Kinetics and mechanism of oxidation of 1-methoxy-2-propanol and 1-ethoxy-2-propanol by Ditelluratocuprate(III) in alkaline medium

Article abstract of DOI:10.1002/cjoc.201190134

The kinetics of oxidation of 1-methoxy-2-propanol and 1-ethoxy-2-propanol by ditelluratocuprate(III) (DTC) in alkaline liquids has been studied spectrophotometrically in the temperature range of 293.2-313.2 K. The reaction rate showed first order dependence in DTC and fractional order with respect to 1-methoxy-2-propanol or 1-ethoxy-2-propanol. It was found that the pseudo-first order rate constant k_obs increased with an increase in concentration of OH^- and a decrease in concentration of TeO₄ ^2-. There is a negative salt effect. A plausible mechanism involving a pre-equilibrium of a adduct formation between the complex and 1-methoxy-2-propanol or 1-ethoxy-2-propanol was proposed. The rate equations derived from mechanism can explain all experimental observations. The activation parameters along with the rate constants of the rate-determining step were calculated. The kinetics of oxidation of 1-methoxy-2-propanol and 1-ethoxy-2-propanol by ditelluratocuprate(III) (DTC) in alkaline liquids has been studied spectrophotometrically in the temperature range of 293.2-313.2 K. The reaction rate showed first order dependence in DTC and fractional order with respect to 1-methoxy-2-propanol or 1-ethoxy-2-propanol. It was found that the pseudo-first order rate constant k_obs increased with an increase in concentration of OH^- and a decrease in concentration of TeO ₄^2-. A plausible mechanism of reaction was proposed. The activation parameters along with the rate-determining step have been also calculated.

Full text of DOI:10.1002/cjoc.201190134

FULL PAPER
Kinetics and Mechanism of Oxidation of 1-Methoxy-2-propanol
and 1-Ethoxy-2-propanol by Ditelluratocuprate(III)
in Alkaline Medium
Shan, Jinhuan(单金缓)
Liu, Yanping(刘艳平)
Zhang, Jiying(张记英)
College of Chemistry and Environmental Science, Hebei University, Baoding, Hebei 071002, China
The kinetics of oxidation of 1-methoxy-2-propanol and 1-ethoxy-2-propanol by ditelluratocuprate(III) (DTC) in
alkaline liquids has been studied spectrophotometrically in the temperature range of 293.2— 313.2 K. The reaction
rate showed first order dependence in DTC and fractional order with respect to 1-methoxy-2-propanol or
1
-ethoxy-2-propanol. It was found that the pseudo-first order rate constant k increased with an increase in con-
obs
－
2－
centration of OH and a decrease in concentration of TeO4 . There is a negative salt effect. A plausible mecha-
nism involving a pre-equilibrium of a adduct formation between the complex and 1-methoxy-2-propanol or
1-ethoxy-2-propanol was proposed. The rate equations derived from mechanism can explain all experimental ob-
servations. The activation parameters along with the rate constants of the rate-determining step were calculated.
Keywords ditelluratocuprate(III) (DTC), 1-methoxy-2-propanol, 1-ethoxy-2-propanol, kinetics, mechanism, oxi-
dation
Introduction
persant or thinners are used in coatings, inks, printing
and dyeing, pesticide, cellulose and acrylic acid industry
etc. In addition, they also can be used as fuel antifreeze,
cleaning agents, the extractant, non-ferrous metal
dressing agent and organic synthetic materials etc.
