# Polyethylene Glycols as Efficient Catalysts for the Oxidation of Xanthine Alkaloids by Ceric Ammonium Nitrate in Acetonitrile: A Kinetic and Mechanistic Approach

Authors: S. Shylaja; K. C. Rajanna; K. Ramesh; K. Rajendar Reddy; P. Giridhar Reddy
Journal: Advances in Physical Chemistry
 (2013)

---

## Abstract

Kinetics of oxidation of xanthine alkaloids, such as Xanthine (XAN), hypoxanthine (HXAN), caffeine (CAF), theophylline (TPL), and theobromine (TBR), have been studied with ceric ammonium nitrate (CAN) using poly ethylene glycols (PEG) as catalysts. Reaction obeyed first order kinetics in both [CAN] and [Xanthine alkaloid]. Highly sluggish CAN-xanthine alkaloid reactions (in acetonitrile media even at elevated temperatures) are enhanced in presence PEGs (PEG-200, -300, -400, -600). An increase in [PEG] increased the rate of oxidation linearly. This observation coupled with a change in absorption of CAN in presence of PEG, [H–(OCH2–CH2)n–O–NH4Ce(NO3)4(CH3CN)] (PEG bound CAN species), is considered to be more reactive than CAN. The mechanism of oxidation in PEG media has been explained by Menger-Portnoy’s enzymatic model.


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## Body

## 1. Introduction

There has been an increasing interest in the kinetics of electron transfer reactions since more than half a century because of their ever green importance in understanding the mechanisms of industrially, pharmaceutically, and biologically important redox reactions [1–11]. A special focus has been paid to single electron transfer (SET) oxidations [1–18]. In this context, ceric ammonium nitrate (CAN) has emerged as one of the most valuable and notable SET oxidants for a variety of reactions [19–30], due to its relative abundance, ease of preparation, low cost, and low toxicity. During the oxidation of organic substrates, the initial formation of a radical or radical cation is usually followed by rearrangement or follow-up reactions that led to other free radical intermediates. Typically, the free radical reacts with another substrate (olefin, etc.) to form a new C–C bond and a product radical. Oxidation of the free radical intermediate to a cation leads to capture of solvent or nitrate expelled from CAN upon its reduction to Ce(III) and these alternative mechanistic pathways result in many of the side products prevalent in oxidations. Therefore, preparative Ce(IV) initiated oxidations cannot be achieved in many instances. Chemical intuition suggests that these pathways can be depressed by understanding the interrelationship between the mechanism of oxidation by Ce(IV), the effect of solvent on the stability of the initially formed radical cation intermediate, and the rates (mechanisms) of various available pathways.Polyethylene glycol (PEG) is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular weight. PEG is a neutral, hydrophilic polyether and less expensive. It avoids the use of acid or base catalysts and reagent can be recovered and reused. Thus, it offers a convenient, inexpensive, nonionic, nontoxic, and recyclable reaction medium for the replacement of volatile organic solvents (see Scheme1).Scheme 1
Structure of polyethylene glycol (PEG).Polyethylene glycol (PEG) is a condensation polymer of ethylene oxide and water with the general formula[H(OCH2CH2)nOH], where n is the average number of repeating oxyethylene groups typically from 4 to about 180. The low molecular weight members from n=2 to 4 are diethylene glycol, triethylene glycol, and tetra ethylene glycol, respectively, which are produced as pure compounds. The wide range of chain lengths provides identical physical and chemical properties for the proper application selections directly or indirectly in the field of chemical and biological sciences. In recent past polyethylene glycols (PEGs) have been used as catalysts and catalyst supports and also have been found to be an inexpensive, non-toxic, environmentally friendly reaction medium, which avoid the use of acid or base catalysts. Moreover PEG can be recovered after completion of the reactions and recycled/reused [31–37] in another batch. Inspired by the striking features of PEG the author wants to use it as a catalyst by avoiding the use of acid in the present study, namely, ceric ammonium nitrate (CAN) triggered oxidation of certain xanthine alkaloid compounds. Acetonitrile is used as solvent in order to facilitate kinetic studies.


## 2. Experimental Details

Poly ethylene glycols were procured from E-Merck and other materials used were similar to those given in previous chapters. Thermostat was adjusted to desired reaction temperature. Flask containing known amount of ceric ammonium nitrate (CAN) in acetonitrile solvent and another flask containing the substrate (Xanthine alkaloid) and suitable amount of PEG solutions were clamped in a thermostatic bath. Reaction was initiated by mixing requisite amount of CAN to the other contents of the reaction vessel. The entire reaction mixture was mixed thoroughly. Aliquots of the reaction mixture were withdrawn into a cuvette and placed in the cell compartment of the laboratory visible spectrophotometer. Cell compartment was provided with an inlet and outlet for circulation of thermostatic liquid at a desired temperature. The CAN content could be estimated from the previously constructed calibration curve showing absorbance versus [CAN]. Absorbance values were in agreement to each other with an accuracy of ±3% error.


### 2.1. Determination of the Order of Reaction and Salient Kinetic Features

(1)
Reactions were conducted under two different conditions. Under pseudo first order conditions [CAF] ≫ [CAN], plots ofln(A0/At), that is, ln[a/(a-x)] versus time, were straight lines with positive slopes, passing through origin indicating first order (x) with respect to [oxidizing agent] (Figure 1).
(2)
This reaction is also conducted under second order conditions with equal concentrations of[CAF]0=[CAN]0. Under these conditions, kinetic plots of [1/(At)] versus time have been found to be linear with a positive gradient and definite intercept on ordinate (vertical axis), indicating overall second order kinetics (Figure 2). Since the order with respect to [CAN] is already verified as one under pseudo conditions this observation suggests that order in [CAF] is also one.
(3)
In PEG mediated reactions an increase in the [PEG] increased the reaction rates depending on the nature of PEG. By and large reaction rates were found high in PEG-200 media over other PEGs (Tables2, 3, 4, 5, and 6).
(4)
In the present study, kinetic data have been collected at three to four different temperatures within the range of 300 to 320 K. Activation parameters such asΔH# and ΔS# have been evaluated by Eyring’s equation. Free energy of activation (ΔG#) is obtained from Gibbs-Helmholtz equation. The data related to activation parameters are compiled in Tables 2 to 6.
(5)
Addition of olefin monomer (acryl amide and acrylonitrile) to the reaction mixture decreased the reaction rate. When heated, the contents of the reaction mixture turned viscous and indicated dense polymer formation. This observation can be explained due to the induced vinyl polymerization of added monomer, showing the presence of free radicals in the system.Figure 1
Pseudo first order kinetic plots of caffeine with MeCN at 310 K. [CAF] = 0.016 mol dm−3; [CAN] = 0.0041 mol dm−3; [PEG-300] = 0.062 mol dm−3.Figure 2
Second order kinetic plots of caffeine with MeCN at 310 K. [CAF] = 0.002 mol dm−3; [CAN] = 0.002 mol dm−3; [PEG-300] = 0.375 mol dm−3.


