Kinetic and mechanistic studies on the oxidation of hydroxylamine, semicarbazide, and thiosemicarbazide by iron(III) in the presence of triazines

Article abstract of DOI:10.1007/s11243-012-9609-0

In the presence of 3-(2-pyridyl)-5,6-bis(4-phenylsulphonicacid)- 1,2,4-triazine disodium salt (PDTS), 3-(4-(4- phenylsulphonic-acid)-2-pyridyl)- 5,6-bis(4-phenylsulphonicacid)- 1,2,4-triazine trisodium salt (PPDTS), or 2,4-bis(5,6- bis(4-phenylsulphonic-acid)-1,2,4-triazin-3-yl)pyridine tetra sodium salt (BDTPS), iron(III) oxidizes hydroxylamine to nitrogen gas, semicarbazide to CO₂ and NH₃ and thiosemicarbazide to a disulfide. The corresponding iron product is the 1:3 complex of iron(II) and PDTS, PPDTS, or BDTPS. The kinetics of these reactions was studied by monitoring the iron( II) product by conventional spectrophotometry. The reaction is first order in iron(III).Kinetic evidencewas obtained for the formation of 1:1:2 ternary complexes of iron(III), substrate, and sulfonated triazine. Evidence for the ternary intermediate complexes was obtained by ion-exchange studies using ⁵⁹Fe-labeled iron(III) solutions. The dissociation of the ternary complex is identified as the rate-determining step. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11243-012-9609-0

Transition Met Chem (2012) 37:453–462
DOI 10.1007/s11243-012-9609-0
Kinetic and mechanistic studies on the oxidation
of hydroxylamine, semicarbazide, and thiosemicarbazide
by iron(III) in the presence of triazines
Susarla Ratnam Nageswara Rao Anipindi
•
Received: 8 February 2012 / Accepted: 11 April 2012 / Published online: 1 May 2012
Ó Springer Science+Business Media B.V. 2012
Abstract In the presence of 3-(2-pyridyl)-5,6-bis(4-phenyl-
sulphonicacid)-1,2,4-triazine disodium salt (PDTS), 3-(4-(4-
phenylsulphonic-acid)-2-pyridyl)-5,6-bis(4-phenylsulphonic-
acid)-1,2,4-triazine trisodium salt (PPDTS), or 2,4-bis(5,6-
bis(4-phenylsulphonic-acid)-1,2,4-triazin-3-yl)pyridine tetra
sodium salt (BDTPS), iron(III) oxidizes hydroxylamine to
nitrogen gas, semicarbazide to CO and NH and thiosemi-
uranium(VI) [1], molybdenum [2], and vanadium(IV) [3]
by potentiometric methods, titrating against iron(II) in
strong phosphoric acid medium. The greater reducing
ability of iron(II) in concentrated phosphoric acid solutions
is due to the fact that phosphoric acid forms a stronger
complex with iron(III) than iron(II). On the other hand,
0
diimines like 2,2 -bipyridine (bpy) and 1,10-phenanthroline
2
3
carbazide to a disulfide. The corresponding iron product is the
:3 complex of iron(II) and PDTS, PPDTS, or BDTPS. The
(phen) are known to stabilize iron(II). In the presence of
1
these two diimines, there is an increase in the redox
3
2
þ
2þ
;
kinetics of these reactions was studied by monitoring the iro-
n(II) product by conventional spectrophotometry. The reac-
tionis firstorderin iron(III). Kinetic evidence wasobtained for
the formation of 1:1:2 ternary complexes of iron(III), sub-
strate, and sulfonated triazine. Evidence for the ternary inter-
mediate complexes was obtained by ion-exchange studies
potential of the Fe(III)/Fe(II) couple ½Fe(bpy) =Fe(bpy)2
3þ
2þ
2
E ¼ 0:97 V; Fe(phen) = Fe(phen) ; E ¼ 1:06 Vꢀ. Thus,
2
iron(III) acts as a better oxidizing agent in the presence of
diimines. The kinetics and mechanism of the oxidation of
iron(II) [4], vanadium(IV) [5], cobalt(II)-EDTA [6], water
[
7], ferrocyanide [8], catechols [9], mercury(I) [10], thio-
3þ
59
using Fe-labeled iron(III) solutions. The dissociation of the
ternary complex is identified as the rate-determining step.
cyanate[11], and titanium(III) [12] by Fe(bpy)2 and
3
þ
Fe(phen) have been studied. The oxidation of iron(II) by
3þ
2
0
00
bis-(2,2 ,6,2 -terpyridine)iron(III) Fe(terpy) was studied
2
by Ford-Smith and Sutin [13]. These reactions are fast and
were monitored by stopped-flow spectrophotometry. Con-
versely, oxidation of H O , indigo carmine, and aromatic
Introduction
2
2
Gopala Rao and coworkers found that in the presence of
phosphoric acid, the formal redox potential of the Fe(III)/
Fe(II) couple decreases while those of the U(VI)/U(IV),
Mo(VI)/Mo(V), V(V)/V(IV), V(IV)/V(III), and Cr(VI)/
Cr(III) couples increase. The same group estimated
amines by iron(III) is extremely slow. However, in the pres-
ence of diimines, the oxidation is sufficiently fast to be mon-
itored by conventional spectrophotometry. Bexandale [14]
studied the oxidation of H O by iron(III) in the presence of
2
2
bpy and postulated a 1:2 complex of iron(III) and bpy as the
reactive oxidizing species. Subba Rao et al. studied the oxi-
dation of indigo caramine [15], phenols [16], and aromatic
amines [17] by iron(III) in the presence of bpy/phen and
identified a 1:2 iron(III)–bpy/phen complex as the reactive
species of iron(III). Komissarov and Davisov [18] investi-
gated the oxidation of methyl ethyl ketone by iron(III) in the
presence of phen and envisaged a similar behavior. Krishna
Rao et al. studied the oxidation of hydrazine [19],
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-012-9609-0) contains supplementary
material, which is available to authorized users.
