH2
Na
+ Br
+
(PTS)
(
here Ts = CH
3
C
6
H
4
SO -)
2
Keyword RuCl -catalysis · Cyclamate · Bromamine-T ·
3
Oxidation-kinetics · Mechanism · Activation parameters
1
Introduction
Cyclamate, chemically known as N-cyclohexylsulfamic
acid, is an artificial sweetener and is used as a non-nutri-
tive sweetener in many dietary and pharmaceutical prod-
ucts [1]. Its potency as a sweetener is around 30 times
greater than that of sucrose. On account of its dietary
and pharmaceutical potentiality, some of the analytical
techniques for the determination of cyclamate includ-
ing UV spectroscopy [2], chromatography [3], auto-
mated turbidimetric technique [4] and flow injection
*
Puttaswamy
pswamy_chem@yahoo.com
1
Department of Chemistry, Bangalore University, Central
College Campus, Bangalore 560 001, India
Vol:.(1234567890)
1
3

Dynamic Impact of RuCl Catalyst on the Oxidation of Cyclamate with Acidified Bromamine-T:…
1393
3
spectrophotometry [5] have been developed. But, to the
best of our knowledge there is no report available in the
literature about oxidation of cyclamate by any oxidants
from its kinetic and mechanistic standpoint. Also, none
have examined the impact of platinum metal ions in the
oxidation of cyclamate. In view of the significance of this
substrate, it is very essential to know its oxidative behav-
iour towards a +1 oxidant kinetically.
kinetically to clarify the mechanistic picture of this redox
system. Consequently, we thought of using platinum group
metal ions as a catalyst to facile the rate of reaction for fur-
ther kinetic analysis.
The catalysis by platinum metal ions in the redox reac-
tions is of great interest due to strong catalytic influence of
these ions in many reactions [16–22]. This catalysis plays
an important role in understanding the mechanism of redox
Aromatic N-halosulfonamides or organic N-haloamines
are mild oxidants containing strongly polarized N-linked
halogen in its +1 oxidation state [6]. Considerable atten-
tion has centred on the chemistry of N-haloamines because
of their diverse properties to act as halonium cations,
hypohalites and N-anions, which behave both as elec-
trophiles and nucleophiles. The chemistry of these com-
pounds has been reviewed by Campbell and Johnson [6],
Banerji et al. [7], Agarwal and Upadhyay [8], Kolaveri
et al. [9], and Jagadeesh and Puttaswamy [10]. The promi-
nent members of this class of compounds are chloramine-T
reactions. Among the platinum group metal ions, RuCl
3
has widely been used as a homogeneous catalyst in vari-
ous redox systems [16–22]. This catalysis involves differ-
ent degrees of complexity, due to the formation of different
intermediate complexes, free radicals and different oxida-
tion states of ruthenium [23, 24]. Literature survey also
indicated that no one has examined the role of RuCl or
3
any other platinum metal ions as catalyst in the oxidation
of cyclamate. Consequently, the authors employed a trace
amount of RuCl catalyst for the oxidation of cyclamate
3
with BAT in acid medium. Surprisingly, it was observed
(
(
p-Me-C H SO NClNa·3H O or CAT) and chloramine-B
that the slow reaction underwent 48-times faster in the pres-
6 4 2 2
C H SO NClNa·1.5H O or CAB). These reagents have
−
5
−3
ence of trace amount of RuCl (Ca. 8.00×10 mol dm ).
6
4
2
2
3
been widely employed as oxidizing agents in the kinetic
and mechanistic studies of oxidation of different functional
groups in acidic and alkaline medium [7–11]. Bromamine-
T (p-Me-C H SO NBrNa·3H O or BAT) is the bromine
This provoked us to choose RuCl as a catalyst in the pre-
3
sent case. In view of the above reasons, we have sought to
study the oxidation of cyclamate with BAT in acid medium
catalyzed by RuCl in order to unfold the mechanistic pic-
6
4
2
2
3
analogue of CAT. Although BAT is found to be a better
oxidizing agent than CAT and CAB, only sporadic infor-
mation exists in the literature on BAT reactions [12–15].
Several preliminary reactions were carried out in our
laboratory to oxidize cyclamate under different experimen-
tal conditions. The reactions studied using CAT as the oxi-
dant in acid and alkaline media were found to be extremely
ture and catalytic impact of RuCl on this redox system
3
through kinetic study.
