Mechanistic investigation on the oxidation of kinetin by Ag(III) periodate complex in aqueous alkaline media: A kinetic approach

Article abstract of DOI:10.1007/s12039-010-0077-9

The oxidation of amino acid, kinetin (KNT) by diperiodatoargentate(III) (DPA) in alkaline medium at a constant ionic strength of 0.5 mol dm^-3 was studied spectrophtometrically. The reaction between KNT and DPA in alkaline medium exhibits 1:3 stoichiometry (KNT: DPA). Intervention of free radicals was observed in the reaction. Based on the observed orders and experimental evidences, a mechanism involving the monoperiodatoargentate(III) (MPA) as the reactive oxidant species has been proposed. The products, furon-2-methanol and para-nitro-purine were identified by spot test and characterized by spectral studies. The rate constants and associated activation parameters for the proposed mechanism as well as the thermodynamic quantities for different equilibrium steps are reported and discussed. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-010-0077-9

J. Chem. Sci., Vol. 122, No. 6, November 2010, pp. 891–900. © Indian Academy of Sciences.
Mechanistic investigation on the oxidation of kinetin by Ag(III)
periodate complex in aqueous alkaline media: A kinetic approach
S D LAMANI, A M TATAGAR and S T NANDIBEWOOR*
PG Department of Studies in Chemistry, Karnatak University, Dharwad 580 003
e-mail: stnandibewoor@yahoo.com
MS received 8 April 2010; revised 18 June 2010; accepted 29 June 2010
Abstract. The oxidation of amino acid, kinetin (KNT) by diperiodatoargentate(III) (DPA) in alkaline
–3
medium at a constant ionic strength of 0⋅5 mol dm was studied spectrophtometrically. The reaction be-
tween KNT and DPA in alkaline medium exhibits 1 : 3 stoichiometry (KNT : DPA). Intervention of free
radicals was observed in the reaction. Based on the observed orders and experimental evidences, a
mechanism involving the monoperiodatoargentate(III) (MPA) as the reactive oxidant species has been
proposed. The products, furon-2-methanol and para-nitro-purine were identified by spot test and charac-
terized by spectral studies. The rate constants and associated activation parameters for the proposed
mechanism as well as the thermodynamic quantities for different equilibrium steps are reported and dis-
cussed.
Keywords. Kinetics of oxidation; mechanism; kinetin; diperiodatoargentate(III); thermodynamic
parameters.
5
1. Introduction
et al have used DPA as an oxidizing agent for the
kinetics of oxidation of various substrates. They
normally found that order with respect to both oxi-
Kinetin is a kind of cytokinin, a class of plant hor-
mone that promotes cell division. Kinetin exists
naturally in the DNA of almost all organisms tested
so far, including human cells, and various plants.
The mechanism of production of KNT in DNA is
thought to be via the production of furfural – an oxi-
dative damage product of deoxyribose sugar in
DNA – and its quenching by the adenine base con-
verting it into N6-furfuryladenine, kinetin. Kinetin
has been thoroughly tested for its powerful anti-
aging effects in human skin cells and other systems.
At present, kinetin is one of the widely used compo-
nents in numerous skin care cosmetics and cosme-
–
dant and substrate was unity and [OH ] was found to
enhance the rate of reaction. It was also observed
that they did not arrive at the possible active species
of DPA in alkali and on the other hand, they pro-
posed mechanism by generalizing the DPA as
(x+1)
6,7
[Ag(HL)L] . However Anil Kumar et al , put an
effort to give an evidence for the reactive form of
DPA in the large scale of alkaline pH. In the present
investigation, we have obtained the evidence for the
reactive species for the DPA in alkaline medium.
Since multiple equilibria between different Ag(III)
species are involved, it would be interesting to know
which of the species is the active oxidant.
1
ceuticals, such as valeant products kinerase. There
are some reports published on other biological ef-
fects of kinetin in human beings, for example, its ef-
fects as anti-platelet aggregation factor reducing
thrombosis formation.