S. Ratnam ꢁ N. R. Anipindi (&)
Department of Physical and Nuclear Chemistry and Chemical
Oceanography, Andhra University, Visakhapatnam 530 003,
India
e-mail: anipindinr@gmail.com
123

4
54
Transition Met Chem (2012) 37:453–462
aroylhydrazides [20], and hydroxylamine [21] by iron(III) in
the presence of phen and obtained evidence for the formation
of a 1:2 complex between iron(III) and phen. Phen and bpy
were extensively used in the spectrophotometric determina-
tion of iron(II) in the older literature. Systematic studies by
Smith and Case have produced a variety of outstanding
chromogenic reagents for iron and copper. An important
aspect of their studies was the discovery that phenyl substit-
uents in positions para to the ferroin nitrogen atoms greatly
enhance the molar absorptivities of the metal chelates.
Prompted by this principle, Case synthesized many com-
pounds which are sensitive metal chromogenic reagents. 3-
was checked by passing 5.0 ml aliquots through a Dowex-
?
50W-X8 cation exchange resin column (H form) and esti-
?
mating the resultant H ions after thoroughly washing the
resin bed with distilled water.
5
9
A sample of Fe (as ferric chloride in HCl t‘ = 45 days
and emits two c-rays of 1.10 and 1.29 MeV) obtained from
Bhabha Atomic Research Centre, Mumbai, was used to
label iron(III) solutions.
Equipment
(
(2-pyridyl)-5,6-diphenyl)-1,2,4-triazine, PDT, 3-(4-(4-phe-
A Shimadzu 1800 UV–visible spectrophotometer fitted
with a UV-240A Cell Positioner having a six cell holder in
which the temperature was controlled to ± 0.1 °C by Peltier
effect was used to record the absorption spectra and
monitor the reaction kinetics. An Orion Ionalyser/901 was
used for pH measurements. The slow reactions were fol-
lowed in CPS running mode and relatively fast reactions
monitored in command chain mode using suitable soft-
ware. Gamma activity measurements were taken with
single channel analyzer SC 800 (ECIL) in conjunction with
nyl-2-pyridyl)-5,6-di-phenyl-1,2,4-triazine, PPDT and 2,4-
bis(5,6-bis(4-phenyl)-1,2,4-triazin-3-yl)pyridine BDTP are
some such compounds. These compounds are insoluble in water,
and therefore, the sulphonated triazines, 3-(2-pyridyl)-5,6-bis(4-
phenyl-sulphonicacid)-1,2,4-triazine disodium salt(PDTS),
3
-(4-(4-phenylsulphonicacid)-2-pyridyl)-5,6-bis(4-phenylsul-
phonicacid)-1,2,4-triazine(PPDTS), and 2,4-bis(5,6-bis(4-
phenylsulphonic-acid)-1,2,4-triazin-3-yl)pyridine tetra sodium
salt(BDTPS) (hereafter referred to generically as tz) are
extensively used in the spectrophotometric determination of
iron(II) in trace quantities. We have observed that hydroxyl-
amine, semicarbazide (scz), and thiosemicarbazide (tsc) are
oxidized by iron(III) in the presence of these sulfonated tria-
zines. The results of kinetic studies on these reactions are pre-
sented in this paper.
0
0
a 3 well-type NaI (Tl) crystal (Nuclear Chicago).
Product analysis
A solution containing substrate NH OH, tz (PDTS/PPDTS/
2
BDTPS), and HNO was prepared in a 25-ml volumetric
3
flask and to this the requisite volume of iron(III) nitrate
solution was added. After a few minutes, a colorless and
odorless gas which extinguished a burning splint was
Experimental
-
1
-3
A 1.0 9 10 mol dm ferric nitrate solution (Merck ‘‘pro
analysi’’) was prepared by dissolving the requisite quantity
in distilled water. This solution was standardized against
evolved. This gas did not respond to the test for N O. From
2
these observations, it was inferred that this gaseous product
-3
-
is nitrogen. The requisite volume of 1.0 9 10 mol dm
3
-
1
EDTA using variamine blue as indicator [22]. A 2.0 9 10
-
iron(III) nitrate solution was added to a mixture of scz, tz, and
nitric acid. A colorless and odorless gas which extinguished a
burning splint and turned lime water milky was evolved. This
product solution on heating with NaOH gave ammonia,
which formed a reddish brown precipitate with Nessler’s
reagent. Hence, it is concluded that CO and NH are the
3
mol dm solution of hydroxylamine was prepared by dis-
solving the requisite quantity of hydroxylamine sulfate
(
BDH, AnalaR) in distilled water. A calculated amount of
Ba(NO ) was added to this solution to precipitate sulfate ion
2
3
as BaSO . The precipitate was removed by filtration and the
4
2
3
hydroxylamine content of the filtrate estimated by a bro-
products of the oxidation of scz by iron(III) in the presence of
tz. This procedure was repeated using tscz instead of semi-
carbazide. The results did not suggest the formation of CO2
-
1
-3
matometric method [23]. 1.0 9 10 mol dm solutions of
scz and tscz were prepared by dissolving the requisite
quantities of semicarbazide hydrochloride and thiosemicar-
bazide (Merck ‘‘pro analysi’’) in distilled water and stan-
dardized iodometrically [24]. PDTS, PPDTS, and BDTPS
received from GFS Chemicals Inc., USA, were used as such
and NH ; however, a sulfurous odor was noticed. This sug-
3
gests the formation of a disulfide. The product solution was
extracted with ether, and the ethereal layer was washed with
-
0.1 mol dm HNO and allowed to stand until the ether had
3
3
-
2
-3
without any further purification. 1.0 9 10
mol dm
evaporated. Elemental analysis of this solid residue by
sodium fusion method gave positive tests for N and sulfur,
further supporting the formation of a disulfide product.