### 2.2. CAN-PEG Binding Studies

UV-Visible Spectrophotometric studies were performed in order to throw light on CAN binding with PEG (Poly ethylene glycol). Absorption spectra of CAN in acetonitrile indicated a band at 459 nm; this band underwent a hypsochromic shift from 459 nm to 441 nm in presence of 0.1 mol PEG, suggesting the interaction of PEG with CAN (Figure3):
(1)H–(OCH2–CH2)n–OH(PEG)+(NH4)[Ce(NO3)5(ACN)](CAN)⇌K[PEG–NH4Ce(NO3)5](ACN)Complex(C)or[PEG-CAN].
The [CAN-PEG] binding constants were evaluated by Benesi- Hildebrand equation according to the method reported in the literature [38], as elaborated in our earlier paper.Figure 3
Absorption of spectra of CAN in presence of PEG.The equilibrium constantK=[C]/[CAN][PEG], where [CAN], [PEG], and [C] are equilibrium concentrations of acceptor (CAN), donor (PEG), and complex, respectively. For the above equilibrium, concentration of [PEG-CAN] complex ([C]) can be correlated to the formation constant (K) by the following relationship. If [CAN]0 and [PEG]0 represent initial concentrations of CAN and PEG, respectively, then
(2)[C]=K[CAN]0[PEG]01+K[PEG]0.
But according to Lambert-Beer’s law absorbance, (A)=ϵcl.In the above equations,l is path length, d is absorbance, ϵ is the molar extinction coefficient, and K is formation constant of the complex, respectively. For one cm path length, above equation can be written as, (A)=ϵc,
(3)[C]=Aϵl=K[CAN]0[PEG]01+K[PEG]0.
Further, taking the reciprocals to the above equation, it rearranges to
(4)[CAN]0A=1K[PEG]0ϵ+1ϵ.
However, the absorbance of CAN and [CAN-PEG] absorb in the same region significantly; therefore the observed absorbance (A) could be written as
(5)A=A(CAN)+A(Complex),A(Complex)=ΔA=A~A(CAN).
Therefore, a plot of ([CAN]0/ΔA) versus 1/[PEG]0 should give a straight line according to the above equation. These plots have been realized in the present study (Figure 4). Formation constant (K) has been calculated from the ratio of intercept to slope, while inverse of the intercept gave molar extinction coefficient (ϵ) and is represented in Table 1.Table 1
Binding constants of [CAN-PEG] at 303°K using Benesi-Hildebrand method.
S. N.
PEG
Benesi-Hildebrand Equation
K
ε
-
Δ
G (kJ/mol)
1
PEG-200
y
=
7
E
-
05
x
+
0.052
743
19.23
16.7
2
PEG-300
y
=
9
E
-
05
x
+
0.055
611
18.18
16.2
3
PEG-400
y
=
5
E
-
05
x
+
0.071
1420
14.08
18.3Table 2
Activation parameters of caffeine in different PEG media.
Type of PEG
PEG % (V/V)
k
′
′ at 300 K
Equation obtained for plot ofln(k′′/T) versus (103/T)
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.4
y
=
-
4.1044
x
+
7.088
0.985
34.1
75.2
138
1.0
0.5
y
=
-
3.9118
x
+
6.686
0.963
32.4
74.7
141
2.0
0.6
y
=
-
3.4191
x
+
5.218
0.968
28.3
74.5
154
3.0
0.6
y
=
-
5.24
x
+
11.28
0.989
43.5
74.7
103
4.0
0.7
y
=
-
4.4927
x
+
8.922
0.999
37.2
74.2
123
5.0
0.9
y
=
-
3.9922
x
+
7.503
0.999
33.1
73.6
135
PEG-300
0.5
0.2
y
=
-
5.7209
x
+
11.77
0.997
47.5
76.6
99.7
1.0
0.3
y
=
-
4.4045
x
+
7.775
1.00
36.5
76.1
132
2.0
0.4
y
=
-
4.1044
x
+
7.088
0.986
34.1
75.5
138
3.0
0.5
y
=
-
3.0174
x
+
3.654
0.998
25.0
75.1
167
4.0
0.6
y
=
-
2.1466
x
+
0.948
0.995
17.8
74.5
189
5.0
0.7
y
=
-
1.8624
x
+
0.153
0.996
15.4
73.5
196
PEG-400
0.5
0.2
y
=
-
4.0901
x
+
6.298
0.992
33.9
75.6
139
1.0
0.4
y
=
-
1.6439
x
-
1.125
0.974
13.6
70.0
188
2.0
0.5
y
=
-
1.3085
x
-
2.031
0.998
10.8
64.8
180
3.0
0.6
y
=
-
1.0727
x
-
2.636
0.999
8.90
61.4
175
4.0
0.7
y
=
-
3.0329
x
+
4.079
0.971
25.1
74.0
163
5.0
0.8
y
=
-
3.0238
x
+
4.168
0.992
25.0
73.6
162
PEG-600
0.5
0.2
y
=
-
4.9831
x
+
9.337
0.980
41.3
77.0
119
1.0
0.3
y
=
-
3.7714
x
+
5.687
0.988
31.3
76.3
150
2.0
0.4
y
=
-
3.0286
x
+
3.490
0.992
25.1
75.5
168
3.0
0.6
y
=
-
1.6344
x
-
0.784
0.966
13.5
70.8
191
4.0
0.7
y
=
-
1.3993
x
-
1.412
0.963
11.6
67.1
185
5.0
0.9
y
=
-
0.6541
x
-
3.627
0.999
5.42
55.5
167Table 3
Activation parameters of Xanthine in different PEG media.
Type of PEG
PEG % (V/V)
k
′
′ at 300 K
Equation
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.09
y
=
-
6.7533
x
+
14.44
0.987
56.0
79.2
77.5
1.0
0.11
y
=
-
7.6965
x
+
17.79
0.989
63.9
78.7
49.6
2.0
0.14
y
=
-
7.2183
x
+
16.39
0.999
60.0
78.3
61.3
3.0
0.16
y
=
-
7.2597
x
+
16.71
0.986
60.2
77.7
58.6
4.0
0.18
y
=
-
7.225
x
+
16.72
0.977
59.9
77.4
58.5
5.0
0.21
y
=
-
6.8661
x
+
15.69
0.971
57.0
77.1
67.1
PEG-300
0.5
0.07
y
=
-
6.3772
x
+
12.96
0.961
52.9
79.8
89.8
1.0
0.09
y
=
-
6.2301
x
+
12.70
0.983
51.7
79.0
91.9
2.0
0.11
y
=
-
7.0542
x
+
15.55
0.987
58.8
79.2
68.2
3.0
0.14
y
=
-
6.7082
x
+
14.77
0.951
55.7
78.1
74.7
4.0
0.14
y
=
-
6.7034
x
+
14.74
0.966
55.6
78.1
75.0
5.0
0.16
y
=
-
6.4505
x
+
14.04
0.958
53.5
77.7
80.8
PEG-400
0.5
0.11
y
=
-
2.0567
x
-
1.059
0.999
17.0
73.4
188
1.0
0.14
y
=
-
1.6434
x
-
2.177
0.980
13.6
67.3
179
2.0
0.14
y
=
-
4.9725
x
+
8.909