In the later period of 20th century the kinetics of
oxidation of various organic and inorganic substrate
have been studied by Ag(III) species which may be
due to its strong versatile nature of two electron
oxidant. Among the various species of Ag(III),
Diperiodatoargentate(III) (DPA) is a powerful
oxidizing agent in alkaline medium with the reduc-
–
Ag(OH)4, diperiodatoargentate(III) and ethylene-
2
tion potential 1⋅74 V. It is widely used as a volu-
bis(biguanide) silver(III) (EBS) are of maximum
metric reagent for the determination of various
attention to the researcher due to their relative sta-
3,4
organic and inorganic species Jayaprakash Rao
8
–
bility. The stability of Ag(OH)4 is very sensitive
towards traces of dissolved oxygen and other impu-
rities in the reaction medium where upon it had not
*For correspondence
891

892
S D Lamani et al
Scheme 1.
drawn much attention. However, the other forms of and then allowed to cool. It was filtered through a
9,10
Ag(III)
in highly alkaline medium and EBS is used in highly NaOH was added slowly to the filtrate, where a
acidic medium. voluminous orange precipitate agglomerated. The
are considerably stable; the DPA is used medium porosity fritted glass filter and 40 g of
In view of pharmaceutical and biological impor- precipitate was filtered as above and washed three to
tance of KTN and lack of literature on the oxidation four times with cold water to remove remaining
3
of KTN, it would be of interest to carryout a detailed K2S2O8. The pure crystals were dissolved in 50 cm
investigation of the oxidation of KNT by DPA in water and warmed to 80°C with constant stirring
aqueous alkaline medium (scheme 1). The thermo- there by some solid was dissolved to give a red solu-
dynamic quantities of various steps shown in tion. The resulting solution was filtered when it was
scheme 1 have been computed to arrive at a suitable hot and on cooling at room temperature, the orange
mechanism.
crystals separated out and were recrystallised from
water, dried and stored in desicator.
The complex was characterized from its UV-
spectrum, which exhibited three peaks at 216, 255,
2. Experimental
All chemicals used were of reagent grade and milli- and 362 nm. These spectral features were identical
12
pore water was used throughout the work. A solution to those reported earlier for DPA. The magnetic
of KNT was prepared by dissolving an appropriate moment study revealed that the complex is diamag-
13
amount of recrystallised sample in millipore water. netic. The compound prepared was analysed for
The purity of KNT was checked by comparing its silver and periodate by acidifying a solution of the
melting point, 268°C with literature data (lit. m.p. material with HCl, recovering and weighing the
270°C). The required concentration of KNT was AgCl for Ag and titrating the iodine liberated when
obtained from its stock solution. A stock solution of excess of KI was used for the required [DPA] solu-
–
IO4 was prepared by dissolving a known weight of tion in the reaction mixture.
KIO4 (S.D. Fine) in hot water. The stock solution
was used after keeping it for 24 h to complete the
2.2 Kinetic studies
equilibrium. Its concentration was ascertained
11
iodometrically at neutral pH maintained by phos- Since the initial reaction was too fast to be moni-
phate buffer. KNO3 and KOH (BDH) were used to tored by usual methods, kinetic measurements
maintain ionic strength and alkalinity of the reaction were performed on a Varian CARY Bio-50-UV-Vis
respectively. An aqueous solution of AgNO3 was spectrophotometer connected to a rapid kinetic
used to study the product effect, Ag(I). The pH of accessory (HI-TECH SFA-12). The kinetics was
the medium in the solution was measured by ELICO followed under pseudo-first order condition where
(LI613) pH meter.
[KNT] > [DPA] at 25 0⋅1°C unless specified.
The reaction was initiated by mixing the DPA to
KNT solution, which also contained required con-
centration of KNO3, KOH and KIO4. The progress
2.1 Preparation of DPA
DPA was prepared by oxidizing Ag(I) in presence of of reaction was followed spectrophotometrically
12
KIO4 as described elsewhere, the mixture of 28 g at 360 nm by monitoring the decrease in absorbance
3
of KOH and 23 g of KIO4 in 100 cm of water along due to DPA with molar absorbency index ‘ε’
3
with 8⋅5 g AgNO3 was heated just to boiling and to be 13900 100 dm mol cm . It was verified
–1
–1
20 g K2S2O8 was added in several lots with stirring that there is a negligible interference from other

Mechanistic investigation on the oxidation of kinetin by Ag(III) periodate complex
893
species present in the reaction mixture at this wave- r and the standard deviation S, of points from the
length.
regression line, was performed with the Microsoft
The reaction was followed to more than 85% Office Excel 2003 programme.
completion of the reaction. Plots of log (absorbance)
versus time lead to the first order rate constant (kobs).
