Homogeneous catalysis of oxovanadium(IV) in the oxidation of substituted 4-oxo acids by bromate in acid medium: A mechanistic study

Article abstract of DOI:10.1016/j.molcata.2010.06.002

Oxovanadium(IV)-catalyzed oxidation of substituted 4-oxo-4-phenyl-butanoic acids (4-oxo acids) by bromate in acid medium leads to the formation of malonic acid and the corresponding benzoic acid. The reaction exhibits first order each in [bromate] and [acid], less than unity order each in [4-oxo acid] and [oxovanadium(IV)], and also shows solvent isotope effect (k(D ₂O)/k(H₂O) ～ 1.8). The reaction is failed to induce the polymerization of acrylonitrile. The decrease in the rate of reaction with increase in dielectric constant of the medium is observed with all the studied 4-oxo acids. Electron-releasing substituents in the phenyl ring accelerate the rate of oxidation to a large extent, the rate retardation by electron-withdrawing substituents, though perceptible, is not very much. The linear free-energy relationship is characterized by a smooth curve, concaves downward, in the Hammett's plot of log k versus σ, however, Hammett' plots are linear with excellent correlation coefficient, at four temperatures, when Brown and Okamoto's σ⁺ values are used. The ρ⁺ values are negative and decrease with increase in temperature. From the intersection of the lines in the Hammett and Arrhenius plots, the isokinetic relationship is discussed. The mechanistic pathway involving the cyclic oxidant-substrate-catalyst ternary complex is proposed, wherein the oxidation state of V(IV) is unaltered. The reaction is an example of the neighboring group participation and intramolecular catalysis.

Full text of DOI:10.1016/j.molcata.2010.06.002

Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Homogeneous catalysis of oxovanadium(IV) in the oxidation of substituted 4-oxo
acids by bromate in acid medium: A mechanistic study
∗
Cherkupally Sanjeeva Reddy , Padma Sunitha Manjari
Department of Chemistry, University College, Kakatiya University, Warangal 506009, Andhra Pradesh, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Oxovanadium(IV)-catalyzed oxidation of substituted 4-oxo-4-phenyl-butanoic acids (4-oxo acids) by
bromate in acid medium leads to the formation of malonic acid and the corresponding benzoic acid. The
reaction exhibits first order each in [bromate] and [acid], less than unity order each in [4-oxo acid] and
Received 24 March 2010
Received in revised form 25 May 2010
Accepted 1 June 2010
[
oxovanadium(IV)], and also shows solvent isotope effect (k(D2O)/k(H2O) ∼ 1.8). The reaction is failed to
Available online 9 June 2010
induce the polymerization of acrylonitrile. The decrease in the rate of reaction with increase in dielectric
constant of the medium is observed with all the studied 4-oxo acids. Electron-releasing substituents
in the phenyl ring accelerate the rate of oxidation to a large extent, the rate retardation by electron-
withdrawing substituents, though perceptible, is not very much. The linear free-energy relationship is
characterized by a smooth curve, concaves downward, in the Hammett’s plot of log k versus ꢀ, however,
Hammett’ plots are linear with excellent correlation coefficient, at four temperatures, when Brown and
Keywords:
4
-Oxo acid
Oxovanadium(IV)
Bromate
Catalysis
+
+
Mechanism
Okamoto’s ꢀ values are used. The ꢁ values are negative and decrease with increase in temperature.
From the intersection of the lines in the Hammett and Arrhenius plots, the isokinetic relationship is
discussed. The mechanistic pathway involving the cyclic oxidant–substrate–catalyst ternary complex is
proposed, wherein the oxidation state of V(IV) is unaltered. The reaction is an example of the neighboring
group participation and intramolecular catalysis.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
carbon atoms separate the carbonyl and the carboxyl groups and so
they behave both as oxocompounds and as acids without the direct
influence of the other group [8–10]. The kinetics of iodination of 4-
oxo acid [11] and the hydrolysis of its ethyl ester [12,13] has yielded
interesting results. Intramolecular catalysis in the iodination of 4-
oxo acid [11] and neighboring group participation in the hydrolysis
of ethyl ester of 4-oxo acid [12,13] have been reported. Hence, 4-
oxo acids are attractive substrates in terms of their mechanistic
aspects.
The chemistry of bromate is quite interesting because of its
ability to act as an oxidizing and brominating reagent [14]. Bro-
mate oxidations of organic and inorganic reactions usually involve
complications such as an induction period [15], autocatalysis [16]
and involvement of two reactions [17,18]. Even bromate oxidations
sometimes involve oscillation reactions [19–22]. Hence, the chem-
istry of bromate ion in an aqueous acid medium is of considerable
interest, given its importance in mechanistic chemistry.
Lack of studies on the oxidation of 4-oxo acids by bromate in
the presence of V(IV) has encouraged us to investigate the kinetic
behavior of the title reaction in order to continue our study on
the role of metal ions in oxidation reactions by bromate. Prelim-
inary experimental results indicate that the reaction of 4-oxo acid
with bromate in acidic medium without a catalyst was sluggish,
but the reaction became facile in the presence of a small amount
of V(IV) catalyst. Therefore, in order to explore the mechanism of
The chemistry of vanadium is of topical interest as it has
a wide number of accessible oxidation states offering a wide
variety of compounds with diverse coordination numbers and
stereochemistries, and is biologically very significant. Vanadium
complexes of hydrazine derivatives of nitrogen heterocyclic are
used in the treatment of arteriosclerosis and to lower blood pres-
sure [1]. Vanadium is involved in phospholipids oxidation, sulfur
metabolism and cholesterol biosynthesis [2]. Peroxo-hetero ligand
vanadates represent a model system for studying some biochem-
ical interaction of vanadium in living matter [3], and there is a
relationship between the chemical composition and the biolog-
ical activity (antitumour activity–toxicity). Several investigations
into biologically relevant vanadium have been made [4–6]. Recent
studies show that peroxovanadium complexes have potent insulin-
like activity, suggesting that they could be orally active alternatives
to insulin [7]. Therefore, vanadium compounds are gaining special
importance.
In 4-oxo-4-phenylbutanoic acid and its phenyl substituted com-
pounds (commonly known as ␥-oxo acids or 4-oxo acids), two
∗ Corresponding author. Tel.: +91 870 2573788; fax: +91 870 2439600.
E-mail address: chsrkuc@yahoo.co.in (C.S. Reddy).
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.06.002