The formation of free radical intermediates was tested
by using acrylonitrile. These tests were performed by
solutions of tz were prepared by dissolving the requisite
quantities in water and stored in amber-colored bottles. A
-
3
2
.0 mol dm solution of NaNO (Merck ‘‘pro analysi’’)
3
was prepared in distilled water. The strength of this solution
1
23

Transition Met Chem (2012) 37:453–462
455
taking requisite quantities of the substrate, triazine, HNO3,
and acrylonitrile in the lower limb of a Thunberg tube and
iron(III) in its upper limb. The tube was evacuated and then
tilted to mix the contents of the two limbs. In the case of
oxidation of scz by iron(III) in the presence of all three
sulfonated triazines, the formation of a precipitate was
observed. Precipitate formation was not observed with
ðnꢂ1Þꢂ
stands for monoprotonated PDTS, PPDTS,
where HL
and BDTPS and n = 2, 3, and 4 for PDTS, PPDTS, and
BDTPS, respectively.
Kinetic procedure
NH OH or tscz.
2
Requisite volumes of HNO , tz, substrate, and NaNO
3
3
Known amounts of NH OH/scz/tscz, PDTS, HNO and
3
2
solutions were taken in an amber-colored 25-ml volumetric
flask, and the contents were diluted with distilled water such
that the total volume after the addition of iron(III) solution
was 10 ml. The volumetric flask was suspended in a water
bath kept at the experimental temperature for sufficient time
to allow the reaction mixture to attain temperature equilib-
rium. Then, a known volume of iron(III) solution, which was
also equilibrated at the experimental temperature was added.
The contents were thoroughly mixed, and a portion of this
solution was transferred quickly to the spectrophotometer.
The absorbance values at different time intervals were
recorded at 562, 563, and 565 nm for PDTS, PPDTS, or
BDTPS, respectively, using CPS recording or command
chain mode depending on the rate of the reaction. In all these
experiments, a solution containing all the reactants except
the substrate was used as the blank. The absorbance at infi-
iron(III) nitrate were taken in an amber colored 25-ml
volumetric flask and the contents were made up to the mark
with distilled water. The reaction mixture was left for
sufficient time for the reaction to complete. All three
substrates give a magenta-colored product. The visible
absorption spectra of the products exhibit a single peak at
5
62 nm with a e value of 28,000 ± 200 [based on the
concentration of iron(III) taken] and these values agree
4
ꢂ
well with that of Fe(PDTS)3 (27,900 at 562 nm [25]).
This indicates that iron(III) is quantitatively reduced to
iron(II). Similarly, the visible absorption spectra of the
products in the presence of PPDTS and BDTPS show peaks
at 563 and 565 nm, respectively, and these values corre-
7
ꢂ and Fe(BDTPS) 13 0ꢂ
spond to those of Fe(PPDTS)3
. The e
values of the products obtained in the presence of PPDTS
and BDTPS are 30,700 ± 200 and 32,200 ± 180 respec-
7
nite time, A , was obtained by allowing the reaction mix-
?
ꢂ
10ꢂ
tively, [e values of Fe(PPDTS)3 and Fe(BDTPS)3 are
0,700 and 32,200 [26], respectively].
tures to stand for sufficient time for the reactions to complete.
The pseudo-first-order rate constants were evaluated from
plots of time, t versus log(A? - A ) where A is the absor-
3
t
t
bance at time t, computed using an MS Excel worksheet.
About 25 % of the kinetic runs were performed in duplicate.
The rate constants were reproducible to within ± 5 %.
Stoichiometry
To determine the stoichiometry of these reactions, known
amounts of substrates (NH OH, scz, or tscz), triazine, and
2
nitric acid were taken and a known excess of iron(III)
nitrate was added to the mixture. The reaction was allowed
to go to completion, indicated by the constancy of absor-
bance values. The amount of iron(II) formed was evaluated
from the absorbance values and e values of the corre-
sponding iron(II) complex. From these values, the amount
of iron(III) reacted was computed. These results suggest
that stoichiometric ratio of [Fe(III)]:[substrate] is 1:1 for all
Results and discussion
The reactions were carried out under pseudo-first-order
conditions isolating the oxidant, that is, [substrate] (and
[tz]) ꢃ [Fe(III)]. Plots of log(A - A ) versus time were
?
t
linear up to more than 80 % completion of the reactions,
indicating first-order behavior with respect to iron(III).
Kinetic runs were performed varying the concentration of
-3
-3
three substrates, namely NH OH, scz, and tscz. The overall
2
iron(III) at [tz] = 1.0 9 10 mol dm , [substrate] = 1.0 9
-3
-
3
-3
?
mol dm , [H ] = 1 9 10
-2
reactions can be represented by the following equations:
10
mol dm , and l =
OH, scz,
-1
-3
3
þ
ðnꢂ1Þꢂ
1.0 9 10 mol dm at 30 °C (substrate = NH
2
2
2
2
Fe þ 6HL
þ 2NH OH
2
or tscz), and kobs values were evaluated. In all the systems,
the kobs values are independent of the initial concentration of
iron(III), see supplementary table S1, further confirming
first-order dependence on oxidant. The reactions were car-
ried out varying the concentration of substrate keeping the
concentrations of iron(III), tz, hydrogen ion, and ionic
strength constant. The rate of these reactions increased with
ð3nꢂ2Þꢂ
þ
!
2FeL
þ N2 þ 2H2O þ 8H
ð1Þ
ð2Þ
ð3Þ
3
3
þ
ðnꢂ1Þꢂ
Fe þ 6HL
þ 2N H CO þ 2H O
3
5
2
ð3nꢂ2Þꢂ
þ
!
2FeL
þ 4NH3 þ N2 þ 2CO2 þ 8H
3
3
þ
ðnꢂ1Þꢂ
Fe þ 6HL
þ 2N3H5CS
ð3nꢂ2Þꢂ
þ
!
