Ru(III) chloride-catalysed oxidation of some α-amino acids by sodium-N-chloro-p-toluenesulfonamide(CAT) in hydrochloric acid medium: Mechanistic investigation and kinetic modeling

Article abstract of DOI:10.1016/j.molcata.2013.12.008

The kinetics of the ruthenium(III) chloride [Ru(III)]-catalysed oxidation of four α-amino acids (AA) viz. glycine (Gly), valine (Val), alanine (Ala) and leucine (Leu) by sodium N-chloro-p-toluenesulfonamide (CAT: Chloramine-T) in the presence of hydrochloric acid medium has been studied at 308 K. The oxidation reaction follows the identical kinetics for all the four amino acids and obeys the rate law, rate=^k′ [_CAT]o [S]ox [ ^H+^]y [Ru(III)], where x and y are fractions. Variation of ionic strength of the medium, addition of the halide ions and the reaction product (PTS: p-toluenesulfonamide) show negligible effect on rate of the reaction. Solvent composition shows positive effect indicating that positive ion-dipolar molecule involved in the rate determining step. The oxidation products were identified. The activation parameters have been evaluated from the Arrhenius plots. The activation energy values are least for the fastest reactions and vice versa. The large negative value of ΔS ^? and the high value of ΔG^? support the proposed mechanism. Under comparable experimental conditions, the rate of oxidation of amino acids increases in the order: Leu > Ala > Val > Gly. This trend may be attributed to hydrophobicity, nucleophilicity and inductive effect of amino acids. The reaction mechanism and the derived rate law are consistent with the observed experimental results.

Full text of DOI:10.1016/j.molcata.2013.12.008

Journal of Molecular Catalysis A: Chemical 383–384 (2014) 203–208
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ru(III) chloride-catalysed oxidation of some ␣-amino acids by
sodium-N-chloro-p-toluenesulfonamide(CAT) in hydrochloric acid
medium: Mechanistic investigation and kinetic modeling
P.A. Prashantha, B.K. Kempe gowdab, S. Anandac, K.S. Rangappac, M.N. Kumarad,∗
a Department of Chemistry, Sai Vidya Institute of Technology, Rajanakunte, Doddaaballapura Road, Bangalore 5600 64, India
b Department of Chemistry, Maharani’s Science College for Women, Mysore 5700 05, India
c Department of Studies in Chemistry, Manasagangotri, University of Mysore, Mysore 5700 06, India
d Department of Chemistry, Yuvaraja’s College, University of Mysore, Mysore 5700 05, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 August 2012
Received in revised form 4 December 2013
Accepted 6 December 2013
Available online 19 December 2013
The kinetics of the ruthenium(III) chloride [Ru(III)]-catalysed oxidation of four ␣-amino acids (AA) viz.
glycine (Gly), valine (Val), alanine (Ala) and leucine (Leu) by sodium N-chloro-p-toluenesulfonamide
(CAT: Chloramine-T) in the presence of hydrochloric acid medium has been studied at 308 K. The
oxidation reaction follows the identical kinetics for all the four amino acids and obeys the rate law,
rate = kꢀ [CAT]o [S]xo [H+]y [Ru(III)], where x and y are fractions. Variation of ionic strength of the medium,
addition of the halide ions and the reaction product (PTS: p-toluenesulfonamide) show negligible effect
on rate of the reaction. Solvent composition shows positive effect indicating that positive ion-dipolar
molecule involved in the rate determining step. The oxidation products were identified. The activation
parameters have been evaluated from the Arrhenius plots. The activation energy values are least for the
fastest reactions and vice versa. The large negative value of ꢀS‡ and the high value of ꢀG‡ support the
proposed mechanism. Under comparable experimental conditions, the rate of oxidation of amino acids
increases in the order: Leu > Ala > Val > Gly. This trend may be attributed to hydrophobicity, nucleophilic-
ity and inductive effect of amino acids. The reaction mechanism and the derived rate law are consistent
with the observed experimental results.
Keywords:
␣-Amino acids
Chloramine-T
Ru(III) chloride
Oxidation
Reaction mechanism
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
CO2, NH3, and aldehydes. Kinetics of oxidation of amino acids by
Chloramines-T (CAT) in both acid and alkaline media has been stud-
Amino acids are simple organic compounds. Their physical and
chemical properties are due to the presence of both acidic and basic
groups present in the same molecule. These simple molecules act
not only as the building blocks of protein but also play a significant
role in metabolism. In metabolism, amino acids are subjected to
many reactions and can supply precursors for many endogenous
substances, e.g. haemoglobin in blood. These small molecules can
undergo many reaction, depending on particular amino acid con-
tains non-polar group or polar group. Amino acids are active site
residues of enzymes, and help also in maintaining the correct con-
formation of enzymes by keeping them in their proper ionic states.