On this backdrop, herein we report for the first time the
oxidation-kinetics of cyclamate with BAT in acid medium,
catalyzed by RuCl in order to throw some light on the
3
mechanistic and catalytic chemistry of this redox sys-
tem. The objectives of the present investigation are to: (i)
develop conditions for the facile oxidation of cyclamate,
(ii) examine the influence of reaction parameters on the
rate, (iii) elucidate the plausible mechanism, (iv) deduce
appropriate rate law, (v) identify the stoichiometry and oxi-
dation products, (vi) ascertain the various reactive species,
−8
−8 −1
slow with reaction rates of 1.88×10 and 1.243×10 s ,
respectively. So, the choice of CAT as an oxidant was con-
sidered inappropriate and was instead replaced by the ben-
zene analogue of CAT, chloramine-B (CAB). The reaction
rates observed were not greatly satisfactory, the rate con-
−
8
−8 −1
stants being 9.45×10 and 7.36×10
s
in acidic and
(vii) find the catalytic efficiency of RuCl on this redox sys-
3
basic media, respectively. The use of BAT, the bromine
analogue of CAT, as an oxidant stemmed from the earlier
discussions. The reaction rates on employing BAT as an
tem (viii) demonstrate the formation of complexes, and (ix)
evaluate the activation parameters.
−
6
oxidant were faster and were found to be 5.12×10 and
−7
−1
8
.31×10
s
in acid and alkaline media, respectively.
2 Experimental
2.1 Materials
The reaction in alkaline medium was found to be slow in
comparison to the one in acidic medium. All these reac-
tions carried out in CAT, CAB and BAT in acidic and
basic media were under similar experimental conditions.
The reaction rates discussed proves varied efficiencies of
N-haloamines as oxidants in the order: BAT>CAB>CAT.
In the background of the above facts, it was apt to use BAT
The oxidant, BAT, was prepared by the partial debro-
mination of dibromamine-T (DBT) by the standard pro-
cedure [25]. An aqueous solution of BAT was prepared
afresh, standardized by iodometric method and presereved
in brown bottles to prevent its photochemical deteriora-
tion. The substrate, sodium cyclamate was obtained from
Tokyo Chemical Industry Co., Ltd. Tokyo, Japan and was
as an oxidant in HClO medium to oxidize cyclamate.
4
However, in fact that BAT is a better oxidizing agent in
the present study; the reaction is still slow to be studied
1
3

1
394
R. Radhakrishnan, Puttaswamy
used as received. An aqueous solution of desired strength
of sodium cyclamate was prepared freshly as and when
required. Ruthenium trichloride (Merck) solution was pre-
pared in 20 mM HCl solution. Allowance for the amount of
acid present in the catalyst solution was made while prepar-
ing the reaction mixture for kinetic runs. The addition of
methanol in varying proportions (0–30% v/v) was devised
to alter the permittivity of the reaction mixture. All chemi-
cals and reagents used in the study were of analytical rea-
gent grade. Aqueous solutions were prepared using double
distilled water. UV–Vis SL159 Scanning Spectrophotom-
eter was used for studying the formation of intermediate
at regular intervals of time. The course of the reaction was
studied for more than two half-lives. The pseudo-first-order
−
1
rate constants (k′ s ) calculated from the linear plots of log
[BAT] versus time were reproducible within 3–5%. k′ val-
ues reported are the mean values of duplicate kinetic runs.
3 Results
3.1 Stoichiometry
Reaction mixtures constituting varying ratios of cyclamate
2
−3
−3
complexes. Regression coefficients (R ) calculations were
and BAT in presence of 1.00×10 mol dm HClO and
4
−
5
−3
performed on an f -100W scientific calculator.
8.00×10 mol dm RuCl were equilibrated at 313 K
x
3
for 24 h with occasional stirring. The iodometric titration
of unreacted BAT in the reaction mixture showed that 1
mol of BAT was consumed per mole of cyclamate. The
observed stoichiometric results could be represented in the
form of the equation given below:
2
.2 Kinetic Measurements
The kinetic procedure followed to study the progress of
the reaction was according to a literature procedure [26].
All the kinetic measurements were carried out under
pseudo-first-order conditions of [Cyclamate] >>[BAT] .
0
0
The kinetic runs were performed in glass stoppered Pyrex
boiling tubes coated black from the outside in order to
eliminate any possibility of photochemical effects. Reac-
tion mixtures containing requisite amounts of cyclamate,
HClO and RuCl were pre-equilibrated at 313 K by plac-
(1)
(here Ts=CH C H SO −).
4
3
3
6
4
2
ing the tube in a water bath set at the required temperature.