2FeL
þ 8H þ ðH NN ¼ CðNH ÞSÞ
3
2
2
2
[substrate], see supplementary table S2. Plots of 1/kobs against
123

4
56
Transition Met Chem (2012) 37:453–462
1
/[substrate] were linear with positive intercepts on the ordi-
nate, indicating the formation of 1:1 complexes between iro-
n(III) and each substrate. The plots of 1/kobs against
1
/[NH OH] in the presence of PDTS are shown in Fig. 1.
2
For the plots of 1/kobs against 1/[scz] and 1/kobs against
/[tscz], see supplementary figures S3 and S4. The reactions
1
were also performed at different concentrations of tz and
fixed concentrations of oxidant, substrate, hydrogen ion, and
ionic strength. The rate data show that the rate of the reaction
increases with [tz], see supplementary table S5. Plots of
2
reciprocal of kobs versus reciprocal of [tz] were also linear
with positive intercepts on the rate axis, supplementary fig-
ure S6, suggesting the formation of 1:2 complexes of iro-
n(III) and tz. Hydrogen ion has a decelerating effect on these
2
redox processes. The plots of 1/kobs versus 1/[tz] for the
Fe(III)-NH OH-PDTS system are shown in Fig. 2.
2
2
The plots of 1/kobs versus [tz] for the other systems are
Fig. 2 Effect of [PDTS] on the rate of the reaction in the oxidation of
-
5
given in supplementary figure S7. The reactions were carried
out at different initial concentrations of hydrogen ion with the
concentrations of all other reactants constant, supplementary
tableS8.Ionicstrengthhasnoinfluenceonthereactionratesin
all these systems, see supplementary table S9, indicating that
the rate-determining step is between ions of opposite charge.
Representative kinetic data for the PDTS system are given in
Table 1 and that for PPDTS and BDTPS in Table 2.
NH
2
OH by Fe(III) in the presence of PDTS at [Fe(III)] = 4.0 9 10
-
3
-3
-3
-2
mol dm , [NH
-
2
OH] = 1.0 9 10 mol dm , [H?] = 2.0 9 10
mol dm , l = 0.1
3
reaction, and ionic strength has no effect on the rate. The
reaction mixtures did not initiate polymerization of acry-
lonitrile for the NH OH and tscz oxidations, whereas
2
polymerization was noticed in the oxidation of scz.
Olson and Simonson [27] concluded from their spectro-
The oxidation of NH OH, scz, and tscz by iron(III) in
2
3
photometric studies that iron(III) exists mainly as Fe at
?
the presence of all three sulfonated triazines is first order in
-
4
-3
mol dm concentrations in slightly acid solutions.
10
iron(III). The plots of 1/k versus 1/[substrate] and 1/kobs
obs
2
versus 1/[tz] were linear with positive intercepts on the x-
Anipindi [28] determined the protonation constants of PDTS
and PPDTS as 3.032 and 3.165, respectively, at 25 °C. In the
present studies, we have determined the protonation constant
of BDTPS as 2.762 also at 25 °C by potentiometry. The
species distribution using these protonation constants indi-
cates that at pH * 2.0, more than 95 % of PDTS, PPDTS,
and BDTPS exist in monoprotonated forms. The protonation
axis. This suggests that the substrates form 1:1 complexes
with iron(III), while the three tz species form 1:2 com-
plexes with the oxidant. Hydrogen ion decelerates the
constants of NH OH [29], scz [30], and tscz [31] are high,
2
and hence the protonated form of the substrate is the pre-
dominant species. However, the unprotonated form of the
substrate is considered as the reactive species because of its
greater nucleophilicity over the protonated form. A mecha-
nism is proposed to explain all the experimental results
considering the formation of a ternary intermediate complex
of iron(III), NH OH, and tz in two steps, as follows:
2
K1
þ
þ
sub þ H ꢀ subH
ð4Þ
ð5Þ
K2
3
þ
þ
III
þ
Fe þ 2Htz ꢀ Fe ðtzÞ þ2H
2
C1
K3
III
C1 þ sub ꢀ Fe ðtzÞ ðsubÞ
ð6Þ
2
C2
Fig. 1 Effect of [NH
NH OH by Fe(III) in the presence of PDTS at [Fe(III)] = 4.0 9 10
mol dm , [PDTS] = 1.0 9 10
2
OH] on the reaction rate in the oxidation of
The dissociation of this intermediate ternary complex
II
occurs in a slow step to give Fe (tz) and the reduced form
-
5
2
-
3
-3
-3
-2
mol dm , [H?] = 2.0 9 10
2
-
3
mol dm , l = 0.1
of the substrate [sub(red)];
1
23

Transition Met Chem (2012) 37:453–462
457
Table 1 kobs values for
iron(III)-NH OH reactions in
the presence of PDTS at 30 °C
5
3
3
?
[H ] 9 10
-3
(mol dm
2
3
10 9
[Fe(III)] 9 10
-3
mol dm
[NH OH] 9 10
2
[PDTS] 9 10
-3
l
2
-3
-1
)
(mol dm
)
(mol dm
)
)
k
obs (s
2
3
4
5
6
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
1.0
1.0
1.0
1.0
1.0
2.0
4.0
6.0
8.0
10.0
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
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
0.4
0.8
1.2
1.6
2.0
2.4
1.0
1.0
1.0
1.0
1.0
1.0
1.0
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
2.0
2.0
2.0
2.0
2.0
2.0
1.0
2.0
3.0
4.0
5.0
6.0
2.0
2.0
2.0
2.0
2.0
2.0
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
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.3
0.4
0.6
0.8
4.00
3.92
4.20
4.08
4.25
5.81
7.29
8.20
8.70
9.02
0.60
1.84
2.99
3.83
4.41
4.80
6.18
2.48
1.00
0.47
0.25
0.15
4.03
3.98
3.95
4.08
4.12
4.06
Slow
2
e
II
þ
K K ½FeðIIIÞꢀ ½Htzꢀ ½subꢀ
C2 ꢂ! Fe ðtz) þ H þ subðredÞ
ð7Þ
2
3
t
e
2
½
C2ꢀ ¼
ð12Þ
k1
e
2
e
þ
2
2
½
H ꢀ þ K2½Htz] þ K2K3½Htz] ½sub]
?
