Kinetics and mechanism of oxidation of benzohydrazide by bromate catalyzed by vanadium(IV) in aqueous acidic medium

Article abstract of DOI:10.1002/kin.20296

The reaction between benzohydrazide and potassium bromate catalyzed by vanadium(IV) was studied under pseudo-first-order condition keeping large excess of hydrazide concentration over that of the oxidant. The initiation of the reaction occurs through oxidation of the catalyst vanadium(IV), VO²⁺, to vanadium(V), VO₂⁺, which then reacts with hydrazide to give N,N′-diacylhydrazine and benzoic acid as the products. The order in [H⁺] is found to be two, and its effect is due to protonation and hydrolysis of oxidized form of the catalyst to form HVO₃. The oxidized form of the catalyst, VO₂⁺, forms a complex with the protonated hydrazide as evidenced by the occurrence of absorption maxima at 390 nm. The rate of the reaction remains unaffected by the increase in the ionic strength. The activation parameters were determined, and data support the mechanism. The detailed mechanism and the rate equation are proposed for the reaction.

Full text of DOI:10.1002/kin.20296

Kinetics and Mechanism
of Oxidation of
Benzohydrazide by Bromate
Catalyzed by Vanadium(IV)
in Aqueous Acidic Medium
S. A. SHEWALE,1 A. N. PHADKULE,2 G. S. GOKAVI1
1Kinetics and Catalysis Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416 004, India
2Department of Chemistry, Sangameshwar College, Solapur 413 001, India
Received 20 April 2007; revised 15 August 2007, 11 September 2007, 28 September 2007, 11 October 2007;
accepted 12 October 2007
DOI 10.1002/kin.20296
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The reaction between benzohydrazide and potassium bromate catalyzed by vana-
dium(IV) was studied under pseudo-first-order condition keeping large excess of hydrazide
concentration over that of the oxidant. The initiation of+the reaction occurs through oxidation
of the catalyst vanadium(IV), VO2+, to vanadium(V), VO2 , which then reacts with hydrazide to
give N,Nꢀ-diacylhydrazine and benzoic acid as the products. The order in [H+] is found to be
two, and its effect is due to protonation and hydrolysis of oxidized form of the catalyst to form
HVO3. The oxidized form of the catalyst, VO+2 , forms a complex with the protonated hydrazide as
evidenced by the occurrence of absorption maxima at 390 nm. The rate of the reaction remains
unaffected by the increase in the ionic strength. The activation parameters were determined,
and data support the mechanism. The detailed mechanism and the rate equation are proposed
for the reaction. ꢁ 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 151–159, 2008
C
aldehydes, ketones, esters and carboxylic acids [1–9],
and ethers [9b]. Conversion of sulfides to sulfoxides
[10], hydroquinones to quinones [9,10], thiols to disul-
fides [11], and iodobenzene to iodoxybenzenes [12]
have also been carried out by using bromate salts. Al-
though the bromate itself is a strong oxidizing agent
having a redox potential of 1.45 V, its oxidations gen-
erally require a catalyst due to the slowness of the
uncatalyzed reactions under normal conditions. The
catalysts used for bromate oxidations are in their lower
oxidation state, which will be oxidized to their corre-
sponding higher oxidation state. The oxidized form of
the catalyst will then effect the further conversion of the
INTRODUCTION
Potassium and sodium bromate are stable solids and
easily handled as compared with liquid bromine and
hypobromous acid solutions. The product of bromate
oxidation is bromide ion, which can be safely recy-
cled, thus making the methods of their oxidations en-
vironmentally benign than the metal ion oxidations.
Bromates have been utilized to oxidize alcohols to
Correspondence to: G. S. Gokavi; e-mail: gsgokavi@hotmail.
com.
c
ꢀ
2008 Wiley Periodicals, Inc.

152
SHEWALE, PHADKULE, AND GOKAVI
substrate into the product. Bromate ion is also used for
volumetric determination of inorganic ions having the
redox potential <0.8 V, and for the ions exceeding this
value of the potential the analysis of the ions becomes
difficult due to their slow oxidation. Kinetics of oxi-
dation of inorganic reductants like vanadium(IV) [13]
and 12-tungstocobaltate(II) [14] has also been studied
by using bromate, and their mechanisms are predicted.