The total volume of the solution was maintained constant
for all the runs by the addition of adequate quantities of
water (50 ml each). To this solution a measured amount of
pre-equilibrated (313 K) BAT solution was added to initiate
the reaction. The reaction mixture was periodically shaken
in order to attain uniform concentration. The progress of
the reaction was monitored by iodometric determination
of unreacted BAT in 5 ml aliquots of the reaction mixture
3.2 Product Analysis
The reaction mixtures as per the stoichiometric ratio
arrived at were again made to react for 24 h at 313 K under
stirred conditions. On completion of the reaction (moni-
tored by TLC), the mixture was neutralized by NaOH and
the reaction products were extracted with ether. An oily
liquid (cyclohexanone) was obtained on drying the organic
layer, whereas the aqueous layer furnished a solid (sul-
famic acid) on drying. Cyclohexanone was characterized
by GC–MS data, which showed a molecular ion peak at 98
amu (Fig. 1). Following which the solid obtained on dry-
ing the aqueous layer was subjected to column chromatog-
raphy on silica gel (60–200 mesh) using gradient elution
from dichloromethane to chloroform. After initial separa-
tion the products were further purified by recrystallization.
The other product of cyclamate oxidation sulfamic acid,
was identified [27] by the reaction of product with nitrous
acid which converted the former to sulfuric acid, this on
reaction with barium chloride formed a white precipitate
of barium sulfate. The reduction product of BAT, p-tolue-
nesulfonamide (PTS) was extracted with ethyl acetate and
detected by paper chromatography [28]. Benzyl alcohol
saturated with water was used as solvent with 0.5% vanillin
in 1% HCl solution in ethanol as spray reagent (R =0.905).
Fig. 1 GC–Mass spectrum of cyclohexanone with its molecular ion
f
peak at 98 amu
PTS was further ascertained by GC-MS analysis with the
1
3

Dynamic Impact of RuCl Catalyst on the Oxidation of Cyclamate with Acidified Bromamine-T:…
1395
3
Fig. 2 GC–Mass spectrum of p-toluenesulfonamide with its molecular ion peak at 171 amu
molecular ion peak at 171 amu (Fig. 2) and also from the
melting point value of 138°C (lit mp 138–139°C). It was
noticed that there was no further oxidation of these prod-
ucts under the prevailing kinetic conditions.
Hence, the present reaction is an important protocol in syn-
thetic chemistry.
3.3 Effect of Varying BAT and Cyclamate
Concentrations on the Rate of Reaction
The oxidation products cyclohexanone and sulfamic
acid, both of which find extensive industrial applications
[
29, 30]. A major part of the world’s supply of cyclohex-
Under
the
0
pseudo-first-order
0
conditions
anone is converted to adipic acid and cyclohexanone
oxime, the former is one of the precursors for Nylon 6,6.
A major reactant in the manufacture of Nylon 6 is capro-
lactum, which is in turn obtained from cyclohexanone. It
is also used as a solvent for cellulose lacquers. The by-
product sulfamic acid is used in the design of many drugs
including anti-cancer, anti-epileptic, weight loss drugs etc
apart from being used as a cleansing reagent in industries.
([Cyclamate] >> [BAT] ), maintaining the concentra-
tions of [Cyclamate] , [RuCl ], [HClO ] and the tem-
0
3
4
perature to be constant, plots of log [BAT] versus time
2
were found to be linear (R > 0.9975) for varying [BAT] ,
0
indicates a first-order dependence of oxidant on the rate
of reaction. Further k′ was not affected by a change in
[BAT] , confirming the first-order dependence of rate on
0
[BAT] . The pseudo-first-order rate constants obtained
0
1
3

1
396
R. Radhakrishnan, Puttaswamy
Table 1 Effect of varying oxidant, substrate, acid and catalyst con-
Table 2 Effect of varying die-
lectric constant of the medium
on the rate of reaction at 313 K
104k′ (s−1)
%
MeOH
D
centrations on the rate of reaction at 313 K
(
v/v)
4
3
10 [HClO ] 10 [RuCl ] 10 k′ (s−1)
3
5
4
1
0 [BAT] 10
0
4
)
3
−
3
−3
−3
0
1
2
3
76.73
72.37
67.48
62.71
2.49
2.24
2.06
1.77
(
mol dm
)
[cyclamate]
(mol dm
(mol dm )
0
mol dm−
3
)
0
0
0
(
0.5
0.75
1.0
2.5
5.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
5.0
5.0
5.0
5.0
5.0
1.2
2.0
5.0
7.0
10.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.75
1.0
2.0
4.0
1.0
1.0
1.0
1.0
1.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
2.5
5.0
8.0
11.0
13.0
2.39
2.36
2.49
2.30
2.33
1.47
2.02
2.49
4.72
5.20
3.26
2.46
2.49
1.77
0.97
1.61
1.86
2.49
2.92
2.95
−
4
−3
[
BAT] =1.0×10
mol dm
;
0
−
3
[
cyclamate] =5. 0 × 10
0
−3
mol
dm
;
[HClO ]=1.0
4
−3
−3
×
10
mol dm ; [RuCl ]=
3
−5 −3
8.0×10 mol dm
indicating a fractional-order dependence on the rate of
the reaction.
