Kinetics and mechanism of oxidation of chloramphenicol - An antibiotic drug by diperiodatocuprate(III) in aqueous alkaline medium

Article abstract of DOI:10.1524/zpch.2011.0002

The kinetics of oxidation of chloramphenicol (CHP) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.10 mol dm^-3 was studied spectrophotometrically. The reaction between DPC and CHP in alkaline medium exhibits 1:2 stoichiometry (CHP: DPC). The main oxidation products were identified by spot test, IR, NMR and GCMS spectral studies. The reaction is of first order in DPC and CHP concentrations. As the alkali concentration increases the rate of reaction increases with fractional order dependence on alkali concentration. Increase in periodate concentration decreases the rate. A suitable mechanism is proposed. The reaction constants involved in the different steps of the mechanism were calculated. The activation parameters with respect to slow step of the mechanism are computed and discussed. Thermodynamic quantities are also determined. by Oldenbourg Wissenschaftsverlag, Munchen.

Full text of DOI:10.1524/zpch.2011.0002

Z. Phys. Chem. 225(2011) 79-94. DOI 10.1524.zpch.2011.0002
© by Oldenbourg Wissenschaftsverlag, München
Kinetics and Mechanism of Oxidation
of Chloramphenicol – an Antibiotic Drug
by Diperiodatocuprate(III) in Aqueous
Alkaline Medium
By Rajeshwari V. Hosahalli, Kirthi S. Byadagi, Sharanappa T. Nandibewoor
*
and Shivamurti A. Chimatadar
P.G. Department of Studies in Chemistry, Karnatak University, Dharwad- 580003, India
(Received May 17, 2010; accepted June 29, 2010)
Chloramphenicol . Diperiodatocuprate(III) . Oxidation . Kinetics
The kinetics of oxidation of chloramphenicol (CHP) by diperiodatocuprate(III) (DPC) in aque-
ous alkaline medium at a constant ionic strength of 0.10 mol dmK3 was studied spectrophoto-
metrically. The reaction between DPC and CHP in alkaline medium exhibits 1:2 stoichiometry
(CHP: DPC). The main oxidation products were identified by spot test, IR, NMR and GCMS
spectral studies. The reaction is of first order in DPC and CHP concentrations. As the alkali
concentration increases the rate of reaction increases with fractional order dependence on alkali
concentration. Increase in periodate concentration decreases the rate. A suitable mechanism is
proposed. The reaction constants involved in the different steps of the mechanism were calcu-
lated. The activation parameters with respect to slow step of the mechanism are computed and
discussed. Thermodynamic quantities are also determined.
1. Introduction
In recent years the study of highest oxidation states of transition metals has
intrigued many researchers. Transition metals in a higher oxidation state can be
stabilized by chelating with suitable polydentate ligands. Metal chelates such as
diperiodatocuprate(III) [1], diperiodatoargentate(III) [2], diperiodatonickel-
ate(IV) [3] are good oxidants in a medium with a appropriate pH value. Periodate
and tellurate complexes of copper in its trivalent state have been extensively
used in the analysis of several organic compounds [4]. The kinetics of self-
decomposition of these complexes was studied in detail [5]. Copper(III) is shown
*
Corresponding author. E-mail: schimatadar@gmail.com
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R. V. Hosahalli et al.
to be an intermediate in the copper(II) catalysed oxidation of amino acids by
peroxydisulphate [6]. The oxidation reaction usually involves the copper(II)-cop-
per(I) couple and such aspects are dealt in different reviews [7,8]. The use of
diperiodatocuprate(III) (DPC) as an oxidant in alkaline medium is new and re-
stricted to a few cases due to its limited solubility in aqueous medium. DPC is
a versatile one-electron oxidant for various organic compounds in alkaline me-
dium and its use as an analytical reagent is well recognized [9].Copper com-
plexes have occupied a major place in oxidation chemistry due to their abun-
dance and relevance in biological chemistry [10–14]. Copper(III) is involved
in many biological electron transfer reactions [15]. When copper(III) periodate
complex is an oxidant, multiple equilibrium steps between different copper(III)
species are involved, it would be interesting to know which of the species is the
active oxidant.