C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
77
4
-oxo acid-bromate reactions in an acid medium and also to study
organic intermediates in the reaction were performed using high
performance liquid chromatography (HPLC). The experiments
were performed with Shimadzu equipment using an ion-exchange
the catalytic action of V(IV), oxovanadium(IV) has been selected
as a catalyst. In the presence of oxovanadium(IV), the reduction
of bromate with excess [4-oxo acid] is autocatalytic (an initial
slow reaction followed by a fast reaction). The oxo acid reacts
with bromate forming Br2 and HOBr, which are reduced further
◦
column at 45 C and a UV detector working at 220 nm. The inter-
mediates and the products were identified from their retention
time (tr). Regression analyses of experimental data yielded the
regression coefficient (r) and standard deviation (s).
−
to Br ions. The kinetics of the reaction of oxo acid with Br2 has
been studied and it was found that kHOBr > kbromine. Tribromide
ions are kinetically inactive, hence, bromate ions are consumed
not only by the reaction with oxo acid but also by the reaction
2.2. Kinetic procedure
−
with Br ions. If bromide ions are masked in the solution by adding
A thermostatic water bath was used to maintain the desired
−
◦
a suitable complexing agent such as mercury(II), Br ions cannot
temperature within ± 0.1 C. The reaction mixture, containing
take part in follow up reactions and the overall reaction loses its
autocatalytic character. This is exactly what is observed with the
addition of mercury(II) acetate. The added Hg(II) ions form non-
ionized mercury(II)-bromocomplexes of high stability constants
4-oxo acid, H SO , mercuric acetate and V(IV) solutions, was
2
4
thermally equilibrated for an hour at the desired temperature
under subdued light. The reaction was initiated by the addition of
temperature-equilibrated bromate solution of requisite concentra-
tion. The total volume of the reaction mixture was 100 ml in each
case. 5 ml of aliquots of the reaction mixture was pipetted out at
different time intervals and quenched with 4% potassium iodide
solution. The progress of reaction was iodometrically monitored
by estimating the unreacted bromate. However, before adopting
iodometric method, it was ensured that the presence of 4-oxo
acids in the quenching solution of potassium iodide did not change
the bromate titer value. All the reactions were carried out under
the conditions using ten (or more)-fold excess of 4-oxo acid over
+
8
−1
8
−1
[
23] (HgBr , K = 2.51 × 10 M ; HgBr , K = 3.80 × 10 M ) and
1
2
2
the kinetic results remained the same over a wide concentration
range (0.001–0.01 M) of mercuric acetate. In order to mask all the
Br ions formed, an optimum concentration (0.005 M) of mercuric
−
acetate is employed to pertain Br(V) oxidation only.
The main objectives of the present study are to: (i) ascertain
the reactive species of the substrate, catalyst and oxidant, (ii) elu-
cidate a plausible mechanism, (iii) deduce an appropriate rate law,
(
iv) identify the oxidation products, (v) find the catalytic efficiency
of V(IV) and (vi) evaluate the related kinetic and thermodynamic
parameters along with their linear free-energy relationships. This
is the first report of the neighboring group participation and
intramolecular catalysis in the oxidation of 4-oxo acids.
[KBrO ] at constant temperature. The pseudo-first order rate con-
3
−
1
stants (k, s ) were computed from the linear plots of log[bromate]t
against time (r ≥ 0.99; s ≤ 0.025) up to 90% completion of the reac-
tion, and the rate constants were reproducible within ± 5%. Rate
constants did not alter in a nitrogen atmosphere and all the rate
constants reported in this paper were obtained without nitrogen.
Freshly prepared solutions of oxo acids in purified acetic acid were
used to avoid any possible side reactions.
2
. Experimental
2.1. Materials
ꢀ
2.3. Stoichiometry and product analysis
4
-Oxo
acid,
4-oxo-4-(4 -methoxyphenyl)butanoic
acid,
ꢀ
ꢀ
4
-oxo-4-(4 -methylphenyl)butanoic
acid,
4-oxo-4-(4 -
The stoichiometry of the reaction was determined by equili-
brating reaction mixture of various [bromate]/[4-oxo acid] ratios
at 40 C for 24 h, keeping all other reagents constant. Estimation
of unconsumed bromate concentration revealed the consumption
ratio of substrate to oxidant as:
chlorophenyl)butanoic acid were obtained from Aldrich chemical
company. 4-Oxo-4-(3 -nitrophenyl)butanoic acid was prepared
from nitrobenzene and succinic anhydride in 1:1 molar ratio using
anhydrous aluminium chloride as a catalyst [24]. All the 4-oxo
ꢀ
◦
The same stoichiometry was observed with all the substituted
4-oxo acids. The ultimate products of oxidation of 4-oxo acid are
benzoic and malonic acids, identified by comparing the retention
time (tr) of the authentic samples. Bromide ion was identified
by adding AgNO3 solution, resulting in the formation of pale yel-
low AgBr precipitate. Benzoic acid, after separation by HPLC, was
estimated quantitatively (92–96%, yield) with a standard curve at
ꢂmax = 235 nm. Malonic acid was further confirmed with its char-
acteristic spot test through conversion into barbituric acid [26].
acids were crystallized twice from water and their purity was
checked by their melting points and UV, IR and NMR spectra.
KBrO3 (Reidal), H SO and Hg(OAc)2 (E. Merck) were of analytical
2
4
reagent grade and were used as received. D O (purity 99.4%) was
2
obtained from the Baba Atomic Research Centre, Mumbai, India.
Acetic acid (BDH) was purified by refluxing with chromic acid
and acetic anhydride for 6 h and then distilled. A stock solution
of vanadium(IV) was prepared by dissolving a known amount of
VOSO ·H O (Sisco) in diluted sulfuric acid and was standardized
4
2
using KMnO [25]. An allowance for the amount of sulfuric acid
3. Results
4
present in the catalyst solution was also made while the reaction
mixture for the studies was prepared. The other solutions were
prepared with either doubly distilled water or purified acetic acid
and were standardized by known methods. Fresh solutions were
used for each kinetic run. The reaction vessels were coated from
outside with black paint to avoid any photochemical reaction.
A Shimadzu multipurpose recording double beam UV–visible
spectrophotometer equipped with a temperature controller was
used for absorption studies. Separation and identification of
3.1. Effect of concentrations
Molecular bromine production anticipated from the
bromate–bromide reaction and its subsequent reaction with
the 4-oxo acid was eliminated by the addition of bromocom-
plexing agent Hg(II). The added mercuric acetate over a wide
concentration range (0.001–0.01 M) had no effect on the rate of
−
reaction. In order to keep back all the Br ions formed, an optimum

