Kinetics and mechanism of oxidation of neutral α-amino acids by sodium N-chloro-p-toluenesulfon-amide in acid medium

Article abstract of DOI:10.1002/kin.10011

Kinetics of oxidation of α-amino acids, glycine, valine, alanine, and phenylalanine, by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) has been investigated in HClO₄ medium at 30°C. The rate shows first-order dependence on both CAT and amino acid concentrations and an inverse first-order on [H⁺]. The variation of ionic strength and the addition of p-toluenesulfonamide and Cl^- ion had no effect on the reaction rate. Decrease of dielectric constant of the medium by increasing the MeOH content decreased the rate. Rate studies in D₂O medium showed the inverse solvent-isotope effect of k_D(2)O/k_H(2)O = 0.50. Proton-inventory studies were carried out using H₂O-D₂O mixtures. The activation parameters have been computed. The proposed mechanism and the derived rate law are consistent with the observed kinetic data. An isokinetic relationship is observed with β = 323 K, indicating enthalpy as a controlling factor. The rate of oxidation increases in the following order: Gly < Val < Phe < Ala.

Full text of DOI:10.1002/kin.10011

Kinetics and Mechanism
of Oxidation of Neutral
-Amino Acids by Sodium
N-Chloro-p-toluenesulfon-
amide in Acid Medium
K. S. RANGAPPA,1 K. MANJUNATHASWAMY,1 M. P. RAGHAVENDRA,1 N. M. MADE GOWDA2
1Department of Studies in Chemistry, University of Mysore, Masasagangothri, Mysore 570 006, India
2Department of Chemistry, Western Illinois University, One University Circle, Macomb, Illinois 61455
Received 14 November 1997; accepted 22 June 2001
ABSTRACT: Kinetics of oxidation of -amino acids, glycine, valine, alanine, and phenylalanine,
by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) has been investigated in
HClO4 medium at 30 C. The rate shows first-order dependence on both CAT and amino acid
concentrations and an inverse first-order on [H+]. The variation of ionic strength and the ad-
dition of p-toluenesulfonamide and Cl ion had no effect on the reaction rate. Decrease of
dielectric constant of the medium by increasing the MeOH content decreased the rate. Rate
studies in D2O medium showed the inverse solvent-isotope effect of kD O/kH O = 0.50. Proton-
2
2
inventory studies were carried out using H2O–D2O mixtures. The activation parameters have
been computed. The proposed mechanism and the derived rate law are consistent with the ob-
served kinetic data. An isokinetic relationship is observed with = 323 K, indicating enthalpy as
a controlling factor. The rate of oxidation increases in the following order: Gly < Val < Phe < Ala.
C
2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 49–55, 2002
saccharin. Generally CAT undergoes a two-electron
change in its reactions resulting in the formation of
the reduction products, p-toluenesulfonaminde or PTS
(p-CH3C6H4SO2NH2) and sodium chloride. The ox-
idation potential of the CAT–PTS couple varies with
pH of the medium (1.139 V at pH 0.65, 0.778 V at pH
7.0, and 0.614 V at pH 9.7). A detailed review of the
chemistry of CAT and related N-haloarylsulfonamides
has been reported [3].
INTRODUCTION
The chemistry of sodium N-chloroarylsulfonamides in
aqueous solutions has received considerable attention
[1,2]. The prominent member of this class of com-
pounds, namely, sodium N-chloro-p-toluenesulfona-
mide or chloramine-T (p-CH3C6H4SO2NClNa 3H2O
or CAT), is a by-product in the manufacture of
Correspondence to: N. M. Made Gowda; e-mail: gn-made@
wiu.edu.
In the present paper, we report the kinetic and
mechanistic study of the oxidation of four neutral ␣-
amino acids, glycine, valine, alanine, and phenylala-
nine, by CAT, performed under the same experimental
Contract grant sponsor: CSIR, New Delhi, India.