3
.5 Effect of Varying Halide Ions Concentrations
on the Rate of Reaction
−
−
The addition of Cl or Br ions in the form of NaCl or
−
3
−3
NaBr solution (4.0 × 10 mol dm ) proved ineffective
in bringing about any significant change on the rate of the
−
−
reaction. This enabled to conclude that the Cl or Br ions
do not play any role in the reaction.
3
.6 Effect of Varying PTS Concentration on the Rate
of Reaction
are listed in Table 1. Under the prevailing experimen-
tal conditions, an increase in [Cyclamate] was found
0
p-Toluenesulfonamide (PTS), the reduction product of BAT,
to increase the rate of the reaction (Table 1). A plot of
−3
−3
was added to the reaction mixture (6.0×10 mol dm )
which left the rate of reaction significantly unaltered. This
was evidence to show that PTS is not involved in any step
prior to the rate-determining step (rds) in the proposed
scheme.
log k′ versus log [Cyclamate] was found to be linear
0
2
(
R = 0.9956) with a slope of 0.28, which stems from
the fact that there is a fractional-order dependence on
[
Cyclamate] . Further, a plot of k′ versus [Cyclamate]
0
0
2
was linear (R = 0.9956) with a y-intercept which
confirmed a fractional-order dependence of rate on
3
.7 Effect of Varying Ionic Strength of the Medium
on the Rate of Reaction
[
Cyclamate] .
0
Keeping all the other experimental conditions constant,
the effect of ionic strength on the rate of reaction was stud-
3
.4 Effect of Varying HClO and RuCl Concentrations
4 3
on the Rate of Reaction
−
3
ied by adding 0.30 mol dm NaClO solution in to the
4
reaction mixture. The rate of reaction was not altered sig-
nificantly. Hence, no efforts were made to keep the ionic
strength of the medium constant for kinetic runs.
The influence of [HClO ] on the reaction rate was deter-
4
mined by varying [HClO ] under the prevailing con-
4
ditions of the reaction (Table 1). There was a retarding
influence on the rate of the reaction as the [HClO ] was
4
increased. The log–log plot of k′ versus [HClO ] was lin-
3.8 Effect of Varying Dielectric Constant
of the Medium on the Rate of Reaction
4
2
ear (R = 0.9769) and possessed an order of −0.57, indi-
cating a negative-fractional-order dependence of rate on
[
HClO ]. An increase in [RuCl ] was found to enhance
Kinetic studies were carried out in H2O–MeOH mixtures
of varying compositions (0–30% v/v), thereby changing
the dielectric constant of the solvent medium. The rate
4
3
the rate of reaction (Table 1). A plot of log k′ versus log
2
[
RuCl ] was linear (R = 0.9821) with a slope of 0.30,
3
1
3

Dynamic Impact of RuCl Catalyst on the Oxidation of Cyclamate with Acidified Bromamine-T:…
1397
3
was found to decrease with increase in MeOH content and
4
4
4
.40
.38
.36
2
a plot of log k′ versus 1/D was linear (R =0.9953) with
a negative slope. The results are reported in Table 2 and
graphically represented in Fig. 3. The values of dielectric
constant of H O–MeOH mixture as reported in the lit-
4.34
2
erature [31] were employed. Blank experiments were per-
formed to ensure that there was no oxidation of MeOH by
BAT, under the prevailing experimental conditions.
4.32
4.30
4.28
4.26
4.24
3
.9 Effect of Varying Temperature on the Rate
of Reaction
0
.0130 0.0135 0.0140 0.0145 0.0150 0.0155 0.0160
/D
1
The reaction was studied at five different temperatures (303,
3
08, 313, 318 and 323 K) in the presence and absence of
Fig. 3 plot of log k′ versus 1/D
RuCl catalyst, maintaining other experimental conditions
3
constant. From the linear Arrhenius plot of log k′ versus 1/T
2
(
Fig. 4; R =0.9856), the values of activation parameters (E ,
a
#
#
#
ΔH , ΔG , ΔS ) were evaluated. All these results are tabu-
4.7
4.6
4.5
lated in Table 3.
3
.10 Polymerization Study
The addition of reaction mixtures to aqueous acrylamide
monomer solutions, kept in dark, was unable to initiate
polymerization. It proves the absence of any free radical spe-
cies generated during reaction. The control experiments were
carried out under similar reaction conditions without the
oxidant.
4.4
4.3
4
.2
.1
4
4
Discussion
3.10
3.15
3.20
3.25
3.30
3
1
0 /T (1/K)
4
.1 Reactive Species
Fig. 4 Plot of log k′ versus 1/T
Irrespective of whether the medium is acidic or basic,
N-haloamines have been proved to be mild oxidants [7].