7
8
C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
Table 1
Effect of variation of [KBrO3] on the rate of V(IV)-catalyzed oxidation of 4-oxo acids by bromate in acid medium at 313 K.
k × 10 (s−1) at [KBrO3] × 104 (M)
4
4
-Oxo acid
5.00
7.50
10.00
15.00
10.00
–
H
2.57 (0.31)a
2.54 (0.30)
2.55 (0.31)
46.06 (4.79)
5.75 (0.68)
0.82 (0.11)
0.32 (0.03)
2.55 (0.31)
46.08 (4.81)
5.73 (0.66)
0.84 (0.12)
0.31 (0.03)
2.56 (0.30)
46.06 (4.79)
5.75 (0.68)
0.82 (0.11)
0.31 (0.03)
p-OCH3
p-CH3
p-Cl
46.03 (4.78)
5.75 (0.66)
0.81 (0.12)
0.31 (0.03)
46.06 (4.80)
5.75 (0.67)
0.83 (0.10)
0.31 (0.03)
m-NO2
Conditions: [substrate] = 0.01 M, [H2SO4] = 1.0 M, [V(IV)] = 1.0 × 10−4 M, [Hg(OAc)2] = 0.005 M, HOAc–H2O = 1:1% (v/v). Reported rate constants are the mean of duplicate
experiments.
a
Values in parentheses are the uncatalyzed rate constants.
Table 2
Effect of variation of [substrate] on the rate of V(IV)-catalyzed oxidation of 4-oxo acids by bromate in acid medium at 313 K.
4
−1
)
[4-oxo acid] (M)
k × 10 (s
–H
p-OCH3
p-CH3
p-Cl
m-NO2
0
0
0
0
0
0
0
.0075
.01
.015
.02
.03
2.19 (0.27)a
2.55 (0.31)
3.21 (0.47)
3.83 (0.63)
4.60 (0.95)
5.08 (1.27)
6.39 (1.91)
37.35 (3.83)
46.06 (4.79)
63.95 (7.19)
76.76 (9.59)
95.95 (14.39)
115.15 (19.19)
151.35 (28.78)
4.79 (0.45)
5.75 (0.67)
7.67 (1.00)
8.84 (1.34)
11.51 (2.01)
13.42 (2.68)
16.17 (4.03)
0.67 (0.07)
0.82 (0.11)
0.97 (0.15)
1.27 (0.21)
1.59 (0.31)
1.91 (0.42)
2.55 (0.63)
0.25 (–)
0.31 (0.03)
0.42 (0.05)
0.50 (0.06)
0.63 (0.10)
0.76 (0.13)
0.95 (0.21)
.04
.06
Conditions: [KBrO3] = 0.001 M, [H2SO4] = 1.0 M, [V(IV)] = 1.0 × 10−4 M, [Hg(OAc)2] = 0.005 M, HOAc–H2O = 1:1% (v/v). Reported rate constants are the mean of duplicate exper-
iments.
a
Values in parentheses are the uncatalyzed rate constants.
Table 3
Effect of variation of [H2SO4] on the rate of V(IV)-catalyzed oxidation of 4-oxo acids by bromate in acid medium at 313 K.
4
−1
)
[H2SO4] (M)
k × 10 (s
–H
p-OCH3
p-CH3
p-Cl
m-NO2
0
0
0
1
1
1
2
.25
.50
.75
.00
.25
.50
.00
.00
0.68
1.27
1.91
2.55
3.03
3.83
4.81
7.03
1.01
12.57
25.58
38.38
46.06
63.95
76.76
95.95
152.82
1.00
1.73
3.46
5.08
5.75
7.67
9.59
12.51
18.19
1.02
0.21
0.40
0.62
0.82
0.95
1.27
1.58
2.30
1.00
–
0.16
0.24
0.31
0.40
0.48
0.63
0.92
1.00
3
a
S
Conditions: [substrate] = 0.01 M, [KBrO3] = 0.001 M, [V(IV)] = 1.0 × 10−4 M [Hg(OAc)2] = 0.005 M, HOAc–H2O = 1:1% (v/v). Reported rate constants are the mean of duplicate
experiments.
a
S = slopes of log k versus log[H2SO4] plots.
concentration (0.005 M) of mercuric acetate was employed. In
the presence of mercuric acetate, the oxidation of 4-oxo acid and
phenyl substituted 4-oxo acids by bromate in acidic medium pro-
ceeds smoothly and is found to be clearly of first order on bromate
as both determinations with respect to time (as evidenced by
good fits of log[bromate]t versus time plots) and concentration (as
(Table 2) with an increase in [substrate]o. The log–log plots of k ver-
sus [substrate]o were linear (r ≥ 0.995, s ≤ 0.03) with slope values
of 0.52–0.63. Further, dependence of k values on the initial sub-
strate concentration were consistent with the Lineweaver–Burk
kinetics in each case (Fig. 1a). The rate of the reaction increased
linearly with an increase in [acid] (Table 3) and the order with
respect to [acid] was found to be unity from the linear plots of log k
versus log[acid]. Zucker–Hammett plots of log k versus Ho indicate
non-involvement of water molecule in the rate determining step.
−
1
shown by the time order rate coefficient (k, s ), which is inde-
pendent of initial bromate concentration), Table 1. The observed
−1
pseudo-first order rate constant (k, s ) increased non-linearly
Table 4
Effect of variation of [V(IV)] on the rate of V(IV)-catalyzed oxidation of 4-oxo acids by bromate in acid medium at 313 K.
4
4
−1
)
[
V(IV)] × 10 (M)
k × 10 (s
–H
p-OCH3
p-CH3
p-Cl
m-NO2
0.50
1.00
2.00
4.00
8.00
1.53
2.55
4.03
6.15
8.04
28.78
46.06
69.18
95.95
129.87
3.83
5.75
9.18
14.22
21.99
0.57
0.82
1.43
2.08
3.22
0.19
0.31
0.47
0.69
1.05
Conditions: [substrate] = 0.01 M, [bromate] = 0.001 M, [H2SO4] = 1.0 M, [Hg(OAc)2] = 0.005 M, HOAc–H2O = 1:1% (v/v). Reported rate constants are the mean of duplicate
experiments.