0.999
41.2
78.1
123
3.0
0.21
y
=
-
4.4045
x
+
7.418
0.999
36.5
77.0
135
4.0
0.28
y
=
-
3.7414
x
+
5.514
0.991
31.0
76.3
151
5.0
0.30
y
=
-
3.5601
x
+
4.983
0.985
29.5
76.3
156
PEG-600
0.5
0.16
y
=
-
3.4399
x
+
3.886
0.957
28.5
78.0
165
1.0
0.18
y
=
-
3.4127
x
+
3.971
0.995
28.3
77.5
164
2.0
0.18
y
=
-
4.5044
x
+
7.594
1.00
37.3
77.5
134
3.0
0.21
y
=
-
4.1343
x
+
6.498
0.994
34.3
77.2
143
4.0
0.30
y
=
-
2.6889
x
+
2.060
0.999
22.3
76.3
180
5.0
0.39
y
=
-
2.2924
x
+
0.992
0.999
19.0
75.7
189Table 4
Activation parameters of hypoxanthine in different PEG media.
Type of    PEG
PEG % (V/V)
k
′
′ at 300 K
Equation
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.04
y
=
-
9.0666
x
+
12.37
0.976
75.2
103
94.7
1.0
0.09
y
=
-
5.8149
x
+
11.36
0.978
48.3
79.2
103
2.0
0.11
y
=
-
5.7982
x
+
11.48
0.962
48.1
78.7
102
3.0
0.18
y
=
-
4.2045
x
+
6.604
0.998
34.8
77.4
142
4.0
0.21
y
=
-
4.1526
x
+
6.623
0.965
34.4
77.3
143
5.0
0.02
y
=
-
4.3082
x
+
7.221
0.981
35.7
76.8
137
PEG-300
0.5
0.14
y
=
-
4.3602
x
+
6.862
1.00
36.1
78.1
140
1.0
0.16
y
=
-
4.344
x
+
6.985
0.972
36.0
77.1
139
2.0
0.18
y
=
-
4.2045
x
+
6.604
0.998
34.9
77.5
142
3.0
0.21
y
=
-
4.2723
x
+
6.939
0.988
35.4
77.1
139
4.0
0.23
y
=
-
3.9753
x
+
6.107
0.982
33.0
76.8
146
5.0
0.25
y
=
-
4.145
x
+
6.761
0.978
34.4
76.7
141
PEG-400
0.5
0.16
y
=
-
4.3194
x
+
6.817
0.971
35.8
77.8
140
1.0
0.18
y
=
-
4.1982
x
+
6.561
0.997
34.8
77.7
143
2.0
0.21
y
=
-
3.9535
x
+
5.901
0.997
32.8
77.2
148
3.0
0.23
y
=
-
3.9681
x
+
6.058
0.999
32.9
77.0
147
4.0
0.25
y
=
-
4.2862
x
+
7.204
0.998
35.5
76.6
137
5.0
0.28
y
=
-
4.0984
x
+
6.692
0.998
32.7
75.0
141
PEG-600
0.5
0.14
y
=
-
3.0095
x
+
2.327
0.963
24.9
78.3
178
1.0
0.21
y
=
-
3.0241
x
+
2.815
1.00
25.0
77.2
174
2.0
0.25
y
=
-
2.6271
x
+
1.673
0.997
21.8
76.7
183
3.0
0.30
y
=
-
2.4203
x
+
1.15
0.995
20.0
76.4
188
4.0
0.35
y
=
-
2.5077
x
+
1.581
0.974
20.8
76.0
184
5.0
0.42
y
=
-
2.283
x
+
1.035
0.999
18.9
75.3
188Table 5
Activation parameters of theophylline in different PEG media.
Type of    PEG
PEG % (V/V)
k
′
′ at 300 K
Equation
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.39
y
=
-
1.426
x
-
1.899
0.993
11.8
66.1
181
1.0
0.49
y
=
-
2.6456
x
+
2.392
0.993
21.9
75.0
177
2.0
0.51
y
=
-
2.8259
x
+
3.021
0.984
23.4
75.0
172
3.0
0.53
y
=
-
3.3595
x
+
4.821
0.964
27.8
74.9
157
4.0
0.56
y
=
-
3.5908
x
+
5.700
0.995
29.8
74.8
150
5.0
0.65
y
=
-
3.8286
x
+
6.599
0.984
31.7
74.3
142
PEG-300
0.5
0.23
y
=
-
5.1973
x
+
10.21
0.952
43.1
76.7
112
1.0
0.30
y
=
-
4.348
x
+
7.607
0.992
36.0
76.2
134
2.0
0.37
y
=
-
3.7544
x
+
5.847
0.979
31.1
75.5
148
3.0
0.39
y
=
-
3.6694
x
+
5.622
0.970
30.4
75.4
150
4.0
0.44
y
=
-
3.2984
x
+
4.504
0.969
27.3
75.3
160
5.0
0.51
y
=
-
3.0744
x
+
3.877
0.998
25.5
75.0
165
PEG-400
0.5
0.16
y
=
-
5.2588
x
+
9.971
0.995
43.6
77.8
114
1.0
0.25
y
=
-
3.8986
x
+
5.902
0.999
32.3
76.7
148
2.0
0.28
y
=
-
4.1044
x
+
6.732
0.986
34.0
76.3
141
3.0
0.30
y
=
-
4.2289
x
+
7.215
0.988
35.1
76.2
137
4.0
0.32
y
=
-
4.4535
x
+
8.039
0.978
36.9
75.9
130
5.0
0.44
y
=
-
3.187
x
+
4.113
0.993
26.4
75.3
163
PEG-600
0.5
0.11
y
=
-
4.8443
x
+
8.296
0.956
40.2
78.6
128
1.0
0.21
y
=
-
3.2573
x
+
3.634
0.953
27.0
77.1
167
2.0
0.25
y
=
-
3.1274
x
+
3.359
0.981
25.9
76.6
169
3.0
0.35
y
=
-
2.1206
x
+
0.333
0.979
17.6
75.8
194
4.0
0.44
y
=
-
1.5663
x
-
1.290
0.980
13.0
68.8
186
5.0
0.60
y
=
-
0.7623
x
-
3.664
0.958
6.33
56.4
167Table 6
Activation parameters of theobromine in different PEG media.
Type of    PEG
PEG % (V/V)
k
′
′ at 300 K
Equation
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.21
y = −5.2713x+10.31
0.999
43.7
80.0
111
1.0
0.23
y = −5.9167x+12.54
0.999
49.1
77.0
93.3
2.0
0.30
y = −4.7454x+8.893
0.996
39.3
76.2
123
3.0
0.32
y = −4.6086x+8.510
0.999
38.2
76.0
126
4.0
0.35
y = −4.4002x+7.950
0.979
36.5
75.8
131
5.0
0.44
y = −3.5467x+5.326
0.981
29.4
75.3
153
PEG-300
0.5
0.16
y = −5.8962x+12.17
0.973
48.9
77.7
96.3
1.0
0.21
y = −4.983x+9.385
0.980
41.3
77.0
119
2.0
0.23
y = −5.2059x+10.27
0.987
43.0
76.6
112
3.0
0.30
y = −5.236x+10.41
0.972
43.4
76.7
111
4.0
0.35
y = −4.632x+8.798
0.956
38.4
75.6
124
5.0
0.42
y = −4.6308x+8.917
0.962
38.4
75.3
123
PEG-400
0.5
0.14
y = −6.9315x+15.48
0.987
57.5
78.1
68.8
1.0
0.21
y = −6.0803x+13.08
0.953
50.4
77.0
88.8
2.0
0.30
y = −4.5372x+8.261
0.971
37.6
76.0
128
3.0
0.35
y = −5.2988x+10.96
0.972
43.9
75.7
106
4.0
0.35
y = −6.2994x+14.29
0.978
52.3
108
186
5.0
0.42
y = −5.7161x+12.49
0.997
47.4
76.1
93.7
PEG-600
0.5
0.18
y = −3.4127x+3.971
0.995
28.3
77.5
164
1.0
0.21
y = −3.0241x+2.815
1.00
25.0
77.2
174
2.0
0.28
y = −2.0791x-0.0386
0.996
17.2
76.3
197
3.0
0.32
y = −2.3784x+1.079
0.998
19.7
79.4
188
4.0
0.42
y = −1.7943x-0.569
0.963
14.8
72.4
192
5.0
0.51
y = −2.9311x+3.403
0.995
24.3
75.0
169Figure 4
Benesi-Hildebrand plot of CAN-PEG-200.


