Chloraminometric and bromaminometric oxidation of sulfanilic acid in alkaline medium: A comparative kinetic and mechanistic study

Article abstract of DOI:10.1002/kin.20138

The kinetics of oxidation of sulfanilic acid (SAA) by chloramine-B (CAB) and bromamine-B (BAB) has been investigated in alkaline medium at 35 ± 0.1°C. The oxidation reaction follows identical kinetics in the case of both the oxidants with first-order dependence on each [oxidant]_o and [SAA]_o and an inverse first-order dependence on [OH^-]. The variation of ionic strength, dielectric constant of the medium, addition of the reaction product (benzene-sulfonamide), and halide ions showed no significant influence on the reaction rate. Proton inventory studies made in H₂O-D₂O mixtures for CAB and BAB have been utilized to calculate the isotopic fractionation factor. The reaction has been studied at different temperatures, and activation parameters for the composite reaction have been computed from the Arrhenius plots. N-Hydroxylaminobenzene-4-sulfonic acid was identified as the oxidation product of SAA from IR and GC-MS analysis. A mechanism consistent with the kinetic results is proposed in which PhSO ₂NHX (X = Cl or Br) interact with the substrate in the rate-limiting step. A suitable rate law is derived. The rate of oxidation of sulfanilic acid is about fourfold faster in BAB compared to CAB. The oxidation of SAA brought about by CAT and BAT was also investigated under identical experimental conditions, and the overall rate of oxidation of SAA increases in the order: BAB > BAT > CAB > CAT This may be attributed to the difference in electrophilicities of Cl⁺ and Br⁺ ions and also the van der Waal's radii of chlorine and bromine.

Full text of DOI:10.1002/kin.20138

Chloraminometric and
Bromaminometric
Oxidation of Sulfanilic Acid
in Alkaline Medium:
A Comparative Kinetic
and Mechanistic Study
PUTTASWAMY, R. V. JAGADEESH
Department of Postgraduate Studies in Chemistry, Central College, Bangalore University, Bangalore 560 001, India
Received 17 December 2004; accepted 14 August 2005
DOI 10.1002/kin.20138
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of oxidation of sulfanilic acid (SAA) by chloramine-B (CAB) and
◦
bromamine-B (BAB) has been investigated in alkaline medium at 35 ± 0.1 C. The oxidation
reaction follows identical kinetics in the case of both the oxidants with first-order dependence
−
on each [oxidant]o and [SAA]o and an inverse first-order dependence on [OH ]. The variation of
ionic strength, dielectric constant of the medium, addition of the reaction product (benzene-
sulfonamide), and halide ions showed no significant influence on the reaction rate. Proton
inventory studies made in H O–D O mixtures for CAB and BAB have been utilized to calcu-
2
2
late the isotopic fractionation factor. The reaction has been studied at different temperatures,
and activation parameters for the composite reaction have been computed from the Arrhenius
plots. N-Hydroxylaminobenzene-4-sulfonic acid was identified as the oxidation product of SAA
from IR and GC-MS analysis. A mechanism consistent with the kinetic results is proposed in
which PhSO NHX (X = Cl or Br) interact with the substrate in the rate-limiting step. A suit-
2
able rate law is derived. The rate of oxidation of sulfanilic acid is about fourfold faster in BAB
compared to CAB. The oxidation of SAA brought about by CAT and BAT was also investigated
under identical experimental conditions, and the overall rate of oxidation of SAA increases in
the order: BAB > BAT > CAB > CAT. This may be attributed to the difference in electrophilicities
+
+
of Cl and Br ions and also the van der Waal’s radii of chlorine and bromine. ꢀC 2005 Wiley
Periodicals, Inc. Int J Chem Kinet 38: 48–56, 2006
INTRODUCTION
pable of producing halonium cations, hypohalites, and
N-anions, behave both as bases and as nucleophiles,
depending on the reaction conditions. The subject has
been extensively reviewed by Campbell and Johnson
The N-metallo-N-haloarylsulfonamides generally
known as N-haloamines are a class of compounds ca-
Correspondence to: Puttaswamy; e-mail: pswamy chem@
[1], Bremner [2], and Banerji et al. [3]. The promi-
yahoo.com.