C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
79
Fig. 1. (a) Plots between 1/k and 1/[substrate] for V(IV)-catalyzed reactions under the conditions of Table 2. (A) p-OCH3, (B) p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2. (b) Plots
between 1/kun and 1/[substrate] for uncatalyzed reactions under the conditions of Table 2. (A) p-OCH3, (B) p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2.
The Ho values were taken from Paul and Long [27] assuming that
Ho values do not change much in a media containing acetic acid
up to 50%. A change in the concentration of added salt like NaClO4
had a marginal effect on the reaction rate. The observed rate
3.3. Test for free radicals
The addition of monomers such as acryl amide or acrylonitrile
to the reaction system, in an atmosphere of nitrogen, did not alter
the reaction rate or indicate polymer formation, ruling out the par-
ticipation of free radical intermediates in the oxidation reaction. To
further confirm the absence of free radicals in the reaction pathway,
the reaction was carried out in the presence of 0.05 M of 2,6-di-t-
butyl-4-methylphenol (butylated hydroxyl toluene or BHT). It was
observed that the BHT was recovered unchanged, almost quantita-
tively.
−1
constants (k, s ) increased with an increase in [oxovanadium (IV)]
Table 4) and slopes of less than unity (0.45–0.66) were obtained
(
when log k was plotted against log[V(IV)] for all the substrates
under study. Further, 1/k versus 1/[catalyst] plots (Fig. 2, r ≥ 0.985;
s ≤ 0.03) resulted variable intercepts on the 1/k axis for the studied
substituted 4-oxo acids. The rates of uncatalyzed reactions are
negligible.
3.2. Effect of solvent polarity
3.4. Kinetic isotope effect
Varying the acetic acid and water composition in the reaction
mixture varied dielectric constant (D) of the medium. The decrease
in dielectric constant of the reaction medium increased the reaction
rate for all the substrates under study (Table 5) and the plots of log k
versus 1/D were linear (Fig. 3). The inertness of the solvent towards
the oxidant has also been tested.
Rate of oxidation increased in the D O medium and
2
k(D O)/k(H O) ∼ 1.80, at all the studied temperatures for all the
2
2
studied 4-oxo acids (Table 6). The solvent isotope effect suggests a
proton-catalyzed reaction.

8
0
C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
Fig. 2. Plots between 1/k and 1/[catalyst] under the conditions of Table 4. (A) p-OCH3, (B) p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2.
Table 5
Effect of varying solvent composition on the rate of V(IV)-catalyzed oxidation of 4-oxo acids by bromate in acid medium at 313 K.
4
−1
)
HOAc–H2O % (v/v)
D
k × 10 (s
–H
p-OCH3
p-CH3
p-Cl
m-NO2
3
4
5
6
7
0–70
0–60
0–50
0–40
0–30
53.18
46.48
39.78
33.08
26.08
1.43
1.91
2.55
3.83
6.41
25.58
35.43
46.06
86.36
160.76
2.42
3.36
5.75
8.39
25.47
0.47
0.58
0.82
1.17
2.70
0.15
0.19
0.30
0.47
0.95
Conditions: [V(IV)] = 1.0 × 10−4 M and other conditions as in Table 4. D = dielectric constant of the medium. Reported rate constants are the mean of duplicate experiments.
3
.5. Effect of temperature
using appropriate equations are presented in Table 7. The aver-
age error limits in the values of ꢃH , ꢃS and ꢃG (at 313 K)
#
#
#
−
1
−1
−1
−1
The oxidation reactions were studied in the temperature range
are ± 3 kJ mole , 8 J K mole and ± 4 kJ mole , respectively. The
plot of ꢃH# versus ꢃS# was linear (Fig. 5, r ≥ 0.997, s ≤ 0.03) and
the isokinetic temperature (␤) obtained is 377.3 K. The ␤ calcu-
lated from the Exner’s plot (Fig. 6, r ≥ 0.987; s ≤ 0.04) of log k
of 298–323 K. Activation energy (Ea) of the reactions was calcu-
lated from the least-square slopes of linear Arrhenius plots (Fig. 4,
r ≥ 0.99; s ≤ 0.02) of log k versus 1/T. The related thermodynamic
323 K
#
parameters viz. enthalpy of activation (ꢃH ), entropy of activa-
against log k3
is 375.2 K, which is in good agreement with the
13 K
#
#
value obtained from the ꢃH against ꢃS# plot.
#
tion (ꢃS ) and Gibbs free-energy of activation (ꢃG ) calculated
3.6. Structural effect
Structural modifications in the phenyl moiety of 4-oxo acid
result in varied reactivity; while electron-releasing substituents
donors) accelerate the rate of oxidation to a large extent, and
(
the rate retardation by electron-withdrawing substituents (attrac-
tors), though perceptible, was not very much (Table 8). The
free-energy relationship is characterized by a smooth curve;
concaves downward, in the Hammett’s plot of log k versus ꢀ
Table 6
Solvent isotope effect on the rate of V(IV)-catalyzed oxidation of 4-oxo acids by
bromate in acid medium at 313 K.
4
−1
4
−1
)
4
–
-Oxo acid
H
k (H2O) × 10 (s
)
k (D2O) × 10 (s
k (D2O)/k (H2O)
2.55
5.75
0.82
0.31
4.59
10.23
1.49
1.80
1.78
1.82
1.80
p-Methyl
p-Chloro
m-Nitro
0.56
Conditions:
[substrate] = 0.01 M,
[KBrO3] = 0.001 M,
[H2SO4] = 1.0 M,
−
4
[
V(IV)] = 1.0 × 10 M, [Hg(OAc)2] = 0.005 M, HOAc–H2O/D2O = 1:1% (v/v). Reported
Fig. 3. Plots between log k and 1/D under the conditions of Table 5. (A) p-OCH3, (B)
p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2.
rate constants are the mean of duplicate experiments.