## 2.1. Determination of the Order of Reaction and Salient Kinetic Features

(1)
Reactions were conducted under two different conditions. Under pseudo first order conditions [CAF] ≫ [CAN], plots ofln(A0/At), that is, ln[a/(a-x)] versus time, were straight lines with positive slopes, passing through origin indicating first order (x) with respect to [oxidizing agent] (Figure 1).
(2)
This reaction is also conducted under second order conditions with equal concentrations of[CAF]0=[CAN]0. Under these conditions, kinetic plots of [1/(At)] versus time have been found to be linear with a positive gradient and definite intercept on ordinate (vertical axis), indicating overall second order kinetics (Figure 2). Since the order with respect to [CAN] is already verified as one under pseudo conditions this observation suggests that order in [CAF] is also one.
(3)
In PEG mediated reactions an increase in the [PEG] increased the reaction rates depending on the nature of PEG. By and large reaction rates were found high in PEG-200 media over other PEGs (Tables2, 3, 4, 5, and 6).
(4)
In the present study, kinetic data have been collected at three to four different temperatures within the range of 300 to 320 K. Activation parameters such asΔH# and ΔS# have been evaluated by Eyring’s equation. Free energy of activation (ΔG#) is obtained from Gibbs-Helmholtz equation. The data related to activation parameters are compiled in Tables 2 to 6.
(5)
Addition of olefin monomer (acryl amide and acrylonitrile) to the reaction mixture decreased the reaction rate. When heated, the contents of the reaction mixture turned viscous and indicated dense polymer formation. This observation can be explained due to the induced vinyl polymerization of added monomer, showing the presence of free radicals in the system.Figure 1
Pseudo first order kinetic plots of caffeine with MeCN at 310 K. [CAF] = 0.016 mol dm−3; [CAN] = 0.0041 mol dm−3; [PEG-300] = 0.062 mol dm−3.Figure 2
Second order kinetic plots of caffeine with MeCN at 310 K. [CAF] = 0.002 mol dm−3; [CAN] = 0.002 mol dm−3; [PEG-300] = 0.375 mol dm−3.


