Kinetics and mechanism of oxidation of sarcosine by diperiodatocuprate(III) complex in alkaline medium

Article abstract of DOI:10.1002/cjoc.201090189

The kinetics of oxidation of sarcosine by diperiodatocuprate(III) (DPC) was studied with spectrophotometry in a temperature range of 292.2-304.2 K. The reaction between diperiodatocuprate(III) and sarcosine in alkaline medium exhibits 1:1 stoichiometry (DPC: sarcosine). The reaction was found to be first order with respect to both DPC and sarcosine. The observed rate constant (k _obs) decreased with the increase of the [IO^-₄], decreased with the increase of the [OH], and then increased with the increase of the [OH] after a turning point. There was no salt effect, and free radicals were detected. Based on the experimental results, a mechanism involving the diperiodatocuprate(III) (DPC) as the reactive species of the oxidant has been proposed. The activation parameters, as well as the rate constants of the rate-determining step, have been calculated.

Full text of DOI:10.1002/cjoc.201090189

FULL PAPER
Kinetics and Mechanism of Oxidation of Sarcosine by
Diperiodatocuprate(III) Complex in Alkaline Medium
Shan, Jinhuan*,a(单金缓)
Wang, Xiaoqiana(王晓倩)
Shen, Haixiaa(申海霞)
Zhao, Ningb(赵宁)
a College of Chemistry and Environmental Science, Hebei University, Baoding, Hebei 071002, China
b School of Pharmaceutical Science, North China Coal Medical University, Tangshan, Hebei 063000, China
The kinetics of oxidation of sarcosine by diperiodatocuprate(III) (DPC) was studied with spectrophotometry in
a temperature range of 292.2— 304.2 K. The reaction between diperiodatocuprate(III) and sarcosine in alkaline me-
dium exhibits 1∶ 1 stoichiometry (DPC∶ sarcosine). The reaction was found to be first order with respect to both
－
DPC and sarcosine. The observed rate constant (kobs) decreased with the increase of the [IO4 ] , decreased with the
increase of the [OH－ ], and then increased with the increase of the [OH－ ] after a turning point. There was no salt ef-
fect, and free radicals were detected. Based on the experimental results, a mechanism involving the diperiodatocu-
prate(III) (DPC) as the reactive species of the oxidant has been proposed. The activation parameters, as well as the
rate constants of the rate-determining step, have been calculated.
Keywords diperiodatocuprate(III), sarcosine, kinetics and mechanism, redox reaction
Introduction
method given by Jaiswal.7 The concentration of DPC
was derived from its absorption at λ＝415 nm. The
ionic strength µ was maintained by adding KNO3 solu-
tion and the pH value of the reaction mixture was regu-
lated with KOH solution. The study was performed with
a TU-1900 UV-Vis spectrophotometer fitted with a
DC-2010 thermostat (±0.1 K, China)
In recent years, the study of the highest oxidation
state of transition metals has intrigued many researchers.
This can provide new and valuable information in some
fields. Transition metals in a higher oxidation state can
generally be stabilized by chelation with suitable
polydentate ligands, while metal chelates, such as dipe-
riodatocuprate(III),1 ditelluratocuprate(III),2 diperioda-
tonickelate(IV),3 are good oxidants in a medium with an
appropriate pH value. These complexes, such as Ag(III),
Cu(III) and Ni(IV) complexes, can be used as an oxida-
tion reagent in organic chemistry and analytical chemis-
try.4,5 As a kind of oxidation reagents, those complexes
have been used widely in kinetic study.6
Sarcosine is an important and interesting compound,
which finds a number of applications in the syntheses of
biochemical reagent, daily chemical, amino acid surfac-
tant, health caring medicine, etc. The study of sarcosine
becomes important because of its biological significance.
In this paper, the kinetics and mechanism of oxidation
of sarcosine by diperiodatocuprate(III) is presented.
Kinetics measurements and product analysis
All kinetics measurements were carried out as pre-
viously.6 The way for product analysis was similar to
previous study.8 After completion of the reaction, the
main reaction products were identified as aldehyde9 by a
spot test, amino acid10 by ninhydrin reagent and Cu(II)
by its spot test.11 When added Nessler’s reagent into the
solution of the product, however, there was not any phe-
nomenon.
Results and discussion
Evaluation of pseudo-first order rate constants
Under the conditions of [sarcosine]0>>[Cu(III)]0, the
plots of ln(At－ A∞) versus time were straight lines, and
details of the evaluation are described in our previous
work.12
Experimental
Reagents and apparatus
Rate dependence on [sarcosine]
All the reagents used were of A.R. grade. All solu-
tions were prepared with doubly distilled water. Solu-
At constant temperature, kobs values increase by in-
－
tion of [Cu(H2IO6)2]3 (DPC) and reductant were al-
creasing the concentration of sarcosine while keeping
－
－
ways freshly prepared before use. The stock solution of
DPC in a strong alkaline medium was prepared by the
the concentration of [Cu(III)], [OH ], [IO4 ] , and µ
constant. The order with respect to [sarcosine] was unity;
*
E-mail: shanjinhuaner@yahoo.com.cn
Received September 9, 2009; revised November 23, 2009; accepted March 6, 2010.
Chin. J. Chem. 2010, 28, 1081— 1084
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1081