C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
81
Table 7
Values of rate constant at different temperatures and activation parameters at 313 K of V(IV)-catalyzed oxidation of 4-oxo acids by bromate in acid medium.
5
−1
)
Temp. (K)
k × 10 (s
–
H
p-OCH3
p-CH3
p-Cl
1.24
2.30
3.83
8.22
m-NO2
2
3
3
3
3
3
98
03
08
13
18
23
–
95.95
154.3
255.5
383.8
508.0
767.6
63.18
60.58
9.59
15.99
32.20
57.57
115.1
191.9
95.73
93.13
−36.5
104.57
0.29
0.76
1.73
3.09
7.67
16.76
134.02
131.42
55.9
5.32
11.51
25.58
57.57
95.95
15.9
31.98
122.54
119.94
32.5
Ea (kJ/mole)
101.47
98.87
ꢃH (kJ/mole)
#
ꢃS (J/mole K)
−24.0
−123.1
#
ꢃG (kJ/mole)
106.67
100.10
109.75
113.92
Conditions: [substrate] = 0.01 M, [KBrO3] = 0.001 M, [H2SO4] = 1.0 M, [V(IV)] = 1.0 × 10−4 M, [Hg(OAc)2] = 0.005 M, HOAc–H2O = 1:1% (v/v).
Fig. 6. Exner’s plot of log k323 K against log k313 K under the conditions of Table 7. (A)
p-OCH3, (B) p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2.
Fig. 4. Arrhenius plots between log k and 1/T showing the isokinetic temperature
under the conditions of Table 7. (A) p-OCH3, (B) p-CH3, (C) –H, (D) p-Cl, and (E)
m-NO2.
Table 8
Effect of substituent on the rate of V(IV)-catalyzed oxidation of 4-oxo acids by bro-
mate in acid medium at 313 K.
(
substituent constant) at all the studied temperatures (Fig. 7).
However, the Hammett’ plots are linear, with excellent correla-
ꢀ+
k × 10 (s
4
−1
)
5 + log k
tion coefficient at four temperatures (Fig. 8, r ≥ 0.995, s ≤ 0.03),
Substituent
ꢀ
+
when exalted substituent constant (Brown and Okamoto’s ꢀ )
–H
0.00
0.00
2.55
46.06
5.75
0.82
0.31
1.4065
2.6633
1.7597
0.9138
0.4913
+
values [28] are used. The reaction constants (ꢁ ) are nega-
p-OCH3
p-CH3
p-Cl
−0.27
−0.17
0.23
−0.78
−0.31
0.11
m-NO2
0.71
0.67
Conditions as in Table 7.
tive and decrease (in magnitude) with increase in temperature
Table 9).
(
4. Discussion
4.1. Role of mercury(II) acetate
The presence of mercuric acetate (in excess of bromate con-
centration) simply means that Br2 oxidation has been completely
Table 9
+
Effect of temperature on the reaction constant (ꢁ ).
Temperature (K)
Reaction constant (ꢁ )
308
−1.50
313
−1.33
318
−1.20
323
−1.09
+
Fig. 5. Plot between ꢃH# and ꢃS : isokinetic relationship under the conditions of
Table 7. (A) p-OCH3, (B) p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2.
#
Conditions as in Table 7.

8
2
C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
Before further discussion, it is necessary to make clear that,
in the present study, no free radicals are present and that the
reaction rates are independent of [mercuric acetate]. These consid-
erations confirm only the complex-forming ability of mercury(II)
with bromide ions in these reactions. Hg(II) ion acts as a scavenger
−
16–18] for any Br ion formed, as unionized HgBr and HgBr [23],
2
+
[
thus ensuring that the oxidation takes place purely through bro-
mate. The reported rate constants therefore, stand for pure bromate
(
uncontaminated with Br ) oxidation.
2
4.2. Active species of reactants
In acid solutions, the reactive species of bromate are likely to
−
be BrO3 and protonated bromate. Hence, the oxidation may be
a reaction involving one of the oxidant species. Based on the ear-
lier propositions made in bromate oxidations (in acid medium) by
Indelli et al. [30], Anbar and Guttmann [31], Wright and Barton [32],
and Rabai et al. [33], the following equilibrium steps (Eqs. (3)–(6))
may be considered.
BrO3 + H+ ꢀ HBrO3
−
(3)
(4)
(5)
+
+
HBrO + H ꢀ H BrO3
3
2
+
+
H BrO3 ꢀ BrO2 + H O
2
2
Fig. 7. Hammett’ plots between log k and ꢀ at different temperatures under the
conditions of Table 7.
+
These authors [30–33] consider the H attack to bromate form-
+
−
ing intermediates HBrO3 and H BrO3 . However, BrO3 is a very
weak base and does not attract the H efficiently [29]. As HBrO3 is
2
+
suppressed; Br2 otherwise would have been formed by interaction
of bromate and bromide (the reduction product of bromate) as:
a very strong acid [34], the H2BrO3+ species will not be formed
at the acid concentrations used to study the bromate–oxo acid
reaction. Another point to be considered is the possibility of
bromate, after two protonations, losing water and forming the
BrO2+ species as considered by other authors [31,35,36]. How-
ever, this possibility has been ruled out by the results [37,38] on
the rate of exchange of isotopic oxygen between the oxyanion
BrO3 + 5Br− + 6H+ ꢀ 3Br + 3H O
−
(2)
2
2
It is pertinent to point out here that, Cortes and Faria [29]
observed acetate ion catalysis in bromate–bromide reaction and
the experimental results were interpreted in a six term rate law,
with and without involvement of acetate ion. The mechanism
proposed considers that when acetate binds to bromate it facil-
itates its second protonation. However, in the present study of
oxovanadium(IV)-catalyzed oxidation of substituted 4-oxo acids
by bromate (in 1:1 HOAc–H O medium containing sulfuric acid)
no catalytic effect of acetate ion is observed when mercuric acetate
is added to the reaction mixture over a wide concentration range
−
XO3 (X = Cl, Br) and water as these rates are significantly low.
−
These results indicate BrO3 ion is the active species of oxidant,
and excludes the involvement of HBrO , H BrO and BrO2+ in the
+
3
2
3
reaction.
2
The fractional order (<1) dependence on [substrate] and a
definite intercept in the 1/k versus 1/[oxo acid] plot (Fig. 1a)
indicates that the substrate is involved in the complex forma-
tion. Uncatalyzed bromate oxidation of 4-oxo acids exhibited first
order each in [4-oxo acid] and [bromate], and 1/kun versus 1/[sub-
strate] plots were linear passing through the origin (Fig. 1b). The
apparent order of less than unity (0.45–0.66) with respect to
(
0.001–0.01 M).
[
V(IV)] and a definite intercept in the 1/k versus 1/[V(IV)] plot
(Fig. 2) may be attributed to a complex formation between 4-oxo
acid and V(IV), which subsequently involves in the reaction with
−
BrO3
.
V(IV) is one of the stable oxidation states of vanadium and,
moreover, it is one equivalent oxidant/reductant in many oxida-
tions to be converted either to V(V) or V(III). The absence of any
effect of added acrylonitrile on the reaction discounts the possibil-
ity of one-electron oxidation/reduction leading to the formation of
free radicals. Therefore, it is assumed that in the oxidation of 4-oxo
acids by bromate, the oxidation state of V(IV) is not altered during
the oxidation reaction. Further, the absorption maximum (ꢂmax)
3
−1
−1
of oxovanadium(IV) at 745 nm with ε = 20 (dm mole cm )
remained the same during the course of the reaction, the
substrate forms preferably an outer sphere complex with the
catalyst V(IV).
◦
The 4-oxo acid is a weak acid (pKa = 5.77 at 40 C in aqueous
solution [39]) and the undissociated form of the substrate can be
taken as the only form in acidic media. 4-Oxo acid enolizes in acid
medium to form the enol and proceeds by a concerted or push-
+
Fig. 8. Hammett’ plots between log k and ꢀ at different temperatures showing the
kiso (s ) and ꢀ+iso under the conditions of Table 7. (A) 323 K, (B) 318 K, (C) 313 K,
−1
and (D) 308 K.