In recent years, the study of the higher 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 stabilised by chelation with suitable
polydentate ligands. Metal chelates such as ditellurato-
Experimental
1
,2
3
cuprate(III),
diperiodatocuprate(III), diperiodatoar-
4
5
Materials
gentate(III) and diperiodatonickelate(IV) are good
oxidants in a medium with an appropriate pH value. The
use of Cu(III) as an oxidation agent is well known in
analytical chemistry in the estimation of glucose, sugars
and organic acids. The use of Cu(III) as an oxidation
agent is also known in organic mixture qualitative
All the reagents used were of A.R. grade. All solu-
tions were prepared with doubly distilled water. Solu-
－
tions of [Cu(H
4
TeO
6
)
2
]
(DTC) and reductants were
always freshly prepared before using. The stock solution
of DTC was prepared and standardized by the method
6
9
,10
analysis. Many biological systems involve electron-
given by Jaiswal and Yadava. Its electronic spectrum
was found to be consistent with that reported by Jaiswal
and Yadava.
transfer processes wherein Cu(III) plays an important
7
role. Based on the studies of oxidation of some organic
compounds by Cu(III) complex, Indian researchers
proposed that in the alkaline medium the formula of
Kinetics measurements and apparatus
－
DTC may be represented by [Cu(H TeO ) ] and the
4
6 2
All kinetics measurements were carried out under
pseudo-first order conditions. Solution (2 mL) con-
mechanism involving a pre-equilibrium of an adduct
formation between the complex and reductant was also
－
2－
8
taining definite concentration of Cu(III), OH , TeO4
and ionic strength and reductant solution (2 mL) of
appropriate concentration were transferred separately to
the upper and lower branch tubes of a λ type two-cell
reactor. The concentration of DTC was derived from its
absorption at 405 nm. The ionic strength µ was main-
proposed. Because Cu(III) is in the higher oxidation
state and the reaction is complicated in this kind of re-
action system, it is of significance to have a further
study on this kind of reaction system. Investigation on
them will certainly provide us with more dynamical
parameters, and will provide theoretical foundation for
the design of reaction route in the organic synthesis and
quantitative analysis in analytical chemistry.
tained by adding KNO solution and the pH of the reac-
3
tion mixture was adjusted with a KOH solution. The
kinetic measurements were performed on a UV-vis
Both of the reductants which serve as solvent, dis-
*
E-mail: shanjinhuaner@yahoo.com.cn
Received October 25, 2010; revised November 25, 2010; accepted December 17, 2010.
Chin. J. Chem. 2011, 29, 639— 642
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
639

Shan, Liu & Zhang
FULL PAPER
spectrophotometer (TU-1900, Beijing Puxi Inc., China),
which had a cell holder kept at constant temperature
(
(
±0.1 ℃) by circulating water from a thermostat
DC-2010, Baoding, China). All other species did not
absorb significantly at this wavelength.
Product analysis
A solution with known concentrations of Cu(III),
－
2－
OH , TeO4 was mixed with an excess of reductants.
The complete fading of DTC color (reddish brown)
marked the completion of the reaction. The product of
oxidation was identified as ketone by its characteristic
1
1
spot test.
Results and discussion
Figure 1 Plots of 1/kobs vs. 1/[1-methoxy-2-propanol] (r≥
－
5
－
1
－
－
2
0
.997). [Cu(III)]＝3.93×10 mol•L , [OH ]＝1.00×10
Evaluation of pseudo-first order rate constants
－
1
2－
－
3
－
1
－
2
mol•L , [ TeO ]＝1.00×10 mol•L , µ＝1.90×10 mol•
4
Under the conditions of [reductant]
0
>>[Cu(III)]
0
, the
－
1
L .
plots of ln(A
t
－A
∞
) versus time were straight lines, in-
dicating the reaction is first order with respect to Cu(III),
where A
t
and A
∞
are the absorbance at time t and at infi-
nite time, respectively. The pseudo-first-order rate con-
stants obs were calculated by the method of
least-squares. Generally, more than 8A values within
three times the half-lives were used to calculate kobs
k
t
.
The kobs values were the average value of at least three
independent experiments, and reproducibility is within
±
5%.
Rate dependence on the [reductant]
－
2－
At fixed concentration of Cu(III), OH , TeO4
and ionic strength µ, the values of kobs were determined
at different temperatures. The plots of ln kobs versus
ln[reductant] were linear (r≥0.996), and from the slope
of such plots, the order with respect to reductant was
found to be fractional. In addition, the kobs was found to
be increased with the increase of reactant concentration.