ꢀ
c 2005 Wiley Periodicals, Inc.
nent member of this group is chloramine-T (CAT,

CHLORAMINOMETRIC AND BROMAMINOMETRIC OXIDATION OF SULFANILIC ACID
49
p-CH3C6H4SO2NClNa·3H2O), and the mechanistic
(99.4%) supplied by the Bhabha Atomic Research Cen-
tre, Mumbai, India. Double distilled water was used
throughout the studies.
aspects of its reaction have been very well documented
[
(
1–3]. The benzene analogue of CAT, chloramine-B
CAB, C6H5SO2NClNa·1.5H2O), and the bromine
analogues of CAT and CAB are (bromamine-T (BAT,
Kinetic Measurements
p-CH3C6H5SO2NBrNa·3H2O) and bromamine-B
(
BAB, C6H5SO2NBrNa·1.5H2O)) are becoming
The reaction was carried out in glass-stoppered Pyrex
boiling tubes whose outer surface was coated black to
eliminate photochemical effects. Solution containing
appropriate amounts of substrate, NaOH and water (to
keep the total volume constant for all runs) were placed
important as mild oxidants. There is very little infor-
mation available in the literature on CAB and BAB
[4–6].
Sulfanilic acid (SAA, p-aminobenzenesulfonic
◦
acid) finds widespread use in the syntheses of various
organic dyes [7]. The sulfanilamide and certain related
substituted amides have significant medicinal impor-
tance as sulfa drugs. Although their use has been super-
seded to a large extent by the antibiotics such as peni-
cillin, terramycin, chloromycetin, and aureomycin, the
sulfa drugs still have their medicinal use and make up
a large proportion of the output from pharmaceutical
industry [8]. Although SAA has been oxidized by var-
ious oxidants [9,10], there seems to be no report in the
literature on the kinetics of oxidation of SAA by posi-
tive halogen compounds. These facts obviate the need
for us to undertake the title investigation.
in the reaction tube and thermostatted at 35 ± 0.1 C. A
known amount of oxidant was also thermostatted at the
sametemperatureandwasrapidlyaddedtothemixture,
and the progress of the reaction was monitored by iodo-
metric determination of unreacted oxidant in a known
aliquot (5 mL each) of the reaction mixture at regular
intervals of time. The course of the reaction was studied
up to two half-lives. Pseudo-first-order rate constants
ꢁ
(k ) calculated from log [oxidant] vs. time plots were
reproducible to within ± 4%. Regression analysis of
the experimental data to obtain regression coefficient
(r) was carried out on an fx-100W scientific calculator.
In continuation of our work on oxidation kinetic
studies using organic N-haloamines, the present paper
reports a systematic investigation on oxidation kinetics
of sulfanilic acid by CAB and BAB in alkaline medium.
The main objectives of the present investigations are (i)
toelucidatetheplausiblemechanism, (ii)toputforward
the appropriate rate law, (iii) to ascertain the reactive
species of the oxidants, (iv) to find the stoichiometry
and to identify the oxidation products, and (v) to assess
the relative reactivity of CAT, CAB, BAT, and BAB
toward SAA.
Reaction Stoichiometry
The reaction mixture containing SAA, alkali, and an
excess of oxidant (CAB or BAB) was kept for 48 h
◦
at 35 C. Determination of the unreacted oxidant iodo-
metrically in the reaction mixture showed that 1 mole
of SAA consumed 1 mole of oxidant, which can be
stoichiometrically represented as
C6H4(NH2)SO3H + PhSO2NXNa + H2O
+
−
→
C6H4(NHOH)SO3H + PhSO2NH2 + Na + X
(
1)
EXPERIMENTAL
Materials
where X = Cl for CAB and Br for BAB.