Shan et al.
FULL PAPER
the plots of kobs versus [sarcosine] are straight lines
which pass the grid origin at different temperatures
(Figure 1).
[OH－ ], µ
ꢀ
and temperature, the experimental results in-
dicate that kobs decreases while increasing the concen-
－
－
tration of [IO4 ] . The order with respect to [IO4 ] was
－
negative fractional and the plot of 1/kobs versus [IO4 ]
was linear (Figure 2). The addition of KNO3 solution, to
adjust the ionic strength of the reaction at constant
－
－
[DPC], [sarcosine], [IO4 ] , [OH ] and temperature,
had no effect on the rate (Table 2). It showed that there
was no salt effect, which was consistent with the com-
mon regulation of the kinetics 13.
Figure 1 Plots of kobs vs. [sarcosine] at different temperatures.
－
－
2
(a) [Cu(III)]＝6.5×10－ mol•L－ 1, [OH ]＝1.0×10 mol/L,
5
[IO4 ] ＝1.0×10 mol•L－ 1, µ＝0.08 mol•L 1; (b) [Cu(III)]＝
－
－
－
3
6.5×10－ 5 mol•L 1, [OH ]＝5.0×10 mol•L－ 1, [IO4 ] ＝1.0×
－
－
－
2
－
10－ 3 mol•L－ 1, µ＝0.08 mol•L－
1
Rate dependence on [OH－ ]
－
－
Figure 2 Plots of 1/kobs vs. [IO4 ] at 298.2 K. (a) In a less
At constant [Cu(III)], [sarcosine], [IO4 ] , µ and
alkaline medium: [Cu(III)]＝6.5×10－ 5 mol•L－ 1, [sarcosine]＝1.0
temperature, kobs values decreased rapidly with the in-
crease in [OH－ ], and then increased slowly with the
increase in [OH－ ]. The concentration of OH－ was
about 0.05 mol•L－ 1 at the turning point at which the rate
was the slowest (Table 1).
×10－ 2 mol•L－ 1, [OH－ ]＝0.01 mol•L－ 1, µ＝0.07 mol•L－ 1; (b) In
a more alkaline medium: [Cu(III)]＝6.5×10－ 5 mol/L, [sarcosine]
＝1.0×10－ 2 mol•L－ 1, [OH－ ]＝0.05 mol•L－ 1, µ＝0.07 mol•L－
.
1
Table 2 The dependence of rate on µ at 298.2 Ka
Table 1 Rate dependence on [OH－ ] at T＝298.2 Ka
µ/(mol•L－
102kobs/s－
)
0.07 0.1
0.2
0.3
0.4
0.5
0.6
1
10[OH－ ]/
1
2.30 2.15 2.13 2.38 2.72 2.47 2.58
0.050 0.080 0.10 0.15 0.20 0.50 1.0 1.5 2.0 2.5
(mol•L－
)
1
1
1
a [Cu(III)]＝6.5×10－ 5 mol•L－ 1, [sarcosine]＝1.0×10－ 2 mol•L－
,
102kobs/s－
2.47 1.72 1.60 1.51 1.42 1.30 1.51 1.70 1.95 2.27
－
－
1
[OH－ ]＝0.05 mol•L－ 1, [IO4 ] ＝1.0×10 3 mol•L
.
－
－
－
－
－
a
5
1
[Cu(III)]＝6.5×10 mol•L 1, [sarcosine]＝5.0×10 3 mol•L
,
－
－
－
1
－
[IO4 ] ＝1.0×10 3 mol•L 1, µ＝0.30 mol•L
.
Reaction mechanism
Rate dependence on [IO－4 ] and ionic strength (µ)
In aqueous periodate solution, Eqs. (1)—(3) were
detected and the corresponding equilibrium constants at
298.2 K were determined by Aveston.14
At constant concentration of [Cu(III)], [sarcosine],
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Chin. J. Chem. 2010, 28, 1081— 1084