Chloramphenicol(2,2-dichloro-N-[(1R,2R)-2-hydroxy-(hydroxymethyl)-
nitrophenyl)-ethyl]acetamide) is a bacteriostatic antimicrobial. Chlorampheni-
col(CHP) is effective against a wide variety of Gram-positive and Gram-negative
bacteria, including most anaerobic organisms. It is considered a prototypical
broad band spectrum antibiotic, alongside the tetracycline’s. The clinical applica-
tion of chloramphenicol was in the treatment of typhoid. The most serious ad-
verse effect associated with chloramphenicol treatment is bone marrow toxicity.
Due to their extensive usage, chloramphenicol may enter the environment via
wastewater effluent and biosolids from sewage treatment plants and via manure
and litters from food-producing animal husbandry. The presence and accumula-
tion of chloramphenicol antibiotics in aquatic environments, albeit at low con-
centrations, may pose threats to the ecosystem and human health by inducing
increase and spread of bacteria drug-resistance due to long-term exposure. This
necessitates development of the various advanced oxidation processes for the
transformation of chloramphenicol in water. In view of potential pharmaceutical
importance of chloramphenicol and lack of literature on the oxidation of this
drug by any oxidant and the complexity of the reaction, a detailed study of the
reaction becomes important. Hence the title reaction is undertaken.
2. Experimental
2.1 Chemicals and materials
All chemicals used were of A. R. grade and double distilled water was used
throughout the work. The solution of chloramphenicol (Sisco Chem) was pre-
pared by dissolving known amount of the samples in distilled water. The purity
of the samples was checked by their melting point 150oC. (Literature: 149oC–
153oC). The copper(III) periodato complex was prepared [16] and standardized
by a standard procedure [17]. The copper(II) solution was made by dissolving
the known amount of copper sulphate (BDH) in distilled water. Periodate solu-
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Fig. 1. First order plots of oxidation of chloramphenicol by DPC at 25oC [CHP] = 5.0 ! 10K4
[OHK] = 0.05; I = 0.10.mol dmK3 = (1) 0.5, (2) 1.0, (3) 2.0, (4) 3.0, (5) 4.0, (6) 5.0.
;
tion was prepared by weighing out the required amount of sample in hot water
and used after 24h. Its concentration was ascertained iodometrically [18] at neu-
tral pH by a phosphate buffer. Since periodate is present in excess in DPC and
also in the reaction mixture, the possibility of oxidation of chloramphenicol by
periodate on alkaline medium in 25oC was checked. However, it was found that
there was no significant reaction under the experimental conditions employed
compared to the DPC oxidation of chloramphenicol. KOH and KNO3 solutions
were employed to maintain the required alkalinity and ionic strength respec-
tively.
2.2 Kinetic measurements
The oxidation of CHP by DPC was followed under pseudo-first order conditions,
where [CHP] > [DPC] at 25 0.1oC. The reaction was initiated by adding DPC
to chloramphenicol which also contained the required concentration of KNO3,
KOH and KIO4. The progress of the reaction was followed spectrophotometri-
cally at 415 nm by monitoring the decrease in absorbance of DPC. Earlier the
extinction coefficient was determined at 415 nm for different concentrations and
was found to be ε = 6235 250 dm3 molK1 cmK1. It was verified that there is
negligible interference from other species present in the reaction mixture at this
wavelength.