Hydrazides, which are derivatives of both carboxylic
acids and hydrazine, have been utilized as starting ma-
terials in organic synthesis [15]. The oxidative transfor-
mation with most oxidants gives corresponding acids
and in some cases [15] esters or amides. Hydrazides
have also been converted into N-N-diacylhydrazines
with [15c] various oxidants. Formation of acids and
their derivatives in the presence of different nucle-
ophiles is the indication of direct two-electron transfer
to the oxidant, whereas cyclization products generally
involve a single electron transfer with intervention of a
free radical. Vanadium(IV) in aqueous acidic medium
is also known to be oxidized by bromate [13] to give
vanadium(V) as the product. Since bromate is not oxi-
dizing hydrazides, in the present study, the redox prop-
erty of vanadium(IV)–bromate system is utilized to
generate vanadium(V) in solution and then oxidizing
the hydrazide.
at constant temperature of 27.0 0.1◦C. The reaction
was initiated by mixing the previously thermostated so-
lutions of oxidant, catalyst, and substrate, which also
contained the required amount of hydrochloric acid,
potassium chloride, and doubly distilled water. The
reaction was followed by titrating the reaction mix-
ture for unreacted oxidant iodometrically and the rate
constants were determined from the pseudo-first-order
plots of log [oxidant] against time. The pseudo-first-
order plots were linear for more than 90% comple-
tion of the reaction, and the rate constants were repro-
ducible within 6%.
The reaction between vanadium(V) and benzo-
hydrazide was also studied under similar conditions
as that of vanadium(IV)-catalyzed reaction. The reac-
tion was followed at 390 nm at which the complex
formed between vanadium(V) and benzoic acid hy-
drazide absorbs. The pseudo-first-order rate constants
of the decomposition of the complex were obtained by
linear log (absorbance) against time plots.
RESULTS
Stoichiometry
In 10 mL of 0.6 mol dm−3 hydrochloric acid, 35
µmol (5 mg) catalyst and 2 mmol (0.272 g) of ben-
zohydrazide were dissolved. To the resulting solution,
1 mmol (0.167 mg) of KBrO3 was added. The re-
action mixture was stirred at 27◦C for 5 min. The
precipitate obtained was filtered, and ether was added
to it. The insoluble part of the precipitate (140 mg)
(product I) was washed with water and recrystallized
from ethyl acetate-petroleum ether mixture. The re-
crystallized product I was dried, and its melting point
(mp) was determined as 238◦C. The soluble part of
the precipitate (product II) in ether was washed with
sodium bicarbonate solution and neutralized with hy-
drochloric acid. The solid obtained after neutralization
(0.036 g) was washed with water and recrystallized
from water. The recrystallized product II was dried,
and the mp was determined as 121◦C. By compari-
son of the reported [15b] mp values, the product I was
found to be N-N-diacylhydrazine (mp 238–240◦C) of
benzohydrazide and product II was found to be benzoic
acid. Benzoic acid separated was filtered and recrys-
tallized by water (mp 121◦C).
EXPERIMENTAL
Materials
Reagent-grade chemicals and doubly distilled water
were used throughout the work. The KBrO3 solution
was prepared by dissolving KBrO3 (BDH) in water
and standardized iodometrically. The hydrazide was
prepared by esterification of the corresponding acids in
ethanol followed by its conversion to hydrazide accord-
ing to the reported procedure [16]. The solutions of hy-
drazide were prepared by dissolving required quantity
in water. Ionic strength was maintained by using KCl,
and to vary hydrogen ion concentration HCl (BDH)
was used.
The catalyst vanadium(IV) was prepared by dis-
solving vanadyl sulfate (BDH) in water and standard-
ized by titrating against potassium permanganate. The
solution of vanadium(V) was prepared by dissolving
ammonium vanadate (BDH) in hot water.