Replacing [Htz] by [tz] and [H ] by [H ] where [tz]
e
e
e
II
The iron(II) species Fe ðtz) reacts with another mole of
2
?
e
II
Htz to give Fe ðtz) in a series of fast steps, while the
?
?
3
and [H ] are the initial concentrations of tz and H ,
respectively,
reduced form of the substrate rapidly rearranges to give the
final products.
2
K2K3½FeðIIIÞꢀ ½tzꢀ ½subꢀ
The rate law for this mechanism is derived as follows:
t
e
½
½
C ꢀ ¼
ð13Þ
ð14Þ
2
e
ꢀ
þꢁ
þ 2
2
2
3
½H ꢀ þ K2½tz] þ K2K3½tz] ½sub]e
½
FeðIIIÞꢀ ¼ Fe
þ½C1ꢀ þ½C2ꢀ
ð8Þ
ð9Þ
t
e
e
e
ꢂ
ꢃ
subꢀ ¼ ½subꢀ þ ½sub ꢀ ¼ ½subꢀ 1 þ k1½Hþꢀ
þ
þ 2
t
e
e
e
e
½
C1ꢀ ½H ꢀ
e
e
¼
þ ½C ꢀ þ ½C ꢀ
1
e
2 e
2
K2½Htz]e
Since the protonation constant of hydroxylamine, scz,
?
and tscz are very high, K [H ] ꢃ 1. Hence, Eq. 14 can be
1
þ 2
½
C2ꢀ ½H ꢀ
½C2ꢀ
e
e
e
written as
¼
¼
þ
þ ½C2ꢀe
ð10Þ
ð11Þ
2
e
K2K3½Htz] ½sub]
K3½sub]e
e
þ
½
subꢀ ¼ ½subꢀ =k1½H ꢀ
ð15Þ
ð16Þ
(
)
e
t
e
þ 2
2
e
2
e
½
H ꢀ þ K2½Htzꢀ þ K2K3½Htzꢀ ½subꢀ
2
e
e
½C2ꢀe
k1K2K3½FeðIIIÞꢀ þ½tzꢀ ½subꢀ
t
e
2
e
Rate ¼ k1½C2ꢀ ¼
K2K3½Htz] ½sub]
e
þ 2
2
2
½
H ꢀ þ K2½tz] þ K2K3½tz] ½sub]
e
123

4
58
Transition Met Chem (2012) 37:453–462
Table 2 kobs values for
iron(III)-NH OH reactions in
the presence of PPDTS and
5
3
3
?
[H ] 9 10
2
4
-1
[
(
Fe(III)] 9 10
-3
[NH
(mol dm
2
OH] 9 10
-3
[tz] 9 10
-3
(mol dm
l
10 9 kobs (s
)
2
-3
(mol dm )
mol dm
)
)
)
PPDTS
BDTPS
BDTPS at 30 °C
2
3
4
5
6
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
1.0
1.0
1.0
1.0
1.0
2.0
4.0
6.0
8.0
10.0
12.0
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
1.0
1.0
1.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1.0
0.5
0.2
0.4
0.6
0.8
1.0
1.2
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
2.0
3.0
4.0
5.0
6.0
2.0
2.0
2.0
2.0
2.0
2.0
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
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.3
0.4
0.6
0.8
3.03
2.86
2.61
2.83
3.07
2.04
2.77
3.05
3.31
3.42
3.57
0.55
2.08
3.90
7.58
9.25
10.30
30.00
13.52
5.64
2.63
1.42
0.85
1.43
1.35
1.39
1.51
1.46
1.37
23.51
24.26
25.73
26.85
27.52
3.01
5.38
7.30
8.85
10.16
11.28
0.63
1.66
2.32
2.72
3.03
3.22
14.83
3.85
1.28
0.56
0.29
0.17
25.45
25.10
24.28
23.23
26.07
24.84
Substituting the value of [sub] in the above equation and
e
ordinate. However, the dependence of rate on hydrogen ion
?
replacing [sub] and [H ] by [sub] and [H ], respectively
?
concentration is not simple. Rearrangement of Eq. 19 gives
e
e
2
þ 2
k1K2K3½FeðIIIÞꢀ þ½tzꢀ ½subꢀ
1
K1½H ꢀ
K1
t
e
Rate ¼
ð1=kobs ꢂ 1=k1Þ Hþ
¼
þ
ð20Þ
þ 3
þ
2
2
2
K1½H ꢀ þ K1K2½H ꢀ½tz] þ K2K3½tz] ½sub]
k1K2K3½tz] ½sub] k1K3½sub]
According to Eq. 20, the plot of (1/k -1/k )/[H ]
e
ð17Þ
?
obs
1
?
2
versus [H ] should be linear with positive intercept on the
ordinate. Such plots were obtained in the oxidation of
This rate law satisfactorily explains all the experimental
observations. The pseudo-first-order rate constant, kobs, is
given by
NH OH by iron(III) in the presence of PDTS and PPDTS and
2
also for the oxidation of scz and tscz in the presence of PDTS,
see supplementary figure S8. The intercept of the plot of 1/
kobs versus 1/[sub] is equal to 1/k and hence the k values
2
k1K2K3½tzꢀ ½subꢀ
kobs ¼
ð18Þ
þ 3
þ
2
2
1
1
K1½H ꢀ þ K1K2½H ꢀ½tz] þ K2K3½tz] ½sub]
This equation on rearrangement gives
were evaluated from the intercepts of these plots. The slope
? ? 2
and intercept of the plot of (1/k - 1/k )/[H ] versus [H ]
obs
1
2
þ 3
þ
give K /k K K [tz] [sub] and K /k K [sub], respectively.
