Indian Journal of Chemistry
Vol. 55A, September 2016, pp. 1074-1079
Kinetics and mechanism of electron transfer
reaction: Oxidation of sulfanilic acid by
hexachloroiridate(IV) in acid medium
Riya Sailani, Deepmala Pareek, Anita Meena,
Kritika Jangid & Chandra L Khandelwal*
Department of Chemistry, University of Rajasthan,
Jaipur 302 055, India
Email: [email protected]
Received 8 December 2015; revised and accepted 21 August 2016
The kinetics of oxidation of sulfanilic acid by [IrCl6]2- in acid
medium has been studied. The reaction is overall second order
being first order with respect to each reactant. However, rate is
retarded by hydrogen ion concentration. Various proposals have
been suggested on the basis of reactivity of Ir(IV) species. The
activation parameters such as energy and entropy of activation
calculated by Eyring equation are found to be 59.99±0.93 kJ mol-1
and -106.44±2.2 J K-1 mol-1 respectively. The oxidation product,
2-keto-azoxy- benzene-4,4′-disulfonic acid has been confirmed
spectrally.
Keywords: Kinetics, Reaction mechanisms, Oxidation, Electron
transfer reactions, Sulfanilic acid, Hexachloroiridate(IV)
Iridium(IV) has been extensively employed both as an
oxidant and as a catalyst in electron transfer reactions.
It is one-equivalent in nature and is reduced to
iridium(III), the reduced form of Iridium(III) being
employed as a catalyst.1-6 The kinetics employing
Ir(IV) both as an oxidant and catalyst has also been
reported.7-9 Hexachloroiridate(IV) is an oxidant of
choice because of easily handling and acts as oneelectron oxidant and is often treated as an outersphere oxidant.10-14 Certain oxidation reactions of
Ir(IV) are trace metal-ion catalyzed. Stanbury et al14.
have reported oxidation of thioglycolic acid
in the presence of chelating agent such as
bathophenenthroline disulfonate for the trace metal
ions. This interesting observation prompted us to
undertake the kinetics of oxidation of sulfanilic
acid by hexachloroiridate(IV) in acid medium to
understand its role as an outer-sphere oxidant as well
as the role of trace metal-ion in this reaction.
Experimental
Iridium(IV) chloride hydrate (Aldrich) was
employed as received without any further treatment.
Aqueous solution of the reagent was prepared by
dissolving its requisite amount in doubly distilled
water. The solution is quite stable and its stability is
further enhanced if it is kept in bottles coated
black from the outside and stored at refrigerated
temperature (~5 ºC).
However, a freshly prepared solution of
hexachloroiridate(IV) was employed as and when
required. Other reagents were of GR or AnalaR grade
and were employed as received. Doubly distilled
water was employed throughout the study; second
distillation was from alkaline permanganate solution
in an all-glass still. Doubly distilled water was further
re-distilled in the presence of EDTA to remove
trace metal-ions.
The stoichiometry of the reaction was determined
by taking sufficiently large excess of iridium(IV) over
that of sulfanilic acid. The excess [Ir(IV)] was
estimated after completion of the reaction employing
UV-visible spectrophotometer at λmax, 485nm31
(ε, 4050 dm3 mol-1 cm-1). The results indicated that
4 moles of Ir(IV) were required for one mole of
sulfanilic acid, corresponding to the stoichiometry of
the reaction as represented by Eq. (1).
NH2
+ 8[IrIVCl6]2- + 2H2O
2
H+
8[IrIIICl6]3- + 8H+ +
O
O
SO3H
HO3S
N
N
SO3H
… (1)
The oxidation product of sulfanilic acid has further
been identified spectrally to be 2-keto-azoxybenzene4,4'-disulfonic acid. The quantitative estimation of
this product was not possible in the light of complete
isolation of this product.
Thin layer chromatograms were conducted on
Merck silica gel G plates in methyl alcohol:
acetonitrile (7:3) and in the column chromatographic
fractionation, silica gel (60-120 mesh) was used.