The plots were linear up to 85% completion of reac-
3. Results
tion and rate constants were reproducible within 3.1 Stoichiometry and product analysis
5%. During the kinetics, a constant concentration
–5
–3
Different sets of reaction mixtures containing vary-
ing ratios of DPA to kinetin in presence of constant
(1⋅0 × 10 mol dm ) of KIO4 was used throughout
the study unless otherwise stated. Thus, possibility
of oxidation of KNT by periodate was tested and
found that there was no significant interference due
to KIO4 under experimental condition. The total
–
amount of OH , KNO3 were kept for 4 h in closed
vessel under nitrogen atmosphere. The remaining
concentration of DPA was estimated by spectropho-
tometrically at 360 nm. The results indicated a 1 : 3
stoichiometry as given in scheme 2.
–
concentration of OH was calculated by considering
the amount present in the DPA solution and addi-
tionally added. Kinetic runs were also carried out in
N2 atmosphere in order to understand the effect of
dissolved oxygen on the rate of reaction. No signifi-
cant difference in the results was obtained under a
N2 atmosphere and in the presence of air. In view of
the ubiquitous contamination of carbonate in the ba-
sic medium, the effect of carbonate was also studied.
Added carbonate had no effect on the reaction rates.
The spectral changes during the reaction are shown
in figure 1. It is evident from the figure 1 that the
concentration of DPA decreases at 360 nm.
The main oxidation products were identified as
para-nitro-purine and furon-2-methanol, and were
characterized by its melting point, IR and GC–MS
respectively. The nature of furon-2-methanol was
confirmed by its IR spectrum, which showed a –OH
–1
stretch at 3386 cm indicating the presence of alco-
hol and para-nitro-pteridine was also confirmed by
the presence of NO2 group, stretching frequency at
–1
1332 cm . Further, the products were subjected to
GC-mass spectral analysis. GC–MS data was ob-
tained on a QP-2010S Shimadzu gas chromatograph
mass spectrometer. The mass spectral data showed a
base peak at 98 m/z thus, confirming the presence of
furon-2-methanol (figure 2) and another product,
was also confirmed by mass spectral data which
showed a peak at 182 m/z, hence confirming the
product para-nitro-pteridine (figure 3). All other
peaks observed in GC–MS can be interpreted in ac-
cordance with the observed structure of para-nitro-
pteridine and furon-2-methanol.
In view of the modest concentration of alkali used
in the reaction medium, attention was also directed
to the effect of the reaction vessel surface on the
kinetics. Use of polythene acrylic wares and quartz
or polyacrylate cells gave the same results, indicat-
ing that the surface did not have any significant ef-
fect on the reaction rates. Regression analysis of
experimental data to obtain regression coefficient
The by-products were identified as ammonia by
14
Nessler’s reagent,
the CO2 was qualitatively
detected by bubbling nitrogen gas through the acidi-
fied reaction mixture and passing the liberated gas
through tube containing lime water. The formation
+
of free Ag in solution was detected by adding KCl
solution to the reaction mixture, which produced
white turbidity due to formation of AgCl. It was
observed that para-nitro-pteridine and furon-2-
methanol did not undergo further oxidation under
the present kinetic condition.
Figure 1. Spectroscopic changes occurring in the oxi-
–5
3.2 Reaction orders
dation of KNT by DPA at 298 K, [DPA] = 5⋅0 × 10 ;
–4
–
–3
KNT] = 5⋅0 × 10 ; [OH ] = 0⋅50; and I = 0⋅50 mol dm
with scanning time interval of (1) 1⋅0, (2) 2⋅0, (3) 3⋅0, (4)
4⋅0, (5) 5⋅0 min.
The reaction orders were determined from the slope
of log kobs vs log (concentration) plots by varying the

894
S D Lamani et al
Figure 2. Mass spectrum of reaction product, furon-2-methanol at 98 amu.
Figure 3. Mass spectrum of reaction product, para-nitro-pteridine at 182 amu.
concentration of kinetin, alkali in turn while keep- 3.4 Effect of [kinetin]
ing all other concentrations and conditions con-
The effect of KNT on the rate of reaction was stud-
stant.
ied at constant concentration of alkali, DPA and
–3
periodate at constant ionic strength of 0⋅50 mol dm .
The substrate KNT was varied in the range of
3.3 Effect of [diperiodatoargentate (III)]
–4
–3
–3
1⋅0 × 10 to 1⋅0 × 10 mol dm . The kobs values in-
creased with increase in concentration of KNT. The
apparent order with respect to [KNT] was found to
be less than unity (table 1) (r ≥ 0⋅998, S ≤ 0⋅02).