1 1 2 3 1 1 3
1
K1½H ꢀ
K1½H ꢀ
1
¼
þ
þ
ð19Þ
Taking the literature [32] values of K , the K (1/K ) values
a
2
1
a
kobs
k K K ½tz] ½sub] k1K3½sub] k1
1
2
3
-6
for NH OH [(1.14, 1.52, and 1.86) 9 10 dm mol at 25,
3
-1
2
2
According to Eq. 19, plots of 1/kobs versus 1/[tz] should
be linear with positive slopes and positive intercepts on the
30, and 35 °C, respectively], scz [(5.43, 3.28, and
-
1
3
dm mol
-1
1.40) 9 10
at 25, 30, and 35 °C,
1
23

Transition Met Chem (2012) 37:453–462
459
2
Table 3 Equilibrium and specific rate constants for Fe(III)–subtrate–
PDTS and Fe(III)-NH
K ½FeðIIIÞꢀ ½tzꢀ ½subꢀ
4
t
ð28Þ
2
OH-PPDTS reactions at 30 °C
½C3ꢀ ¼
e
þ
3
2
K1½H ꢀ þ K4½tz] ½sub]
The rate of the reaction can other be expressed as
3
System
K
2
K
(
3
1
k (dm
3
dm mol
-1
)
-1 -1
mol , s
)
2
2
-5
-3
2
Fe(III)-NH
PDTS
2
OH-
OH-
1.10 9 10
4.58 9 10
9.17 9 10
1.01 9 10
4.15 9 10
k K ½FeðIIIÞꢀ ½tzꢀ ½subꢀ
2
4
t
Rate ¼ k2½C3ꢀ ¼
ð29Þ
ð30Þ
ð31Þ
e
3
þ
2
K1½H ꢀ þ K4½tz] ½sub]
The pseudo-first-order rate constant, kobs, is given by
-5
-3
Fe(III)-NH
PPDTS
2
3.05 9 10
-
4
-3
-3
Fe(III)-scz-PDTS
Fe(III)-tscz-PDTS
0.33
1.32 9 10
1.45 9 10
2
-
3
-3
k K ½tzꢀ ½subꢀ
0.48 9 10
1.87 9 10
30.30 9 10
2
4
kobs ¼
þ 3
2
K1½H ꢀ þ K4½tz] ½sub]
or
-
respectively], and tscz [(4.92, 4.77, and 4.64) 9 10 dm
1
3
þ
3
1
¼
K ½H ꢀ
1
-
1
1
mol at 25, 30, and 35 °C, respectively] and from th?e
slopes and intercepts of the plots of (1/k -1/k )/[H ]
þ
2
kobs
K4k2½tz] ½sub] k2
obs
1
?
2
versus [H ] , the values of K and K were computed and
2
3
Equation 31 stipulates that plots of 1/k versus 1/[sub]
obs
2
and 1/kobs versus 1/[tz] should be linear with the same
the values of k , K and K at 30 °C are presented in
1
1
2
Table 3. For these values at 25 and 35 °C, see
supplementary Table S9.
magnitude of intercept on the 1/k axis. Plots of 1/kobs
obs
?
3
versus [H ] should also be straight lines with positive
intercept on the ordinate, of the same magnitude as the above
plots. The experimental results are in consonance with these
requirements for the iron(III)-scz-PPDTS, iron(III)-tscz-
PPDTS, and all the three iron(III)–substrate–BDTPS
For the other systems, that is, oxidation of scz and tscz
by iron(III) in the presence of PPDTS and iron(III) oxi-
dation of all three substrates in the presence of BDTPS,
?
? 2
plots of (1/kobs-1/k )[H ] versus [H ] were not linear.
1
Hence, a slightly different mechanism involving that the
formation of a ternary intermediate complex of iron(III),
substrate, and tz in a single equilibrium step is proposed
and the rate law derived as follows:
systems. The values of the specific rate constants k were
2
evaluated from the intercept of the plot of 1/k versus
obs
?
H ] . Using the literature values of K and from the slope of
3
[
1
this plot, the values of K were calculated. The values of
4
K4
specific rate constant k and the equilibrium constant K at
1
Fe þ 2Htz þ sub ꢀ Fe ðtz) ðsub) þ2Hþ
3
þ
III
5ꢂ
4
ð21Þ
ð22Þ
2
C3
3
0 °C are given in Table 4. These values at 25 and 35 °C are
available in supplementary Table S10.
Slow
II
þ
C3 ꢂ! Fe ðtz) þ H þ subðredÞ
Nitrogen was identified as the oxidation product of
hydroxylamine. The formation of this product is shown in
Eqs. 7 and 22 of the two reaction schemes. The ternary
3
k2
The rate law for this mechanism is derived as below:
ꢀ
þꢁ
3
intermediate complexes containing iron(III), NH OH, and tz,
2
½
FeðIIIÞꢀ ¼ Fe
eþ ½C3ꢀe
ð23Þ
ð24Þ
t
II
:
C or C give Fe ðtz) and NH2O in the rate-limiting steps:
2
3
2
þ 2
½
C3ꢀ ½H ꢀ
e
e
Slow
II
:
¼
þ ½C3ꢀe
C2ðor C3Þ ꢂ! Fe ðtz) þ NH2O
ð32Þ
2
2
K ½Htz] ½sub]
k
4
e
e
1
(
)
þ 2
2
e
However, Waters et al. [32] in their studies on the
½
H ꢀ þ K4½Htzꢀ ½subꢀ
e
e
¼
½C3ꢀe
ð25Þ
ð26Þ
2
e
oxidation of hydroxylamine by cerium(IV) also observed
:
K ½Htz] ½sub]
4
e
the formation of NH O according to the following steps:
2
2
e
K4½FeðIIIÞꢀ ½Htzꢀ ½subꢀ
t
e
e
½
C3ꢀ ¼
e
2
þ
2
e
½
H ꢀ þ K4½Htz] ½sub]
Table 4 Equilibrium and specific rate constants for Fe(III)–scz/tscz–
PPDTS and Fe(III)–sub–BDTPS reactions at 30 °C
e
On substituting the value of [sub] from Eqs. 14 and 26
e
-4
3
9 10 (dm
3
System
K
(
4
9 10
k
mol , s )
2
transforms to
3 -1
dm mol
-1 -1
)
2
K4½FeðIIIÞꢀ ½Htzꢀ ½subꢀ
t
e
t
Fe(III)-scz-PPDTS
Fe(III)-tscz-PPDTS
1.53
2.96
0.44
0.43
0.50
0.19
0.54
½
C ꢀ ¼
ð27Þ
3
e
þ 3
2
K ½H ꢀ þ K ½Htz] ½sub]
1
4
e
t
?