Reaction Order
KINETIC STUDIES
The uncatalyzed reaction did not occur under the exper-
imental conditions. The catalyzed reaction was carried
out under pseudo-first-order conditions keeping the
The reaction was studied under pseudo-first-order con-
ditions keeping hydrazide concentration large excess
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KINETICS AND MECHANISM OF OXIDATION OF BENZOHYDRAZIDE
153
Table I Effect of Reactant Concentrations on the Rate
of Reaction at 27◦C
Table II Effect of Catalyst Concentration on the
Reaction at 27◦C
102 [Hydrazide]
103 [KBrO3]
104 [VIV] (mol dm−3
)
104 k (s−1
)
obs
(mol dm−3
)
(mol dm−3
)
104 k s−1
obs
0.5
0.8
1.0
2.0
3.0
4.0
5.0
2.3
3.8
5.8
12.1
19.1
24.3
31.9
0.5
0.8
1.0
2.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.0
2.0
5.0
5.6
5.5
5.6
5.5
5.4
5.8
5.8
5.8
5.8
5.8
102[Hydrazide] = 1.0 mol dm−3, 103[KBrO3] = 1.0 mol dm−3
,
[HCl] = 0.1 mol dm−3, and I = 0.5 mol dm−3
.
104[VIV] = 1.0 mol dm−3, [HCl] = 0.1 mol dm−3, and I =
0.5 mol dm−3
.
than unity (1.09) as determined from the plot of log
kobs against log [H+]. The plot of kobs against [H+]2 is
linear (R2 = 0.9978) with an intercept indicating that
the reaction involves both hydrogen ion-dependent and
-independent paths to different extents. The rate of hy-
drogen ion-dependent path is considerably greater than
that of the independent path as the values of slope and
concentration of hydrazide large excess at a constant
concentration of HCl at 0.1 mol dm−3 and at a constant
ionic strength of 0.5 mol dm−3 (Table I). The pseudo-
first-order plots were found to be linear on vary-
ing the concentration of oxidant between 0.5 × 10−3
and 5.0 × 10−3 mol dm−3 keeping the concentration
of catalyst and hydrazide constant at 1.0 × 10−4 and
1.0 × 10−2 mol dm−3, respectively (Table I), indicat-
ing the order in oxidant concentration is unity. The
pseudo-first-order rate constants, kobs, were fairly con-
stant as the concentration of hydrazide was varied
between 0.5 × 10−2 and 5.0 × 10−2 mol dm−3 keep-
ing all other concentrations constant (Table I); there-
fore, the order in hydrazide concentration is also unity.
To understand the effect of dissolved oxygen, sep-
arate kinetic runs were carried out for the standard
run ([KBrO3] = 1.0 × 10−3 mol dm−3, [hydrazide] =
intercept were found to be 4.2 × 10−2 and 2.6 × 10−4
,
respectively.
Effect of Ionic Strength and Temperature
The effects of ionic strength and temperature were
studied keeping [hydrazide], [KBrO3], [catalyst], and
[HCl] constant at 1.0 × 10−2, 1.0 × 10−3, 1.0 × 10−4
,
and 0.1 mol dm−3, respectively. Potassium chloride
was used to vary the ionic strength. The rate of the reac-
tion remains unaffected with increasing ionic strength
(from 0.1 to 0.5 mol dm−3). The effect of temperature
on the reaction was studied at 20, 27, 30, and 40◦C
and the pseudo-first-order rate constants were found
to be 3.0 × 10−4, 5.8 × 10−4, 7.3 × 10−4, and 19 ×
10−4 s−1, respectively. The activation parameters ꢀH#,
1.0 ×10−2 mol dm−3, [VIV] = 1.0 × 10−4 mol dm−3
,
[HCl] = 0.1 mol dm−3, and I = 0.5 mol dm−3 at 27◦C)
under nitrogen atmosphere. There was no change in
the value of pseudo-first-order rate constant indicat-
ing no effect of the dissolved oxygen on the reac-
tion. The effect of catalyst concentration was stud-
ied between the concentration range of 5.0 × 10−5 to
5.0 × 10−4 mol dm−3, and the plot kobs against [cata-
lyst] was found to be linear indicating the first-order
dependence of reaction on [catalyst] (Table II).