2001 John Wiley & Sons, Inc.
c
DOI 10.1002/kin.10011

50
RANGAPPA ET AL.
conditions including the HClO4 medium and the tem-
perature. The main objective of this investigation was
twofold: (1) to ascertain whether the neutral ␣-amino
acids follow a common mechanism of oxidation, which
they do, and (2) to compare these new kinetic results
with those of the earlier studies [1,2] so that the main
differences could be identified. The following differ-
ences between the earlier studies and the present work
are noted:
differences described above justify the present
study.
EXPERIMENTAL
Chloramine-T (Merck) was purified by a standard
method[3]anditspuritywascheckedbyiodometryand
by its 1H and 13C NMR spectra. An aqueous solution of
the compound was prepared and preserved in a brown
bottle to prevent its photochemical deterioration. The
chromatographically pure glycine (S.D. Chem.), DL-
valine, L-alanine, and L-phenylalanine (Loba Chemie)
were further assayed by the acetous perchloric acid
method [4]. All other chemicals used were of analytical
reagent grade. Solvent-isotope studies were made with
D2O (99.4%) supplied by BARC, Trombay, Mumbai,
India. Triply distilled water was used in the prepara-
tion of aqueous solutions. The dielectric constant of the
medium was varied by the addition of MeOH and the
values of dielectric constant for MeOH–water mixtures
reported in literature [5,6] were employed.
i. Unlike the present case, in the earlier studies
[1,2], oxidation kinetics were performed on indi-
vidual amino acids under different reaction con-
ditions, including acid and base media, resulting
in different experimental rate laws and mecha-
nisms. For example, one of the previous studies
[1] involved the oxidation of L-threonine by CAT,
in HCl, HClO4, and H2SO4 media, while the
present study involves the oxidation of four neu-
tral amino acids in HClO4 medium by CAT. The
threonine oxidation shows the following kinetic
orders, which differ from the present data: a first-
order in [CAT], a negative first-order in [H+],
and a fractional-order each in [threonine] and
[Cl ].
Kinetic Procedure
The reaction was carried out in glass-stoppered Pyrex
boiling tubes whose outer surface was coated black to
eliminate photochemical effects. Requisite amounts of
the solutions of the amino acid and perchloric acid were
taken in the tube and thermostated at 30 C. A measured
amount of CAT solution, also thermostated at the same
temperature, was rapidly added with stirring to the mix-
ture in the tube. The progress of the reaction was moni-
tored by the iodometric determination of the unreacted
CAT in aliquots of the reaction mixture withdrawn at
different intervals of time. The course of the reaction
was studied for at least two half-lives. The pseudo first-
order rate constants, kobs, calculated from the plots of
log [CAT] vs. time were reproducible within 3%.
Regression analysis of experimental data, to obtain
regression coefficient r and the standard deviation s,
was performed with an EC-72 statistical calculator.
ii. Similar differences can be found between the ki-
netic results and mechanisms for several amino
acids reported earlier [2].
iii. A part of the earlier study [2] involved the ki-
netics of oxidation of glycine, valine, alanine,
and phenylalanine with CAT performed in HCl
medium, but not in HClO4 medium. Hence the
two sets of kinetic data in two acid media for the
same four amino acids are different.
iv. There are differences in the reaction stoichiome-
tries and in the products formed. Under the exper-
imental conditions used in this work, the mole-
to-mole ratio of the amino acid to CAT is 1 : 1
with NH4+ ion, CO2, and the corresponding alde-
hyde as oxidation products of the substrate. In
contrast, under the experimental conditions used
in the previous studies [2], the stoichiometry in-
volved 2 mol of CAT consumed per mole of the
amino acid, yielding a nitrile and CO2 as oxida-
tion products.
v. In the present study, a proton-inventory experi-
ment has been performed showing the involve-
ment of H+ in the transition state. The earlier
studies [1,2] do not report this experiment.
vi. The applicabilities of the Taft equation and
single-parametercorrelationshavebeenexplored
to elucidate stereochemical information of the
reactions in the present study. These data are
lacking in the earlier investigations [1,2]. The
Stoichiometry and Product Analysis
The reaction mixtures containing the amino acid and
perchloric acid with excess of CAT were kept for 24 h
at 30 C. Iodometric determination of the unconsumed
CAT showed that 1 mol of the oxidant was consumed
per mole of the amino acid forming the correspond-
ing aldehyde. The stoichiometry of the reaction can be
represented by Eq. (1).