Replacing [Htz ] by [tz}, [H ] by [H ], and [sub]
?
?
Fe(III)-NH
2
OH-BDTPS
2.50
e
e
?
where [sub], [H ], and [sub] are the initial concentrations
Fe(III)-scz-BDTPS
Fe(III)-tscz-BDTPS
15.15
1.82
?
of tz, H , and sub, respectively,
123

4
60
Transition Met Chem (2012) 37:453–462
ꢂe
:
þ
step results in the formation of a thiosemicarbazide free
radical intermediate:
NH O ꢂ! HNO þ H
2
ð33Þ
ð34Þ
2
HNO ! N2O þ H2O
This can probably be expected at higher concentrations of
slow
III
II
:
þ
Fe ðtzÞ ðtsczÞ ꢂ! Fe ðtzÞ þNH ꢂCSꢂN ꢂNH þ H
2
2
2
2
ð38Þ
oxidant or with strong oxidizing agents. The stoichiometry
observed in the present investigations indicates that the steps
Dimerization of the thiosemicarbazide free radical will
give a disulfide; this process is likely to be rapid, which
would explain why the reaction mixture did not initiate the
polymerization of acrylonitrile. McAuley [35] observed the
formation of disulfide in the oxidation of thiomalic acid by
iron(III).
3
3 and 34 do not occur to a significant extent. The formation
:
of free radical ðNH2O Þ intermediates is postulated in the
rate-determining steps, even though the reaction did not
induce polymerization of acrylonitrile. Rapid dimerization
of the free radicals is probably responsible for this. Gutch and
Waters [33] studied the oxidation of NH OH by ceric
2
It is interesting to note that the K values pertaining to the
4
:
ammonium nitrate and obtained esr evidence for NH O
2
formation of the 1:1:2 ternary complex of iron(III), substrate,
and sulfonated triazine are very high (Table 4), indicating
that the ternary complexes are fairly stable and hence can be
isolated. To isolate these intermediate ternary complexes, an
radicals. Bengtsson et al. [34] studied the oxidation of
hydroxylamine by iron(III). Their results led to two rate
expressions and were consistent with a mechanism dependent
on the relative iron(III)-to-hydroxylamine concentrations; an
excess of iron(III) gives a stoichiometry of 2:1 for the total
5
9
iron(III) nitrite solution labeled with Fe was prepared and
the reaction mixtures were subjected to ion-exchange stud-
ies. Reaction mixtures containing labeled iron(III) (2–4 9
reaction and N O as the oxidation product, whereas at equal
2
-
5
-3
-4
-3
amounts or excess of hydroxylamine, the stoichiometry
10
mol dm ), PPDTS (5.0 9 10 mol dm ), scz or
-
3
-3
mol dm ), and HNO3 (2–3 9 10
-2
reduces to 1:1 with N as the main product and involves a
2
tscz (1–2 9 10
þ
-3
dihydroxy species of iron(III), namely Fe(OH) .
mol dm ) were prepared. After allowing the reaction
mixtures to stand at ambient temperature for above 15 m, the
reactions were cooled by adding ice and the cooled solutions
2
In the oxidation of scz, the final products are produced
by the decomposition of the ternary intermediate complex,
III
ꢂ
C or C , Fe (tz) (scz):
were passed through a Dowex 1X8 (200–400 mesh, NO3
2
2
2
slow
form) anion exchange resin column. The resin bed was
-3
III
II
:
Fe ðtzÞ ðsczÞ ꢂ! Fe ðtzÞ þNH CONHN H
Polymerization of acrylonitrile by the reaction mixtures
gives support to the formation of a free radical
intermediate.
ð35Þ
-4
washed with a mixture of PPDTS (5.0 9 10 mol dm
2
2
2
)
-
3
-3
and scz/tscz (1–2 9 10 mol dm ) of pH * 1.5. The
eluates from the column did not show gamma activity,
indicating the absence of neutral or cationic species of iron
3
?
including free Fe . The resin bed was then washed with
fast
:
_
NH CONHN H þ H O ꢂ! N H þ NH COOH
ð36Þ
2
2
2
3
2
NaNO solutions (at pH * 1.5) of different concentrations
3
-
and containing PPDTS (5.0 9 10 mol dm ) and scz/tscz
4
-3
:
The free radical, NH2CONH N H, undergoes hydrolytic
-3
-3
mol dm ). The presence of scz/tscz and
(1–2 9 10
_
cleavage giving N H free radical and NH COOH (carbamic
2
3
2
PPDTS in the eluent was to prevent the dissociation of iro-
_
acid), which being very unstable decomposes to give N2H3
n(III)–PPDTS–substrate intermediate complexes. The elu-
-3
_
and CO . The N H radicals formed undergo dimerization to
2
2
3
ents obtained on elution with 3.2 mol dm NaNO showed
3
give N H , which decomposes giving N and NH . In a fast
3
gamma activity in the case of both semicarbazide and thio-
semicarbazide. For comparison, a solution containing
4
6
2
?