ꢀG#, and –ꢀS# were found to be 68.5 6 kj mol−1
,
Table III Effect of Hydrogen Ion Concentration on the
Reaction at 27◦C
10 [HCl] (mol dm−3
)
104 k (s−1
)
obs
0.2
0.4
0.8
1.0
2.0
3.0
4.0
3.0
3.5
4.8
Effect of Hydrogen Ion Concentration
5.8
19.1
42.5
68.1
The effect of hydrogen ion was studied to understand
the nature of reactant species present in the solution.
The [H+] was varied from 2.0 × 10−2 to 0.4 mol dm−3
(Table III). The [H+] accelerates the rate of the reac-
tion, and the order in [H+] is found to be slightly more
102[Hydrazide] = 1.0 mol dm−3, 103 [KBrO3] = 1.0 mol dm−3
,
104[VIV] = 1.0 mol dm−3, and I = 0.5 mol dm−3
.
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154
SHEWALE, PHADKULE, AND GOKAVI
105.4
respectively.
6 kJ mol−1, and 123.8
7 J K−1 mol−1
,
vanadium(V) and hydrochloric acid were varied
between 0.2 × 10−3 to 1.0 × 10−3 mol dm−3 and
1.0 × 10−2 to 1.0 × 10−1 mol dm−3, respectively. The
pseudo-first-order rate constants were determined by
linear (R2 > 0.993) log (Abs.) against time plots. The
pseudo-first-order rate constants for the decomposi-
tion of the complex were found to be constant when
the concentration of the hydrazide was varied be-
tween 0.5 × 10−2 and 5.0 × 10−2 mol dm−3 at constant
concentration of vanadium(V), hydrochloric acid, and
Spectrophotometric Examination
of Reaction Mixture
During the kinetic study, it was observed that a brown
color complex was formed in the reaction mixture
containing the highest catalyst concentration of 5.0 ×
10−4 mol dm−3. Therefore, the reaction mixture was
examined spectrophotometrically for formation of a
complex in the course of the reaction. There was no
change in the UV–vis spectrum of the mixture of the
catalyst and the hydrazide. But when the vanadium(V),
the oxidized form of the catalyst, was added to hy-
drazide the color of the solution changed from yel-
low to brown. Therefore, the spectra of only vana-
dium(V) (1.0 × 10−4 mol dm−3), hydrazide 1.0 ×
10−2 mol dm−3, and in the presence of hydrazide 1.0 ×
10−2 mol dm−3 in 0.1 mol dm−3 hydrochloric acid at an
ionic strength of 0.5 mol dm−3 were recorded between
305 and 500 nm using an Elico SL159 spectrophotome-
ter. A new absorption peak at 390 nm was observed
for the mixture containing hydrazide and vanadium(V)
(Fig. 1).
ionic strength of 1.0 × 10−3, 0.1, and 0.5 mol dm−3
,
respectively. The linearity of the of the pseudo-first-
order plots and the constancy of pseudo-first-order rate
constants as the hydrazide concentration was varied,
indicating the first-order dependence of the reaction
on both vanadium(V) and the hydrazide concentra-
tion. The effect of the hydrogen ion concentration was
also studied on the decomposition of the vanadium(V)-
hydrazide complex, and it was found that the rate of
decomposition was independent of the hydrogen ion
concentration.