Kinetics and Mechanism of Oxidation of Sarcosine
2k1K1[R－ ]
－
4－
2IO4 ＋2OH－
H I O
10
ꢀꢀꢁ
ꢂꢀꢀ
lg β 15.05
(1)
(2)
＝
1
2
2
kobs
＝
(10)
[H3IO22 － ][OH－ ]＋K1
IO4 ＋OH－ ＋H2
H IO
－
2－
ꢀꢀꢁ
ꢂꢀꢀ
lg β 6.21
＝
2
3
6
Then substitute Eq. (5) for [H3IO62－ ] in Eq. (10):
IO4 ＋2OH－ ＋H2
H IO
－
3－
6
ꢀꢀꢁ
ꢂꢀꢀ
lg β 8.67 (3)
＝
3
2
2k1K1[R－ ]
j([OH－ ])[OH－ ][IO4 ]ex＋K1
kobs
＝
(11)
(12)
－
The distribution of all species of periodate in alka-
line solution can be calculated from Eqs. (1)—(3). In
－
－
the [OH ] range used in this work, the dimer and [IO4 ]
species of periodate can be neglected. The main species
of periodate are H2IO36－ and H3IO62－ . Eqs. (4) and
(5) can be obtained from Eqs. (1—3)
1
1
j([OH－ ])[OH－ ]
－
＝
＋
[IO4 ]ex
2k1[R－ ]
2k K1 [R－ ]
kobs
1
Mechanism II (in a more alkaline medium):
－
β3[OH ]
[H2IO36－ ]＝
[IO4 ]ex＝f ([OH－ ])[IO4 ]ex
－
－
K2
ꢂꢀꢀꢀ
1＋β2＋β3[OH－ ]
[Cu(H2IO6 )2 ]3－ ＋OH－
ꢀꢀꢀꢁ
(13)
[Cu(HIO6 )] ＋H2IO36－ ＋H2O
－
(4)
β2
[H3IO62－ ]＝
[IO4 ]ex＝j([OH－ ])[IO4 ]ex
－
－
MPA
(5)
β2＋β3[OH－ ]
k2
－
－
－
[Cu(HIO6 )] ＋CH3NHCH2COO ⎯⎯⎯→
where [IO4 ]ex represents the concentration of original
overall periodate ion.
Based on the above discussion, two simultaneous
reaction mechanisms were proposed.
slow
(14)
(15)
Cu(II)＋complex
Mechanism I (In a less alkaline medium):
Cu*(III)＋complex ⎯⎯⎯→
fast
－
Cu(II)＋HCOH＋NH2CH2COO
K1
3－
ꢀꢀꢀꢁ
[Cu(H IO ) ] ＋3H O
ꢂꢀꢀꢀ
2
6
2
2
(6)
[Cu(H2IO6 )(H2O)2 ]＋H3IO62－ ＋OH－
The total concentration of Cu(III) at time t can be
written as:
MPC
[Cu(III)] [DPC] [MPC]
＝
＋
t
e
e
k1
－
Subscripts t and e stand for total concentration and
concentration at equilibrium, respectively.
[Cu(H2IO6 )(H2O)2 ]＋CH３ NHCH COO ⎯⎯⎯→
２
slow
Cu(II)＋complex
As the rate of the disappearance of [Cu(III)]t was
monitored, the rate of the reaction can be derived as:
(7)
Cu*(III)＋complex ⎯⎯⎯→
－d[Cu(III)]
dt
2k1K1[OH－ ][R－ ]
[H3IO62－ ][OH－ ]＋K1
fast
＝
×[Cu(III)]t＝
(8)
－
(16)
(17)
Cu(II)＋HCOH＋NH2CH2COO
kobs[Cu(III)]t
The total concentration of Cu(III) is
[Cu(III)] [Cu(H IO )3－
]
e
[Cu(H IO )(H O) ]
2 6 2 2 e
t
2k2K2[R－ ][OH ]
－
＝
＋
2
6
2
kobs＝
K2[OH－ ]＋[H2IO36－
]
here “e” and “t” mean the equilibrium concentration of
the all kinds of Cu(III) complexes and the total concen-
tration of Cu(III) complexes, respectively. Eq. (7) is the
rate-determining step, and the rate equation can be ex-
pressed as below:
Then substitute Eq. (4) for [H2IO 36－ ] in Eq. (17):
1
1
f ([OH－ ])
－
＝
＋
[IO4 ]ex
(18)
2k2[R－ ] 2k2K2[R－ ][OH－ ]
kobs
－d[Cu(III)]t
dt
2k1K1[R－ ]
＝
×[Cu(III)]t
[H3IO62－ ][OH－ ]＋K1
In this report, Eqs. (10) and (17) can explain why
(9)
kobs values decreased rapidly with the increase in [OH－ ]
＝kobs[Cu(III)]t
Chin. J. Chem. 2010, 28, 1081— 1084
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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1083