## 2.2. CAN-PEG Binding Studies

UV-Visible Spectrophotometric studies were performed in order to throw light on CAN binding with PEG (Poly ethylene glycol). Absorption spectra of CAN in acetonitrile indicated a band at 459 nm; this band underwent a hypsochromic shift from 459 nm to 441 nm in presence of 0.1 mol PEG, suggesting the interaction of PEG with CAN (Figure3):
(1)H–(OCH2–CH2)n–OH(PEG)+(NH4)[Ce(NO3)5(ACN)](CAN)⇌K[PEG–NH4Ce(NO3)5](ACN)Complex(C)or[PEG-CAN].
The [CAN-PEG] binding constants were evaluated by Benesi- Hildebrand equation according to the method reported in the literature [38], as elaborated in our earlier paper.Figure 3
Absorption of spectra of CAN in presence of PEG.The equilibrium constantK=[C]/[CAN][PEG], where [CAN], [PEG], and [C] are equilibrium concentrations of acceptor (CAN), donor (PEG), and complex, respectively. For the above equilibrium, concentration of [PEG-CAN] complex ([C]) can be correlated to the formation constant (K) by the following relationship. If [CAN]0 and [PEG]0 represent initial concentrations of CAN and PEG, respectively, then
(2)[C]=K[CAN]0[PEG]01+K[PEG]0.
But according to Lambert-Beer’s law absorbance, (A)=ϵcl.In the above equations,l is path length, d is absorbance, ϵ is the molar extinction coefficient, and K is formation constant of the complex, respectively. For one cm path length, above equation can be written as, (A)=ϵc,
(3)[C]=Aϵl=K[CAN]0[PEG]01+K[PEG]0.
Further, taking the reciprocals to the above equation, it rearranges to
(4)[CAN]0A=1K[PEG]0ϵ+1ϵ.
However, the absorbance of CAN and [CAN-PEG] absorb in the same region significantly; therefore the observed absorbance (A) could be written as
(5)A=A(CAN)+A(Complex),A(Complex)=ΔA=A~A(CAN).
Therefore, a plot of ([CAN]0/ΔA) versus 1/[PEG]0 should give a straight line according to the above equation. These plots have been realized in the present study (Figure 4). Formation constant (K) has been calculated from the ratio of intercept to slope, while inverse of the intercept gave molar extinction coefficient (ϵ) and is represented in Table 1.Table 1
Binding constants of [CAN-PEG] at 303°K using Benesi-Hildebrand method.
S. N.
PEG
Benesi-Hildebrand Equation
K
ε
-
Δ
G (kJ/mol)
1
PEG-200
y
=
7
E
-
05
x
+
0.052
743
19.23
16.7
2
PEG-300
y
=
9
E
-
05
x
+
0.055
611
18.18
16.2
3
PEG-400
y
=
5
E
-
05
x
+
0.071
1420
14.08
18.3Table 2
Activation parameters of caffeine in different PEG media.
Type of PEG
PEG % (V/V)
k
′
′ at 300 K
Equation obtained for plot ofln(k′′/T) versus (103/T)
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.4
y
=
-
4.1044
x
+
7.088
0.985
34.1
75.2
138
1.0
0.5
y
=
-
3.9118
x
+
6.686
0.963
32.4
74.7
141
2.0
0.6
y
=
-
3.4191
x
+
5.218
0.968
28.3
74.5
154
3.0
0.6
y
=
-
5.24
x
+
11.28
0.989
43.5
74.7
103
4.0
0.7
y
=
-
4.4927
x
+
8.922
0.999
37.2
74.2
123
5.0
0.9
y
=
-
3.9922
x
+
7.503
0.999
33.1
73.6
135
PEG-300
0.5
0.2
y
=
-
5.7209
x
+
11.77
0.997
47.5
76.6
99.7
1.0
0.3
y
=
-
4.4045
x
+
7.775
1.00
36.5
76.1
132
2.0
0.4
y
=
-
4.1044
x
+
7.088
0.986
34.1
75.5
138
3.0
0.5
y
=
-
3.0174
x
+
3.654
0.998
25.0
75.1
167
4.0
0.6
y
=
-
2.1466
x
+
0.948
0.995
17.8
74.5
189
5.0
0.7
y
=
-
1.8624
x
+
0.153
0.996
15.4