DISCUSSION
Oxidation of inorganic substrates by bromate ion has
been studied [13,14], and the complexity in their mech-
anism strongly depends on the redox potential of the
metal ion. The metal ions having E◦ values more than
1.1 V are not directly oxidized by the bromate ion and
involve [14] an induction period. The dependence of
the rate of the reaction on acidity of the solution, for
such reactions, is also complicated due to the forma-
tion of species like HBrO2. On the other hand, the
mechanism for the reduction of metal ions with E◦
value less than 1.1 V does not involve any induction
period [13], and metal ions are directly oxidized by
bromate ion. For such reactions, the rate laws obtained
are also simple and independent of the hydrogen ion
concentration. The examples are bromate ion oxidation
of [CoIIW12O40]6− and VIV with complex and simple
rate law, respectively. The stoichiometry of these re-
ported reactions also predicts HOBr2, Br2, and HOBr
as the products of the reduction of bromate. In the
present investigation, the reaction between hydrazide
and bromate occurs only in the presence of catalytic
amount of VIV. Therefore, the reaction is initiated by
the oxidation of the catalyst (VIV) to its higher oxi-
dation state (VV). The reaction between bromate and
VIV has been studied [13] earlier and found to follow
a simple second-order rate law with an order of unity
in both the reactant concentrations. The reaction was
independent of the hydrogen ion concentration and the
ionic strength variation. The mechanism also involves
Reaction Between Oxidized Form of the
Catalyst, Vanadium(V), and Hydrazide
The formation of the complex between vanadium(V)
and hydrazide was very rapid; therefore, the re-
action between hydrazide and vanadium(V) was
studied at 390 nm by following the decomposi-
tion of the complex formed. The concentration of
hydrazide was kept at 1.0 × 10−2 mol dm−3 and
Figure
1 UV–vis spectra of vanadium(V) and the
mixture of vanadium(V) and the hydrazide. [VV] =
1.0 × 10−4 mol dm−3, [hydrazide] = 1.0 × 10−2 mol dm−3
,
[HCl] = 0.1 mol dm−3, and I = 0.5 mol dm−3
.
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS AND MECHANISM OF OXIDATION OF BENZOHYDRAZIDE
155
intervention of BrO2, HBrO2, and HOBr with bromine
as the final products. In our work, the orders in bromate
and the catalyst were found to be unity, which parallels
the reported results. This indicates that the first step
of the reaction between the catalyst and the bromate
will be the oxidation of VIV generating BrO2. But the
hydrogen ion dependence and the final product of the
bromate reduction in the present study differ from that
of the reported work. Therefore, identification of the
product of bromate reduction was necessary. During
the kinetic study, generation of bromine either in situ
or at the end of the reaction was not observed, whereas,
with the addition of silver nitrate to the reaction mix-
ture, after completion of the reaction in sulfuric acid,
precipitation of silver bromide occurred, confirming
bromide as the product of the reaction. This might be
due to the fact that even if HOBr or Br2 are formed
during the course of the reaction, because of their re-
spective oxidation potentials of 1.34 and 1.07 V, they
can easily oxidize the hydrazide in acidic solutions
with the formation bromide as the final product.
3–4. Therefore, under the reaction conditions, only for-
mation of HVO3 as a result of hydrolysis of VO+2 ion
is considered. In acidic solutions, protonation of hy-
drazide [18] occurs and VO+2 ion undergoes hydrogen
ion-dependent hydrolysis to form HVO3. The values
of slope and intercept obtained from the plot of kobs
against [H+]2 are 4.2 × 10−2 and 2.6 × 10−4, respec-
tively, and the ratio is found to be around 1.6 × 102.
A very low value of intercept indicates that the rate
law consists of a negligible [H+]-independent term,
and the linearity with high slope shows the presence
of a term with second order in [H+]. Thus, neglect-
ing the hydrogen ion-independent term in comparison
with that of hydrogen ion-dependent term, the effect of
hydrogen ion on the reaction can be explained consid-
ering a prior protonation of hydrazide and formation of
HVO3.