R0CH(NH2)COOH + RNCl + H3O+
→ R0CHO + NH4+ + CO2 + RNH2 + Cl (1)

OXIDATION OF ␣-AMINO ACIDS BY SODIUM N-CHLORO-p-TOLUENESULFONAMIDE
51
Here R0 = H for glycine, (CH3)2CH for valine, CH3
Effect of [HClO4]
for alanine, and C6H5CH2 for phenylalanine while R
represents p-CH3C6H4SO2.
The rate decreased with increase in [HClO4] for each
amino acid and the plot of log kobs vs. log [HClO4]
was linear (r > 0.9979, s 0.04; Table I) with a slope
of 1.
Ammonia and CO2 were identified by the con-
ventional tests. The aldehydes were characterized by
preparing the DNP derivatives and by their spectral
data in comparison with the authentic aldehyde sam-
ples. The reduction product of CAT, RNH2, among
the reaction products was detected by TLC using
dichloromethane and petroleum ether (7:3 v/v) as the
solvent system and iodine as the detecting reagent
(Rf = 0.34).
Effect of RNH2 and Cl Ion
Addition of the reaction products, p-toluenesulfo-
namide and chloride ions, the latter in the form of NaCl,
did not affect the rate. It may thus be inferred that the
sulfonamide is not involved in a pre-equilibrium with
the oxidant.
RESULTS
Effect of Ionic Strength
Effect of Varying Reactant Concentration
on the Rate
Variation of the ionic strength of the medium by adding
NaClO4 (0.20–1.0 mol dm 3) had no effect on the
reaction rate. This effect supports the involvement
of at least one nonionic species in the rate-limiting
step.
The kinetics of oxidations of neutral ␣-amino acids
by CAT were investigated at several initial concen-
trations of the reactants in perchloric acid medium
at 30 C. With the [substrate]0 in excess and at con-
stant [HClO4], plots of log [CAT] vs. time were linear
(r > 0.9930, s 0.05) for each [CAT]0. The reaction
showed a first-order dependence of rate on [CAT]
(Table I). Values of kobs increased with increase in
[amino acid]. The plots of log kobs vs. log [amino
acid]0 were linear (r > 0.9990, s < 0.02) with unit
slopes (Table I). Also, the second-order rate constants
k2 = kobs/[S]0 were constant confirming the first-order
dependence on [S].
Effect of Varying the Solvent Composition
The solvent composition and dielectric constant (D)
of the reaction medium were varied by adding MeOH
(0–40% Vol%). The rate decreased with increase in the
MeOH content. Plots of log kobs vs. 1/D were linear
(r > 0.9896, s 0.05) with a negative slope. Control
experiments performed showed that MeOH was not
oxidized by CAT under the experimental conditions.
Table I Effect of Varying Reactant Concentrations on the Rate of Oxidation of Neutral -Amino Acids at 30 C
1
104kobs(s
)
103[CAT]0
(mol dm
102[S]0
(mol dm
102[H+]
(mol dm
3
3
3
)
)
)
Glycine
Valine
Alanine
Phenylalanine
2.00
2.50
3.00
3.50
4.00
4.50
3.00
3.00
3.00
3.00
3.00
3.00
3.00
2.0
5.00
5.00
5.00
5.00
5.00
5.00
1.00
3.0
7.00
9.00
10.0
5.00
5.00
5.00
5.00
5.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
2.00
4.00
8.00
10.0
20.0
2.06
2.25
2.40
2.44
2.53
2.67
0.64
1.75
3.83
4.80
5.92
7.98
4.29
1.75
1.20
0.63
4.72
5.05
5.45
5.60
5.68
5.88
1.50
3.98
7.89
9.80
11.52
17.50
9.11
4.20
3.07
1.66
5.28
5.75
5.92
5.96
6.03
6.20
1.52
4.03
8.46
10.6
12.8
21.5
11.0
4.50
2.92
1.60
5.38
5.45
5.63
5.68
5.73
5.93
1.28
3.55
7.98
9.74
11.0
19.6
9.13
4.15
3.20
1.50
3.0
3.00

52
RANGAPPA ET AL.