C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
83
pull mechanism, and the rate of enolization was determined by
bromination method [40].
Thus, the enolization reaction is represented as:
Scheme 1.

8
4
C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
Fig. 9. Plots between 1/k and 1/[acid] under the conditions of Table 3. (A) p-OCH3, (B) p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2.
In oxidation reactions, the keto group of the substrate can react
4.4. Rate law and its verification
either directly or through the enol form. Oxidation rates faster
than the rates of enolization have been observed with ceric ion
Taking all the steps of Scheme 1 for the oxidation of 4-oxo acid
and stoichiometry of the reaction, the following rate law Eq. (14)
may be written in terms of loss of concentration of bromate ion.
[
41], manganic ion [42], cobaltic and vanadate ions [43], mercuric
perchlorate [44] and alkaline diperiodatoargentate(III) [45] as oxi-
dants, indicating that the keto group of the substrate reacts directly.
All of these oxidants undergo one-electron reduction, and the reac-
tions proceeded via a free radical mechanism [41–45].
+
−d[Br(V)]T
kdKeKcKf [H ][V(IV)][oxo acid][Br(V)]T
Rate =
=
(14)
dt
1 + KeKcK [V(IV)[oxo acid]
f
The rates of oxidation and enolization were found to be equal in
the oxidation reactions by manganic pyrophosphate [46], thallium
triacetate [47] and diperiodatonickelate(IV) [48]. These reactions
were zero order in [oxidant], indicating the enol formation as the
rate determining step.
In the present investigation, the rate of enolization (measured by
the bromination method [40]) is greater than the rate of oxidation
by a factor of ∼12–15 and the enolization step can be visualized as
proceeding via the enol form of the keto group of 4-oxo acid. Hence,
the step involving enol formation can be assumed to be a fast step
and thus is not the rate determining step of the reaction.
The obtained rate law could explain the fractional order each in
substrate] and [catalyst], and first order each in [H ] and [bromate].
+
[
+
Rate
Br(V)]T
kdKeKcKf [H ][V(IV)][oxo acid]
=
k =
(15)
(16)
[
1 + KeKcK [V(IV)][oxo acid]
f
ꢀ
ꢁ
1
k
1
1
1
+
=
+
kdKeKcK [V(IV)][oxo acid]
kd [H ]
f
At constant (1.0 M) acid concentration, Eq. (16) reduces to Eq.
17)
(
1
k
1
1
kd
=
+
(17)
kdKeKcK [V(IV)][oxo acid]
f
4.3. Mechanism
According to Eq. (16), other conditions being constant, 1/k ver-
sus 1/[H ] should be linear passing through origin and the same
is observed (Fig. 9). Similarly, according to Eq. (17), the plots of
Based on aforesaid reasons and in accordance with the experi-
mental results, the mechanism as shown in Scheme 1 is proposed
for the V(IV)–catalyzed bromate oxidation of 4-oxo acids.
The mechanistic pathway is demonstrated as the enol form of
the 4-oxo acid forms an outer sphere complex with V(IV). Bromate
+
1
/k versus 1/[oxo acid] (at constant catalyst concentration), and 1/k
against 1/[catalyst] (at constant substrate concentration) should be
linear with a definite (the same) intercept on the 1/k axis. Such a
realization (Figs. 1a and 2) supports the validity of the rate law
(Eq. (14)) and hence the validity of the proposed reaction mecha-
nism (Scheme 1). The proposed mechanism is further corroborated
by the solvent influence on the reaction rate. The intermediate
complex C2 is less polar than the reactants due to dispersal of
charge; hence, decreasing polarity of the solvent media is expected
to stabilize the complex C2 in preference to the reactants there by
enhancing the reaction rate. Such a solvent influence has actually
been observed (Table 5).
reacts with the complex C , giving the cyclic bromate ester (com-
1
plex C ) through the carbon–oxygen–bromine bond. V(IV)–enol
2
complex formation enhances the reactivity of the –ene bond of the
complex C1 to form the cyclic bromate ester. The decomposition
of the complex C2 involving C–C and O–Br bonds, in a slow step,
results open structures, benzoic acid and formyl acetic acid. The oxi-
dation state of V(IV) is unaltered during the course of the reaction.
The formyl acetic acid on oxidation with the formed HOBr converts
−
into malonic acid along with the formation of Br ion. Formation of
formyl acetic acid as the intermediate was confirmed by comparing
with the retention time (tr) of the authentic sample. Benzoic and
malonic acids were the final products of oxidation. It is pertinent
to point out here that malonic acid is stable and does not undergo
further oxidation. The proposed mechanism is also in accordance
with the observed stoichiometry. This reaction is an example of the
neighboring group participation and intramolecular catalysis.
From the intercept and slope data of Fig. 1a and also Fig. 2,
the decomposition constant (kd) and, enolization and complexa-
tion constants (Ke and KcK ), can be evaluated and the obtained
f
values are presented in Table 10. Using the values of Ke, Kc, K and
f
−1
kd, the rate constants (k, s ) under different experimental condi-
tions were calculated according to Eq. (15) and compared with the
experimental data. There is a good agreement (within ± 5% error)