The pseudo-first order rate constants, kobs were determined from log(absorb-
ance) versus time plots (Fig. 1). The plots were linear up to 80% completion of
the reaction. The rate constants were reproducible within 5% and are the aver-
age of atleast three independent kinetic runs. During the kinetics a constant con-
centration of KIO4 viz. 5.0 ! 10K5 mol dmK3 was used throughout the study
unless otherwise stated and excess periodate is present in DPC, the possibility
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R. V. Hosahalli et al.
Fig. 2. Spectral changes during the oxidation of chloramphenicol by DPC at 25.0°C; [DPC] =
5.0 ! 10K5, [CHP] = 5.0 ! 10K4, [OHK] = 0.05 and I = 0.10 . mol dmK3 (scanning time
interval is 1.0 min).
of the oxidation of chloramphenicol by periodate in alkaline medium at 25oC
was checked. It was found that there is no significant reaction between CHP and
periodate under the experimental conditions employed compared to the DPC
oxidation of chloramphenicol. The concentration of periodate and OHK was cal-
culated by considering the amount present in the DPC solution and that addition-
ally added. Kinetic runs were also carried out in N2 atmosphere in order to
understand the effect of dissolved oxygen on the reaction. No significant differ-
ence in the results was obtained in the N2 atmosphere and in presence of air. In
view of the ubiquitous contamination of carbonate in the basic medium, the
effect of carbonate was also studied. Added carbonate has no effect on the reac-
tion rates. The spectral changes during the reaction are shown in Fig. 2. It is
evident from the figure that the concentration of DPC decreases at 415nm.
2.3 Stoichiometry and product analysis
Different sets of reaction mixtures containing excess of DPC to chloramphenicol
in presence of constant amount of OHK and KNO3 were kept for 6h in a closed
vessel under inert atmosphere. The remaining DPC concentration was estimated
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Scheme 1. Stoichiometric equation for the oxidation of chloramphenicol by alkaline diperioda-
tocuprate.
by spectrophotometrically at 415nm. The results, indicated that one mole of CHP
requires two moles of DPC as shown in Scheme 1.
After completion of the reaction, the reaction mixture was acidified, concen-
trated and extracted with ether. The ether layer was subjected to column chroma-
tography [19] and fractions are subjected to spectral investigations. From the IR,
(Fig. 3) GCMS (Fig. 4) and NMR (Fig. 5) spectra, the main oxidation product
was identified as p-nitrobenzaldehyde. IR spectrum showed C = O stretching for
aldehydic functional group at 1708 cmK1,while –NO2 stretching observed at
1349 cmK1. The presence of p-nitrobenzaldehyde was also confirmed by GC-
MS analysis (Fig. 4), obtained on a Shimadzu 17A gas chromatograph with a
Shimadzu XP- 5000A mass spectrometer using EI ionization technique. The
mass spectrum showed the base peak at 151 amu consistent with the molecular
ion of 151 amu. All other peaks observed in the GC-MS can be interpreted in
accordance with the structure of p-nitrobenzaldehyde. Further p-nitrobenzalde-
1
hyde was characterized by its HNMR spectra (Fig. 5) (DMSO): 10.17 (s.1H,
CHO), 8.18 (d, 2H, ArH), 7.67(d, 2H, ArH).Another product 2-Amino, hydroxy-
ethanol was confirmed by GCMS spectra which showed the molecular ion peak
at 77amu. Chloro-acetic acid is confirmed by spot test [20].
3. Results
3.1 Reaction orders
The reaction orders were determined from the slope of log kobs versus log (con-
centration) plots by varying the concentrations of chloramphenicol, alkali and
periodate in turn while keeping all other concentrations and conditions constant.
3.2 Effect of [diperiodatocuprate (III)]
The oxidant DPC concentrations was varied in the range of 5.0 ! 10K6 to
5.0 ! 10K5 mol dmK3 and fairly constant kobs values indicates that order with
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R. V. Hosahalli et al.
Fig. 3. FT-IR spectra of p-nitrobenzaldehyde, the product of oxidation of chloramphenicol by
DPC.