Effect of Temperature
The reaction was studied at different temperatures
(298–313 K). From the linear Arrhenius plots of log
kobs vs. 1/T (r > 0.9998, s 0.01), the activation en-
ergy Ea was calculated. Values of the other activation
parameters, 1H= and 1S=, were computed from the
Ea values (Table II).
Effect of Solvent Isotope
The reaction was studied in D2O medium in the pres-
ence of HClO4. The rate decreased with increase
in D2O content. The inverse solvent-isotope effect
kobs(D2O)/kobs(H2O) was found to be 0.50, showing a
retardation of the reaction rate in D2O medium. Proton-
inventory studies were carried out performing the re-
action in H2O–D2O mixtures with varying deuterium
atom fractions n. The proton-inventory plots relating
the rate constants konbs with n are given in Fig. 1.
Test for Free Radicals
Figure 1 Proton-inventory plots for the oxidation of ␣-ami
Addition of the reaction mixture to aqueous acrylamide
solution did not initiate polymerization, showing the
absence of free radical species.
no acids by CAT in H2O–D2O mixtures at 30 C:
3
[CAT]0 = 3.00 10
mol dm 3; [amino acid]0 = 5.00
10 mol dm 3; [HClO4] = 6.00 10 mol dm 3; A:
2
2
Gly; B: Val; C: Ala; D: Phe.
DISCUSSION
free acid [7], monochloramine-T, RNHCl:
A detailed study of monochloramines shows that sim-
ilar equilibria exist in aqueous solutions of these com-
pounds. Chloramine-T ionizes in aqueous solution and
the anion picks up a proton in acid medium to give the
RNClNa
RNCl + Na+
(2)
(3)
Ka
RNCl + H+
RNHCl,
3
Ka = 2.82 10 5mol dm at 18 C
Table II Kinetic and Activation Data for the Oxidation
Although the free acid has not been isolated, there
is sufficient experimental evidence for its formation in
solution [8]. It can further undergo disproportionation/
hydrolysis giving RNH2, dichloramine-T (RNCl2), and
HOCl:
of Neutral -Amino Acids by CAT in HClO4 Medium
Amino Temp 104kobs
Acid
Ea
1H=
1S=
(K) (s 1) (kJ mol 1) (kJ mol 1) (J mol 1K
)
1
Glycine 298 1.38
303 2.40
95.8
73.6
72.2
65.7
93.3
71.0
69.7
63.2
6.8
Kd
308 4.25
313 7.68
2RNHCl
RNH2 + RNCl2
Kd = 6.1 10 2at 25 C (4)
Valine
298 3.98
303 5.45
308 7.68
313 13.1
73.3
77.0
99.0
Kh
RNHCl + H2O
RNHCl + H+
RNH2 + HOCl,
Kh = 4.88 10 8at 25 C (5)
Alanine 298 4.00
303 5.92
RNH2Cl+
(6)
308 8.79
313 14.1
Phenyl- 298 4.20
alnine 303 5.63
308 9.47
The further protonation of RNHCl to RNH2Cl+ in
a strong acid, HClO4, is to be expected. The inverse
first-order dependence of the rate on [H+] supports
the involvement of RNHCl as the reactive species,
as the equilibrium [Eq. (6)] predicts a retardation of
313 13.8

OXIDATION OF ␣-AMINO ACIDS BY SODIUM N-CHLORO-p-TOLUENESULFONAMIDE
53
the rate with an increase in [H+]. If RNCl2 were to
be the reactive species in the present investigations,
then the rate law would predict a second-order depen-
dence of the rate on [CAT], which is contrary to the
experimental observations. If HOCl were primarily in-
volved, a first-order retardation of the rate by the added
p-toluenesulfonamide (RNH2) would be expected.