Product Analysis
Chloramine-T (E. Merck) was purified by the method
of Morris and co-workers [11]. Preparation of
chloramine-B, bromamine-B, and bromamine-T has
been described earlier [12–14]. Further, the purity of
all the four oxidants was assayed iodometrically to de-
termine the active halogen content. Aqueous solution
of the oxidants was standardized by the iodometric
procedure and preserved in brown bottles to prevent
photochemical deterioration [11]. An aqueous solu-
tion of AnalaR grade sulfanilic acid (BDH, England)
of desired strength was freshly prepared whenever re-
quired. All the other reagents used were of analytical
grade. Solvent isotope studies were made with D2O
The reactions were allowed to progress under stirred
conditions and were allowed to progress for 48 h at
35 C. After completion of the reaction, the products
wereneutralizedwithHClandextractedwithether. The
organic products were subjected to spot tests and chro-
matographic analysis (TLC technique), which revealed
the formation of N-hydroxylaminobenzene-4-sulfonic
acid as the oxidation product of SAA; the separation
of which from benzenesulfonamide is achieved using
column chromatography.
◦
The reaction product of oxidant, benzenesul-
fonamide (PhSO2NH2), was detected by thin-layer

50
PUTTASWAMY AND JAGADEESH
Figure 1 GC-Mass spectrum of benzenesulfonamide with its molecular ion peak at 157 amu.
chromatography [15] using petroleum ether : chloro-
form : 1-butanol (2 : 2 : 1, v/v/v) as the solvent system
and iodine as the spray reagent (Rf = 0.88). The deter-
mined Rf value agrees with the value reported in the
literature [15]. It was further confirmed by its melting
point of 150 C (lit. mp 150–152 C). Further, benzene-
sulfonamide was subjected to GC-mass spectrum anal-
ysis. GC-MS data were obtained on a 17A Shimadzu
gas chromatograph with a QP-5050A Shimadzu mass
spectrometer using the EI ionization technique. The
mass spectrum showed a molecular ion peak at 157
amu confirming benzenesulfonamide (Fig. 1).
The presence of N-hydroxylaminobenzene-4-
sulfonic acid was confirmed by spot test [16] and
by IR spectrum: IR (KBr) showed band at (ꢀ) 3431
−
1
−1
cm for OH stretching and 3064 cm for NH
stretching. It was further confirmed by GC-MS anal-
ysis. The mass spectrum showed a parent ion peak
at 189 amu confirming N-hydroxylaminobenzene-4-
sulfonic acid (Fig. 2). All other peaks observed in
GC-MS can be interpreted in accordance with the ob-
served structure. Further, quantitative determination
of the product, N-hydroxylaminobenzene-4-sulfonic
acid, was made by mixing known quantities of oxidant
◦
◦
Figure 2 GC-Mass spectrum of N- hydroxylaminobenzene-4-sulfonic acid with its molecular ion peak at 189 amu.

CHLORAMINOMETRIC AND BROMAMINOMETRIC OXIDATION OF SULFANILIC ACID
51
and substrate in the presence of 2.0 × 10− mol dm
2
−3
Effect of [NaOH] on the Rate of Reaction. At
constant [OX]o, [SAA]o, and temperature, the reaction
rate was decreased with an increase in [NaOH]
NaOH. The product recovered was found to be 84%. It
was further observed that N-hydroxylaminobenzene-
-sulfonic acid does not undergo oxidation any further
ꢁ
4
(Table I). Plots of log k vs. log [NaOH] were linear
under the present set of experimental conditions.
(r > 0.9994) with negative slopes of unity for each of
the two oxidants, thus indicating an inverse first-order
dependence on [NaOH].
RESULTS
Effect of [BSA] on the Rate of Reaction. Ad-
dition of the reaction product, benzenesulfonamide
The kinetics of oxidation of sulfanilic acid by CAB and
BAB was investigated at several initial concentrations
of reactants in NaOH medium. A similar oxidation ki-
neticbehaviorwasobservedforboththeoxidants(OX).
−3
−3
(BSA or PhSO2NH2; 2.0 × 10 mol dm ) to the
reaction mixture did not affect the rate significantly,
indicating its noninvolvement in pre-equilibrium to
the rate-limiting step (RLS).
Effect of Ionic Strength on the Rate of Reaction.
The effect of ionic strength on the reaction rate was
carried out in a range of concentration 0.10–0.40 mol
Reaction Kinetics
Effect of Reactants on the Rate of Reaction.