Fig. 4. GC-MS spectra of the product p-nitrobenzaldehyde showed molecular ion peak and base
peak at m.z 151 amu.
respect to DPC concentration was unity (Table 1). This was also confirmed by
linearity of the plots of log(absorbance) versus time up to 80% completion of
the reaction as shown in Fig. 1.
3.3 Effect of [chloramphenicol]
The effect of chloramphenicol on the rate of reaction was studied at constant
concentrations of alkali, DPC and periodate at a constant ionic strength of
0.10 mol dmK3. The substrate CHP was varied in the range of 1.0 ! 10K4 to
1.0 ! 10K3 mol dmK3. The kobs values increased with increases in the concentra-
tion of CHP (Table 1). The order with respect to CHP concentration was found
to be unity. This was also confirmed by the plot of kobs versus CHP concentration
(Fig. 6.), which is a straight line passing through the origin.
3.4 Effect of [alkali]
The effect of alkali concentration on the reaction was studied in the range 0.01
to 0.1 mol dmK3 at constant concentrations of chloramphenicol, DPC and perio-
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1
Fig. 5. H NMR spectra of p-nitrobenzaldehyde, the product of oxidation of chloramphenicol
by DPC.
date at constant ionic strength of 0.10 mol dmK3 at 25oC. The rate constants
increased with the increase in the alkali concentration (Table 1). The order was
found to be less than unity (0.32).
3.5 Effect of [periodate]
The effect of periodate was studied by varying the periodate concentration from
1.0 ! 10K5 to 1.0 ! 10K4 mol dmK3 keeping all other reactant concentrations
constant. It was found that the added periodate had a retarding effect on the rate
of reaction (Table 1).
3.6 Effect of ionic strength and dielectric constant
The effect of ionic strength was studied by varying the potassium nitrate concen-
tration. Ionic strength was varied from 0.01 to 1.0 mol dmK3 at constant concen-
tration of DPC, chloramphenicol, periodate and alkali. It was found that increas-
ing ionic strength had negligible effect on the rate constant. The effect of
dielectric constant (D) was studied by varying the t-butanol-water (v.v) content
in the reaction mixture with all the other conditions being kept constant. Decreas-
ing the dielectric constant of the medium had no effect on the rate of the reaction.
3.7 Effect of initially added products
The externally added products, p-nitrobenzaldehyde and copper(II) (CuSO4) in
the concentration range 1.0 ! 10K6 to 1.0 ! 10K5, did not have any significant
effect on the rate of reaction.
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R. V. Hosahalli et al.
Table 1. Effect of variation of [DPC], [CHP] and [OHK] on the oxidation of chloramphenicol
by DPC at 25oC and I = 0.10 mol dmK3
.
[DPC]!105 [CHP] ! 104 [OHK]
[H3IO62K] ! 105 kobs ! 103 (sK1
)
k2 = kobs.[CHP]
(mol dmK3
)
(mol dmK3
)
(mol dm3) (mol dm3)
(dm3molK1sK1
)
Found
2.33
2.20
2.33
2.25
2.31
2.53
0.50
1.51
2.53
3.54
4.55
5.06
1.33
2.20
2.53
2.70
2.81
2.85
3.09
2.78
2.53
2.32
2.14
2.06
Calc.
0.5
1.0
2.0
3.0
4.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
7.0
9.0
10.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.03
0.05
0.07
0.09
0.10
0.05
0.05
0.05
0.05
0.05
0.05
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
7.0
9.0
10.0
2.31
2.10
2.43
2.20
2.34
2.53
0.52
1.55
2.53
3.54
4.55
5.06
1.30
2.24
2.53
2.72
2.84
2.81
3.10
2.75
2.53
2.30
2.16
2.09
-
-
-
-
-
-
5.00
5.03
5.06
5.05
5.05
5.06
-
-
-
-
-
-
-
-
-
-
-
-
3.8 Polymerization study
The intervention of free radicals in the reaction was examined as follows: The
reaction mixture, to which a known quantity of acrylonitrile monomer was ini-
tially added, was kept for 2h in an inert atmosphere. On diluting the reaction
mixture with methanol, a white precipitate was formed, indicating the interven-
tion of free radical in the reaction. The blank experiments of either DPC or
chloramphenicol alone with acrylonitrile did not induce any polymerization un-
der the same conditions as those induced for reaction mixture. Initially added
acrylonitrile decreased the rate of reaction indicating free radical intervention.