Thus, their oxidation may help in understanding enzyme kinetics.
The oxidation of these molecules is interested because of different
products are obtained with different oxidants [1–6]. The kinetics
of oxidation of several amino acids by a number of oxidants has
been reported [7–11]. The products of oxidations were generally
ied [12–14]. Catalysed oxidation of amino acids by Chloramine-T
in aqueous acid medium as well as in alkali medium have been
reported in the literature [15–19]. These reports are not sufficient to
understand Ru(III) catalysed reaction in acidic medium. Therefore,
we reported in this paper detailed reaction mechanism of Ru(III)
chloride catalysed oxidation of ␣-Amino acids by Chloramine-T in
aqueous HCl medium.
Sodium N-chloro-p-toluenesulfonamide (CAT: Chloramine-T)
is the most important member of organic haloamine family and
behaves as a mild oxidizing agent in both acidic and basic media.
Mechanistic aspects of many of its reaction have been well docu-
mented [20,21]. However, literature survey revealed that, there are
less efforts made on the oxidation of chosen amino acids by CAT.
Ruthenium(III) chloride [Ru(III)] is the platinium group metal
ion and has been widely used as a homogeneous catalyst in various
redox reactions [22] The mechanism of catalysis is quite compli-
cated due to the formation of different intermediate complexes,
free radicals and different oxidizing states of Ru(III). Although many
complexes of Ru(III) with various organic and inorganic substances
have been reported [22–27]. A literature survey shows a very few
∗
Corresponding author. Tel.: +91 821 2419292; fax: +91 821 2419239.
E-mail address: kumaramanikya@yahoo.com (M.N. Kumara).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.12.008

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P.A. Prashanth et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 203–208
Table 1
kinetic investigations on the oxidation reactions of amino acids
involving Ru(III) as a homogeneous catalyst. Therefore, in order to
explore the mechanism of amino acid-CAT reactions in hydrochlo-
ric acid medium and also study the catalytic action of Ru(III) in
this reaction, we selected Ru(III) as a catalyst in the present work.
Based on the above facts, in the present communication, we report
the results of the detailed catalysed oxidation on the kinetics and
mechanistic investigation of Ru(III)-catalysed oxidation of amino
acids by CAT in HCl medium at 308 K. Objectives of the present
study are to: (i) elucidate a plausible mechanism, (ii) deduce an
appropriate rate law, (iii) identify the reaction products, (iv) assess
the relative rates of oxidation of amino acids towards Chloramine-T
and (v) elucidate activation parameters.
Effect of varying reactant concentration on the reaction rate with
[H+] = 1.0 × 10−1 mol dm−3; [Ru(III)] = 3 × 10−4 mol dm−3; T = 308 K.
104[CAT]o
(mol dm−3
103[S]o
kꢀ × 104 (s−1
)
)
(mol dm−3
)
Glycine
Valine
Leucine
Alanine
2.0
3.5
5.0
7.5
10.0
5.0
5.0
5.0
5.0
5.0
5.0
10.0
10.0
10.0
10.0
10.0
2.50
5.00
7.50
10.00
12.50
15.00
6.63
6.58
6.60
6.65
6.68
3.43
4.76
5.72
6.60
7.00
7.84
16.21
16.28
16.23
16.25
16.20
12.58
14.52
15.51
16.23
17.43
19.21
23.15
23.13
23.03
23.03
23.12
13.72
17.28
20.45
23.03
24.81
27.43
20.36
20.42
20.47
20.43
20.42
13.77
16.53
18.53
20.47
21.41
22.89
2. Experimental
2.1. Materials
where: Rꢀ = H for glycine; (CH3)2CH for valine; (CH3)2CHCH2
for leucine and CH3 for alanine. R represents the aromatic moiety
CH3C6H5SO2.
l-Glycine, l-Valine, l-Leucine, and l-Alanine were purchased
from Sigma Chemicals (St. Louis, MO) were of acceptable grades
of purity and were used as received. Aqueous solution of the com-
pounds are employed. Chloramine-T (Merck) was purified by the
method of Morris et al. [28]. An aqueous solution of CAT was pre-
pared, standardized iodometrically and stored in amber coloured
stoppered bottles until further use. The concentration of stock solu-
tions was periodically determined. A solution of RuCl3 (Merck) in
0.2 mol dm−1 HCl was used as the catalyst. Allowance was made
for the amount of HCl present in catalyst solution, while prepar-
ing solution for kinetic runs. Reagent grade chemicals and doubly
distilled water were used throughout.