The VIV-catalyzed oxidation of benzohydrazide by
bromate is found to be first order in oxidant, substrate,
and the catalyst. The order in hydrogen ion concentra-
tion is more than unity. Since the uncatalyzed reaction
between bromate and hydrazide do not occur under the
experimental conditions, the catalyzed reaction pro-
ceeds with the interaction of the catalyst and the ox-
idant. Therefore, the initiation of the reaction is the
oxidation of the catalyst to its higher oxidation state by
bromate, which then effects the oxidation of the sub-
strate. The reaction is also not affected by the change in
the ionic strength, and the hydrogen ion concentration
effect is due to the reaction between protonated [18]
benzohydrazide with unhydrolyzed oxidized form of
the catalyst, VO+2 . The protonation of hydrazide and
hydrolysis of VV occur in prior equilibria. Thus, the
mechanism of the reaction involves oxidation of the
catalyst (VIV) to its higher oxidation state (VV) by bro-
mate ion-generating BrO2. At comparatively high con-
centration of catalyst, 5.0 × 10−4 mol dm−3, a brown
color was developed in the reaction mixture and disap-
peared. It was assumed that the VV formed as a result
of prior redox reaction between the catalyst and the ox-
idant might be forming a complex with the substrate. In
a separate experiment, a mixture containing equal con-
centrations of VV and benzohydrazide was examined
spectrophotometrically. An absorption maxima for the
mixture was observed at 390 nm, which was absent in
the solutions containing only VV or hydrazide under
similar conditions (Fig. 1), which confirms the for-
mation of a complex between the oxidized form of the
catalyst and the substrate. The reaction between VIV by
bromate is fast [13], and the second-order rate constant
The plot of kobs against [H+]2 plot (R2 = 0.9978)
was found to be linear with an intercept that is due
to the simultaneous involvement of both hydrogen-
dependent and the independent paths in the reaction.
The hydrogen ion-dependent path of the reaction is
due to prior protonation equilibria of the reactants, and
the hydrogen ion-independent path is due to the par-
ticipation of unprotonated reactant species. Potassium
bromate is a strong electrolyte and in aqueous solution
it dissociates to give BrO−3 ion which is also a strong
acid, thus its protonation would not be possible under
the experimental conditions. Another possibility of ex-
plaining the presence of protonation prior equilibria is
the involvement of an induction period. The bromate
oxidations of one-electron reducing agents were also
found to involve induction period [14] due to initial hy-
drogen ion-dependent reduction of bromate according
to the equilibrium shown in Eq. (1). But in the present
investigation, there was no
Red + BrO−3 + 3H+
Ox + HBrO2 + H2O (1)
ꢀ
ꢁ
such induction period excluding the possibility of equi-
librium (1). Therefore, the effect of [H+] on the re-
action is due to protonation prior equilibria of hy-
drazide and the oxidized form of the catalyst, VO2+
ion. The monomeric form of vanadium (V), VO+2 ion,
is reported to be involved in various hydrogen ion-
dependent equilibria [17] like formation of HVO3,
VO34−, HVO24−, H2VO4−, and H3VO4. In the present
study, the range of pH of the solutions used is between
1 and 2 and the equilibria involving the vanadate ion,
VO34−, are possible only in aqueous solutions of pH
for the oxidation is reported to be 3.86 dm3 mol−1 s−1
.
Therefore, under the pseudo-first-order conditions em-
ployed in our study using catalytic amount of VIV and
large excess of bromate, the oxidation is expected to
International Journal of Chemical Kinetics DOI 10.1002/kin

156
SHEWALE, PHADKULE, AND GOKAVI
Scheme 1
be very fast. Separate kinetic runs were also carried
out to determine the rate constants of VV-hydrazide
complex formation and its decomposition. Even un-
der second-order conditions at equal concentration of
VV and hydrazide, the formation of the complex was
immediate as noticed by the rapid increase in the ab-
sorbance at 390 nm. Thus, the rate constant for the de-
composition of the complex was measured by follow-
ing the absorbance at 390 nm. The rate constant, under
pseudo-first-order conditions keeping large excess of
step of Scheme 1. In the following steps, the protonated
hydrazide and the VO+2 ion form a complex. The forma-
tion of the complex has been supported by the spectral
changes occurring during the course of the reaction
between VV and hydrazide (Fig. 1). The complex for-
mation even in equimolar concentrations of the VV
and the hydrazide (second-order conditions) occurs
rapidly. The complex further decomposes to give hy-
drazide free radical regenerating the catalyst back with
a rate constant 1.7 × 10−4
s
−1. Considering the oxi-
hydrazide than VV, was found to be 1.7 × 10−4 s−1
.