This was also confirmed by the plots of kobs vs
The oxidant DPA concentration was varied in the
–5
–4
–3
range of 1⋅0 × 10 to 1⋅0 × 10 mol dm and fairly
constant kobs value indicate that order with respect to
[DPA] was unity (table 1). This was also confirmed
by linearity of the plots of log (absorbance) vs time
to 85% completion of reaction.
0⋅4
[KNT] which is linear rather than the direct plot of
kobs vs [KNT] (figure 4).

Mechanistic investigation on the oxidation of kinetin by Ag(III) periodate complex
895
– –
Table 1. Effect of variation of [DPA], [KNT], [OH ] and [IO ] on the oxidation of kinetin
4
–3
by diperiodatoargentate(III) in aqueous alkaline medium at 298 K and I = 0⋅50 mol dm .
5
10 [DPA]
4
10 [KNT]
–
[OH ]
5 –
10 [IO ]
4
–3
(mol dm )
–3
(mol dm )
–3
(mol dm )
–3
(mol dm )
3
10 k
3
10 k
obs
cal
1⋅0
3⋅0
5⋅0
8⋅0
10⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
0⋅5
0⋅5
0⋅5
0⋅5
0⋅5
1⋅0
1⋅0
1⋅0
1⋅0
1⋅0
2⋅2
2⋅1
2⋅0
2⋅3
2⋅1
2⋅0
2⋅0
2⋅0
2⋅0
2⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
1⋅0
3⋅0
0⋅5
0⋅5
0⋅5
0⋅5
0⋅5
1⋅0
1⋅0
1⋅0
1⋅0
1⋅0
0⋅9
1⋅6
2⋅0
2⋅3
2⋅5
0⋅87
1⋅6
1⋅9
2⋅2
2⋅4
5⋅0
8⋅0
10⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
0⋅05
0⋅1
0⋅2
0⋅3
0⋅5
1⋅0
1⋅0
1⋅0
1⋅0
1⋅0
0⋅69
1⋅1
1⋅6
1⋅8
2⋅0
0⋅68
1⋅0
1⋅4
1⋅7
1⋅9
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
5⋅0
0⋅5
0⋅5
0⋅5
0⋅5
0⋅5
0⋅5
0⋅8
1⋅0
3⋅0
5⋅0
2⋅4
2⋅2
2⋅0
1⋅4
1⋅2
2⋅4
2⋅2
1⋅9
1⋅4
1⋅1
3.6 Effect of [periodate]
The effect of increasing concentration of periodate
was studied by varying the periodate concentration
–5
–4
–3
from 1⋅0 × 10 to 1⋅0 × 10 mol dm keeping all
other reactants concentration constant. It was found
that added periodate had retarding effect on the rate
of reaction.
3.7 Effect of ionic strength (I) and dielectric con-
stant of medium (D)
The addition of KNO3 at constant [DPA], [KNT],
0⋅40
–
–
Figure 4. Plots of k vs [KNT]
obs
and k vs [KNT]
obs
[OH ] and [IO4] was found that increasing ionic
strength of the reaction medium did not effect the
rate of reaction. Varying the t-butyl alcohol and
water percentage varied dielectric constant of the
medium ‘D’. The D values were calculated from the
equation D = DwVw + DBVB, where Dw and DB are
dielectric constants of pure water and t-butyl alcohol
respectively and Vw and VB are the volume fractions
of components of water and t-butyl alcohol respec-
(conditions as in table 1).
3.5 Effect of [alkali]
The effect of increase in concentration of alkali on
the reaction was studied at constant concentration of
KNT, DPA and periodate at constant ionic strength
–3
of 0⋅50 mol dm at 25°C. The rate constants in-
creased with increase in alkali concentrations (table tively in the total mixture. The decrease in dielectric
1), indicating positive fractional order dependence of constant of the reaction medium had no effect on the
rate of reaction.
rate on alkali concentration (r ≥ 0⋅996, S ≤ 0⋅02).

896
S D Lamani et al
diluting the reaction mixture with methanol, a white
3.8 Effect of initially added products
precipitate was formed, which indicated the inter-
15
The externally added product Ag(I), para-nitro-
pteridine and furon-2-methanol did not have any
significant effect on the rate of reaction.
vension of free radicals in the reaction.