Spots on TLC plates were visualized by spraying
with 2% ceric ammonium sulphate in 2N H2SO4 or
with iodine vapours.
The IR spectrum of the product showed absorption
bands at 1120 cm-1 and 1040 cm-1 indicating two
stretching bands of –C6H4SO3H. Also, absorption
NOTES
Results and discussion
The concentration of Ir(IV) was varied from 2×10-5
to 1.0×10-4 mol dm-3 at three different but fixed
concentrations of sulfanilic acid (heretofore written as
SA) to be 1.0×10-2, 2.0×10-2 and 3.0×10-2 mol dm-3
respectively and also keeping constant concentrations
of hydrogen ion to be 0.1 mol dm-3 at 45 °C. The
pseudo-first order rate constants (k', s-1) were found to
be independent of gross initial concentrations of
Ir(IV), confirming first order dependence with respect
to the oxidant (Table 1).
The concentration of sulfanilic acid was varied
from 5.0×10-3 to 3.0×10-2 mol dm-3 keeping constant
concentrations of other reaction ingredients, viz,
[IrCl6]2- = 2.0×10-4 mol dm-3 and [HClO4] = 0.1 mol dm-3
at 45 °C. Pseudo-first order rate constants were
evaluated and a plot of these rate constants versus the
concentration of sulfanilic acid yielded a straight line
passing through the origin (Fig. 1) confirming first
order with respect to substrate (Table 1).
The effect of hydrogen ion concentration was
studied by employing perchloric acid. The
concentration of hydrogen ion was varied from
0.1 to 0.5 mol dm-3 at fixed concentrations of
other reactants i.e., [SA] = 2.0×10-2 mol dm-3,
[IrCl6]-2 = 2.0×10-4 mol dm-3 and [LiClO4] = 1.0 mol dm-3
8
7
6
5
104 k (s-1)
at 1320 cm-1 and 1440 cm-1 respectively indicated the
presence of –N=N→O strong stretching bands while
the band for >C=O was observed at 1727 cm-1. The
bands at 1500 cm-1 and 1230 cm-1 indicated the -N=Nand –C-N= stretching frequency of azoxy group.
1
H NMR spectrum was also obtained in DMSO-d6
employing 300 MHz spectrometer using TMS as
reference. In 1H NMR spectrum, singlet as obtained
at δ 2.49 ppm shows the presence of –OH group.
Peaks in the range δ 5-8 ppm were due to aromatic
rings. The mass spectrum of this product displayed a
peak at m/z 372, after the loss of two hydrogens
from its molecular ion of 374, in agreement with
earlier reports.30
To study the kinetics, the reaction mixtures
were taken in glass stoppered Erlenmeyer flasks,
painted black from the outside and were suspended
in a water-bath thermostated at 45±0.1 ºC unless
stated otherwise. The reactions were initiated by
adding thermally pre-equilibrated solution of Ir(IV).
The kinetics of the reaction was monitored
spectrophotometrically measuring absorbance of the
remaning [Ir(IV)] at 485 nm periodically. Since the
reaction was studied under pseudo-first order
conditions, pseudo-first order plots were made.
The rate constants in triplicate were reproducible to
within ± 4%.
1075
4
3
2
1
0
0
5
10
15
20
25
30
35
103 [SA] (mol dm-3)
Fig. 1 – Variation of concentration of sulfanilic acid in
oxidation of sulfanilic acid by hexachloroiridate(IV). {[IrCl6]-2
= 2.0×10-4 mol dm-3; [HClO4] = 0.1 mol dm-3; temp. = 45 °C}.