dation of catalyst by bromate as a fast equilibrium and
decomposition of the complex formed between VO2+
ion and the hydrazide as the rate-determining step, the
rate law for the reaction can be written as in Eq. (12)
and the expression for the kobs by Eq. (13). Accord-
ing to Eq. (13), the values of pseudo-first-order rate
constants (kobs) vary linearly with the catalyst concen-
tration and the plot of kobs against [H+]2 is linear. The
Furthermore, the decomposition was also found to be
independent of VV, hydrazide, and hydrogen ion con-
centration. On the basis of kinetic data obtained, the
mechanism in terms of the active species of reactants
and the oxidized form of the catalyst is summarized
in Scheme 1. According to Scheme 1, the reaction is
initiated by the rapid oxidation of VIV by bromate ion
thus generating VO2+ and BrO2 in the first step. The
VO+2 ion then gets hydrolyzed to form an vanadate
ion, HVO3. The protonated form of the substrate is
formed in another prior equilibria as shown in the third
value of K2 is reported [17] to be 5.0 × 10−4 mol dm−3
,
which can be neglected at high [H+] but comparable
at low [H+], thus making the overall order in hydro-
gen concentration to be slightly more than unity as
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS AND MECHANISM OF OXIDATION OF BENZOHYDRAZIDE
157
Scheme 2
observed.
first-order rate constant when the reaction was carried
out in nitrogen atmosphere indicating no effect of the
dissolved oxygen on the reaction. The stoichiometric
analysis of the reaction mixture led to two major prod-
ucts N,Nꢀ-diacylhydrazine and benzoic acid. The for-
mation of these two products in the present reaction can
be explained due to the competitive nucleophilic attack
on the carbonyl carbon of the hydrazide free radical by
either another hydrazide molecule or water molecule
producing N,Nꢀ-diacylhydrazine or benzoic acid, re-
spectively, which is shown separately in Scheme 2. The
formation of both the products generates NHNH2 free
radical. The oxidized form of the catalyst is completely
utilized in the rapid formation of the complex, which
reduces its concentration in the solution to an insignif-
icant extent, and the low concentration of the NHNH2
free radical formed makes the further possible interac-
tion between them negligible. Therefore, the •NHNH2
free radical produced preferably reacts with strong
ꢀ
ꢁ
k1K1K2K3K4[H+]2
Rate =
([H+]) + K2(1 + K3[H+])
ꢂ
ꢃ
× BrO−3 [Hydrazide][VO2+
]
(12)
ꢀ
ꢁ
k1K1K2K3K4[H+]2
kobs
=
[VO2+] (13)
([H+]) + K2(1 + K3[H+])
Hydrazide-free radicals have been predicted in oxi-
dations of hydrazide by one-electron oxidizing agents
[19,20]. In one such study on oxidation of amino
benzohydrazide by hydrogen peroxide mediated by
myeloperoxidase [20], a ferric enzyme, it has been
observed that the hydrazide-free radical produced
also undergo slow oxidation by the dissolved oxygen.
Therefore, the reaction was carried out in nitrogen
atmosphere to know the effect of dissolved oxygen.
It was found that there was no change in the pseudo-
International Journal of Chemical Kinetics DOI 10.1002/kin

158
SHEWALE, PHADKULE, AND GOKAVI
one-electron oxidant BrO2 generated in the first
equilibrium of Scheme 1, leading to the formation
of HBrO2 and NHNH. The final product nitrogen is
formed as a result of fast reactions of NHNH with
intermediates of bromate, HBrO2, and HOBr as shown
in Scheme 1. The probable structure of the complex
between the oxidized form of the catalyst and the
hydrazide can be coordination of hydrazide ligand
through oxygen and nitrogen atoms to the VV. The
moderate values of ꢀH# and ꢀS# were both favorable
[21] for electron transfer processes. The negative value
of ꢀS#, within the range for free radical reactions [22],
has been ascribed to the nature of electron pairing
and unpairing processes and to the loss of degrees of
freedom formerly available to the reactants upon the
formation of rigid transition state.