3.10 Effect of temperature
3.9 Polymerization study
The kinetics was studied at four different tempera-
tures (15, 20, 25 and 30°C) under varying concentra-
tions of KNT, alkali and periodate keeping all other
conditions constant. The rate constants (k), of the
slow step of the reaction mechanism were obtained
from the slopes and intercepts of the plots of 1/kobs
The intervension of free radicals in the reaction was
examined as follows: The reaction mixture, to which
a known quantity of acrylonitrile monomer initially
added, was kept for 2 h in an inert atmosphere. On
–
2–
vs 1/[KNT], 1/kobs vs 1/[OH ] amd 1/kobs vs [H3IO6 ]
at four different temperatures. The values are given
in table 2.
Table 2. Activation parameters and thermodynamic
quantities for the oxidation of KNT by diperiodatoargen-
tate(III) in aqueous alkaline medium with respect to the
slow step of scheme 2.
4. Discussion
(a) Effect of temperature
12
The literature survey reveals that the water soluble
3 –1
10 k (s )
Temperature (K)
diperiodatoargentate(III) (DPA) has a formula
7–
2
[Ag(IO6)2] with dsp configuration of square pla-
nar structure, similar to diperiodatocopper(III) com-
plex with two bidentate ligands, periodate to form a
planar molecule. In the alkaline medium, the disso-
293
298
303
308
2⋅2
2⋅8
3⋅9
4⋅7
–
ciative equilibria (1–3) of the IO4 were detected and
(b) Activation parameters
the corresponding equilibrium constants were
16
determined at 298.2 K by Aveston.
Parameter
Value
–1
E (kJ mol )
39⋅4
37 21
–169 10
87
4⋅3 0⋅2
2
a
–
2IO4 + 2OH
–
4–
H2I2O10 logβ1 = 15⋅05
ꢀꢀꢁꢀ
ꢂꢀꢀꢀ
#
ΔH (kJ mol )
–1
(1)
(2)
(3)
#
ΔS (JK mol )
–1
–1
# –1
ΔG (kJ mol )
– –
IO4 + OH + H2O
2–
H3IO6 logβ2 = 6⋅21
4
ꢀꢀꢁꢀ
ꢂꢀꢀꢀ
logA
–
IO4 + 2OH
–
3–
H2IO6 logβ3 = 8⋅67.
ꢀꢀꢁꢀ
ꢂꢀꢀꢀ
(c) Effect of temperature on K , K and K for the oxi-
1 2 3
dation of KNT by diperiodatoargentate(III) in aqueous
alkaline medium.
The distribution of all species of periodate in aque-
ous alkaline solution can be calculated from equilib-
5
–4
Temperature
(K)
K
K × 10
2
K × 10
3
3 –1
(dm mol )
1
–
ria (1–3). In the [OH ] range used in this work the
3
(dm mol ) (mol dm )
–1
–3
4–-
–
amount of dimer (H2I2O10 ) and IO4 species of perio-
date can be neglected. The main species of periodate
293
298
303
308
0⋅3
0⋅4
0⋅5
0⋅6
1⋅0
1⋅4
1⋅6
1⋅8
2⋅6
2⋅3
1⋅9
1⋅6
3– 2–
are H2IO6 and H3IO6 , which are consistant with the
17
result calculated from Crouthamel’s data. Hence
–
3–
DPA could be as [Ag(H3IO6)2] or [Ag(H2IO6)2] in
alkaline medium. Therefore, under the present con-
dition, diperiodatoargentate(III), may be depicted as
(d) Thermodynamic quantities using K , K and K
1 2
3
Thermodynamic
quantities
Values
from K
Values
Values
–
[Ag(H3IO6)2] . The similar speciation of periodate in
from K
from K
3
1
2
18
alkali was proposed for diperiodatonickelate(IV).
–1
ΔH (kJ mol )
35⋅3 4⋅0
28⋅5 0⋅7 –23⋅8 0⋅8
The reaction between diperiodatoargentate(III)
complex and kinetin in alkaline medium has the
stoichiometry 1 : 3 (KNT : DPA) with a first order
–1
ΔS (JK mol ) 111⋅1 23 117⋅1 13 118⋅3 22
–1
–1
ΔG (kJ mol )
2⋅0 0⋅12 –6⋅6 0⋅3 –59⋅3 3⋅1

Mechanistic investigation on the oxidation of kinetin by Ag(III) periodate complex
897
Scheme 2. Detailed scheme for the oxidation of KNT by alkaline DPA.