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Fig. 6. Plot of kobs versus [CHP] (Conditions as in Table 1).
3.9 Effect of temperature
The kinetics was studied at four different temperatures under varying concentrations
of chloramphenicol, alkali and periodate, keeping other conditions constant. The
rate of reaction was found to increases with increase in temperature. The rate con-
stants (k) of the slow step of Scheme 1 were obtained from the slopes and intercepts
of [CHP].kobs versus 1.[OHK] and [CHP].kobs versus [H3IO62K] (Fig. 7.) plots at
four different temperatures. The energy of activation corresponding to these rate
constants was evaluated from the Arrhenius plot of log k versus 1.T and from which
other activation parameters were calculated (Table 2).
4. Discussion
The water soluble copper(III) periodate complex is reported [21]
[Cu(HIO6)2(OH)2]7K. However in an aqueous alkaline medium and at high pH
range employed in the study, periodate is unlikely to exist as HIO64K is evident
from its involvement in the multiple equilibria [22–24] (1)–(3), depending on
the pH of the solution.
K1
#
H5IO6
H4IO6
H3IO6
H4IO6K+ H+
K1 = 5.1!10K4 mol dmK3
(1)
(2)
(3)
K2
#
H3IO62K+ H+ K2 = 4.9!10K9 mol dmK3
K
K3
#
2K
H2IO63K+ H+ K3 = 2.5!10K12 mol dmK3
Periodic acid exists as H5IO6 in acid medium and as H4IO6K near pH 7.
Thus, under the conditions employed in alkaline medium, main species are
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R. V. Hosahalli et al.
Fig. 7. (a) Plots of [CHP].kobs versus 1.[OHK] at four different temperatures (Conditions as
in Table 1). (b) Plots of [CHP].kobs versus [H3IO62K] at four different temperatures (Conditions
as in Table 1).
expected to be H3IO62Kand H2IO63K. Thus, at pH employed in this study,
the soluble copper(III) priodate complex exist as diperiodatocuprate(III),
[Cu(OH)2(H3IO6)2]2K, a conclusion also supported by literature [23,24].
The reaction between the diperiodatocuprate(III) complex and chlorampheni-
col in alkaline medium has stiochiometry 1:2 (CHP: DPC) with first order de-
pendence on DPC and CHP concentrations and an apparent order less than unit
order in alkali concentration, a negative fractional order dependence on the perio-
date concentration. No effect of added products was observed. Based on the
experimental results, a mechanism is proposed for which all the observed orders
in each constituent such as [oxidant], [reductant]. [OHK] and [IO4K] may be well
accommodated. Lister [25] proposed three forms of copper(III) periodate
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Table 2. Effect of temperature on the oxidation of chloramphenicol by DPC in aqueous alkaline
medium.
(a) Rate constant with respect to slow step of Scheme 2.
Temperature (K)
k (dm3 molK1sK1
2.345
)
288
298
6.562
308
10.12
318
14.68
Parameter
Values
Activation parameters
Ea
46 1 kJ molK1
43 1 kJ molK1
ΔH#
ΔS#
K84 4 J KK1molK1
ΔG#
68 2 kJ molK1
log A
8
0.02 sK1
(b) Effect of temperature on first and second equilibrium step of Scheme 2.