−
3
Under pseudo-first-order conditions of [SAA]o ꢂ
dm with NaClO , keeping all other experimental
4
−
[OX]o at constant [OH ] and temperature, plots of
conditions constant. It was noticed that the ionic
log [OX] vs. time were linear (r > 0.9910), showing a
strength had negligible effect on the reaction rate,
indicating the involvement of nonionic species in the
rate-limiting step. Hence, no attempt was made to
keep ionic strength constant during kinetic runs.
first-order dependence of the reaction rate on [OX]o.
ꢁ
The values of pseudo-first-order rate constants (k )
are listed in Table I. Further, the rate constant did
not alter with the change in [OX]o, confirming the
first-order dependence on [OX]o. The rate was found
to increase with increase in [SAA]o (Table I), and
Effect of Dielectric Constant of the Medium
on the Rate of Reaction. The dielectric constant
(D) of the medium was varied by adding methanol
(0–20% v/v) to the reaction mixture. The rates were
not significantly affected in the case of both the
oxidants.
ꢁ
plots of log k vs. log [SAA] were linear (r > 0.9984)
ꢁ
with unit slopes. Further, plots of k vs. [SAA] gave
straight lines (r > 0.9961) passing through the origin,
confirming the first-order dependence on [SAA]o.
Table I Effect of Varying Concentrations of Oxidant, Substrate, and NaOH on the Rate of Reactiona
ꢁ
4
−1
)
k × 10 (s
4
2
−
2
[
(
Oxidant]o × 10
[SAA]o × 10
[OH ] × 10
−
3
−3
−3
mol dm
)
(mol dm
)
(mol dm
)
CAB
BAB
2
5
.50
.00
1.0
1.0
1.0
1.0
1.0
0.5
1.0
1.5
2.0
3.5
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
2.0
4.0
6.0
2.10
2.08
2.03
2.00
2.15
1.11
2.03
2.90
4.00
6.65
8.44
4.21
2.03
1.09
0.70
7.60
7.68
7.67
7.70
7.84
3.83
7.67
10.6
14.7
22.5
34.5
15.7
1
1
2
1
1
1
1
1
1
1
1
1
1
0.0
5.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
7.65
3.45
2.46
a
Temperature = 308 K.

52
PUTTASWAMY AND JAGADEESH
Table II Temperature Dependence on the Rate of Reaction and Activation Parameters for the Oxidation of SAA by
a
CAB, BAB, CAT, and BAT in Alkaline Medium
ꢁ
4
−1
)
k × 10 (s
Temperature (K)
CAB
BAB
CAT
BAT
2
3
3
3
3
98
03
08
13
18
0.72
1.14
2.03
2.85
5.00
3.75
5.56
7.67
10.1
15.4
50.7
48.1 ± 0.15
93.8 ± 0.41
−149 ± 0.20
5.46 ± 0.02
0.45
0.70
1.21
2.14
3.49
2.35
3.71
5.40
8.00
12.0
61.9
59.4 ± 0.01
95.0 ± 0.90
115 ± 0.40
6.40 ± 0.09
Ea (kJ mol 1)
−
71.8
83.7
ꢃ=
−1
ꢀ
ꢀ
ꢀ
H
G
(kJ mol )
69.3 ± 0.10
97.4 ± 0.20
−91.6 ± 0.90
8.45 ± 0.09
81.2 ± 0.05
98.3 ± 0.46
56.9 ± 0.20
10.2 ± 1.0
ꢃ=
−1
(kJ mol
)
ꢃ=
−1
−1
)
S
(J K mol
log A
a
= 1.0 × 10−3 mol dm ; [SAA]
−3
= 1.0 × 10−2 mol dm−3; [NaOH] = 2.0 × 10−2 mol dm−3
[
Oxidant]
o
o
.
−
4
−1
−4 −1
Effect of Halide Ions on the Rate of Reaction.
Addition of halide ions, Cl or Br , in the form of
(D O) is 1.04 × 10
s
and 4.20 × 10
s
, respec-
2
−
−
ꢁ
ꢁ
tively. The formal solvent isotope effect k (H O)/k
2
−
2
−2
−3
their sodium salts (1.0 × 10 –4.0 × 10 mol dm
)
D O) = 1.95 and 1.83 for these two oxidants. Proton
2
showed no effect on the rate of reaction. This suggests
that no interhalogen or chlorine (or bromine) is formed
and that there is a direct interaction of oxidizing
species with the substrate.
inventory studies were made by carrying out the reac-
tion in H O–D O mixtures with CAB and BAB, and
these results are given in Table III.