Since no sucheffect wasnoticed, HOCl canbe ruledout
as the oxidizing species. Hence, RNHCl is the reactive
species of CAT responsible for the oxidation of amino
acids. Furthermore, the variation of ionic strength of
the medium and the addition of RNH2 to the reaction
mixture have no effect on the rate while the decrease in
dielectric constant of the medium retards the rate. Bear-
ing these facts in mind, a general scheme involving a
direct interaction of the amino acid (S) with RNHCl in
the rate-determining step is proposed in Scheme 1.
Amino acids are known to exist in different equi-
libria depending on the pH of the medium. In al-
kaline, neutral, and acidic media, amino acids exist
as anions. (R0CH(NH2)COO or S ), zwitterions (+R0
transient state X is formed, the chlorine atom probably
undergoes a Walden-type of inversion as the carboxy-
late anion attacks and kicks out the sulfonamide anion
species, which is resonance stabilized and which later
abstracts the acidic proton off the amino acid nitrogen
and forms the sulfonamide product as in fast step (iii).
In the subsequent fast steps (iv) and (v), the complex X0
undergoes intramolecular rearrangements and a nucle-
ophilic attack by water to form end products including
the aldehyde.
Application of the steady-state concept to the in-
termediate, RNHCl, in Scheme I leads to the rate law
[Eq. (7)], which is in agreement with the kinetic data.
d [CAT]
K1k2[S] [CAT]
rate =
=
(7)
dt
[H+]
The general mechanisms, reported by earlier work-
ers [2], for the oxidation of several amino acids by CAT
in different acid media differ from the above general
Scheme I proposed for the oxidation of four amino
acids in HClO4 medium. They have proposed two gen-
eral, mechanistic pathways leading to products involv-
ing the interaction of the substrate with different reac-
tive species of CAT in the rate-determining step [2].
The first pathway involves RNHCl while the second
one involves Cl2 or H2OCl+ as reactive species. The
mechanistic differences can be attributed to the follow-
ing: As CAT in aqueous solutions can furnish such re-
active species as RNCl , RNHCl, RNH2Cl+, H2OCl+,
Cl2, and RNCl2, varying reaction conditions like acid
medium and nature of the amino acid can initiate an in-
teraction of the substrate with a known reactive species
of CAT, especially in the slow step.
CH(NH3+)COO or S), and cations (R0CH(NH3
COOH or SH+), respectively.
)
In Scheme 1, the fast pre-equilibrium step (i) involv-
ing deprotonation of RNH2Cl+ forms the reactive oxi-
dant species, RNHCl. In the slow step, an electrophilic
attack by Cl+ of RNHCl on the carboxylate anion of
the substrate, resulting in the formation of an N-chloro-
O bridged transition state, X, is envisaged. After the
(i) In the present study, tests [9] for the applicability
of the Taft equation as well as single-parameter
correlations for the amino acids were made. A
linear regression of results yielded the following
equations:
log k2
=
=
0.274 * 2.003;
r = 0.8369 (8)
0.252Es 2.004;
r = 0.8705 (9)
log k2
log(k2 Es) = 0.393 * 2.003;
r = 0.4593 (10)
The reasonable correlations between log k2
and * [Eq. (8)] and log k2 and Es [Eq. (9)] in-
dicate that the steric effects are more important
than electronic factors. However, an examination
of Eq. (10) shows a poor correlation for the com-
bined effect of polar and steric factors. A negative
SCHEME 1

54
RANGAPPA ET AL.
value of reaction constant ( * = 0.274 or
0.393) indicates that the presence of electron-
donating groups increase the rate of reaction. It
is seen from Table I that the rate of oxidation
of amino acids increases in the following order:
Gly < Val < Phe < Ala.
enable us to calculate the fractionation factors of
the transition state (8i ). However, from a qual-
itative point of view, curvatures of the proton-
inventory plots in Fig. 1 were compared with
those of the standard curves in the literature [14]
and it was concluded that a H+ ion is involved in
the transition state formation.
