Indian Journal of Chemical Technology
Vol. 20, January 2013, pp. 70-76
Kinetics and mechanism of uncatalyzed and selenium dioxide catalyzed oxidation
of nicotinic acid hydrazide by bromate
R S Yalgudre & G S Gokavi*
Kinetics and Catalysis Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416 004, India
Received 27 June 2011; accepted 23 August 2012
The uncatalyzed and selenium dioxide catalyzed oxidation of nicotinic acid hydrazide, (NIH) by bromate has been
studied in hydrochloric acid medium. The –NH2 of hydrazoic moiety and pyridine nitrogen of the NIH forms protonated
species which are involved in two ion pair complexes with the oxidant in prior equilibria. In case of uncatalyzed reaction the
complex with the protonated hydrazoic moiety decomposes to give corresponding acyl diimide intermediate while that of
the pyridine nitrogen decreases the rate of reaction. In presence of selenium dioxide as catalyst, the NIH reduces the catalyst
to H2SeO2 species which is oxidized by the oxidant to complete its catalytic cycle. The product of the reaction is found to be
nicotinic acid and there is no intervention of any free radicals. A rate law derived for both the reactions satisfy the kinetic
data obtained and UV-spectrophotometer examination of the reaction mixture also support the mechanisms proposed.
Keywords: Bromate, Catalysis, Nicotinic acid hydrazide, Selenium dioxide
The ligands containing either a hydrazone or
hydrazine moiety are found to exhibit antibacterial
activity1 and such activity is also enhanced by
complexation2 of hydrazides with metal ions of the
first transition series. Therefore, aroylhydrazones are
used as ligands in inorganic coordination chemistry.
Further, hydrazones containing pyridine ring are also
utilized as analytical reagents for transition and
lanthanide ions due to their high sensitivity3 towards
these metal ions. Nicotinic acid hydrazide is one such
pyridine containing hydrazide which is an anologue of
isoniazid, the anti tuberculosis drug. These hydrazides
are also found to effectively inhibit4 the
myeloperoxidase enzyme activity. Hydrogen peroxide
is generated during digestion of pathogens in presence
of myeloperoxidase enzyme, which, in turn, reacts
with chloride ion to produce hypochlorous acid. The
role of hydrazides is to react with the intermediates
like hypochlorous acid, thus inhibiting the tissue
damage at the sites of inflammation. In synthetic
organic chemistry, hydrazides are used as starting
materials for preparation of esters, amides5 and N-Ndiacylhydrazines6 in presence of various oxidizing
agents.
The oxidant of the present study, bromate, is also a
strong oxidizing agent but its rate of oxidation of
____________
*Corresponding author.
E-mail: [email protected]
organic substrates is slow which requires a catalyst5,7
to inititiate the reaction. The catalyst of the present
study is selenium dioxide which is a mild and
selective oxidant8 used in synthetic organic chemistry.
Due to its mild nature, selenium dioxide is also used
as a catalyst for various organic transformations8 in
presence of cooxidants like hydrogen peroxide. Such
catalysis include oxidation of amines9, anilines10,
alkenes11, aldehydes12 and Bayer-Villiger13 reactions.
Therefore, the activity of selenium dioxide will be
facilitated by the presence of another cooxidant like
bromate. In continuation of our earlier work14,15, in
this study the oxidation of nicotinic acid hydrazide by
bromate is investigated kinetically to know the
probable pathway of its oxidative degradation.
Experimental Procedure
Materials and methods
Double distilled water was used throughout the
work. All the chemicals used for experiments were of
reagent grade. The stock solution of KBrO3 was
prepared by dissolving KBrO3 (BDH) in water and
standardized iodometrically. The solution of nicotinic
acid hydrazide(SD fine) was prepared by dissolving
requisite amount in water. The ionic strength was
maintained using sodium perchlorate and to vary
hydrogen ion concentration HCl (BDH) was used.
Acetic acid and acrylonitrile were used directly as
received to study the effect of solvent polarity on the
YALGUDRE & GOKAVI: KINETICS & MECHANISM OF UNCATALYZED & SeO2 CATALYZED OXIDATION OF NIH
reaction medium and free radical formation
respectively. The solution of catalyst selenium (IV)
was obtained by dissolving selenium dioxide (SD
fine) in distilled water.