_
step the oxidation of N H forms N H and H . The N H
2 2
2
3
2
2
-
2.0 9 10 mol dm each cis-bis|(oxalate)chromate(III),
3
-3
molecule undergo further decomposition, to give nitrogen
gas according to the following fast step:
ꢂ
3ꢂ
ꢂ
Crðox) ðH O) , and tris(oxalato)chromate(III), Crðox)
2
2
2
3
was passed through a Dowex 1X8 (200–400 mesh, NO3
þ
ꢂ
_
N H ! N þ 3H þ 3e
ð37Þ
However, the observed stoichiometry, [Fe(III)]:[scz] =
2
3
2
form) anion exchange column. It was noticed Crðox)
2
ꢂ
ðH2O) desorbs from the column on elution with
2
-
3
.0 mol dm NaNO , while elution with 3.0 mol dm
-3
1
1
:1, suggests that the contribution from Eq. 37 is not
3
3
3
ꢂ
appreciable.
NaNO
3
elutes Crðox) from the resin bed, as established
For the Fe(III)-tscz-tz system, formation of the mixed
III
from the visible absorption spectra of the eluates. This
indicates that the species separated from the resin bed on
-
elution with 3.2 mol dm NaNO will have a charge of 3-.
ligand complex, Fe (tz) (tscz), involves coordination of
2
3
the hydrazinic nitrogen and sulfur of the tscz ligand;
however, electron transfer probably takes place preferably
3
A large volume of eluent is required to elute this species
completely. The species thus separated slowly turned from
colorless to magenta when brought to ambient temperature.
from sulfur because of its low electronegativity. The
III
decomposition of Fe (tz) (tscz) in the rate-determining
2
1
23

Transition Met Chem (2012) 37:453–462
461
Table 5 Activation parameter for some oxidation reactions of iron(III) in the presence of triazines
Substrate
PDTS
PPDTS
BDTPS
#
#
#
#
#
#
DH
(
DS
(J mol deg
DH
(kJ mol
DS
(J mol deg
DH
(kJ mol
DS
(J mol deg )
-1
)
-1
-1
)
-1
)
-1
-1
)
-1
)
-1
-1
kJ mol
Hydroxylamine
Semicarbazide
33
56
46
-143
-123
-84
21
23
26
-170
-173
-153
70
95
79
62
32
38
Thiosemicarbazide
The visible absorption spectrum of this magenta species, has
the stability of the intermediate complexes, Fe(III)–PPDTS
and Fe(III)–BDTPS, is higher than that of the Fe(III)–
PDTS intermediate complexes. Hence, the PDTS systems
kmax at 563 nm, indicating the formation of the final product,
iron(II)–PPDTS complex. These results support the forma-
III
#
tion of ternary intermediate complexes, Fe ðPPDTS)
might be expected to exhibit higher DH values than the
PPDTS and BDTPS systems. However, the experimental
2
3
ꢂ
III
3ꢂ
ðscz)
or Fe ðPPDTS) ðtscz) . The magenta species
2
results show that the reactions in the presence of BDTPS
#
exhibit very high DH compared to PDTS. Because of the
adsorbed at the top of the resin column did not desorb from
-
the resin even on washing the resin bed with 6.0 mol dm
3
bulky nature of the BDTPS, higher reorganization energy
may be required for the formation of the transition state,
leading to higher enthalpy values. Between scz and tscz,
tscz forms a stronger complex with iron(III) than scz,
which is reflected in the enthalpy of the transition state.
NaNO . These results suggest that the magenta species is
3
highly negatively charged. However, on washing the resin
-
3
bed with 0.05 mol dm EDTA (disodium salt) solution
pH * 4.0), the magenta color disappeared and the eluents
showed gamma activity. We suggest that the intermediates,
(
#
III
3ꢂ
III
3ꢂ
The entropies of activation DS for these reactions also fall
Fe ðPPDTS) ðscz) or Fe ðtscz)(PPDTS) react with
2
2
#
in line with this analysis. The DS values are the lowest in
the PPDTS systems, which indicates that the transition
EDTA to give a complex that is more easily desorbed. The
gamma activity of these two species separated with
-
3
-3
states are more highly ordered compared to the reactions in
#
the presence of PDTS. The very large positive DS values
3
.2 mol dm NaNO and 0.5 mol dm EDTA solutions
3
5
9
accounted for most of the total activity of Fe taken. Similar
results were obtained for all the reactions carried out in the
presence of BDTPS. The reaction mixtures were passed
noticed for the BDTPS systems indicate that the transition
state is highly disordered, which can again also be attrib-
uted to steric factors. From these results, it can be con-
cluded that the size and p-electron delocalization ability of
the a,a-diimine ligand play a vital role in these reactions.
ꢂ
through Dowex 2X8 (NO form) resin column. The solution
3
that passed through the column did not show any c-activity,
indicating the absence of either neutral or cationic species.
The resin bed was washed with the solutions of different con-
-
3
centrations of Ca(NO ) of 2.0 mol dm onwards. Eluents
3
2
-3
References
obtained on elution with 3.0 mol dm Ca(NO ) showed c-
3
2
activity. The anionic species eluted could be a highly negative
5ꢂ
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III
charged iron(III) intermediate, probably Fe ðscz)(BDTPS)
2
III
5ꢂ
or Fe ðtscz)(BDTPS) . Similar results to those obtained
2
4
5
. Kimura M, Wada G (1978) Inorg Chem 17:2241
. Birk JP, Weaver SV (1972) Inorg Chem 11:95
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-
3
obtained on elution with 0.01 mol dm
EDTA solution
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8
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#
1
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#
DH and DS values were computed for all these reac-
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1
2
lated in Table 5. A comparison of the energies of activation
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1
#
PPDTS, and BDTPS shows that the DH values follow the
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1
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1
16. Subbaiah KV, Subba Rao PV, Murthy PSN, Murthy RVS (1980)
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23