N-haloamines are compounds containing halonium cati-
ons; the presence of only one halogen brings about an
efficient two-electron transfer in their reactions. Bro-
mamine-T, like its chlorine analogue chloramine-T,
behaves as a strong electrolyte in aqueous solutions and
depending on its pH, it furnishes different types of reac-
tive species [32–34].
these contradict the experimental observations. Whereas,
if HOBr were the primary oxidizing species, the addition
of TsNH to the reaction medium should have caused a
2
decrease in the rate of reaction; but the experimental evi-
dence obtained stands against this expectation by leaving
the reaction rate unaffected. Studies pertaining to pH-
dependent relative concentrations of acidified haloamine
solutions bearing comparable concentrations have shown
the possibility of free acid being the most likely oxidizing
The possible oxidizing species in acid medium are the
+
free acid (TsNHBr), DBT (TsNBr ), HOBr and H O Br.
2
2
Hence, in acidified solution of BAT there are four possi-
ble species which can oxidize the substrate. From these
four possibilities, the active reactive species of BAT can
species [34]. Furthermore, the formation of the diproto-
+
nated species (TsN H Cl) with a protonation constant of
2
−2
−1
1
.02×10
M
at pH 2 has been reported for CAT in acid
be decided based on observed kinetic results. If TsNBr
2
solutions [35, 36].
was involved in the reaction mechanism, it would have led
+
TsNHCl + H ⇌ TsN H Cl
+
to a second-order dependence of rate on [BAT] and also
2
(2)
0
an inverse effect of TsNH (PTS) on the rate. But both of
2
1
3

1
398
R. Radhakrishnan, Puttaswamy
Table 3 Effect of varying temperature on the rate of reaction and
4.2 Reaction Scheme
values of activation parameters for the oxidation of cyclamate by
BAT in acid medium in the presence and absence of RuCl catalyst
3
In the light of the above facts, Scheme 1 is put forward
6
−1
4
−1
3
Temperature (K)
10 k′ (s
)
10 k′ (s
)
10 Kc
for the RuCl catalyzed oxidation of cyclamate by BAT
3
Uncatalyzed
RuCl catalyzed
3
in acid medium to substantiate the experimental observa-
tions. A detailed mechanistic interpretation of RuCl cata-
3
3
3
3
3
03
08
13
18
23
2.47
3.37
5.11
6.21
9.59
53.9
51.3± 0.01
108± 0.31
−183± 0.12
1.36
1.54
2.49
3.84
4.99
35.1
32.5± 0.01
98.4± 0.24
−210± 0.36
2.26
2.54
4.14
6.39
8.29
54.7
52.1± 0.01
91.1± 0.20
−124± 0.25
3
lyzed BAT-cyclamate redox reaction in HClO medium is
4
illustrated in Scheme 1. According to Scheme 1, step (i)
+
involves a fast deprotonation of TsN H Br leading to the
2
formation of the active oxidant species TsNHBr in the acid
E (kJ mol−1)
retarding step. Following this, the active catalyst RuCl
3
a
#
−1
forms a partial bond with the nitrogen of TsNHBr, which
yields Complex-I (step (ii)) along with elimination of a
molecule of water. An attack of the substrate cyclamate, on
Complex-I occurs by the breakage of an N–Sπ bond of the
former, leading to the formation of Complex-II (step (iii)).
Following this fast equilibrium step, Complex-II, through
a slow and rds (step (iv)) undergoes deprotonation result-
ing in the formation of Complex-III with the simultane-
ΔH (kJ mol
)
ΔG (kJ mol−1)
#
ΔS (JK mol−1)
#
−1
−
−
4
−3
−3
−3
[
[
BAT] =1.0×10 mol dm ; [cyclamate] =5.0×10 mol dm−3;
0
4
0
3
−5
−3
HClO ]=1.0×10 mol dm ; [RuCl ]=8.0×10 mol dm
3
Values at the last row are the catalytic constants (K ) at different tem-
c
peratures and activation parameters with respect to RuCl catalyst
3
2
−
ous elimination of PTS and [RuCl H O] . A nucleophilic
5
2
Since organic N-haloamines express similar chemical
behaviours, the above equilibrium can be applied to BAT
as well. If the diprotonated species is taken as the reactive
attack on the electrophilic nitrogen centre of Complex-III
by a molecule of water may yield the ultimate products
cyclohexanone and sulfamic acid as shown in the Scheme 1
+
species of BAT it will lead to a positive effect of [H ] on
(step (v)).
the rate of oxidation, which is contrary to the observed neg-
+
ative effect of [H ] on the rate. Hence, in the present case a
4
.3 Spectroscopic Evidence for Complex Formation
+
fractional negative-order dependence on [H ] suggests the
backward reaction as represented by Eq. (2) to be feasible.