2
2
Test for Free Radicals. Addition of the reaction
mixture to the acrylamide monomer did not initiate
polymerization indicating the absence of any free rad-
icals produced during the course of reaction.
Effect of Temperature on the Rate of Reaction.
The effect of temperature on the rate was studied by
performing the kinetic experiments at various temper-
◦
atures (25–45 C), while keeping other experimental
conditions constant. From the linear Arrhenius plots
ꢁ
of log k vs. 1/T (r > 0.9908), values of activation
DISCUSSION
parameters for the overall reaction were computed
for CAB, BAB, CAT, and BAT. These results are
summarized in Table II.
Reaction Mechanism
In general, organic haloamines undergo two electron
changes in their reactions. The oxidation potential of
haloamine/sulfonamide couple is pH dependent, and
the value decreases with increase in the pH of the
medium. Since the organic haloamines have the same
chemical properties, it is expected that similar equi-
libria to exist in solution of these compounds [17,18].
Depending on the pH of the medium, CAB or BAB
furnishes different equilibria in acid and alkaline so-
lutions [11,17,19,20]. The possible oxidizing species
in acid medium are PhSO2NHX, PhSO2NX2, HOX,
Solvent Isotope Studies. The rate studies in D2O
ꢁ
ꢁ
medium for CAB and BAB revealed that while k
4
−1
−4 −1
(
H2O) = 2.03 × 10−
s
and 7.67 × 10
s
, and k
Table III Proton Inventory Studies for the Oxidation of
a
SAA by CAB and BAB in H O-D O Mixtures
2
2
ꢁ
4
−1
)
k × 10 (s
n
Atom Fraction
of Deuterium (n)
CAB
BAB
+
and H2OX . In alkaline solutions of haloamine, di-
0
0
0
0
0
.00
.24
.49
.74
.92
2.03
1.74
1.42
1.21
1.04
7.67
6.32
5.25
4.62
4.20
haloamine does not exist, and the possible oxidizing
−
−
species are PhSO NX and OX anions, which would
2
be transformed into more reactive oxidizing species
PhSO2NHX and HOX, during the course of the re-
action in alkali retarding steps. Several workers have
−
a
−3
−2
−3
−2
observed the retarding effect of OH ions on the rate
[
Oxidant]
o
= 1.0 × 10 mol dm ; [SAA]
o
= 1.0 × 10 mol
−
3
−3
dm ; [NaOH] = 2.0 × 10 mol dm ; temperature = 308 K.
of haloamine reactions with a number of substrates

CHLORAMINOMETRIC AND BROMAMINOMETRIC OXIDATION OF SULFANILIC ACID
53
Scheme 1
[
5,17,21–27] and have suggested that the reactivity
Step (ii) of Scheme 1 determines the overall rate
Rate = k2[PhSO2NHX][SAA] (2)
of weakly alkaline solutions of haloamines is due to
the formation of the conjugate acid PhSO2NHX from
−
−
PhSO2NX in a OH retarding step. Based on the pre-
ceding discussion and the observed kinetic results, a
mechanism (Scheme 1) is proposed for the oxidation
of sulfanilic acid by CAB and BAB in alkaline medium.
In Scheme 1, Y represents the complex intermediate
species whose structure is shown in Scheme 2, in which
a detailed mechanistic interpretation of SAA oxidation
by CAB and BAB in alkaline medium is proposed.
If [OX]t represents the total concentration of the
oxidant, then
−
[OX]t = [PhSO2NX ] + [PhSO2NHX]
(3)
from which, solving for [PhSO2NHX] and substituting
its value in Eq. (2), rate law (4) can be derived.