dependence on [DPA] and an apparent order of less observed orders in each constituent such as [oxi-
–
–
than unit order in [substrate], [alkali] and negative dant], [OH ], and [IO4] may be well accommodated
fractional order in [periodate]. No effect of added (scheme 2).
products was observed. Based on the experimental In view of the observed experimental results,
results, a mechanism is proposed for which all the monoperiodatoargentate(III) (MPA) is considered to

898
S D Lamani et al
be the active species. The fractional order depend-
Spectroscopic evidence for the complex formation
– –
ence of kobs on [OH ] suggests that OH takes part in between oxidant and substrate was obtained from
–5 –3
the pre-equilibrium step 1 with DPA to give a depro- UV-Visible spectra of [DPA] (5⋅0 × 10 mol dm ),
–4
tonated diperiodatoargentate(III). The plot of 1/kobs [KNT] (5⋅0 × 10 mol dm ), [OH ] (0⋅5 mol dm )
–3
–
–3
–
vs [IO4] is linear with positive intercept indicating a and mixture of DPA and kinetin. A bathochromic
dissociative equilibrium in which the DPA loses a shift of about 5 nm from 234 to 239 nm in the spectra
periodate ligand from its coordination sphere, form- of DPA to mixture of substrate and DPA was
ing a reactive monoperiodatoargentate(III) complex observed. However, the Michaelis–Menten plot
(MPA) in the second step, which is evidenced by proved the complex formation between DPA and
–
decrease in the rate with increase in [IO4]. It may be kinetin. Such type of complex between an oxidant
expected that lower Ag(III) periodate species such and substrate has also been observed in other
19
as MPA will be more important in the reaction than studies scheme 2 leads to the rate law (4)
2–
DPA. The inverse fractional order in H3IO6 concen-
–
kK K K [DPA][OH ][KNT]
1
tration might also due to this reason. The fractional
2 3
Rate=
, (4)
order with respect to KNT presumably results from
the complex formation between MPA and KNT
prior to the slow step. Indeed it is to be noted that a
plot of 1/kobs vs 1/[KNT] was linear and shows an
intercept in agreement with the complex formation.
The KNT-DPA complex (I) is formed by chelation
of DPA molecule at amine group of KTN. As oxy-
gen is more electronegative compared to nitrogen,
nitrogen donates a lone pair of electron to monope-
riodatoargentate(III). This complex (I) decomposes
in a slow step to give 1H-imidazole pyridine-4-yl
amine radical (II), furon-3-yl-methanol (III) and
Ag(II) species. Imidazole pyridine 4-yl-amine radi-
cal (II) combines with Ag(II) species in a fast step to
give 1H-imidazole pyridine 4-yl-amine cation (IV)
and Ag(I) species. This amine cation (IV) reacts
with water molecule in a fast step, to give 1H imida-
zole N-pyrimidine 4-yl-hydroxyl amine (V), which
further reacts with another mole of MPA in a fast
step to give 1H-imidazole 4-nitroso-pyrimidine (VI)
and Ag(I) species. In further fast step the 1H-
Imidazole-4-nitroso pyrimidine (VI) reacts with an-
other molecule of MPA to give the product 1H-
imidazole-nitro pyrimidine (VII) and Ag(I). All
these results may be interpreted in the form of
scheme 2. The probable structure of MPA and com-
plex(C) (I) are given below.
2– – 2–
[H ]IO + K [OH ][H IO ] +
3
⎢
⎡
⎤
⎥
6
1
3
6
– –
K K [OH ] + K K K [OH ][KNT]
1
⎢
⎣
⎥
⎦
2
1 2 3
Rate
k
obs
=
=
[DPA]
–
kK K K [OH ][KNT]
1
. (5)
2 3
2– – 2–
[H IO ] + K [OH ][H IO ] +
6
⎡
⎤
3
6
1
3
⎢
⎥
– –
K K [OH ] + K K K [OH ][KNT]
1
⎢
⎣
⎥
⎦
2
1 2 3
This explains all the observed kinetic orders of differ-
ent species. The rate law (5) can be rearranged in the
following form, which is suitable for verification.
2–
]
6
[H IO
3
1
=
–
kK K K [OH ][KNT]
1
k
obs
2 3
(6)
2–
]
6
[H IO
3
1
1
+ .