This implies that in the present case TsNHBr is the reactive
oxidizing species involved in the oxidation of cyclamate.
Electronic spectral studies carried out by Cady and Con-
nick [37], and Connick and Fine [38] reveal that species
The evidence required to prove the formation of Complex-I
between oxidant and catalyst species (step (ii) of Scheme 1)
were obtained from the UV–Vis spectra of BAT, RuCl
3
and (BAT+RuCl ) mixture. Absorbance maxima in acid
3
medium were observed at 227.1 and 361.8 nm respectively
2
−
−
such as [RuCl (H O)] , [RuCl (H O) ] , [RuCl (H O) ],
5
2
4
2
2
3
2
3
for BAT and RuCl whereas in (BAT+RuCl ) mixture it
3
,
3
+
2−
[
RuCl (H O) ] and [RuCl(H O) ] are absent in an aque-
2
2
4
2
5
was observed at 342.8 nm which is a hypsochromic shift of
19 nm as expected from the structure of Complex-I. These
are illustrated in the Fig. 5. Further evidence for the for-
mation of Complex-II has been evinced from the UV–Vis
ous solution of RuCl . A detailed study [39, 40] on the pos-
3
sible oxidation states of RuCl in aqueous acidic medium
3
has revealed the following equations:
absorbance maxima at 331.6 nm of (BAT+RuCl +Cycla-
ꢀ
ꢁ
3
3
−
+
RuCl ⋅ xH O + 3HCl ⇌ RuCl
3
+ xH O + H
2
(3)
(4)
mate) mixture where the UV inactive cyclamate brings
2
6
about a hypsochromic shift of 11.2 nm in the absorbance
ꢀ
ꢁ
ꢀ
ꢂ
ꢃꢁ
3−
2−
−
+ Cl
maxima of (RuCl +BAT) as in Fig. 5. The wavelengths
3
RuCl6
+ H O ⇌ RuCl H O
2
5
2
obtained from the previous data were now found to further
−
In the present study the addition of Cl ions fails to
shift lower, thus proving this complex to be unstable.
bring about any significant change on the rate of reaction,
indicating that the equilibrium represented by Eq. (4) does
not contribute greatly in the reaction. Hence, the complex
4
.4 Deduction of Rate Law
2
−
ion [RuCl (H O)] is assumed to be the reactive catalytic
5
2
The rate law for Scheme 1 can be deduced as follows:
The total effective concentration of BAT is:
species that interacts with BAT to form an intermediate
complex.
ꢀ
ꢁ
+
ꢂ
ꢃ
ꢂ
ꢃ
[
BAT] = Ts N H Br + [TsNHBr] + Complex - I + Complex - II
t
2
(
5)
1
3

Dynamic Impact of RuCl Catalyst on the Oxidation of Cyclamate with Acidified Bromamine-T:…
1399
3
K1
+
(
(
i) TsNH Br
TsNHBr + H+
( Fast )
2
-
2
Br
-2
K2
TsNH
+
[RuCl H O]
ii) TsNHBr
5
2
+
H O
2
(
Fast )
RuCl5
Complex-I
O
-2
-2
Br
O-
S
+
N H
S
OH
+
K3
( Fast )
(
iii)
TsNH
RuCl5
N H
+
OH
Br
O
O
TsNH
RuCl5
Complex-I
cyclamate
O
-2
H
+
Complex-II
NH
Br
S
OH
O
(
iv)
O
k4
+
-2
TsNH
RuCl5
NH
S
OH + [RuCl H O]
(slow and rds )
5
2
+ TsNH2
O
Complex-III
Complex-II
O
O
S
.
.
NH
O H
OH
NH
+
S
OH + H O
2
(v)
O
O
Complex-III
O
+
NH SO H
2
3
cyclohexanone
sulfamic acid
Scheme 1 A detailed mechanistic description for RuCl catalyzed oxidation of cyclamate by BAT in acid medium
3
From steps (i), (ii) and (iii) of Scheme 1, the values of
ꢀ
ꢀ
ꢁ
+
Complex - II
ꢁꢀ
[
TsN H Br], [TsNHBr] and [Complex-I] were found to be:
2
[
TsNHBr] =
ꢁ
(7)
ꢀ
ꢁ
K K Cyclamate RuCl
2
3
3
+
ꢂ
ꢃ
ꢀ
ꢁ
Complex - II H
+
ꢀ
K Cyclamate
ꢁ
(6)
Ts N H Br =
2
ꢂ
ꢃꢂ
ꢃ
ꢀ
ꢁ
Complex - II
ꢀ
Complex - I =
ꢁ
(8)
K K K Cyclamate RuCl
1
2
3
3
3
1
3

1
400
R. Radhakrishnan, Puttaswamy
From the value of intercept, the decomposition constant
−
4 −1
2
.5
A
(k ) could be derived as 8.33 × 10 s .