Procedure and kinetic measurements
The reaction was studied under pseudo-first-order
conditions keeping hydrazide concentration large
excess over that of oxidant (KBrO3) at constant
temperature of 25.0 ± 0.1oC. The reaction was
initiated by mixing the previously thermostated
solutions of the oxidant, substrate and catalyst which
also contain the required amount of hydrochloric acid,
potassium chloride and distilled water. The reaction
was followed by titrating the reaction mixture for
unreacted oxidant iodometrically and the rate
constants were determined from the pseudo-first-order
plots of log [oxidant] against time. The pseudo-firstorder plots were linear for more than 90% of the
reaction and rate constants were reproducible within
±6% for both uncatalyzed and selenium dioxide
catalyzed reactions.
Stoichiometry and product analysis
The stoichiometry of bromate oxidation predicts
either Br2 or HOBr as the product of reaction but the
hydrazides can be very easily oxidized by both of
them in acidic solutions due to the oxidation potential4
of HOBr or Br2 as 1.34 and 1.07 V respectively. The
test for formation of bromide ion was carried out in
sulphuric acid solution instead of hydrochloric acid,
for both uncatalyzed and catalyzed reactions, by
adding silver nitrate to the reaction mixture after
completion of the reaction. The precipitation of silver
bromide confirms the formation of bromide ion as one
of the product of the reaction. Therefore, the product
of the reaction under the present experimental
conditions is bromide ion. It is also noticed during the
kinetic studies and the stoichiometric analysis that no
bromine is evolved, further confirming the bromide
ion as the only product.
Further, in 10 mL of 0.6 mol dm-3 hydrochloric acid
1 mmol nicotinic acid hydrazide (0.1371 g) was
dissolved. To the resulting solution 2 mmol (0.3349 g)
of KBrO3 was added in presence of 1.0 × 10-5 mol dm-3
selenium dioxide for catalyzed reaction. The reaction
mixture was stirred at 25oC for 2 days in case of
uncatalyzed reaction and 1 day for catalyzed reaction.
The respective nicotinic acid separated was filtered and
recrystallized form ethanol-water mixture. The GCMS
analysis of the solution of the product in methanol
71
shows peak at 123. The m.p. of the recrystallized
product is found to be 236oC (lit m. p. 236-237oC16).
From the GCMS analysis and the m. p. determination
the product of the reaction is confirmed to be nicotinic
acid for both uncatalyzed and catalyzed reactions.
Therefore, the stoichiometry of the reaction is found to
be two moles of oxidant per three moles of the
hydrazide as shown in following equation:
2BrO3- + 3RCONHNH2
2Br- + 3RCOOH+3H2O + 3N2
... (1)
where R = C5H5N
Results and Discussion
Effect of reactants concentration
The reaction was carried out under pseudo-firstorder conditions keeping the concentration of
nicotinic acid hydrazide large excess at a constant
HCl concentration (0.1 mol dm-3) and at a constant
ionic strength of 0.5 mol dm-3 (Table 1). The pseudofirst-order plots are found to be linear on varying the
concentration of oxidant between 0.5 × 10-3 and 5.0 ×
10-3 mol dm-3, keeping the concentration of nicotinic
acid hydrazide (NIH) constant at 1.0 × 10-2 mol dm-3
(Table 1) for both uncatalyzed and catalyzed
reactions, indicating that the order in oxidant
Table 1Effect of [bromate], [NIH] and [H+] on the uncatalyzed
and selenium dioxide catalyzed oxidation of nicotinic acid
hydrazide by bromate at 298 K
[I = 0.5 mol dm-3 and [SeO2] ×105 = 1.0 mol dm-3]
[BrO3]×103 [NIH] ×102
mol dm-3
mol dm-3
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.6
0.8
1.0
2.0
3.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.0
2.0
5.0
7.0
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
[H+]×10
mol dm-3
kuncat×104
s-1
kcat×104
s-1
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
1.0
2.0
3.0
4.0
1.0
0.88
0.70
0.57
0.31
0.21
0.71
0.70
0.71
0.71
0.70
0.70
0.71
0.27
0.38
0.71
4.0
8.0
16
8.70
8.20
7.70
6.10
2.80
2.30
7.80
78.0
7.71
7.72
7.83
7.81
7.82
1.01
4.02
7.81
30.1
68.3
180.0
72
INDIAN J. CHEM. TECHNOL., JANUARY 2013
concentration is unity. The effect of nicotinic acid
hydrazide was studied by varying the concentration of
nicotinic acid hydrazide between 5.0 × 10-3 and 7.0 ×
10-2 mol dm-3 keeping all other concentrations
constant (Table 1). The pseudo-first-order rate
constants(kobs) are found to decrease as the
concentration of nicotinic acid hydrazide increases for
both uncatalyzed and catalyzed reactions. The effect
of catalyst concentration was studied between the
concentration range 1.0 × 10-6 and 5.0 × 10-5 mol dm-3
and the plot of kcat against [catalyst] is found to be
linear, indicating an order of unity in [catalyst].