(ii) The rate of reaction decreases in D2O medium
and the values of the ratio kobs(D2O)/kobs (H2O)
are 0.49, 0.52, 0.49, and 0.50 for glycine, valine,
alanine, and phenylalanine, respectively. The
deuterated species RNClD, DOC1, and DO are
expected to be present in heavy water under these
conditions. Theinversesolvent-isotopeeffectcan
be correlated with the greater acidity of D3O+
ions compared to H3O+ ions (by a factor of
2–3), resulting in a greater retardation of the rate
in D2O medium [10,11]. Proton inventory stud-
ies could throw some light on the nature of the
transition state. The dependence of rate constants
konbs on n, the atom fraction of deuterium, in sol-
vent mixtures containing H2O and D2O is given
[12,13] by the Grass–Butler relationship [Eq. 11]
(iii) The rate decreased with decrease in dielectric
constant (D) of the medium. A plot of log kobs
vs. 1/D was linear with a negative slope. The ef-
fect of D on the rate for a reaction involving two
ions is given [15] by the standard relationship,
Eq. (12)
log k = log k0 ZA ZBe2/DkT dAB (12)
where k0 is the rate constant in a medium of in-
finite dielectric constant, ZAe and ZBe are the
charges, dAB is the activated complex size, k is the
Boltzmann constant, and T is the absolute tem-
perature. From the slope = ZA ZB e2/kT dAB
,
the values of dAB were computed as 2.0, 1.67,
˚
1.51, and 1.67 A for glycine, valine, alanine,
and phenylalanine, respectively. The values are
found to be reasonable in comparison with those
of other reactions of similar nature [16].
5TS(1 n + n8i )
ko0bs/konbs
=
(11)
5
RS(1 n + n8j )
(iv) The data in Table II show that the energy of acti-
vation is the highest for the slowest reaction, in-
dicating that the reaction is enthalpy controlled.
Furthermore, values of 1H= and 1S= can be
correlated linearly (r = 0.9990, s = 0.02) result-
ing in an isokinetic relation (Fig. 2), indicating
where 8i and 8j are the isotopic fractionation
factors (equilibrium constants for the H–D ex-
change) for isotopically exchangeable hydrogen
sites in the transition state (TS) and the reactant
state (RS), respectively. A knowledge of isotopic
fractionation factors of reactants (8j ) would
3
FIGURE 2 Isokinetic plots of (a) 1H= vs. 1S= and (b) log kobs(313 K) vs. log kobs(298 K): [CAT]0 = 3.00 10 mol
2
2
3
dm 3; [amino acid]0 = 5.00 10 mol dm 3; [HClO4] = 6.00 10 mol dm
.

OXIDATION OF ␣-AMINO ACIDS BY SODIUM N-CHLORO-p-TOLUENESULFONAMIDE
55
that a common mechanism operates in the ox-
idation of the selected amino acids by CAT.
The slope gives the value of the isokinetic tem-
University of Mysore, Manasagangothri, Mysore, and
Drs. J. W. Hartman and T. K. Vinod, Department of Chem-
istry, Western Illinois University, Macomb, IL, for helpful
discussion.
perature
as 323 K, which is much higher
than the experimental temperature. The rela-
tionship was proved to be genuine through the
Exner criterion [17] by plotting log kobs(313 K)
vs. log kobs(298 K) (r = 0.9955, s = 0.04,
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= T1(1 q)/(T1T2)
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CONCLUSION
In the present kinetic study of the oxidation of neu-
tral ␣-amino acids, Gly, Val, Ala, and Phe, by CAT in
HClO4 medium, a general mechanism involving the in-
teraction of the substrate (S) with the reactive species of
CAT (RNHCl) in the slow step, which is different from
the ones previously reported [1,2] for several amino
acid substrates, has been proposed. This mechanism is
supported by the experimental data such as the reaction
stoichiometry, the oxidation products, the rate law, the
proton-inventory plots, the activation parameters, and
the Taft relationship.
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The authors are indebted to Dr. D. S. Mahadevappa, Professor
Emeritus (Rtd.), Department of Studies in Chemistry,