According to equilibrium 3 of Scheme 1
[HVO3] = K2[VO+2 ]F/[H+]
(A4)
ꢄ
ꢂ
ꢃ
ꢅ
Therefore, [VO+2 ]T = [VO2+]F + K2 VO2+ F /[H+]
Then, [VO+2 ]F = [VO2+]T/{1 + (K2/[H+])}
[VO+2 ]F = [VO2+]T[[H+]/([H+] + K2)]
(A5)
Substituting for concentration of [VO+2 ] in Eq. (A2)
Rate = (k1K2K4[VO+2 ]T[H+]
× [RCONHNH+3 ])/([H+] + K2) (A6)
Then from equilibrium 4 of Scheme 1, the total con-
centration of benzohydrazide can be obtained as
[RCONHNH2]T = [RCONHNH2]F + [RCONHNH+3 ]
CONCLUSION
(A7)
[RCONHNH2]T = [RCONHNH2]F
+ K3[H+][RCONHNH2]F
The reaction between KBrO3 and benzoic acid hy-
drazide in the presence of VIV as a catalyst oc-
curs through rate-determining conversion of the cat-
alyst to its higher oxidation state which then effects
the oxidation of the substrate leading to the forma-
tion of hydrazide free radical. Nucleophilic attack
on the carbonyl oxygen by either another hydrazide
molecule or by water leads to the formation of N,Nꢀ-
diacylhydrazine and benzoic acid, respectively. In the
present study, the protonated form of the hydrazide as
well as unhydrolyzed form of the oxidized catalyst is
the reactive species. The probable transition state was
considered to be an interaction between oxygen and ni-
trogen of the hydrazide moiety with the oxidized form
of the catalyst leading to the formation of a complex
with absorption maxima at 390 nm.
[RCONHNH2]T = [RCONHNH2]F(1 + K3[H+])
(A8)
[RCONHNH2]F = [RCONHNH2]T/(1 + K3[H+])
(A9)
and that of [RCONHNH+3 ] is given by
[RCONHNH+3 ] = K3[H+][RCONHNH2]T/
(1 + K3[H+])
(A10)
Substituting the [RCONHNH+3 ] from Eq. (A10) in
Eq. (A6) we get
Rate = k1K2K3K4[VO+2 ]T[H+]2[RCONHNH2]T/
APPENDIX: DERIVATION OF RATE LAW
{([H+] + K2)(1 + K3[H+])}
(A11)
The rate of reaction is given by
Since, [VO+2 ]T is obtained form fast equilibrium 2 of
Scheme 1 and the [VO+2 ]T is given by
Rate = k1[Complex]
(A1)
(A2)
[VO2+]T = K1[BrO−3 ][VO2+
]
(A12)
Rate = k1 K4[VO+2 ][RCONHNH3+]
Then, from Eqs. (A11) and (A12), we get
The total oxidized form of the catalyst, VO+2 would be
Rate = k1K1K2K3K4[BrO−3 ][VO2+][H+]2
× [RCONHNH2]T/{([H+] + K2)
[VO+2 ]T = [VO2+]F + [HVO3]
(A3)
where T and F indicate total and free [VO+2 ].
× (1 + K4[H+])}
(A13)
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS AND MECHANISM OF OXIDATION OF BENZOHYDRAZIDE
159
Since [RCONHNH2]T is highly excess and considering
its concentration as constant, we get
9. (a) Banerjee, A.; Dutt, S.; Sengupta, D.; Adak, M. M.;
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15. (a) Gladstone, W. A. F. J Chem Soc (C) 1969, 1571;
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Rate = k1K1K2K3K4[BrO−3 ][VO2+][H+]2/
([H+] + K2)(1 + K3[H+])
Then
kobs = Rate/[BrO−3 ] = k1K1K2K3K4[VO2+][H+]2/
([H+] + K2)(1 + K3[H+])
or
kobs = k1K1K2K3K4[VO2+][H+]2
/{([H+] + K2)(1 + K3[H+])} (A14)
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International Journal of Chemical Kinetics DOI 10.1002/kin