4
D
Scheme 1 and rate law (11) can explain the following
2.0
observed experimental results:
A
B
C
D
RuCl
3
RuCl +BAT+cyclamate
1.5
3
RuCl +BAT
3
BAT
4.5 Effect of Dielectric Constant
C
1.0
B
Since, most of the organic reactions require a medium
to make the reaction feasible, it is apt to use solvents of
varying nature; the effects of these on the rate should
thus be assessed. This can be achieved by changing the
solvent composition by varying the methanol content in
the reaction mixture [41–44]. For limiting case of zero
angle approach between two dipoles or an ion dipole sys-
tem, Amis [44] has shown that a plot of log k′ versus 1/D
gives a straight line, with a positive slope for a reaction
involving a positive ion and a dipole, and a negative slope
for a negative ion–dipole or dipole–dipole interactions.
In the present investigations, a plot of log k′ versus 1/D
was linear with a negative slope. The observation can be
thus directed towards a negative ion–dipole interaction
as observed in the rate-determining step (Scheme 1. step
ꢁ
0
.5
0.0
200
250
300
350
400
450
Wavelength (nm)
Fig. 5 UV–Vis spectra of a RuCl , b RuCl +BAT+Cyclamate, c
3
3
RuCl +BAT, d BAT
3
+
By substituting for [TsN H Br], [TsNHBr] and [Complex-
2
I] from Eqs. (6), (7) and (8) in Eq. (5), the value of [Com-
plex-II] was obtained as:
ꢀ
ꢁꢀ
ꢀ
ꢁ
K K K [BAT] RuCl Cyclamate
1
2
3
t
3
Complex - II =
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁꢀ
ꢁ
(9)
+
K H + K + K K RuCl + K K K RuCl Cyclamate
2
3
1
1
2
3
1
3
3
From the slow and rds (step (iv)) of Scheme 1,
(iv)), wherein anionic Complex-II reacts with a molecule
of water.
ꢀ
ꢁ
Rate = k Complex - II
4
(10)
On substitution of [Complex-II] from Eq. (9) in
4
.6 Effect of Ionic Strength
Eq. (10), the rate law is given as,
The ionic strength of the solution as well as the charges borne
by the reactive species has a pronounced effect on the rate of
ionic reactions. The study of the influence of ionic strength
on the rate of reaction gives valuable information about the
type of the species involved in the rate-determining step, i.e.
whether they are ionic or non-ionic in nature [45]. The effect
of primary salt on the reaction is explained by Bronsted and
Bjerrum equation:
ꢀ
+ K + K K RuCl + K K K RuCl Cyclamate
ꢁꢀ
ꢁ
K K K k [BAT] RuCl Cyclamate
3
1
2
3
4
t
Rate = ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁꢀ
ꢁ
+
H
1
1
2
3
1
2
3
3
(11)
Rate law (11) predicts a first-order dependence of reac-
tion rate on [BAT] , fractional order on both [Cyclamate]
0
0
and [RuCl ] and an inverse-fractional-order with respect
3
+
to [H ], in agreement with our experimental results.
Since rate = k′[BAT] , then
t
√
�
log k = log k + 1.018Z Z
I
(14)
K K K k [RuCl ][Cyclamate]
1
0
A
B
�
k =
2
3
4
3
+
[H
] + K + K K [RuCl ] + K K K [RuCl ][Cyclamate]
1
Here I is the ionic strength of the reaction medium: k′ and
1
2
3
1
2
3
3
k , the specific rate constants in the bulk of the reaction and
(12)
0
infinite dilution; and Z and Z , the ionic charges of the reac-
Equation (12) can be transformed as,
A
B
ꢁ
ꢂ
+
[H ]
ꢀ
1
1
1
1
�
k =
1
+
Cyclamate] K K K k [RuCl ] K K k [RuCl ] K3k4
+
+
(13)
[
k4
1
2
3
4
3
2
3
4
3
Based on Eq. (13), a plot of 1/k′ versus 1/[Cyclamate]
tants A and B. The variation in k´ with ionic strength should
depend on the sign of Z and Z . From the above equation,
2
should be linear as it is actually found (R = 0.9945).