Effect of hydrogen ion concentration
The effect of hydrogen ion was studied in order to
understand the nature of reactant species present in
the solution. The [H+] was varied between 4.0 × 10-2
and 0.4 mol dm-3 (Table 1). Increasing [H+]
accelerates the rate of reaction and the order in [H+] is
found to be about 1.9 for uncatalyzed and about 2.3
for catalyzed reaction as determined from the plot of
log kobs against log [H+].
Effect of ionic strength
The effect of ionic strength was studied keeping
[NIH], [KBrO3], [catalyst] and [HCl] constant at 1.0 ×
10-2 mol dm-3, 1.0 × 10-3 mol dm-3, 1.0 × 10-5 mol dm-3
and 0.1 mol dm-3 respectively. Sodium perchlorate
was used to vary the ionic strength. The rate of
uncatalyzed reaction decreases with increasing ionic
strength from 0.05 mol dm-3 to 0.5 mol dm-3, while
that of selenium dioxide catalyzed reaction remain
unaffected.
Effect of solvent polarity
The effect of solvent polarity on both the
uncatalyzed and catalyzed reactions was studied by
varying percentage of acetic acid from 2% to 40% v/v
keeping nicotinic acid hydrazide and bromate
concentration, constant at 1.0 × 10-2 mol dm-3 and
1.0 × 10-3 mol dm-3 respectively. The dielectric
constants of the reaction mixture were calculated by
using the D values for pure solvents. It is found that
the decrease in the dielectric constant of the medium
increases the rate of the reaction for both uncatalyzed
and catalyzed reactions. The plots of log kobs against
(1/D), where D is the dielectric constant, are found to
be linear with negative slopes.
Effect of temperature
The effect of temperature on both the uncatalyzed
and catalyzed reactions was studied at 293, 298, 303
Table 2Effect of temperature and activation parameters of
uncatalyzed and catalyzed oxidation of nicotinic acid hydrazide by
bromate
[BrO3] ×103 = 10[NIH] = 10[HCl] = 0.1 mol dm-3, I = 0.5 mol dm-3 and
[SeO2] × 105 = 1.0 mol dm-3]
Temp.,K
kuncat ×104, s-1
kcat ×104, s-1
293
0.46
4.91
Ea, kJ mol-1
∆G#, kJ mol-1
∆H#, kJ mol-1
−∆S#, JK-1 mol-1
298
0.70
7.73
303
1.0
9.44
Uncatalyzed
64.9 ± 0.5
94.7 ± 0.5
61.5 ± 0.5
111.3 ± 4
313
2.5
19.0
Catalyzed
50.5 ± 0.6
89.6 ± 0.6
47.6 ± 0.6
140.8 ± 5
and 313 K. The activation parameters ∆H#, ∆G# and
∆S# along with corresponding pseudo-first-order rate
constants are given in Table 2.
Test for free radical intervention
In order to understand the intervention of free
radicals in the reaction the reaction was studied in
presence of added acrylonitrile for both uncatalyzed
and catalyzed reactions. It is found that there is no
induced polymerization of the acrylonitrile in both the
reactions, as there was no formation of the precipitate
due to polymerization of acryllonitrile and also it does
not affect the rate of the reaction.