The plots of 1/kobs vs. 1/[reductant] at different tem-
peratures were straight lines (Figure 1 and Figure 2), the
rate constants (k) of the rate-determining step were cal-
culated from intercepts.
Figure 2 Plots of 1/k vs. 1/[1-ethoxy-2-propanol] (r≥0.996).
obs
5
－
－
1
－
－
2
－
1
[
[
Cu(III)]＝3.93×10 mol•L , [OH ]＝1.00×10 mol•L
,
2－
－
3
－
1
－
2
－
1
TeO4 ]＝1.00×10 mol•L , µ＝1.90×10 mol•L .
－
Rate dependence on the [OH ]
2
4
－
At constant [Cu(III)], [reductant], [ TeO ], µ and
temperature, kobs values increased with the increase in
－
－
[
OH ]. The order with respect to [OH ] was fractional
－
and the plot of 1/kobs vs. 1/[OH ] was linear with a posi-
tive intercept (Figure 3).
Rate dependence on the ionic strength
With other conditions fixed, the reaction rate was
decreased by the addition of KNO
indicating there is negative salt effect which is consis-
tent with the common regulation of the kinetics.
3
solution (Table 1),
－
Figure 3 Plots of 1/k vs. 1/[OH ] at 298.2 K. (1) [Cu(III)]＝
obs
1
1
2
－
－
－
5
2
3.93 ×10
mol•L , [1-methoxy-2-propanol] ＝3.00 ×10
－
1
2－
4
－
3
－
1
－
2
mol•L , [ TeO ]＝1.00×10 mol•L , µ＝3.80×10 mol•
2－
Rate dependence on the [ TeO4
]
－
－
－
1
5
1
L
(r ＝0.998); (2) [Cu(III)] ＝3.93 ×10
mol•L , [1-
－
－
2
－
1
2－
4
At constant [Cu(III)], [reductant], [OH ], µ and
temperature, the experimental results indicate that kobs
ethoxy-2-propanol]＝3.00×10 mol•L , [ TeO ]＝1.00×
－
3
－
1
－
2
－
1
1
0
mol•L , µ＝3.80×10 mol•L (r＝0.9998).
6
40
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© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 639— 642

Oxidation of 1-Methoxy-2-propanol and 1-Ethoxy-2-propanol by Ditelluratocuprate(III)
Table 1 Rate dependence on ionic strength µ
The distribution of all species of tellurate in aqueous
alkaline solution can be calculated from equilibriums
2
－
1
1
0 µ/(mol•L )
－
(
1)— (2). In alkaline medium such as [OH ]＝ 0.01
1
.50 3.00 4.50 6.00 7.50
－
1
2－ 2－ 3－
mol•L , [H TeO6 ] ∶ [H TeO ] ∶ [H TeO ] ＝
4
5
6
3
6
－
(
1) 1-methoxy-2-propanol 3.00 2.42 2.32 2.27 1.91
2) 1-ethoxy-2-propanol 4.16 2.90 2.61 2.34 2.24
3
－
1
1000∶ 89∶ 1, so in the concentration of OH range
1
0 kobs/s
－
3
－
(
used in this work, H TeO and H TeO6 can be
5
6
3
2
6
－
－
5
－
1
neglected, the main tellurate species was H TeO
.
(
3
1
[
1) [Cu(III)]＝3.93×10 mol•L , [1-methoxy-2-propanol]＝
4
－
－
－
2
－
1
－
－
2
－
1
2－
4
The fractional order in OH indicated that OH
.00×10 mol•L , [OH ]＝1.00×10 mol•L , [ TeO ]＝
－
3
－
1
－
5
－
1
takes part in a pre-equilibrium with Cu(III) before the
.00 ×10
mol•L . (2) [Cu(III)] ＝3.93 ×10
mol•L
,
2
4
－
－
2
－
1
－
rate-determing step. The plot of 1/kobs versus [ TeO
]
1-ethoxy-2-propanol]＝3.00×10 mol•L , [OH ]＝1.00×
－
2
－
1
2－
4
－
3
－
1
is line with a positive intercept which indicates a
1
0
mol•L , [ TeO ]＝1.00×10 mol•L .