Shan et al.
FULL PAPER
－
1
up to 0.05 mol•L . After that point, it increased gradu-
Conclusion
ally with the continuous increase in [OH－ ]. Eqs. (12)
Among various species of Cu(III) in alkaline liquids,
monoperiodatocuprate is considered as the active spe-
cies for the title reaction. The rate constant of slow step
and other equilibrium constants involved in the mecha-
nism are evaluated and activation parameters with re-
spect to the slow step of the reaction were computed.
－
and (18) show that the plots of 1/kobs versus [IO4 ]ex
should also be linear. Eqs. (10) and (17) show that the
order in sarcosine should be first order and the kobs ver-
sus [sarcosine] should be linear. The rate equations de-
rived from the reaction mechanisms are consistent with
our experimental results. The activation energy and the
thermodynamic parameters (298.2 K) were evaluated by
the previously published method15 (Table 3).
≠
The positive value of ∆ S suggests that the reactant is
more order than the intermediate complex.
Table 3 Rate constants (k) and the activation parameters for the rate-determining step at T＝ 298.2 Ka
Thermodynamic activation parameters (298.2 K)
k1/(mol－ 1•L•s－
)
1
Temperature/K
－
－
－
1
Ea/(kJ•mol－
)
∆ H/(kJ•mol 1) ∆ S/(J•K 1•mol
)
K1/(mol2•L－
)
K2
1
≠
≠
2
In the less alkaline medium
292.2
295.2
298.2
301.2
304.2
1.358
1.699
2.242
3.362
5.198
7.0× 10－
5
82.81
80.33
31.96
61.22
In the more alkaline medium
292.2
295.2
298.2
301.2
304.2
2.317
3.733
5.021
6.282
9.511
1.731× 10－
5
82.44
79.96
a In the less alkaline medium, the plot of ln k vs. 1/T has the following intercept (a), slope (b), and relative coefficient (r): r＝
－
0.99, a＝
34.31, b＝
cient (r): r＝
－
9959.78. In the more alkaline medium, the plot of ln k vs. 1/T has the following intercept (a), slope (b), and relative coeffi-
0.99, a＝ 37.83, b＝ 9915.70.
－
－
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© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2010, 28, 1081— 1084