73.5
196
PEG-400
0.5
0.2
y
=
-
4.0901
x
+
6.298
0.992
33.9
75.6
139
1.0
0.4
y
=
-
1.6439
x
-
1.125
0.974
13.6
70.0
188
2.0
0.5
y
=
-
1.3085
x
-
2.031
0.998
10.8
64.8
180
3.0
0.6
y
=
-
1.0727
x
-
2.636
0.999
8.90
61.4
175
4.0
0.7
y
=
-
3.0329
x
+
4.079
0.971
25.1
74.0
163
5.0
0.8
y
=
-
3.0238
x
+
4.168
0.992
25.0
73.6
162
PEG-600
0.5
0.2
y
=
-
4.9831
x
+
9.337
0.980
41.3
77.0
119
1.0
0.3
y
=
-
3.7714
x
+
5.687
0.988
31.3
76.3
150
2.0
0.4
y
=
-
3.0286
x
+
3.490
0.992
25.1
75.5
168
3.0
0.6
y
=
-
1.6344
x
-
0.784
0.966
13.5
70.8
191
4.0
0.7
y
=
-
1.3993
x
-
1.412
0.963
11.6
67.1
185
5.0
0.9
y
=
-
0.6541
x
-
3.627
0.999
5.42
55.5
167Table 3
Activation parameters of Xanthine in different PEG media.
Type of PEG
PEG % (V/V)
k
′
′ at 300 K
Equation
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.09
y
=
-
6.7533
x
+
14.44
0.987
56.0
79.2
77.5
1.0
0.11
y
=
-
7.6965
x
+
17.79
0.989
63.9
78.7
49.6
2.0
0.14
y
=
-
7.2183
x
+
16.39
0.999
60.0
78.3
61.3
3.0
0.16
y
=
-
7.2597
x
+
16.71
0.986
60.2
77.7
58.6
4.0
0.18
y
=
-
7.225
x
+
16.72
0.977
59.9
77.4
58.5
5.0
0.21
y
=
-
6.8661
x
+
15.69
0.971
57.0
77.1
67.1
PEG-300
0.5
0.07
y
=
-
6.3772
x
+
12.96
0.961
52.9
79.8
89.8
1.0
0.09
y
=
-
6.2301
x
+
12.70
0.983
51.7
79.0
91.9
2.0
0.11
y
=
-
7.0542
x
+
15.55
0.987
58.8
79.2
68.2
3.0
0.14
y
=
-
6.7082
x
+
14.77
0.951
55.7
78.1
74.7
4.0
0.14
y
=
-
6.7034
x
+
14.74
0.966
55.6
78.1
75.0
5.0
0.16
y
=
-
6.4505
x
+
14.04
0.958
53.5
77.7
80.8
PEG-400
0.5
0.11
y
=
-
2.0567
x
-
1.059
0.999
17.0
73.4
188
1.0
0.14
y
=
-
1.6434
x
-
2.177
0.980
13.6
67.3
179
2.0
0.14
y
=
-
4.9725
x
+
8.909
0.999
41.2
78.1
123
3.0
0.21
y
=
-
4.4045
x
+
7.418
0.999
36.5
77.0
135
4.0
0.28
y
=
-
3.7414
x
+
5.514
0.991
31.0
76.3
151
5.0
0.30
y
=
-
3.5601
x
+
4.983
0.985
29.5
76.3
156
PEG-600
0.5
0.16
y
=
-
3.4399
x
+
3.886
0.957
28.5
78.0
165
1.0
0.18
y
=
-
3.4127
x
+
3.971
0.995
28.3
77.5
164
2.0
0.18
y
=
-
4.5044
x
+
7.594
1.00
37.3
77.5
134
3.0
0.21
y
=
-
4.1343
x
+
6.498
0.994
34.3
77.2
143
4.0
0.30
y
=
-
2.6889
x
+
2.060
0.999
22.3
76.3
180
5.0
0.39
y
=
-
2.2924
x
+
0.992
0.999
19.0
75.7
189Table 4
Activation parameters of hypoxanthine in different PEG media.
Type of    PEG
PEG % (V/V)
k
′
′ at 300 K
Equation
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.04
y
=
-
9.0666
x
+
12.37
0.976
75.2
103
94.7
1.0
0.09
y
=
-
5.8149
x
+
11.36
0.978
48.3
79.2
103
2.0
0.11
y
=
-
5.7982
x
+
11.48
0.962
48.1
78.7
102
3.0
0.18
y
=
-
4.2045
x
+
6.604
0.998
34.8
77.4
142
4.0
0.21
y
=
-
4.1526
x
+
6.623
0.965
34.4
77.3
143
5.0
0.02
y
=
-
4.3082
x
+
7.221
0.981
35.7
76.8
137
PEG-300
0.5
0.14
y
=
-
4.3602
x
+
6.862
1.00
36.1
78.1
140
1.0
0.16
y
=
-
4.344
x
+
6.985
0.972
36.0
77.1
139
2.0
0.18
y
=
-
4.2045
x
+
6.604
0.998
34.9
77.5
142
3.0
0.21
y
=
-
4.2723
x
+
6.939
0.988
35.4
77.1
139
4.0
0.23
y
=
-
3.9753
x
+
6.107
0.982
33.0
76.8
146
5.0
0.25
y
=
-
4.145
x
+
6.761
0.978
34.4
76.7
141
PEG-400
0.5
0.16
y
=
-
4.3194
x
+
6.817
0.971
35.8
77.8
140
1.0
0.18
y
=
-
4.1982
x
+
6.561
0.997
34.8
77.7
143
2.0
0.21
y
=
-
3.9535
x
+
5.901
0.997
32.8
77.2
148
3.0
0.23
y
=
-
3.9681
x
+
6.058
0.999
32.9
77.0
147
4.0
0.25
y
=
-
4.2862
x
+
7.204
0.998
35.5
76.6
137
5.0
0.28
y
=
-
4.0984
x