C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
85
Table 10
Evaluated decomposition constant (kd), enolization and formation constants (Ke, Kc,
Kf) of V(IV)-catalyzed oxidation of 4-oxo acids by bromate in acid medium at 313 K
using Eq. (17).
4
−1
−5
Substituent
kd × 10 (s
)
Ke, Kc, Kf × 10
(
decomposition constant)
(formation constants)
p-OCH3
p-CH3
143.55 ± 1.0
23.71 ± 0.5
9.45 ± 0.3
5.33 ± 0.5
4.84 ± 0.41
4.43 ± 0.30
4.06 ± 0.23
3.20 ± 0.11
–
H
p-Cl
m-NO2
3.12 ± 0.08
1.33 ± 0.02
Conditions as in Tables 2 and 4.
between them. This further supports the envisaged reaction mech-
anism (Scheme 1).
4.5. Structural influences and linear free-energy relationships
The order of reactivity among the phenyl substituted 4-oxo
acids is: p-methoxy ꢁ p-methyl > –H > p-chloro > m-nitro. The plot
of log k versus Hammett’s sigma values is a smooth curve; concaves
downward, at all the studied temperatures (Fig. 7). The curvature
may be attributed to (i) a gradual change in the reaction mechanism
when one passes from electron-donating to electron-withdrawing
substituents, (ii) a change in the rate determining step and (iii) a
change in the nature of transition state.
Fig. 10. Plot between log(formation constants) and ꢀ under the conditions of
Table 10. (A) p-OCH3, (B) p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2.
of this view. The ꢃH# value is seen to decrease with increas-
ing electron-donating ability of the substituents (Table 7). At the
same time ꢃS# becomes more negative, reflecting a more solvated
transition state. For electron attractors like nitro group, bond for-
mation is apparently more important, since substitution of nitro
group increases ꢃH# resulting in decreased solvolysis rate, ꢃS#
becomes positive. From these observations it is clear that there is
a direct conjugation between the substituent and the reaction cen-
tre. When there is a direct conjugation between the substituent and
the reaction centre, the plot of log k value corresponding to Ham-
mett’s sigma value does not fall on line. Higher values of sigma
Variation in the rate within a reaction series may be caused by
#
a change in the enthalpy of activation (ꢃH ) or entropy of activa-
#
tion (ꢃS ) or both, however, these quantities vary extensively in
#
#
a parallel fashion. A plot of ꢃH versus ꢃS is linear according to
the following equation.
ꢃH# = ꢃHo + ˇꢃS#
#
␤
is called the isokinetic temperature. It has been asserted that
#
#
apparently linear correlations of ꢃH with ꢃS are sometimes
misleading and the evaluation of ␤ by means of the above equa-
tion lacks statistical validity [49]. Exner advocates an alternative
method for the treatment of experimental data [50]. If the rates
of several reactions in a series have been measured at two tem-
peratures and log k2 (at T ) is linearly related to log k1 (at T ), i.e.,
+
viz., exalted (Brown and Okamoto’s ꢀ ) values [28] are to be used
for fitting all the points on the line. This situation is observed in
the present study and the plot of log k versus ꢀ+ is found to be
linear with a slope value of −1.33 at 313 K (Fig. 8). The reaction
2
1
log k = a + b log k , he proposed that ␤ can be evaluated from the
2
1
+
equation:
constant (ꢁ ) decreases (in magnitude) with an increase in tem-
+
perature (Table 9). The negative value of reaction constant (ꢁ )
T T (b − 1)
1
2
ˇ =
suggests the sensitivity of the reaction to the effects of electronic
perturbation. It also provides information about the nature of the
transition state involved during the reaction. A reaction involv-
ing positive charge development in the transition state is aided by
electron-releasing substituents and ꢁ+ value will be negative [53].
In the present kinetic study, the acceleration of reaction rate with
the electron-releasing substituents and the negative value of the
T b − T
2
1
We have calculated the isokinetic temperature (␤) to be 375.2 K
by plotting log k at 323 K versus log k at 313 K (Fig. 6). The value of
␤
(375.2 K) is higher than the experimental temperature range of
2
98–323 K, indicating that the rate is governed by the enthalpy of
activation [51]. The linearity in the Exner’s plot suggests that these
reactions follow a similar mechanism [52]. This is also corroborated
+
reaction constant (ꢁ ) indicate explicitly that the mechanism of oxi-
#
dation of 4-oxo acids by bromate, in the presence of V(IV) catalyst,
involves the development of positive charge in the transition state.
by near constancy in ꢃG values.
From the preceding discussion, it may be construed that the
curve in plot of log k versus ꢀ (Fig. 7), is a result of the change in
the nature of the transition state when one goes from electron-
releasing to electron-withdrawing substituents, i.e., electronic
perturbation effect. The transition state (Scheme 1) involves both
the C–C bond-breaking and the C–O bond-making. If both the
steps are influenced in the same way by the substituents, a linear
Hammett’s plot is expected. However, if the two processes influ-
ence in different ways, a curvature in the Hammett’s plot will be
obtained. Electron-releasing substituents may lead to a transition
state, where the C–C bond-breaking has progressed to a greater
extent than the C–O bond formation, while with electron-attracting
substituents C–O bond formation may be more extensive than C–C
bond-breaking. The activation parameters do also speak in favor
+
The reaction constant (ꢁ ) depends on the concentration of the enol
form of the substrate and the electron-releasing substituents tend
to delocalize the positive charge on enol form of the substrate and
hence facilitate the formation of carbocation in the transition state.
Oxidation reactions involving the formation of carbocation nor-
+
+
mally have a large negative ꢁ values. The ꢁ value (−1.09 to −1.50)
in the present study may be attributed to the nature of observed rate
constant (k), which is composite of several terms like enolization,
complexation and oxidation.
The linear plot of log(formation constants) versus ꢀ (Fig. 10)
and the linearity in the plot of log kd versus ꢀ+ (Fig. 11), rather than
log kd versus ꢀ (Fig. 12) are further evidences for the electronic
perturbation effects in the transition state.