+
+
kK K [KNT] kK [KNT]
2
k
3
3
According to (6), other conditions being constant,
plots of 1/kobs vs 1/[KNT] (r ≥ 0⋅9998, S ≤ 0⋅016),
–
1/kobs vs 1/[OH ] (r ≥ 0⋅996, S ≤ 0⋅019) and 1/kobs vs
2–
[H3IO6 ] (r ≥ 0⋅9312, S ≤ 0⋅017) should be linear and
are found to be so (figures 5a–c). The slopes and in-
tercepts of such plots leads to the value of K1, K2, K3
3
as 0⋅40 dm mol , 1⋅4 × 10 mol dm ,
–1
–5
–3
and
k
4
3
–1
–3 –1
2⋅3 × 10 dm mol and 2⋅8 × 10 s at 298 K and
–3
I = 0⋅5 mol dm respectively. The values of K1 and
20
K2 are in good agreement with earlier literature.
These constants were used to calculate the rate con-
stant and compared with the experimental kobs values
and found to be in reasonable agreement with each
other which fortifies scheme 2.

Mechanistic investigation on the oxidation of kinetin by Ag(III) periodate complex
899
The thermodynamic quantities for the first, sec- From the slopes and intercepts, the values of K1, K2
ond and third equilibrium steps of scheme 2 can be and K3 were calculated at four different temperatures
–
–
evaluated as follows: [KNT], [OH ] and [IO4] (table and these values are given in table 2. The vant
1) were varied at four different temperatures. The Hoff’s plots were made for variation of K1, K2 and
plot of 1/kobs vs 1/[KNT] (r ≥ 0⋅9998, S ≤ 0⋅016) K3 with temperatures (logK1 vs 1/T) (r ≥ 0⋅9949,
–
1/kobs vs 1/[OH ] (r ≥ 0.996, S ≤ 0⋅019), and 1/kobs vs S ≤ 0⋅00898) (logK2 vs 1/T) (r ≥ 0⋅9362, S ≤ 0⋅006)
2–
[H3IO6 ] (r ≥ 0⋅9912, S ≤ 0⋅017) should be linear. and (logK3 vs 1/T) (r ≥ 0⋅9985, S ≤ 0⋅006) and the
values of enthalpy of reaction ΔH, entropy of reac-
tion ΔS and free energy of reaction ΔG, were calcu-
lated for the first, second and third equilibrium
steps. These values are given in table 2. A compari-
–1
son of ΔH (35⋅3 0⋅8 kJ mol ) from K1 with that of
#
–1
ΔH (37⋅0 1⋅0 kJ mol ), the rate limiting step,
supporting the fact that the first step of scheme 2
is fairly fast since it involves a low activation
21
energy.
The negligible effect of ionic strength and dielec-
tric constant might be due to involvement of neutral
#
substrate in the reaction (scheme 2). The values ΔH
#
and ΔS were both favourable for electron transfer
processes. The favourable enthalpy was due to re-
lease of energy on solutions changes in the transition
#
state. The negative value of ΔS suggests that the in-
termediate complex is more ordered than the reac-
22
tant. The observed modest enthalpy of activation
and a higher rate constant for the slow step indicates
that the oxidation presumably occurs via an inner-
sphere mechanism. This conclusion is supported by
21
earlier observations.
5. Conclusion
Among the various species of DPA in alkaline
medium, MPA, i.e. [Ag(H2IO6)(H2O)2] is considered
as active species for the title reaction. The results
indicated that, the role of pH in the reaction medium
is crucial. Rate constant of slow step and other equi-
librium constants involved in the mechanism were
evaluated and activation parameters with respect to
slow step of reaction were computed. The overall
mechanistic sequence described is consistent with
product studies, mechanistic and kinetic studies.
Appendix I
Derivation of rate equation:
According to scheme 2,
Figure 5. Verification of rate law (4) for the oxidation
of KNT by DPA. (a) 1/k vs 1/[KNT] at four different
obs
temperatures (condition as in table 1). (b) 1/k
–
vs
obs
–
kK K K [KNT][DPA][OH ]
1
1/[OH ] at four different temperatures (condition as in
2–
table 1). (c) 1/k vs [H IO ] at four different tempera-
obs
2 3
(I)
Rate = k[C] =
3
6
2–
[H IO ]
3
tures (condition as in table 1).
6

900
S D Lamani et al
2–
–
kK K K [DPA][KNT][OH ]
1
[DPA] =[DPA] +[Ag(H IO )(H IO )]
6
T
f
3
6
2
2 3
Rate =
.