Temperature (K)
K4 ! 103(dmK3 molK1
1.63 0.02
)
K5 ! 102(mol dmK3
2.12 0.02
)
288
398
308
318
2.11 0.03
1.62 0.03
2.68 0.03
1.12 0.04
3.12 0.04
0.65 0.02
(c) Thermodynamic quantities with respect to first and second step of Scheme 2.
Thermodynamic quantities
Values from K4
16.7 1.2
Values from K5
K29.6 0.5
K1
ΔH kJ mol
ΔS J KK1molK1
ΔG kJ molK1
43 2.4
K135.5
3
3.1 0.01
11.75 0.5
in alkaline medium as diperiodatocuprate(III)(DPC), monoperiodatocu-
prate(III)(MPC) and tetraperiodatocuprate(III). The latter is ruled out, as its equi-
librium constant is 8.0 ! 10K11 at 40oC. In the present kinetic study, DPC and
MPC are to be considered as active forms of copper(III) periodate complex. It
may be expected that lower periodate complex such as MPC is more important
in the reaction than the DPC. The results of increase in the rate with increase in
alkali concentration suggests that equilibia of copper(III) periodate complexes
are possible as in Eqs (4) and (5).
K4
#
[Cu(H3IO6)2]K+ [OHK]
[Cu(H2IO6)(H3IO6)]2K+ H2O
(4)
K5
#
[Cu(H2IO6)(H3IO6)]2K+ 2H2O
[Cu(H2IO6)(H3IO6)] + [H3IO6]2K
(5)
In the present study, the oxidation reaction proceeds via the formation of a
free radical derived from chloramphenicol, in the slow step. The formation of
such free radical is also observed in literature [26]. This free radical in a subse-
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R. V. Hosahalli et al.
quent fast steps decomposes to give p-nitrobenzaldehyde and another free radi-
cal. In the next fast step thus formed free radical reacts with another mole of
MPC in presence of OHK to form 2, 2-Dichloro-N-(1, 2-dihydroxy-ethyl)-acet-
amide. In the further fast steps 2, 2-Dichloro-N-(1,2-dihydroxy-ethyl)-acetamide
undergoes hydrolysis to give the final products 2,2-amino, hydroxy-ethanol and
dichloroacetic acid. So, the detailed mechanistic Scheme for the oxidation of
CHP by diperiodatocuprate(III) is presented in Scheme 2.
Since the Scheme 2 is in accordance with the generally well-accepted princi-
ple of non-complementary oxidations taking place in sequence of one-electron
steps, the reaction between the substrate and oxidant would afford a free radical
intermediate. A free radical scavenging experiment revealed such a possibility.
This type of radical intermediate has also been observed in literature [27,28].
Scheme 2 leads to rate law (12) as follows. According to Scheme 2,
Kd[DPC]
Rate =
= k[Cu(H2IO6)(H O)2][CHP] =
2
dt
kK5[Cu(H2IO6)(H3IO6)]2K[CHP]
(6)
(7)
=
=
2K
[H3IO6
]
kK4K5[CHP][DPC]f[OHK]f
=
2K
[H3IO6
]
The total [DPC] can be written as
[DPC]t = [DPC]f + [Cu(H2IO6)(H2IO6)]2K + [Cu(H2IO6)(H2O)2] =
(8)
(9)
[H3IO62K]+K4[H3IO62K][OHK]+K4K5[OHK]
= [DPC]f
{
2K
}
[H3IO6
]
Where, [DPC]t and [DPC]f refer to total and free DPC concentrations respec-
tively. Therefore free [DPC] is given by,
[DPC]t[H3IO62K][OHK]
[H3IO62K] + K4[H3IO62K] + K4K5[OHK]
[DPC]f =
(10)
Similarly, the total [OHK] can be written as
[OH−]t = [OH−]f + [Cu(H2IO6)(H2IO6)]2− + [Cu(H2IO6)(H2O)2]
K4K5[DPC]
[OH−]t = [OH−]f 1 + K4[DPC] +
{
2−
}
[H3IO6
]
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Scheme 2. Detailed Scheme for the oxidation of chloramphenicol by alkaline diperiodatocu-
prate.