Scheme 2

54
PUTTASWAMY AND JAGADEESH
−
d[OX]t
k1k2[OX]t [SAA][H2O]
k1[H2O] + k−1[OH−]
the reaction proceeds through a single transition state,
Eq. (8) takes the form shown by Eqs. (9) and (10):
Rate =
=
(4)
dt
−
ꢁ
ꢁ
−2
j
If it is assumed that k−1[OH ] ꢂ k1[H2O], rate law (4)
k = k (1 − n + nꢁ )
(9)
n
o
is reduced to Eqs. (5) and (6)
ꢁ
ꢁ
n
1/2
(
k /k ) = [1 + n(ꢁj − 1)]
(10)
o
k1k2[OX]t [SAA][H2O]
Rate =
(5)
(6)
ꢁ ꢁ
Equation (10) shows that a plot of (k /k ) vs. n
o n
should be linear. In the present investigations, plots
of (k /k ) vs. n were indeed found to be linear (r >
1/2
−
k−1[OH ]
ꢁ
k [OX]t [SAA]
ꢁ
ꢁ
1/2
Rate =
o
n
−
k−1[OH ]
0
−
.9926) with slopes of (ꢁj − 1) = − 0.29 and =
0.27 for CAB and BAB, from which fractionation
Scheme1andratelaw(6)accountfortheobservedfirst-
order dependence of rate each on [OX]o and [SAA]o
and the inverse first-order dependence on [OH ]. The
following results support the above conclusions.
For the dipole–dipole interaction, Laidler [28] has
proposed Eq. (7):
−
factor ꢁj of OH is 0.71 and 0.73, respectively, for
these two oxidants. Kresge and Allred [33] have ob-
tainedavalueof0.80fromNMRstudiesfortheisotopic
−
−
fractionation factor of OH ion, which was confirmed
by the works of Gold and Crist [34]. There is a fair
agreement between the present value and the values
−
reported [33–36] for the fractionation factor of OH
ꢁ
ln k = ln ko + 3/8 kT (2/D − 1)
ion.
ꢁ
ꢀ
ꢁ
From Tables I and II, it is evident that the k (BAB) >
k (CAB) is obtained under identical set of experimen-
2
A
3
A
2
B
3
B
2
ꢃ
3
×
µ /r + µ /r − µ /r
(7)
ꢁ
ꢃ
tal conditions. This may be attributed to the difference
in electrophilicities of bromine and chlorine. In these
reactions electronegativity of halogens play an impor-
tant role. Electronegativity of chlorine is 2.8 and that of
bromine is 2.7. As the electronegativity increases there
is a decrease in electropositive nature. Since the posi-
tive halogen atoms forms the reactive species in these
oxidation reactions, the electropositive nature is in the
order Br > Cl. Therefore, reactivity of CAB and BAB
toward SAA is in the order: BAB > CAB. In addition,
it is also partially due to the moderate difference in van
der Waal’s radii of bromine and chlorine.
It was felt desirable to compare the rate of oxida-
tion of SAA by CAT and BAT with that of CAB and
BAB. Similarly in the case of benzene analogues of
haloamines, the reactivity is in the order: BAT > CAT,
and the overall reactivity follows: BAB > BAT > CAB
Here ko is the rate constant in a medium of infinite
dielectric constant, while µ represents the dipole mo-
ment, r refers to the radii of the reactants and activated
complex, k is the Boltzmann constant, and T is the ab-
solute temperature. It is seen that the rate should be
greater in a medium of lower dielectric constant, when
3
3
A
3
B
3
3
A
3
B
r >r + r . On the other hand, r ≈ r + r implies
ꢃ
=
ꢃ
the absence of dielectric effect of solvent on the rate,
which was found to be true in the present case.
The mechanism proposed was also supported by the
observed solvent isotope effect since the rate is retarded
in D2O medium leading to a solvent isotope effect,
ꢁ
ꢁ
k (H2O)/k (D2O) = 1.95 and 1.83 for CAB and BAB,
respectively. For a reaction involving a fast equilibrium
+
−
H or OH ion transfer, the rate was found to increase
+
−
in D2O medium since D3O and OD are stronger acid
+
−
and base, respectively, than H and OH ions [29,30].