+
6.692
0.998
32.7
75.0
141
PEG-600
0.5
0.14
y
=
-
3.0095
x
+
2.327
0.963
24.9
78.3
178
1.0
0.21
y
=
-
3.0241
x
+
2.815
1.00
25.0
77.2
174
2.0
0.25
y
=
-
2.6271
x
+
1.673
0.997
21.8
76.7
183
3.0
0.30
y
=
-
2.4203
x
+
1.15
0.995
20.0
76.4
188
4.0
0.35
y
=
-
2.5077
x
+
1.581
0.974
20.8
76.0
184
5.0
0.42
y
=
-
2.283
x
+
1.035
0.999
18.9
75.3
188Table 5
Activation parameters of theophylline in different PEG media.
Type of    PEG
PEG % (V/V)
k
′
′ at 300 K
Equation
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.39
y
=
-
1.426
x
-
1.899
0.993
11.8
66.1
181
1.0
0.49
y
=
-
2.6456
x
+
2.392
0.993
21.9
75.0
177
2.0
0.51
y
=
-
2.8259
x
+
3.021
0.984
23.4
75.0
172
3.0
0.53
y
=
-
3.3595
x
+
4.821
0.964
27.8
74.9
157
4.0
0.56
y
=
-
3.5908
x
+
5.700
0.995
29.8
74.8
150
5.0
0.65
y
=
-
3.8286
x
+
6.599
0.984
31.7
74.3
142
PEG-300
0.5
0.23
y
=
-
5.1973
x
+
10.21
0.952
43.1
76.7
112
1.0
0.30
y
=
-
4.348
x
+
7.607
0.992
36.0
76.2
134
2.0
0.37
y
=
-
3.7544
x
+
5.847
0.979
31.1
75.5
148
3.0
0.39
y
=
-
3.6694
x
+
5.622
0.970
30.4
75.4
150
4.0
0.44
y
=
-
3.2984
x
+
4.504
0.969
27.3
75.3
160
5.0
0.51
y
=
-
3.0744
x
+
3.877
0.998
25.5
75.0
165
PEG-400
0.5
0.16
y
=
-
5.2588
x
+
9.971
0.995
43.6
77.8
114
1.0
0.25
y
=
-
3.8986
x
+
5.902
0.999
32.3
76.7
148
2.0
0.28
y
=
-
4.1044
x
+
6.732
0.986
34.0
76.3
141
3.0
0.30
y
=
-
4.2289
x
+
7.215
0.988
35.1
76.2
137
4.0
0.32
y
=
-
4.4535
x
+
8.039
0.978
36.9
75.9
130
5.0
0.44
y
=
-
3.187
x
+
4.113
0.993
26.4
75.3
163
PEG-600
0.5
0.11
y
=
-
4.8443
x
+
8.296
0.956
40.2
78.6
128
1.0
0.21
y
=
-
3.2573
x
+
3.634
0.953
27.0
77.1
167
2.0
0.25
y
=
-
3.1274
x
+
3.359
0.981
25.9
76.6
169
3.0
0.35
y
=
-
2.1206
x
+
0.333
0.979
17.6
75.8
194
4.0
0.44
y
=
-
1.5663
x
-
1.290
0.980
13.0
68.8
186
5.0
0.60
y
=
-
0.7623
x
-
3.664
0.958
6.33
56.4
167Table 6
Activation parameters of theobromine in different PEG media.
Type of    PEG
PEG % (V/V)
k
′
′ at 300 K
Equation
R
²
Δ
H
#
Δ
G
#
-
Δ
S
# J/Kmol
kJ/mol
PEG-200
0.5
0.21
y = −5.2713x+10.31
0.999
43.7
80.0
111
1.0
0.23
y = −5.9167x+12.54
0.999
49.1
77.0
93.3
2.0
0.30
y = −4.7454x+8.893
0.996
39.3
76.2
123
3.0
0.32
y = −4.6086x+8.510
0.999
38.2
76.0
126
4.0
0.35
y = −4.4002x+7.950
0.979
36.5
75.8
131
5.0
0.44
y = −3.5467x+5.326
0.981
29.4
75.3
153
PEG-300
0.5
0.16
y = −5.8962x+12.17
0.973
48.9
77.7
96.3
1.0
0.21
y = −4.983x+9.385
0.980
41.3
77.0
119
2.0
0.23
y = −5.2059x+10.27
0.987
43.0
76.6
112
3.0
0.30
y = −5.236x+10.41
0.972
43.4
76.7
111
4.0
0.35
y = −4.632x+8.798
0.956
38.4
75.6
124
5.0
0.42
y = −4.6308x+8.917
0.962
38.4
75.3
123
PEG-400
0.5
0.14
y = −6.9315x+15.48
0.987
57.5
78.1
68.8
1.0
0.21
y = −6.0803x+13.08
0.953
50.4
77.0
88.8
2.0
0.30
y = −4.5372x+8.261
0.971
37.6
76.0
128
3.0
0.35
y = −5.2988x+10.96
0.972
43.9
75.7
106
4.0
0.35
y = −6.2994x+14.29
0.978
52.3
108
186
5.0
0.42
y = −5.7161x+12.49
0.997
47.4
76.1
93.7
PEG-600
0.5
0.18
y = −3.4127x+3.971
0.995
28.3
77.5
164
1.0
0.21
y = −3.0241x+2.815
1.00
25.0
77.2
174
2.0
0.28
y = −2.0791x-0.0386
0.996
17.2
76.3
197
3.0
0.32
y = −2.3784x+1.079
0.998
19.7
79.4
188
4.0
0.42
y = −1.7943x-0.569
0.963
14.8
72.4
192
5.0
0.51
y = −2.9311x+3.403
0.995
24.3
75.0
169Figure 4
Benesi-Hildebrand plot of CAN-PEG-200.