2–
[H IO ] + K [OH ][H IO ] +
⎢
–
2–
⎡
⎤
⎥
+[Ag(H IO )(H O) ]+ C
3
6
2
2
3
6
1
3
6
– –
K K [OH ] + K K K [OH ][KNT]
⎢
⎣
⎥
⎦
1
2
1 2 3
–
K K [OH ]
1
⎡
⎤
⎥
⎥
⎥
⎥
⎥
–
1+ K [OH ] +
1
2
+
⎢
2–
]
6
[H IO
3
⎢
=[DPA] +
f
,
References
⎢
⎢
⎢
–
K K K [KNT][OH ]
3
1
2
2–
]
6
1. Mok D W S and Mok M C 1994 Cytokinins: chemistry,
activity and function (Boca Raton: CRC Press Inc)
2. Sethuram B 2003 Some aspects of electron transfer
reactions involving organic molecules (New Delhi:
Allied Publishers) p. 151
[H IO
3
⎢
⎣
⎥
⎦
where T and f refer to total and free concentrations
2–
]
6
[DPA][H IO
3
3. Jaiswal P K and Yadav K L 1970 Talanta 17 236
4. Jaiswal P K 1972 Analyst 1 503
[DPA] =
f
(II)
2–
–
[H IO ] + K [OH ][H IO ] +
⎢
2–
⎡
⎤
⎥
3
6
1
3
6
5. Jayprakash R P, Sethuram B and Navaneeth Rao T
1985 React. Kinet. Catal. Lett. 29 289
– –
K K [OH ] + K K K [OH ][KNT]
⎢
⎣
⎥
⎦
1
2
1 2 3
6. Kumar A, Kumar P and Ramamurthy P 1999 Polyhe-
dron 18 773
2–
7. Kumar A, Vaishali P and Ramamurthy P 2000 Int. J.
Chem. Kinet. 32 286
[OH] =[OH] +[Ag(H IO )(H IO )]
T
f
3
6
2
6
+[Ag(H IO )(H O) ]
8. Krishenbaum L J and Mrozowski L 1978 Inorg.
Chem. 17 3718
3
6
2
2
9. Cotton F A and Wilkinson G 1980 Advanced inorganic
chemistry (New York: John Wiley and Sons) p. 974
10. Banerjee R, Das R and Mukhopadhyay S 1992
J. Chem. Soc. Dalton Trans. 1317
–
K K [DPA][OH ]
1
–
–
2
=[OH ] + K [DPA][OH ]+
.
f
1
2–
]
6
[H IO
3
11. Panigrahi G P and Misro P K 1978 Indian J. Chem.
A16 762
In view of the low concentration of [DPA] and
2–
[H3IO6 ] used:
12. Cohen G L and Atkinson G 1964 Inorg. Chem. 3 1741
13. Jeffery G H, Bassett J, Mendham J and Denney R C
1996 Vogel’s text book of quantitative chemical
analysis (Singapore: Longmans Publishers Pvt Ltd).
5th edn, pp. 391 and 467
– –
[OH ] = [OH ] .
(III)
T
f
14. Vogel A I 1973 A text book of practical organic
chemistry including quantitative organic analysis
(London: ELBS Longman) 3rd edn, p. 679
15. Frost A A and Pearson R G 1970 Kinetics and
mechanism (New Delhi: Wiley Eastern Ltd)
16. Aveston J 1969 J. Chem. Soc. A p 273
17. Crouthamel C E, Hayes A M and Martin D S 1951 J.
Am. Chem. Soc. 73 82
Similarly,
[KNT] = [KNT] +[C]
T
f
–
K K K [KNT][DPA][OH ]
1
2 3
=[KNT] =
f
.
2–
]
6
[H IO
3
18. Cotton F A and Wilkinson G 1996 Advanced inor-
ganic chemistry (New York: Wiley Eastern) p. 153
19. Shetti N P and Nandibewoor S T 2009 Z. Phys.
Chem. 223 299
–
In view of the low concentration of [DPA], [OH ],
2–
and [H3IO6 ] used:
20. Vereesh T M, Hiremath C V and Nandibewoor S T
2007 J. Phys. Org. Chem. 20 55
[KNT] =[KNT] .
T
(IV)
21. Farokhi S A and Nandibewoor S T 2003 Tetrahedron
59 7595
f
22. Weissberger A and Lewis E S (eds) 1974 Investiga-
tion of rates and mechanism of reactions in technique
of chemistry (New York: Wiley) vol. 4, p. 421
Substituting (II), (III), (IV) in (I) and omitting the
subscripts, we get,