A
B
1
3

Dynamic Impact of RuCl Catalyst on the Oxidation of Cyclamate with Acidified Bromamine-T:…
1401
3
a plot of log k′ versus square root of I would be linear yield-
the catalyst providing an alternate reaction path by afford-
ing a lower activation energy barrier (Table 3) for the reac-
tants to cross over it and enabling the reaction to be rela-
tively facile.
ing a slope equal to 1.018Z Z and an intercept log k . This
A
B
0
equation predicts that (i) if both ions have same charges, then
Z Z is positive and so k′ increases with I; (ii) if Z and Z
A
B
A
B
have opposite signs, then Z Z is negative and so k′ decreases
A
B
with I; and (iii) if either Z or Z bears no charge on it, then
4.9 Activation Parameters
A
B
Z Z is zero, so k′ is independent of the ionic strength of
A
B
the solution. In the present investigations, variation in ionic
The proposed mechanism is also supported by the mod-
erate values of energy of activation and other activation
parameters. The transition state can be predicted to be in a
−3
strength of medium by adding 0.30 mol dm NaClO solu-
4
tion left the reaction rate unaltered. This can be explained by
the species involved in the rate determining step, wherein
Complex (II) is negatively charged and the water molecules
in the medium are neutral (step (iv) of Scheme 1). Therefore,
the observed ionic strength effect is in accordance with the
Bronsted-Bjerrum concept [45] and proposed scheme.
#
#
highly solvated state from the positive ΔH and ΔG values
obtained. The large negative entropy of activation may be
interpreted as due to a fraction of collisions which become
more stringent and form a rigid associative complex, result-
ing in a slow decomposition of the Complex (II). The rate
of the reaction remains to be irresponsive to the addition
of p-toluenesulfonamide which is in turn justified by the
species appearing only in the rate-determining step and not
prior to it. The addition of bromide ions also proved to be
ineffective on the rate of the reaction, which is in agree-
ment with the fact that bromide ions are not involved in
the reaction mechanism. Thus, the proposed mechanism as
well as the derived rate law is in complete agreement with
all experimental findings.
4
.7 Catalytic Activity
Moelwyn-Hughes [46] pointed out that the RuCl catalyzed
3
and uncatalyzed reactions proceed in a parallel way abiding
by the equation:
ꢀ
ꢁ
x
k = k + K catalyst
1
(15)
0
c
Here k is the observed pseudo-first-order rate constant
1
obtained in the presence of RuCl catalyst and k is that for
3
0
the uncatalyzed reaction (in the absence of RuCl catalyst)
3
5
Conclusion
K is the catalytic constant and x is the order of the reaction
c
with respect to RuCl , which is found to be 0.30 in the pre-
3
The kinetics of oxidation of cyclamate with BAT in acid
sent investigations. The values of the catalytic constants K
c
medium catalyzed by RuCl obeys the experimental rate
3
at five different temperatures 303, 308, 313, 318 and 323 K
x
+ −y
z
law: −d[BAT]/dt=k [BAT] [cyclamate] [H ] [RuCl ]
0
0
3
were calculated using the relationship:
here x, y and z are found to be 0.28, −0.57 and 0.30 respec-
tively. Cyclohexanone and sulfamic acid were identified
as the oxidation products of cyclamate. Overall activation
ꢀ
ꢁꢂ
x
K = k −k [Ru(III)]
c
(16)
1
0
The values of K were found to alter with change in tem-
c
2
parameters and also with respect to RuCl catalyst have
perature. A linear plot of log K versus 1/T (R =0.9851)
3
c
been deduced. The RuCl catalyzed reaction was found to
was employed to evaluate the activation parameters with
3
be 48-fold faster than the uncatalyzed reaction. This justi-
respect to RuCl . The results of which are tabulated in
3
fies the use of RuCl catalyst for cyclamate-BAT redox
Table 3.
3
2
−
system in acid medium. TsNHBr and [RuCl (H O)] are
5
2
found to be the reactive species of oxidant and catalyst,
respectively. Spectroscopic evidence for the formation of
complexes between oxidant, catalyst and substrate has been
provided. The best fitted mechanism and rate law based on
the kinetic results have been arrived at.
4
.8 Comparison of RuCl Catalyzed and Uncatalyzed
3
Reactions
According to rate constants reported in Table 3, it can be
seen that oxidation of RuCl catalyzed reaction is 48-fold
3
faster than the uncatalyzed reaction. This justifies the need
Acknowledgements We gratefully acknowledge Professor M. A.
Pasha, Department of Chemistry, Bangalore University, Bangalore for
his helpful discussions about the Scheme 1.
of RuCl as a catalyst in the oxidation of cyclamate using
3
BAT in acid medium, which otherwise as observed the
reaction was found to be extremely sluggish to be studied
kinetically. Complex-I is formed between the catalyst and
oxidizing species, which increases the efficiency of oxi-
dation in comparison to that in the absence of the former.
This complex aids the attack on the substrate resulting in
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