Mechanism of the reaction and the rate laws
Uncatalyzed reaction
The order in [H+] is found to be 1.9 for uncatalyzed
reaction indicating two prior protonation equilibria.
Potassium bromate is a strong electrolyte and in
aqueous solution it exists as BrO3- which is also a
strong acid thus its protonation would not be possible
under the experimental conditions. Another possibility
of explaining the presence of protonation prior
equilibria is the involvement of an induction period.
The bromate oxidations of one-electron oxidants are
also found to involve induction period17 due to initial
hydrogen ion dependent reduction of bromate
according to the equilibrium shown in following
equation:
Red + BrO3- + 3H+
Ox + HBrO2 + H2O … (2)
But in the present investigation there is no such
induction period which is also not observed by
Thomson18 during oxovanadium(IV) oxidation by
bromate. Therefore, the hydrogen ion dependence of
the reaction is not due to the equilibrium (2). Other
possible protonation in the present system would be
YALGUDRE & GOKAVI: KINETICS & MECHANISM OF UNCATALYZED & SeO2 CATALYZED OXIDATION OF NIH
that of the nicotinic acid hydrazide(NIH). There are
two possible protonation sites in nicotinic acid
hydrazide analogous to that of isoniazid19, the
pyridine nitrogen and –NH2 group of the hydrazide.
The pK of the pyridine nitrogen is reported19 to be
1.8 and that of the –NH2 group is 3.5. Since the
reaction is carried out in acidic medium the nicotinic
acid hydrazide will be present in the diprotonated
form. The protonation equilibria of both the sites can
be represented using the following equations:
Further, the π →π∗ and n→π* transitions of the
hydrazide are observed20 between 225-260 nm and
270-290 nm respectively and both these transitions
are sensitive to the pH of the solution. Therefore, to
get further information regarding protonation the UVVIS spectra of nicotinic acid hydrazide was examined
in presence of HCl. The spectrum of aqueous solution
of nicotinic acid hydrazide shows peak at 264 nm but
in presence of 0.1 mol dm-3 HCl the peak shifts at
268 nm with increase in intensity and also a new peak
at 216 nm is observed analogous to that observed for
isonicotinic acid hydrazide21. It is also observed that
there are two isosbestic points at 228 and 246 nm,
indicating the existence of three absorbing, NIH,
NIH2+ and NIH32+ species in the solution21. Since, the
order in [H+] is more than unity, the diprotonated
NIH32+ of the nicotinic acid hydrazide is the active
species in the reaction.
The mechanism of the uncatalyzed reaction
involves interaction between diprotonated nicotinic
acid hydrazide, NIH32+, and bromate in a prior
equilibrium forming a complex which further
decomposes to give the products. The kuncat values are
found to decrease as the [NIH] increases while the
values remain almost constant as the [oxidant]
increases. Since the reaction proceeds with the
interaction of both the reactants the values kuncat are
73
expected to increase as the [reactants] increase. The
decrease in the values of kuncat would indicate
formation of two complexes between the reactants,
one of them does not decompose while the other
decomposes to give products. The oxidant in the
present study is an anion which can undergo an ionpair complex with positively charged sites of the
substrate, nicotinic acid hydrazide. In acidic medium
the active species of the nicotinic acid hydrazide is
NIH32+ as shown in equilibrium (3) which contain
protonated pyridine nitrogen as well as protonated –
NH2 group of the hydrazide moiety. The ion-pair
complex between the protonated –NH2 group leads to
the further reaction while that with protonated
pyridine nitrogen does not undergo further reaction.