2.4. Product analysis
By micro Kjeldal procedure, ammonia which present in
the reaction mixture was estimated quantitatively. In a typi-
cal experiment 2.5 × 10−5 mol dm−3 of [CAT] were mixed with
2 × 10−3 mol dm−3of amino acid in
a total volume of 20 ml
under experimental conditions. The ammonia formed was dis-
tilled and absorbed in 2% boric acid solution. It was then
titrated against 0.01 mol dm−3 of HCl using a mixed indica-
tor (methyl red bromocresol green). The solution consumed
2.5 ml of 0.01 mol dm−3 HCl corresponding to the formation of
2.5 × 10−5 mol dm−3 of NH3.
2.2. Kinetic measurements
The aldehydes were characterized by their 2,4-DNP deriva-
tives. The p-toluenesulfonamide (product) was detected by paper
chromatography using benzyl alcohol saturated with water as the
solvent and 0.5% vanillin in 1% HCl solution in ethanol as spray
reagent (Rf = 0.915).
The reactions were carried out under pseudo first-order condi-
tions by taking a known excess of [Substrate]o over [Oxidant]o at
308 K. The reactions were carried out in stoppered Pyrex boiling
tubes whose outer surfaces were coated black to eliminate pho-
tochemical effects. For each run, requisite amounts of solutions of
substrate, HCl, RuCl3 and water (to keep the total volume constant
for all runs) were taken in the tube and thermostated at 313 K
until thermal equilibrium was attained. A measured amount of
CAT solution, which was also thermostated at the same temper-
ature, was rapidly added with stirring to the mixture in the tube.
The progress of the reaction was monitored iodometrically [29] for
two half-lives, by withdrawing aliquots of the reaction mixture at
3. Results
Ru(III) chloride catalysed oxidation of ␣-amino acids (AA) by
Chloramine-T (CAT) have been studied at several initial concentra-
tions of the reactants in the presence of HCl and at 308 K.
regular time intervals. The pseudo first-order rate constant (kobs
)
3.1. Dependence of rate on [Oxidant] and [AA]
calculated were reproducible within 3% error. Regression anal-
ysis of experimental data to obtain regression coefficient (r) was
performed using an EC-72 statistical calculator.
The reactions were carried out in the presence of Ru(III) catalyst,
under the conditions of [AA]o ꢁ [CAT]o. The plots of log[CAT] versus
time are linear (r > 0.9930), indicating a first order dependence of
the reaction rate on [CAT]o. The pseudo first-order rate constants
(kꢀ) obtained are given in Table 1.
Under the similar experimental conditions, the reaction rate
increased with increase in [AA]o. Plots of log kꢀ versus log[AA]o were
linear (Fig. 1; r > 0.9745). Fractional slopes of the plots indicate a
fractional order dependence on [AA]o (Table 1). This implies that,
rate is proportional to [AA]. Therefore, rate = ks [AA]0.35–0.55. This
explains that the influence of substrate concentration on the rate
is low. Here ks is kinetic rate coefficient for the substrate. From
the Plots of kobs versus [AA], the value of ks was obtained. This
confirms the existence of equilibrium between substrate (S) and
oxidant (Chloramine-T) and also appeared before the slow step.
Further, plots of kꢀ versus [AA]o having an Y-intercept, con-
firming the fractional order dependence on [AA]o.
2.3. Stoichiometry
Reaction mixtures containing AA(1.0 × 10−2 mol dm−3), HCl
(1.0 × 10−1 mol dm−3), catalyst (3.0 × 10−4 mol dm−3) and excess
of CAT (5.0 × 10−4 mol dm−3) were equilibrated at 308 K for 48 h.
Estimation of unreacted CAT was done by iodometric method to
calculate the stoichiometry ratios. Stoichiometric investigations
revealed that one mole of oxidant was sufficient to oxidize 1 mole
of AA leading to products like aldehydes, ammonia, carbon diox-
ide and reaction product (PTS: p-toluenesulfonamide). The overall
reaction can be represented by Eq. (1):
RꢀCH(NH2)COOH + RNClNa + H2O → RꢀCHO + RNH2 + NH3
+ CO2 + NaCl
(1)

P.A. Prashanth et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 203–208
205
1.6
1.4
1.2
1
1.8
Gly
1.6
Gly
Val
Leu
Ala
Val
Leu
Ala
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0.4
0.6
0.8
1
1.2
0.8
1
1.2
1.4
1.6
1.8
2
3 + log[AA]
5 + log[Ru(III)]
Fig. 1. Plots of log k1 vs. log[AA].