Table 1 – Pseudo-first order rate constants and second order rate
constants in the reaction of sulfanilic Acid with Ir(IV) in acid
perchlorate medium. {[HClO4] = 0.1 mol dm–3, temp. = 45 °C}
102[SA]
(mol dm–3)
105[Ir(IV)]
(mol dm–3)
104 (k')
(s–1)
102 (k)
(dm3 mol-1 s-1)
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
3.0
3.0
3.0
3.0
3.0
0.5
0.75
1.0
1.5
2.0
2.5
3.0
2.0
4.0
6.0
8.0
10.0
2.0
4.0
6.0
8.0
10.0
2.0
4.0
6.0
8.0
10.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
3.84
3.84
3.84
3.84
3.84
7.68
7.68
7.68
7.68
7.68
11.52
11.52
11.52
11.52
11.52
1.92
2.88
3.84
5.76
7.68
9.6
11.51
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
3.84
INDIAN J CHEM, SEC A, SEPTEMBER 2016
1076
(ionic strength was kept constant by employing
lithium perchlorate) at 30, 35, 40, 45 and 50 ºC
respectively. The rate decreases with increasing
hydrogen ion concentration.
The effect of ionic strength was studied using
lithium perchlorate keeping the concentrations of
other reactants i.e., [SA] = 2.0×10-2 mol dm-3; [IrCl6]2= 20×10-4 mol dm-3 and [H+] = 0.1 mol dm-3 at 45 ºC.
The rate increases with increasing ionic strength
(Fig. 2). Also, effect of ionic strength was studied
by employing sodium perchlorate and sodium
nitrate respectively; the rate increases in both these
cases also. These results indicate that perchlorate and
nitrate ions behave almost similarly, negating any
specific anion effect. Similarly, there is no specific
cation effect.
The effect of temperature was studied at 30, 35, 40,
45 and 50 °C at fixed concentrations of other
reactants, i.e., [SA] = 2.0×10-2 mol dm-3; [IrCl6]2- =
2.0×10-4 mol dm-3, [H+] = 0.1 mol dm-3 and [LiClO4]
= 1.0 mol dm-3. The activation parameters such as
energy and entropy of activation were calculated by
employing Eyring equation.16 The energy and entropy
of activation were calculated to be 59.99±0.93 kJ mol-1
and −106.44±2.2 J K-1mol-1 respectively.
[IrCl6]2- is inert17-21(a) to substitution and as such
this reaction does not appear to occur via an innersphere mechanism. In all probability, an outer-sphere
complex is formed which on decomposition yields the
[IrCl଺ ]ଶି + SA
௞భ
௞షభ
ି ଷି
[IrClଶି
+ Hା
଺ . SA ]
௞మ
ି ଷି
[IrClଶି
՜ [IrCl଺ ]ଷି + SAᇱ
଺ . SA ]
… (2)
… (3)
Fୟୱ୲
[IrCl6 ]2- + SA' ሱۛሮ [IrCl6 ]3- +Product of sulfanilic acid
… (4)
Other fast steps in the stoichiometry preceding
step (4) are as in Scheme 1.
SA' is an intermediate oxidation product of
sulfanilic acid, which on further interaction with
another molecule of [IrCl6]2- yields 2-keto-azoxy
benzene-4,4'-disulphonic acid as the end product.
Applying steady state treatment to the intermediate,
such a mechanism leads to the rate law (5) or (6).
ௗ[I୰(IV)]
ௗ௧
଼௞భ ௞మ [I୰(IV)][SA]
௞షభ [Hశ ]ା௞మ
12
−
10
where [Ir(IV)] and [SA] are the gross analytical
concentrations of the oxidant and substrate respectively.
8
104 k (s-1)
oxidation products. Since [IrCl6]3- does not affect the
rate, any equilibrium preceded by the rate controlling
step involving [IrCl6]3- is ruled out. Moreover, rate is
not affected by Cl- which further eliminates any
possibility of complex formation between oxidant and
substrate through dissociation of [IrCl6]2-. However,
rate is retarded by hydrogen ion concentration, which
in all probability is not co-related to the oxidant.
Thus considering first order with respect to the
oxidant and substrate each and retarding effect
of hydrogen ion concentration, the following
mechanism I consisting of steps (2)-(4) to account
for experimental observations can be envisaged.