Such stable tetra alkylammonium salts of bromate22-24
have been prepared and used for various synthetic
applications. Therefore, the pyridinium salt of the
bromate in the present study is quite stable and does
not undergo any oxidative transformation, thus
converting the oxidant into an inactive form. In order
to understand the interaction between the reactants,
the UV-VIS spectra of nicotinic acid hydrazide in
presence of bormate in acidic medium was
investigated. It is observed that the intensity of the
absorbance between 200 nm and 300 nm increases as
the bromate ion concentration increases21 indicating
the complex formation between the two. The
mechanism of the reaction is shown in Scheme 1 in
terms of active species of the reactants. The rate law
for the uncatalyzed reaction and the corresponding
expression for the kuncat can be represented by Eq. 5
and Eq. 6 respectively. The following Eqs (5) and (6)
are derived by considering the protonation equilibria
of the nicotinic acid hydrazide and the formation of
ion pair complexes as shown in Scheme 1:
Rate = k1K3[H+]2[BrO3-][NIH]/([H+]2 + K1[H+] +
K1K2)(1 + (K3+K4)[NIH])
... (5)
kuncat = k1K3[H+]2/([H+]2 + K1[H+] + K1K2) (1 +
(K3+K4)[NIH])
... (6)
The decrease in the kuncat values as the
concentration of nicotinic acid hydrazide increases is
due to the formation of an inert ion-pair complex
between the protonated pyridine nitrogen and the
bromate ion. The reported values of equilibrium
constants K1 and K2 are 1.58 × 10-2 dm3 mol-1 and
3.16 × 10-4 dm3 mol-1 respectively. The values are
small and thus if the denominator of Eq (6) is
74
INDIAN J. CHEM. TECHNOL., JANUARY 2013
Fig. 1Plots of (1/kuncat) and (1/kcat) against [NIH]
[[NIH] × 102 = [BrO3-] × 103 = [HCl] ×10 = [SeO2] × 105=1.0 mol
dm-3 and I = 0.5 mol dm-3]
Scheme 1Uncatalyzed oxidation of nicotinic hydrazide (NIH)
by bromate
protonation equilibria are due to that of nicotinic acid
hydrazide as shown in Eqs (3) and (4), and bromate ion
does not take part in any of the hydrogen ion dependent
reaction as explained earlier. Therefore, the third
protonation is due to either the catalyst itself or any
intermediates of the catalyst generated during the
reaction. Selenium dioxide in aqueous solution exists
as selenous acid and H2SeO3, the step wise dissociation
constants of this acid are as shown below:
H2SeO3
K5
H++ HSeO3-
… (7)
HSeO3-
K6
H+SeO32-
… (8)
+ 2
neglected the plot of kuncat against [H ] is expected to
be linear. Such a plot is found to be linear without any
intercept thus verifying the derived rate law Eq. (6) on
the basis of Scheme 1. Further, according to Eq. (6)
the plot of (1 / kuncat) against [NIH] is also fund to be
linear with an intercept (Fig. 1), thus supporting the
mechanism predicted.
The reaction shown in Scheme 1 involves formation
of two ion-pair complexes and the complex 1
decomposes in a two-electron transfer slow step
generating acyl diimide as intermediate. Further, the
nucleophilic attack of water molecule on the carbonyl
carbon of acyl diimide intermediate gives nicotinic
acid and another intermediate NH=NH. The fast
oxidation of NH=NH by the HOBr will complete the
observed stoichiometry of the reaction.
Selenium dioxide catalyzed reaction
The hydrogen ion dependence of the selenium
dioxide catalyzed reaction is about 2.3 an order more
than that of the uncatalyzed reaction. Therefore, three
protonation prior equilibria are taking part in the
mechanism of the catalyzed reaction. Two of the
The dissociation constant25 for first equilibrium is
2.4 × 10-3 mol dm-3 whereas that of the second
dissociation constant is 4.76 × 10-9 mol dm-3. The
values of dissociation constants indicate that first
deprotonation occurs, to a significant extent, in
aqueous acidic solutions but due to very low value,
and the second deprotonation does not occur under
the acidic condition of the present study. Therefore,
under the present conditions of the reaction selenium
dioxide exists as selenous acid which will be in
equilibrium with the HSeO3- and the catalysis by [H+]
indicates selenous acid as the reactive species.