Fig. 3. Plots of log k1 vs. log[catalyst].
Table 2
3.3. Dependence of the rate on catalyst [Ru(III)]cholride
Effect of varying [H+] and ruthenium concentrations on the reaction rate with
[S]o = 1.0 × 10−2 mol dm−3; [CAT]o = 5 × 10−4 mol dm−3; T = 308 K.
The rate of reaction increased with increase in [Ru(III)] and the
plots of log kꢀ versus log[Ru(III)] were linear (Fig. 3) with unit slope
(r > 0.987) confirming first orders in [Ru(III)] (Table 2).
102[H+]
104[ruthanium]
kꢀ × 104 (s−1
)
(mol dm−3
)
(mol dm−3
)
Glycine
Valine
Leucine
Alanine
3.4. Dependence of the rate on [Cl−], [PTS] and ionic strength
4.0
6.0
8.0
3.0
3.0
3.0
3.0
3.0
3.0
1.0
2.0
3.0
4.0
5.0
6.0
3.41
4.51
5.43
6.68
7.35
8.91
2.05
4.28
6.60
8.98
11.31
14.25
9.61
11.81
14.35
16.20
17.83
20.57
4.91
10.32
16.23
22.41
27.61
34.63
15.13
18.25
20.93
23.03
25.61
29.23
11.34
16.71
23.03
26.80
34.11
39.51
14.30
16.73
19.02
20.42
21.83
24.91
7.41
14.06
20.47
27.35
32.91
38.41
Added Cl− in the form of NaCl at fixed [H+] and ionic strength
did not affect the rate. Addition of the reaction product (PTS) had no
effect on the rate indicating product did not involve in any prior in
the rate determined step (rds). Similarly, variation in ionic strength
of medium (0.1–0.5 mol dm−3) did not affect the rate of reaction,
indicating involvement of a non-ionic species in rate determined
step (rds).
10.0
12.0
16.0
10.0
10.0
10.0
10.0
10.0
10.0
3.5. Effect of varying dielectric constant (D)
The solvent composition was varied by adding methanol
(0–40%). The rate of the reaction increased with increase in
methanol content of the reaction medium (Table 3).
Blank experiments performed showed that the oxidation of
methanol by CAT during the period of study was negligible.
3.2. Dependence of the rate on [HCl]
At fixed [CAT]o and [AA]o, the rate of reaction increased with
increase in [HCl] (Table 2) and the plots of log kꢀ versus log[HCl]
were linear (Fig. 2) with fractional slopes (r > 0.9942) indicating
fractional orders in [acid]. Since, variation of [Cl−] had no effect
on the rate of reaction, it gives rate proportional to [H+] or rate =
3.6. Effect of solvent composition on reaction rate
kH+ [H+]0.33−0.51. This confirms the formation of protonated species
of CAT. Protonation of CAT takes place in an equilibrium step and
appears before slow step.
The effect of varying solvent composition on the rate of reaction
has been described in detail earlier in various monographs [30–33].
For the limiting case of zero angle of approach between two dipoles
or an ion-dipole systems, Aims [33] has shown that a plots of log kꢀ
verses 1/D, (where D is dielectric constant of the medium) gives a
straight line (Fig. 4; r > 0.9947) for the reaction involving an ion-
dipole interaction.
This observation indicates the ion-dipole nature of the rate-
determining step in the reaction sequence and also points to
extending of charge in the transition state.
1.7
Gly
1.5
1.3
1.1
0.9
0.7
0.5
0.3
Val
Leu
Ala
Table 3
Effect of varying dielectric constant (D) of the medium on reaction rate with
[S]o = 1.0 × 10−2 mol dm−3;[CAT]o = 5 × 10−4 mol dm−3
;
[H+] = 1.0 × 10−1 mol dm−3
;
[Ru(III)] = 3 × 10−4 mol dm−3; T = 308 K.
%[MeOH] (v/v)
D
kꢀ × 104 (s−1
)
Glycine
Valine
Leucine
Alanine
00.0
10.0
20.0
30.0
40.0
76.73
72.37
67.48
62.71
58.06
6.68
6.89
7.18
7.51
7.97
16.20
17.61
18.95
20.13
21.43
23.03
25.72
27.52
29.91
30.83
19.42
20.58
22.21
24.49
26.46
0.4
0.6
0.8
1
1.2
1.4
3 + log[H+]
Fig. 2. Plots of log k1 vs. log[H+].