In view of low concentration of [DPC] and [H3IO62K] above equation reduced to
[OHK]t = [OHK]f
(11)
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R. V. Hosahalli et al.
Substituting Eqs. (10) and (11) in Eq. (7) and omitting t and f we get,
kK4K5[CHP][DPC][OHK]
[H3IO62K] + K4[H3IO62K][OHK] + K4K5[OHK]
Rate =
(12)
kK4K5[CHP][OHK]
Rate
[DPC]
= kobs
=
[H3IO62K] + K4[H3IO62K][OHK] + K4K5[OHK]
The rate law (12) can be arranged in to the following form, which is suitable for
verification.
2K
2K
[CHP]
kobs
[H3IO6
]
[H3IO6
]
1
k
=
+
+
(13)
kK4K5[OHK]
kK5
According to Eq. (13), other conditions being constant, plots of [CHP].kobs ver-
sus 1.[OHK]and [CHP].kobs versus [H3IO62K] should be linear and found to be
so (Fig.7). The slopes and intercept of such plots lead to the values of K4, K5 and
k are 2.11 ! 10K1 dm3 molK1, 1.62 ! 10K2 mol dmK3 and 6.56 dm3 molK1 sK1
respectively at 25oC. The values of K4 and K5 are in good agreement with earlier
literature [29]. The equilibrium constant K4 is far greater than K5. This may be
attributed to the greater tendency of DPC to undergo hydrolysis compared to the
dissociation of hydrolyzed species in alkaline medium. Using these rate constants
under the experimental conditions were calculated according to Equation (12)
and compared with experimental data (Table 1). The experimental and calculated
rate constants are in good agreement. This fortifies the proposed mechanism.
The thermodynamic quantities for the first and second equilibrium steps of
Scheme 2 can be calculated as follows. The [H3IO62K] and [OHK] (as in Table 2)
were varied at four different temperatures. The plots of [CHP].kobs versus 1.
[OHK] and [CHP].kobs versus [H3IO62K] (Fig.7) should be linear. From the
slopes and intercepts, the values of K4 and K5 were calculated at different temper-
atures and these values are given in Table 2. Van't Hoff plots were made for K4
and K5 with temperatures (log K4 versus 1.T and log K5 versus 1.T) and the
values of enthalpy of reaction ΔH, entropy of reaction ΔS and free energy of
reaction ΔG, were calculated (Table 2) for the first and second equilibrium steps
of Scheme 2. A comparison of the latter values with those obtained from the
slow step of the reaction shows that these values mainly refer to the rate limiting
step, supporting the fact that the reaction before rate determining step is fairly
slow involves high activation energy.
The moderate values of ΔH# and ΔS# were both favorable for the electron
transfer process. The values of ΔS# within the range of radical reaction has been
ascribed to the nature of electron pairing and unpairing processes and to the loss
of degrees of freedom formerly available to the reactants upon the formation of
rigid transition state [30]. The observed modest enthalpy of activation and rela-
tively low value of the entropy of activation as well as a higher rate constant of
slow step indicate that the oxidation presumably occurs via inner-sphere mecha-
nism. The conclusion is supported by earlier observation [31].
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93
5. Conclusion
Among various species of DPC in alkaline medium, monodiperiodatocuprate(III)
(MPC), (Cu(H2IO6)(H2O)2) is considered as active species for the title reaction.
The results demonstrate that in carrying out the reaction, the role of pH in the
reaction medium is crucial. Rate constant of the slow step and other equilibrium
constants involved in the mechanism were evaluated and activation parameters
with respect to slow step of the mechanism were computed. The overall mecha-
nistic sequence described here is consistent with product, mechanistic and kinetic
studies.
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