Hydrazides are very good reductants with reduction
potential of benzoic acid hydrazide4 is reported to be
0.19 V and our preliminary experiments in the
absence of bromate indicates that when selenium
dioxide is allowed to react with nicotinic acid
hydrazide, red colloidal selenium is precipitated
which on standing turns grey. But in presence of
oxidant no such precipitation is observed. Therefore,
YALGUDRE & GOKAVI: KINETICS & MECHANISM OF UNCATALYZED & SeO2 CATALYZED OXIDATION OF NIH
the mechanism of the reaction involves reduction of
the selenium dioxide by hydrazide converting it into
an intermediate species which is then oxidized back
by bromate thus completing the catalytic cycle.
During reduction of selenium dioxide by olefins26 in
acetic acid- acetic anhydride it has been proposed that
HSeO2- is generated. Therefore, HSeO2- can be
considered as the intermediate produced after the
reduction of selenous acid by hydrazide. Further,
bromate has been used27 as an analytical volumetric
reagent for the titration of small amount of colloidal
selenium. In such titration the colloidal selenium is
oxidized to selenous acid and further oxidation to
selenic acid by bromate does not takes place.
Therefore, considering the kinetic data obtained and
the reported results, the mechanism of hydrazide
oxidation is initiated by the reduction of selenous acid
to HSeO2-. Since the reaction is carried out in acidic
medium the intermediate, HSeO2-, is in the form of
H2SeO2 which is oxidized by bromate in a fast step to
selenous acid.
The selenium dioxide mechanism of nicotinic acid
hydrazide by bromate can be summarized as in
Scheme 2. The corresponding rate law is given by
Eq. (9) and the expression for kcat by Eq. (10) as
shown below:
Rate =
k cat =
k 2 K 7 [H + ]3 [H 2SeO3 ][NIH] [BrO3− ]
([H + ]2 + K1[H + ]+K1K 2 )(K 5 +[H + ])
k 2 K 7 [H + ]3 [H 2SeO3 ]
([H + ]2 +K1[H + ]+K1K 2 )(K 5 +[H + ])
75
… (9)
… (10)
The decrease in the rate of reaction as [NIH]
increases is due to the ion pair formation of bromate
with [NIH32+] as explained in case of uncatalyzed
reaction. Then on substituting [BrO3-] by considering
ion pair formation and substituting in Eq. (9), we get
the final rate law Eq. (10). The expression for kcat will
be given by Eq. (11):
k cat =
k 2 K 7 [H + ]3 [H 2SeO 3 ]
([H + ]2 +K1[H + ]+K1K 2 )(K 5 +[H + ])(1+(K 3 +K 4 )[NIH]
… (11)
The comparison of the pseudo-first-order rate
constants of both uncatalyzed and catalyzed reactions
indicates that the uncatalyzed reaction occurs to the
negligible extent in presence of catalyst. Therefore,
while deriving Eq. (11) the contribution of the
uncatalyzed reaction is neglected. The derived
Scheme 2Selenium dioxide catalyzed oxidation of nicotinic hydrazide (NIH) by bromate
76
INDIAN J. CHEM. TECHNOL., JANUARY 2013
Eq. (11) for the pseudo-first-order rate constant based
on the mechanism of Scheme 2 explains the order of
more than two in [H+] and an order of unity in the
[catalyst]. The equilibrium constants K1 and K2 are
small and [H+] is also small, therefore as an
approximation if we neglect the denominator of Eq.
(11) then it is possible to verify the equation after
rearranging, and by plotting kcat against [H+]3 this is
found to be linear (Fig. 1) with an intercept, thus
verifying the rate [Eq. (11)]. The plot of 1/ kcat against
[NIH] is also found to be linear, further supporting the
proposed mechanism.
The increase in ionic strength of the reaction
decreases the rate of uncatalyzed reaction, while the
rate of the catalyzed reaction remains unaffected. The
uncatalyzed reaction occurs between BrO3- and
NIH32+, thus decreasing the rate with increase in ionic
strength while the catalyzed reaction involves neutral
H2SeO3 species of the catalyst. The test for free
radicals is found to be negative for both uncatalyzed
and catalyzed reactions, therefore the reaction
proceeds without any intervention of free radicals.
This observation is also supported by the product
analysis in which corresponding nicotinic acid is the
only product obtained. The N-N–diacylhydrazine
would have been obtained15 along with nicotinic acid
as a result of free radical intervention in the reaction.