Mechanism I
݇=
6
=
௞షభ [Hశ ]
଼௞భ ௞మ
ଵ
+ ଼௞
… (5)
… (6)
భ
where ‘k’ is an observed second order rate constant
(Table 1).
The double reciprocal of Eq. (6) on further
re-arrangement yields Eq. (7).
4
2
ଵ
௞
0
0
0.2
0.4
0.6
0.8
1
1.2
[LiClO4] (mol dm-3)
Fig. 2 – Variation of concentration of LiClO4 in the reaction of
sulfanilic acid with hexachloroiridate(IV). {[SA] = 2.0×10-2 mol dm-3;
[IrCl6] -2 = 2.0×10-4 mol dm-3; [HClO4] = 0.1 mol dm-3;
temp. = 45 °C}.
=
௞షభ [Hశ ]
଼௞భ ௞మ
ଵ
+ ଼௞
… (7)
భ
A plot of 1/k versus [H+] was plotted from Eq. (7),
which yielded a straight line with non-zero intercept
(Fig. 3). ‘k1’ was calculated from the intercept to be
(2.25±0.2)×10-1, (2.88±0.2)×10-2, (4.0±0.4)×10-2,
(4.75±0.4)×10-2 and (6.25±0.5)×10-2 dm3 mol-1 s-1 at
30, 35, 40, 45 and 50 °C respectively and [LiClO4] =
1.0 mol dm-3.
NOTES
1077
H
HO3S
+ IrCl62-
NH2
2-
N IrCl62-
HO3S
H
H
IrCl62-/H2O
H+ + HO S
3
N
HO3S
Fast
+ IrCl63-
NH + IrCl63- +H+
OH
2 IrCl62- Fast
O
N + 2IrCl63- + 2H+
HO3S
HO3S
NH2 + IrCl62-
NH + IrCl63- +H+
HO3S
IrCl62-/H2O
Fast
H
H+ + HO3S
IrCl63- +
N
OH
O
HO3S
N
H
HO3S
+
N
OH
Fast
O
HO3S
N
N
SO3H + H O
2
H2O2IrCl62O
HO3S
N
O
N
SO3H+ 2IrCl
H
Scheme I
Scheme 1
36
+ 2H+
INDIAN J CHEM, SEC A, SEPTEMBER 2016
1078
valid in view of k2' being very close to the diffusion
control limit (~ 1010 dm3 mol-1 s-1). This may also
account for the large favorable variation in free
energy of the reaction step (11). Under such
conditions the rate law (12) is reduced to Eq. (13),
300
1
250
k-1 (mol dm-3 s)
200
2
150
−
3
100
4
50
5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
[H+] (mol dm-3)
ௗ[I୰C୪ల ]మష
ௗ௧
An alternative mechanism where hydrogen ion
dependence can be co-related to sulfanilic acid
without invoking any ion-pair type complex can also
be proposed. Since [IrCl6]2- is a well established
one-electron outer-sphere oxidant, its reaction is
non-complementary with sulfanilic acid and takes
place through successive electron transfer by the
following (Mechanism II),
Mechanism II
SA
௄
SAି + H ା
[IrCl଺ ]ଶି + SAି
[IrCl଺ − SA]ଷି
… (8)
௞భᇲ
ᇲ
௞షభ
௞మᇲ
[IrCl଺ − SA]ଷି
[IrCl଺ ]ଷି + SA
… (9)
… (10)
Fୟୱ୲
[IrCl଺ ]ଶି + SA ሱۛሮ [IrCl଺ ]ଷି + Oxidation product
of sulfanilic acid
… (11)
followed by other fast steps without having any
bearing on overall kinetics of the reaction.
Applying steady state approximation to the free
radical intermediate, the following rate law (12) is
obtained.