In the present case, the observed solvent isotope effect
is greater than unity which is due to the greater basic-
ity of OD compared to OH . Further, for chemical
reactions carried out in H2O and D2O mixtures, the de-
pendence of rate on atom fraction of deuterium (n) can
be shown [31,32] by Eq. (8):
>
CAT. Further, it is also observed that the reactivity
is higher in benzene analogues of haloamines com-
pared with toluene analogues. It is due to the fact that
there is a slight difference in the electrophilicities of
toluene derivatives due to inductive effect of methyl
group and hence the observation made in the present
case indicates the +I effect due to CH3 group of toluene
derivatives. From the above facts, it can be generalized
that bromamines are strong oxidants compared to chlo-
ramines and also benzene derivatives of haloamines are
more reactive than toluene derivatives for the oxidation
of SAA. In the present study, it is substantiated by the
overall reactivity is BAB > BAT > CAB > CAT.
Furthermore, these results are also compared
with the results of oxidation kinetics of several
−
−
TS
RS
ꢁ
ꢁ
o
k /k = π(1 − n + nꢁi )/ π(1 − n + nꢁj )
(8)
n
i
j
Here ꢁi and ꢁj are isotopic fraction factors for iso-
topically exchangeable hydrogenic sites in the transi-
tion state (TS) and reactant state (RS), respectively. If

CHLORAMINOMETRIC AND BROMAMINOMETRIC OXIDATION OF SULFANILIC ACID
55
ꢃ
ꢃ
ꢁ
ꢁ
Figure 3 Isokinetic plots of (a) ꢀH vs. ꢀS (b) log k(
vs. log k
. Experimental conditions are as in Table II.
308 K)
(298 K)
substrates studied using N-haloamines. The ratio of
are almost the same in the cases of all the oxidants used,
suggesting that the oxidation of SAA by CAT, CAB,
BAT, and BAB proceeds by a similar mechanism. The
constancy of rate constant on addition of neutral salts,
benzenesulfonamide, and halide ions also supports the
proposed mechanism and the derived rate law.
ꢁ
ꢁ
k (bromamines)/k (chloramines) in the case of indigo
carmine [37], isoniazid [5], indoles [38], dialkylsul-
fides [39], conjugate alcohols [40], phenethyl alcohols
[41], and monosaccharides [42] lies between 2 and 10.
In the present case this ratio is about 4. This indicates
that the relative rates of oxidation by these oxidants are
dependent on the type of substrates chosen.
CONCLUSION
The proposed mechanism is also substantiated by
the moderate values of energy of activation observed
and other activation parameters. The data in Table II
show that the energy of activation is highest for the
slowest reaction and vice versa as expected, indicat-
ing that the reaction is enthalpy controlled. The val-
The oxidation of sulfanilic acid to N-
hydroxylaminobenzene-4-sulfonic acid by CAB
and BAB in alkaline medium followed the identical
kinetics with the first-order dependence of the reaction
rate each on [oxidant]o and [substrate]o, and the
ꢃ=
ꢃ=
ues of ꢀH and ꢀS for the oxidation of SAA by
all the four oxidants are linearly related (r = 0.9992;
Fig. 3) and the isokinetic temperature β = 358 K. The
genuine nature of the isokinetic relationship was veri-
−
inverse first-order dependence on [OH ]. Activation
parameters for overall reaction were deduced. Proton
inventory studies have been made. The proposed
mechanism and the derived rate law are consistent
with the observed kinetics. The relative reactivity of
CAT, CAB, BAT, and BAB toward SAA was compared
and discussed.
ꢁ
fied by the Exner criterion [43] by plotting log k
(308K)
ꢁ
vs. log k(
298K)
; this plot is linear (r = 0.9980; Fig. 3).
The values of β calculated from the equation β = T1
(
1–q)/(T1/T2) – q, where q is the slope of the Exner plot
and T1 > T2. The value of β was found to be 368 K.
The values of β evaluated from both the plots are much
higher than the range of temperature employed in the
present work (298–318 K). This indicates a common
enthalpy controlled pathway for all the reactions. The
The authors are thankful to Professor B.S. Sheshadri and
Professor M.A. Pasha for their valuable suggestions.
ꢃ
ꢃ=
fairly high positive values of ꢀH and ꢀG suggest
thatthetransitionstateishighlysolvated, whilethehigh
negative entropy of activation indicates the formation
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ꢃ=
of a rigid transition state. Further, the values of ꢀG

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