Colloidal selenium is not formed in any of the
catalyzed kinetic runs although it has been obtained in
absence of oxidant. Hence, it is assumed that the
H2SeO2 formed during the reduction of selenous acid
is oxidized by bromate in a fast step.
The increase in relative permittivity of the reaction
medium with acetic acid increases the rate of both
uncatalyzed and catalyzed reactions and the plots of
log kobs against (1/D)( D = dielectric constant of the
medium) are linear with a negative slope. The charge
separation in the transition state formed and its larger
size increases its stability28 in the medium of higher
relative permittivity, thus increasing the rate of
reaction with increase in acetic acid content. The
negative value of ∆S# can be ascribed to the lesser
degree of freedom formerly available to the reactants.
The moderate value of enthalpy of activation is due to
the electron transfer process. The activation energy of
the uncatalyzed reaction is higher than that of the
catalyzed reaction as expected.
Acknowledgement
One of the authors (RSY) gratefully acknowledges
University Grants commission, New Delhi for the
award of teacher fellowship under FIP- UGC-XI plan.
References
1 Carcelli M, Mazza P, Pelizzi C, Pelizzi G F & Zani F,
J Inorg Biochem, 57 (1995) 43.
2 Gudasi K B, Patil M S, Vadavi R S, Shenoy R V, Patil S A &
Nethaji M, Spectrochimica Acta Part A, 67 (2007)172.
3 Galic N, Peric B, Kojic-Prodic B & Cimerman Z, J Mol
Struct, 559 (2001) 187.
4 Amos R I J, Gourlay B S, Schiesser C H, Smith J A & Yates
B F, Chem Commun, (2008) 1695.
5 Kocevar M, Mihorko P & Polanc S, J Org Chem, 60 (1995)
1466.
6 Kulkarni P P, Kadam A J, Desai U V, Mane R B &
Wadgaonkar P P, J Chem Res(S), 2000, 184.
7 Reddy C S & Kumar T V, Transition Met Chem, 32 (2007)
246.
8 Maity A C, Synlett, 2008, 465.
9 Shunichi M & Tatsuki S, Tetrahedron Lett, 28 (1987) 2383.
10 Christin G, Beate P, Elisabeth I & Karola R, Synthesis (2008)
1889.
11 Manktala R, Dhillon R S & Chhabra B R, Indian J Chem,
45B (2006) 1591.
12 Halina W, Monica B, Krystian K & Jacek M, Tetrahedron,
57 (2001) 9743.
13 Guzman J A, Mendoza V, Gracia E, Garibay C F, OlivaresL
Z & Maldonado L A, Synth Commun, 25 (1995) 2121.
14 Kadam S D, Supale A R & Gokavi G S, Z Phys Chem,
222 (2008) 635.
15 Shewale S A, Phadkule A N & Gokavi G S, Int J Chem
Kinetics, 40 (2008) 151.
16 Handbook of Chemistry and Physics, 67th edn (CRC Press,
Florida, USA), 1987.
17 Mohammad A, Transition Met Chem, 28 (2003) 345.
18 Thomson R C, Inorg Chem, 10 (1971) 1892.
19 Wheate N J, Vora V, Anthony N G & McInnes F J, J Incl
Phenom Macrocycl Chem, 68 (2010) 359.
20 Anoussakdise G, Ristos M & Uri C, Candian J Chem,
51 (1973) 811.
21 Yalgudre R S & Gokavi G S, Ind Eng Chem Res, 51 (2012)
5135.
22 Grancicova O, React Kinet Catal Lett, 94 (2008) 99.
23 Nath U, Das S S, Deb D & Das P J, New J Chem, 28 (2004)
1423.
24 Das S S, Nath U, Deb D & Das P J, Synth Commun,
34 (2004) 2359.
25 Lurie Ju, Handbook of Analytical Chemistry (MIR
Publishers, Moscow), 1971.
26 Trachtenberg E N, Nelson C H & Carver J R, J Org Chem,
35 (1970) 1653.
27 Coleman W C & McCrosky C R, Ind Eng Chem Anal,
9 (1937) 1458.
28 Amis E S, Solvents Effect on Reaction Rates and Mechanism
(Academic Press, New York), 1996.