−
ଶ
8‫݇ܭ‬ଵᇱ ݇ଶᇱ [IrClଶି
݀[IrCl଺ ]ଶି
଺ ]T [SA]T
= ᇱ
ᇱ
݇ିଵ [IrCl଺ ]ଷି + (݇ଶ [IrCl଺ ]ଶି )(‫ ܭ‬+ [H ା ])
݀‫ݐ‬
… (12)
Since the rate is not retarded by [IrCl6]3-, an
inequality (k2' [IrCl6]2-) (K + [H+]) >> k-1' [IrCl6]3- is
଼௞భᇲ ௄[I୰C୪ల ]మష
T [SA]T
[Hశ ]ା௄
… (13)
It has earlier been reported22-26 that in the oxidation
of one-equivalent oxidants such as Mn3+(aq) with
quinols and Co3+ (aq) with quinols and catechols, the
rate determining step involves the oxidation of these
compounds to their free radicals.
The rate law (13) is further reduced to Eq. (14),
଼௞ ᇲ ௄
Fig. 3 – Variation of HClO4 in the reaction of sulfanilic acid
with hexachloroiridate(IV). {[SA] = 2.0×10-2 mol dm-3; [IrCl6]-2
= 2.0×10-4 mol dm-3; [LiClO4] = 1.0 mol dm-3; temp. = 1, 30°;
2, 35°; 3, 40°; 4, 45°; 5, 50 °C}.
=
݇ ᇱ = [Hశభ]ା௄
… (14)
where ݇ ᇱ , is the second order rate constant.
The rate law (14) is similar to the rate law (6) with
the only difference of k2 = K. Thus, Mechanisms I and
II are essentially similar.
The rate law provides no information on whether
electron-transfer is inner-sphere or outer-sphere.
Nevertheless, an examination of the magnitude of the
rate constant may provide some information. Since
the rate constant of the redox reaction step is greater
than the rate of substitution, the reaction is certainly
an outer-sphere. The oxidant is highly inert to
substitution, aquation of first Cl- being of 10-6 s-1.
The fast step involves ion-pairing of substrate
and IrCl62-, i.e., an outer-sphere followed by
slow electron transfer within the complex.
Outer-sphere complex is diffusion controlled with
the formation of an intermediate with rate constant
being ~10-2 dm3 mol-1 s-1. If perturbation of the
intermediate is larger than that expected for an
outer-sphere complex or ion-pair, an inner-sphere
type mechanism is a possibility.
However, it is difficult to distinguish between innersphere and outer-sphere mode of electron transfer.
Since the complexes of second and third
transition series of metals having coordination
number seven are known,21(b), 27-28 there is a possibility
of reactivity of Ir(IV) in aqueous media. However,
hexachloroiridate(IV) is known to be stable29-32
towards substitution or hydrolysis over a wide
range of acidity. Although the solution of [IrCl6]2- is
quite stable for long periods (>24 h), fresh solution
of the oxidant was always employed in kinetics
studies, since the possibility of [IrCl6 (OH)]3- to
NOTES
1079
7
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be the reactive form is not likely under the
experimental conditions.
As far the mode of electron transfer from the
substrate to the oxidant is concerned, the proposed
Scheme 1 can account for the reaction events.
Thus, the oxidation product of sulfanilic acid is
2-keto-azoxybenzene-4,4'-disulfonic acid, corresponding
to the stoichiometry of the reaction. Similar oxidation
product has earlier been reported in oxidation of
sulfanilic acid by hexacyanoferrate(III)33 an oxidant
of one-equivalent nature.
In the present study, the reaction of sulfanilic acid
with hexachloroiridate(IV) is second order in which
the rate is retarded by hydrogen ion concentration
which in all probability is not co-related to the
oxidant. Since [IrCl6]3- does not affect the rate, any
equilibrium preceded by the rate controlling step
involving [IrCl6]2- is ruled out. In all probability, an
outer-sphere complex is formed which on
decomposition yields the oxidation product, viz.,
2-keto-azoxybenzene-4,4'-disulfonic acid.
Acknowledgement
This work was supported in part by the University
Grant Commission, New Delhi, India, through senior
research fellowship to Riya Sailani as financial support.
8
9
10
11
12
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14
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