dissociation equilibrium in which the Cu(III) loses a
2－
2－
decreases while increasing the [ TeO ]. The order with
tellurate ligand H TeO6 from its coordination sphere
4
4
2－
respect to TeO4 was derived to be an inverse fraction,
which reveals that TeO is produced in equilibrium
and forms a reactive monotelluratocuprate(III) complex
(MTC). The fractional order in reductant indicated
complex formation between reductant and MTC, and
the plot of 1/kobs versus 1/[reductant] was straight line
with a positive intercept providing kinetic evidence for
the formation of 2∶ 1 complex. Hence the following
inner-sphere electron transfer mechanism involving a
pre-equilibrium of an adduct formation between the
complex and reductant is proposed for the reaction.
2－
4
before the rate-determining step. A plot of 1/kobs versus
TeO4 ] was straight line with a positive intercept (Fig-
2－
[
ure 4).
－
In alkaline solution studied, (H TeO ) was pro-
2
6 4
tonated easily and it coordinated with central ion form-
－
ing [Cu(H TeO ) ] . In view of the experiments, the
4
6 2
mechanism was poposed as follows (R respectively
stands for methoxy, ethoxy):
K
－
－
1
[
Cu(H TeO ) ] ＋OH ꢂꢀ ꢀꢀ ꢀꢀꢁꢀ
4
6 2
(
3)
2
6
－
[
Cu(H TeO )]＋H TeO ＋H O
3
6
4
2
K
2
[
Cu(H TeO )]＋RCH CHOHCH ꢂꢀ ꢀꢀ ꢀꢀ ꢁꢀ
2
－
3
6
2
3
Figure 4 Plots of 1/kobs vs. [ TeO4 ] at 298.2 K. (1) [Cu(III)]＝
(4)
－
5
－
1
－
2
[(Cu(H TeO )(RCH CHOHCH )]
3
.93×10 mol•L , [1-ethoxy-2-propanol]＝3.00×10 mol•
3
6
2
3
－
1
－
－
2
－
1
－
2
－
1
L , [OH ]＝1.00×10 mol•L , µ＝3.80×10 mol•L (r＝
－
5
－
1
0
.998); (2) [Cu(III)]＝3.93×10 mol•L , [1-ethoxy-2-pro-
k
[
(Cu(H TeO )(RCH CHOHCH )]⎯⎯⎯→
－
2
－
1
－
－
2
－
1
3
6
2
3
panol]＝3.00×10 mol•L , [OH ]＝1.00×10 mol•L , µ＝
slow
(
5)
－
2
－
1
＋
3
.80×10 mol•L (r＝0.999).
RCH·COHCH ＋[Cu(H TeO )]
2
3
4
6
Reaction mechanism
－
K
f
Cu*(III)+OH ＋RCH·COHCH ⎯⎯⎯→
2
3
fast
Acrylamide was added under the protection of ni-
trogen atmosphere during the course of reaction. The
appearance of white polyacrylamide was consistent with
free radical intermediates in the oxidation by Cu(III)
complexes. Blank experiments in reaction system gave
no polymeric suspensions.
(6)
Cu(II)＋RCH COCH +H O
2
3
2
3
＋
where Cu*(III) stands for any kind of form of Cu
complexes which existed in equilibrium and R' stands
for reductant. Subscripts T and e stand for total and
equilibrium concentration respectively.
In the alkaline medium, the electric dissociation
equilibrium of telluric acid was given earlier (here
[
Cu(III)] ＝[Cu(H TeO )] ＋
T
3
6
e
w
pK ＝ 14).