8
6
C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
Fig. 13. Plot between ꢁ+ against 1/T under the conditions of Table 9. (A) 323 K, (B)
18 K, (C) 313 K, and (D) 308 K.
3
shown in Fig. 4. From the plots of ꢃH# against ꢃS , and ꢁ+ against
#
1
/T (Fig. 13), the Tiso value is found to be 377.3 K, which is in good
Fig. 11. Plot between log kd and ꢀ+ under the conditions of Table 10. (A) p-OCH3,
B) p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2.
agreement with the value obtained (375.2 K) from the slope of the
Exner’s plot (Fig. 6). It is interesting that although the rate constant
is composite involving multi-reaction steps—an anti-compensation
effect is not observed.
(
4.6. Isokinetic relationships
+
The Hammett’ reaction constant (ꢁ ) decreases (in magnitude)
5
. Conclusions
with increase in temperature, from −1.50 at 308 K to −1.09 at 323 K
(
Table 9). The Hammett’ lines intersect at a point corresponding to
+
−3 −1
(Fig. 8).
From the Hammett and Arrhenius plots associated with isoki-
an ꢀ
value of −1.30 and a k value of 23.02 × 10
s
iso
iso
netic relationship, it is noted that the ꢁ+ values are negative and
decrease with increase in temperature. Electron-releasing sub-
stituents accelerate the reaction rate and electron-withdrawing
substituents retard it. The structure-reactivity correlations are
made on the basis of the results obtained and suggest a carboca-
tionic reaction center in the transition state. It can be concluded
that V(IV) acts as an efficient catalyst in the oxidation of 4-oxo
acids by bromate in acid medium. This reaction is an example of
the neighboring group participation and intramolecular catalysis.
The experimental protocol suggests that this reaction could find
utility as a regioselective route for the synthesis of carboxylic acids,
specially malonic and substituted benzoic acids.
+
An oxo acid with a substituent having this ꢀ value (−1.30) will be
oxidized by bromate with the same rate at all temperatures. Such
an insensitivity of the reaction rate to temperature is a pointer to
the existence of an isokinetic phenomenon. Introduction of a nitro
+
group in the phenyl ring, which has a higher ꢀ value results in an
increase in the rate constant with temperature.
A reaction series which exhibit a common point of intersection
in the Hammett’ plot (Fig. 7) is expected to have a common point of
intersection in the Arrhenius plot also [52]. The Arrhenius plots are
Acknowledgements
We wish to thank two anonymous reviewers and editor for their
critical and useful comments which refined the manuscript a lot.
Financial assistance from the University Grants Commission, New
Delhi, India, under the Special Assistance Programme, is gratefully
acknowledged.
Appendix A. Supplementary data
−
d[Br(V)T ]
+
Rate
=
= kd[Complex C ][H ]
2
+
dt
−
=
kdK [Complex C ][BrO ][H ]
kdK KcKe[V(IV)][oxo acid][H ][BrO
f
1
3
+
−
]
3
=
f
But, bromate is present in complexed and uncomplexed forms.
Hence, the total bromate concentration is given as:
−
−
−
[
[
Br(V)]T = [BrO ] + [Complex C ]
3
2
−
3
=
[BrO ] + K KcKe[V(IV)][BrO ][oxo acid]
3
f
Fig. 12. Plot between log kd and ꢀ under the conditions of Table 10. (A) p-OCH3, (B)
p-CH3, (C) –H, (D) p-Cl, and (E) m-NO2.
Br(V)]T = [BrO3 ]{1 + K KcKe[V(IV)][oxo acid]}
f

C.S. Reddy, P.S. Manjari / Journal of Molecular Catalysis A: Chemical 328 (2010) 76–87
87
[
17] Ch. Sanjeeva Reddy, E.V. Sundaram, Tetrahedron 45 (1989) 2109–2126.
[18] Ch. Sanjeeva Reddy, T. Vijayakumar, Trans. Met. Chem. 32 (2007)
46–256.
19] P. Ruoff, M. Varga, E. Koros, Acc. Chem. Res. 21 (1988) 326–332.
20] A.M. Zhabotinsky, Chaos 1 (1991) 379–386.
[
Br(V)]T
−
BrO3 = 1
2
+ K KcKe[V(IV)][oxo acid]
f
[
[
[
In terms of total bromate concentration
21] L. Onel, G. Bourceanu, I. Bitter, M. Wittmann, Z. Noszticzius, J. Phys. Chem. A
+
110 (2006) 990–996.
−d[Br(V)]
kdKf KcKe[H ][V(IV)][oxo acid][Br(V)]T
Rate =
=
[22] M.A. Nath, R.P. Rastogi, G.M. Perzada, J. Braz. Chem. Soc. 20 (2009) 1–4.
[23] J.C. Bailar, The Chemistry of Coordination Compounds, Reinhold, New York,
dt
1 + K KcKe[V(IV)][oxo acid]
f
1966, p. 4.
+
[24] J.D. Reinheimer, S. Taylor, J. Org. Chem. 17 (1952) 1505–1509.
Rate
Br(V)T ]
kdKf KcKe[H ][V(IV)][oxo acid]
[
[
25] J.C. Bailar, H.J. Emeleus, R. Nyholm, A.F.T. Dickenson, Comprehensive Inorganic
Chemistry, vol. 3, Pergamon Press Ltd., New York, 1975, p. 496.
26] F. Feigl, V. Anger, Spot Tests in Organic Analysis, Elsevier, Amsterdam, 1975, p.
459.
=
k =
[
1 + K KcKe[V(IV)][oxo acid]
f
ꢀ
ꢁ
1
k
1
1
1
[27] M.A. Paul, F.A. Long, Chem. Rev. 57 (1957) 1–45.
[28] H.C. Brown, Y. Okamoto, J. Am. Chem. Soc. 80 (1958) 4079–4987.
=
+
+
kdK KcKe[V(IV)][oxo acid]
kd [H ]
f
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