206
P.A. Prashanth et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 203–208
The greater reactivity of leucine in the given amino acid series is
1.6
1.5
1.4
1.3
1.2
1.1
1
due to hydrophobicity, nucleophilicity and inductive effect (pres-
ence of long chained alkyl group). Inductive effect is due to the
presence of 2-methyl substituted propyl group attached to adja-
cent carbon of amino group in leucine. Therefore leucine is more
hydrophobic in nature [9] compared to other AAs in the series.
In addition to hydrophobic effect, 2-methyl substituted propyl
group present in leucine is also an electron donating group, which
enhances the electron density at the reaction centre and thereby
the electrophile (Cl+) present in the oxidant moiety can attack the
reaction centre effectively. One more effect, i.e., nucleophilic effect
is predominant in leucine [9]. The least reactivity of glycine is due
to H-atom present in the adjacent carbon of amino group. Due
to the presence of H-atom, there was no inductive effect on car-
bon atom linked to the amino group in glycine. In addition to this,
glycine has a least hydrophobicity value among the series of AAs.
Moreover, nucleophilicity of glycine is very poor compare to all
other amino acids in the present series [9]. Hence, the order is:
Gly < Val < Ala < Leu.
Gly
Val
Leu
Ala
0.9
0.8
0.7
0.011
0.013
0.015
0.017
0.019
1/D
Fig. 4. Plots of log k1 vs. 1/D.
1.5
1.4
1.3
1.2
1.1
The activation energy values (Table 4) are least for the fastest
reactions and vice versa. The rate determining step (Scheme 1)
involves an interaction between dipole-anion which required mod-
erate activation energy. The nearly constant of ꢀG‡ values for all
the four amino acids shows similar mechanism is operative in the
oxidation of amino acids.
Gly
Val
Leu
Ala
1
0.9
0.8
0.7
0.6
3.9. Test for free radical
Addition of reaction mixture to an aqueous acrylamide
monomer did not initiate polymerization, showing the absence of
formation of free radical species in situ in the reaction sequence.
The controlled experiments were also performed under the same
reaction conditions but without CAT.
0.00305
0.00315
0.00325
0.00335
1/T ( K -1
)
Fig. 5. Plots of log k1 vs. 1/T.
4. Discussion
3.7. Effect of varying temperature on the reaction rate
Chloramine-T undergoes two-electron change in its reactions.
It is a strong electrolyst in aqueous solution and depending on the
pH of the medium [34]. The possible oxidizing species of CAT in
acid solution are the conjugated acid TsNHCl, TsNCl2, HOCl and
H2OCl [20]. If TsNCl2 were to be the reactive species a second-
order dependence of the rate on [Oxidant]o would be expected
which is contrary to the experimental observation. If HOCl were
to be involved as an active species, a first-order retardation of rate
by the added TsNH2 would be expected. Science no such effect was
noticed, the species HOCl was ruled out. Based on the knowledge of
equilibrium, disproportions, and hydrolysis constant of reactions,
Bishop and Jennings [35] as a first approximation, have calculated
the concentrations of different species in 0.5 mol dm−3 CAT solution
at different pH. Therefore, the conjugate acid, TsNHCl, is the pre-
dominant species under acidic conditions. Furthermore, variation
of ionic strength of the medium or addition of reduction product
of CAT, TsNH2, has virtually no effect on the rate. The dependence
of rate on [H+] indicates protonation of the oxidant. Further, the
formation of the protonated species with a protonation constant
of 1.02 × 102 mol dm−3 at 25 ◦C has been reported (Eq. (3)) for CAT
acid solution [36]:
The reaction was carried at different temperatures (303–318 K),
keeping other experimental conditions constant. From the linear
Arrhenius plots log kꢀ versus 1/T (Fig. 5; r > 0.9925), values of activa-
tion parameters (Ea, ꢀH‡, ꢀS‡ and ꢀG‡) for the composite reaction
were evaluated. These dates are summarized in Table 4.
3.8. Activation parameters in presence of Ru(III) chloride
(catalyst)
The experimental observation indicates that (Table 2), rate of
oxidation of amino acids by CAT in presence of HCl and Ru(III)
chloride increased in the order: Gly < Val < Ala < Leu. This reactivity
trends was explained as follows.