－
[
Cu(H TeO )(RCH CHOHCH )]＋[Cu(H TeO ) ]
3
6
2
3
4
6
2
e
2
6
－
－
2－
6
H TeO ＋OH ꢀ H TeO ＋H O
4
4
2
Reaction (5) was the rate-determining step.
lg β
1
=3.049
(1)
(2)
[
[
Cu(H TeO )(R ')]＝
3
6
2－
－
3－
6
H TeO ＋OH ꢀ H TeO ＋H O
－
4
6
3
2
K K [Cu(III)] [OH ][R' ]
(7)
1
2
T
2－
－
－
H TeO6 ]＋K [OH ]＋K K [OH ][R' ]
4
1
1
2
lg β
2
＝ － 1
Chin. J. Chem. 2011, 29, 639— 642
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
641

Shan, Liu & Zhang
FULL PAPER
As the rate of the disappearance of Cu(III) was
monitored, the rate law of the reaction can be derived
as:
2
6
－
－
1
1＋K [R']
[H TeO
]
2
4
＝
＋
(13)
kobs 2kK [R'] 2kK K [OH ][R']
2
1
2
d[Cu(III)]T
－
＝2k[Cu(H TeO )(R')]
(8)
3
6
From the equation (11), the plots 1/kobs vs. 1/[R'] are
straight lines and the rate constants of the rate-deter-
mining step at different temperature were obtained from
the intercept of the straight line. Equations (12) and (13)
suggest that the plots of 1/kobs vs. 1/[OH ] and 1/kobs vs.
H TeO ] are straight lines. Activation energy and
the thermodynamic parameters were evaluated by the
method given earlier (Table 2).
dt
－
d[Cu(III)]T
dt
2kK K [Cu(III)] [OH ][R']
1
2
T
－
ꢀ
＝
－
2
6
－
－
－
[H TeO ]＋K [OH ]＋K K [OH ][R']
4
1
1
2
2－
[
4 6
ꢀꢀꢀꢀ＝k [Cu(III)]
obs
T
(
9)
－
Conclusion
2
kK K [OH ][R']
1 2
k ＝
(10)
obs
2－
－
－
[
H TeO6 ]＋K [OH ]＋K K [OH ][R']
4
1
1
2
Base on the above discussion and results, we can
know that the rate constants of the rate-determining step
and the activation parameters for 1-methoxy-2-propanol
and 1-ethoxy-2-propanol are very contiguous. In the
reaction system, both the 1-methoxy-2-propanol and
1-ethoxy-2-propanol formed the same intermediate
compounds with Cu(III). At the same time we also ob-
served the rate of 1-methoxy-2-propanol is a little
quicker than that of 1-ethoxy-2-propanol. In the reaction
system, Compared with CH O— , CH CH O— has an
Rearranging equation (10) leads to equations (11)—
(13):
2
－
－
1
1
[H TeO6 ]＋K [OH ] 1
4
1
＝
＋
(11)
(12)
－
kobs 2k
2kK K [OH ]
[R']
1
2
2
6
－
3
3
2
1
1＋K [R'] [H TeO
]
1
2
4
＝
＋
－
2
excess CH and steric effect is more larger. There is
negative salt effect which is consistent with the common
regulation of the kinetics.
kobs 2kK [R'] 2kK K [R'] [OH ]
2
1
2
Table 2 Rate constants (k) and activation parameters of the rate-determining step (T＝298.2 K)
T/K
2
93.3
298.2
303.2
1.1378
1.3257
308.2
1.4005
1.8394
313.2
1.7745
2.2699
1
-methoxy-2-propanol
-ethoxy-2-propanol
0.6678
0.8231
0.8802
2
－
1
1
0 k/s
1
1.0060
≠
－
1
－
1
≠
－
1
－
－
1
Thermodynamic
activation parameters
1-methoxy-2-propanol
E ＝36.96 kJ•mol , ∆H ＝34.48 kJ•mol , ∆S ＝－168.74 J•K •mol
a
－
1
≠
－
1
≠
－
1
1
1-ethoxy-2-propanol
E ＝40.17 kJ•mol , ∆H ＝37.69 kJ•mol , ∆S ＝－156.48 J•K •mol
a
The plots of ln k vs. 1/T have following intercept (a), slope (b) and relative coefficient (r): 1-methoxy-2-propanol: a＝10.17, b＝
－
4445.78, r＝0.999; 1-ethoxy-2-propanol: a＝11.64, b＝－4831.30, r＝0.996.
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42
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© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 639— 642