Table 4
Temperature dependence on reaction rate and values of composite acti-
vation parameters for the oxidation of amino acids by [CAT]o = 5.0 ×
10−4 mol dm−3
;
[S]o = 1.0 × 10−2 mol dm−3
;
[H+] = 1.0 × 10−1 mol dm−3
;
[Ru(III)] =
3 × 10−4 mol dm−3
.
Temperature (K)
kꢀ × 104 (s−1
)
Glycine
Valine
13.62
16.20
19.21
Leucine
Alanine
TsNHCl + H+ ꢀ TsN+H2Cl
(2)
303
308
313
318
4.35
6.68
9.83
17.18
23.03
26.37
19.98
20.42
23.56
Since, variation of [Cl−] had no effect on the rate of reaction,
it gives rate proportional to [H+] or rate = kH+ [H+]0.33−0.51. This
confirms the formation of protonated species of CAT. Protonation
of CAT takes place in an equilibrium step and appears before slow
step. With all the experimental observations protonated oxidant
(TsN+H2Cl) is the active oxidizing species.
14.39
57.54
55.62
94.59
−127.91
22.18
26.80
24.22
92.58
29.61
20.05
17.47
91.71
27.18
22.79
20.39
92.01
Ea (kJ mol−1
)
ꢀH‡ (kJ mol−1
ꢀG‡ (kJ mol−1
)
)
ꢀS‡ (JK−1 mol−1
)
−220.12
−239.09
−230.65

P.A. Prashanth et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 203–208
207
Scheme 1. Ru(III) catalysed oxidation of amino acids by CAT.
Literature survey revealed that the following equilibrium exists
Since,
for Ru(III) in acidic solutions [37–39].
rate = k3[X][Ru(III)]
(8)
[RuCl6]3− + H2O ꢀ [RuCl5(H2O)]2− + Cl−
(3)
Substituting for [X] from Eq. (7) in Eq. (8) yields the following
rate law (Eq. (9))
Singh et al. employed the above equilibrium in ruthenium(III)
chloride catalysed oxidation of primary alcohols by bromamine-T
and glycols by N-bromoacetamide in HClO4 medium. In the present
study however addition of Cl− ion in the form of NaCl at fixed
[H+] had no effect on the rate, indicating that [RuCl6]3−, is most
likely catalyzing species that interacts with the substrates to form
a complex intermediate. Similar kinetic results were observed in
the Ru(III) catalysed oxidation of chloroacetic acids by BAT [40]
and BAB [41].
d[CAT]t
dt
K1K2[AA][H+][CAT]t[Ru(III)]
1 + K1[H+] + K1K2[AA][H+]
rate =
=
(9)
The rate law (Eq. (9)) is good agreement with the observed kinetic
data.
5. Conclusion
Based on the preceding discussion, a detailed mechanistic inter-
pretation (Scheme 1) for the Ru(III)-catalysed oxidation of amino
acid by CAT in acidic medium has been proposed to substantiate
the observed experimental results. In scheme 1, X and Xꢀ represents
the complex intermediate species. A detailed mechanistic interpre-
tation of Ru(III)-catalysed oxidation of amino acid by CAT in acid
medium is depicted.
The kinetics of the ruthenium(III) chloride catalysed oxidation
of ␣-amino acids viz., glycine, valine, leucine, alanine by CAT in
aqueous acid medium at 308 K was discussed. Relative reactivity
of AAs are in the order of Leu > Ala > Val > Gly. This trend may be
attributed to hydrophobicity and inductive effects. A negative ꢀS‡
value indicates the formation of transition state fair and rapid with
lower degree of freedom. Further, the high value of free energy of
activation suggests a solvated activated complex. Aldehydes are the
main products of the reaction.
The total effective concentration of CAT is [CAT]t, then from
scheme 1
+
[CAT]t = [TsNHCl] + [TsNH2Cl] + [X]
(4)
Acknowledgements
+
One of authors (M.N.K) is grateful to the UGC F.No. 41-317/2012
(SR) Government of India, New Delhi, for financial support.
Solving for [TsNHCl], [TsNH2Cl] and [X] in terms of [X1] from
steps (i)–(iii),
One gets
Appendix A. Supplementary data
+
+
[TsNH2Cl]
K1[H+]
[X]
K2[AA]
[TsNHCl] =
and [TsNH2Cl] =
(5)
(6)
(7)
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molcata.2013.
12.008.
Thus,
[X]
[X]
[CAT]t =
+
+ [X]
K2[AA]
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