## 3. Results and Discussion

### 3.1. Mechanism of CAN Oxidation of Xanthine Alkaloids in MeCN Medium

Earlier reports on CAN oxidation studies from our laboratory and elsewhere show that a variety of CAN species such asCe(NO3)62-, Ce(NO3)5-, Ce(OH)(NO3)4-, Ce(NO3)4, and Ce(OH)3+ may exist in nitric acid medium [39–43]. However, CAN species in MeCN medium could be entirely different. Since MeCN is large excess over [CAN], MeCN may penetrate into the coordination spheres of Ce(IV) and form solvated CAN species according to the following equilibrium:
(6)(NH4)2Ce(NO3)6+CH3CN(CAN)⇌[(NH4)Ce(NO3)5(CH3CN)]+NH4NO3(SolvatedCAN).
Solvated CAN may be able to oxidize the substrate to afford uric acid as product, when Xanthine alkaloid is added to the reaction mixture (see Scheme 2).Scheme 2
CAN oxidation of xanthine alkaloids in ACN medium.


### 3.2. Mechanism of Oxidation in PEG Media

Progress of the reaction has been studied in the presence of a set of poly oxy ethylene compounds (PEGs) with varied molecular weights ranging from 200 to 6000 units, and it was found that the reaction is enhanced remarkably in all PEGs. Reaction times were reduced from 24 hrs to few hours. The catalytic activity was found to be in the decreasing order: PEG-200 > PEG-300 > PEG-400 > PEG-600. UV-Visible Spectroscopic results presented in Figure5 clearly indicated a bathochromic/hypsochromic shift from 459 nm to around 442 nm, followed by hypochromic shift clearly indicate CAN and PEG interactions to afford “PEG bound CAN” [PEG-CAN] according to the following equilibrium  (see Figure 6).Figure 5
Eyring’s plot: PEG-300 catalysed oxidation of caffeine by CAN.Figure 6The plots ofkm (rate constant of PEG reaction) versus CPEG (concentration of PEG) indicated a rate maxima nearly in the vicinity of 1.50 mol dm−3 PEG-200, 0.99 mol dm−3PEG-300, 0.75 mol dm−3, PEG-400, 0.500 mol dm−3, and PEG-600. Mechanism of PEG mediated CAN-xanthine alkaloids reactions was explained in the lines of micellar catalysis because PEG resembles the structure of non-ionic micelles such as Triton-X. Menger and Portnoy model is used to explain PEG effects, which closely resemble that of an enzymatic catalysis [44–48]. According to this model, formation of PEG bound reagent (PEG-Ce(IV)) could occur in the preequilibrium step due to the interaction of Ce(IV) with PEG. The complex thus formed may possess higher or lower reactivity to give products. A general mechanism is proposed by considering the bulk phase and micellar phase reactions as shown in Scheme 3, where km and k0 or (kw) represent rate constants for PEG and bulk phases, respectively, and K is the [PEG-Ce(IV)] binding constant. For the above mechanism, rate law could be derived according to the following sequence of steps in the lines of micellar catalyzed reactions. From Scheme 3 rate (V) of the reaction comes out as
(7)V={k0[CAN]+km[PEG_CAN]}CS,VCS=k′=k0[CAN]+km[PEG_CAN].
Considering the total concentration of (CS) as the algebraic sum of free species and PEG bound CAN complex (PEG-CAN) species,
(8)CCAN=[CAN]+[PEG-CAN].
From PEG-CAN binding equilibrium,
(9)K=[PEG-CAN][PEG][CAN]or[CAN]=[PEG-CAN]K[PEG].
Substitution of [CAN] in (7) gives
(10)CCAN=[PEG-CAN]K[PEG]+[PEG-CAN]=[PEG-CAN]+K[PEG][PEG-CAN]K[PEG]or[PEG-CAN]=K[PEG]CCAN1+K[PEG].
Similarly free substrate [CAN] is written as, [CAN]=CCAN-[PEG-CAN],
(11)[CAN]=CCAN-K[PEG]CCAN1+K[PEG].
After simplification, the above equation reduces to
(12)[CAN]=CCAN1+K[PEG].
Substitution of [PEG-CAN] and [CAN] in rate equation (7) gives
(13)k′=k0CCAN1+K[PEG]+kmK[PEG]CCAN1+K[PEG],(14)orkφ=k0+kmK[PEG]1+K[PEG],(15)kφ=k0+kmK[PEG]1+K[PEG],
where kφ=(k′/[CAN]), the second order rate constant in PEG media. Subtracting k0 from both the sides of equation and rearranging,
(16)kφ-k0=(km-kw)K[PEG]1+K[PEG].
However, since the reactions are too sluggish in the absence of [PEG], the rate constant (k0) would be much smaller than (kmK[PEG]), that is, (k0⋘kmK[PEG]). Therefore the (k0) term could be neglected in the above equation. On the basis of the foregoing discussion, the most plausible mechanism for PEG catalysed reaction could be given as in Scheme 4. The rate law for Scheme 4, could then be considered as
(17)kφ=kmK[PEG]1+K[PEG].
This rate law resembles Michaelis-Menten type rate law that is used for enzyme kinetics. Interestingly the plots of rate constant (kφ), that is, second order rate constant of PEG mediated reaction versus [PEG], indicated Hill type curves (i.e., a gradual increase with an increase in [PEG] passing through a maximum point in the profile). This observation points out that beyond certain concentration, PEG bound [CAN] inhibits the reaction rates. This could be attributed to the fact that [CAN] is tightly bound to PEG and surrounded by PEG environment, giving less scope for rate accelerations. In view of this reaction kinetics are studied in detail at various PEG concentrations in order to have an insight into the variation in the enthalpies and entropies of activation with [PEG].Scheme 3
CAN oxidation mechanism in presence of PEG.Scheme 4


### 3.3. Effect of Structure on Enthalpy and Entropy Changes

The enthalpy and entropy of activation (ΔH# and ΔS#) are the two parameters typically obtained from the slope and intercepts of Eyring’s plot of ln(k′′/T) versus (1/T) as shown in Figure 5. The positive values for ΔS# suggest a dissociative mechanism, while negative ΔS# values indicate an associative mechanism. Values near zero are difficult to interpret [26, 49, 50]. Almost similar magnitude of ΔG# in a series of closely related reactions generally indicates a similar type of mechanism operative for closely related reactions under study. Overall free energy of reaction (ΔG) may be considered to be the driving force of a chemical reaction. When ΔG<0 the reaction is spontaneous; when ΔG=0 the system is at equilibrium and no net change occurs; and when ΔG>0 the reaction is not spontaneous. Entropies of activation data compiled in Tables 1 to 6 of the present study are highly negative, which are in accordance with an associative mechanism leading to a well-organized transition state. These results probably support the association of PEG with CAN, which brings about changes in the transition state and cause simultaneous association and dissociation of species causing disorderness in the transition state leading to a chemical reaction. Similar type of trends is recorded in all the PEGs used in this study.


## 3.1. Mechanism of CAN Oxidation of Xanthine Alkaloids in MeCN Medium

Earlier reports on CAN oxidation studies from our laboratory and elsewhere show that a variety of CAN species such asCe(NO3)62-, Ce(NO3)5-, Ce(OH)(NO3)4-, Ce(NO3)4, and Ce(OH)3+ may exist in nitric acid medium [39–43]. However, CAN species in MeCN medium could be entirely different. Since MeCN is large excess over [CAN], MeCN may penetrate into the coordination spheres of Ce(IV) and form solvated CAN species according to the following equilibrium:
(6)(NH4)2Ce(NO3)6+CH3CN(CAN)⇌[(NH4)Ce(NO3)5(CH3CN)]+NH4NO3(SolvatedCAN).
Solvated CAN may be able to oxidize the substrate to afford uric acid as product, when Xanthine alkaloid is added to the reaction mixture (see Scheme 2).Scheme 2
CAN oxidation of xanthine alkaloids in ACN medium.


## 3.2. Mechanism of Oxidation in PEG Media

Progress of the reaction has been studied in the presence of a set of poly oxy ethylene compounds (PEGs) with varied molecular weights ranging from 200 to 6000 units, and it was found that the reaction is enhanced remarkably in all PEGs. Reaction times were reduced from 24 hrs to few hours. The catalytic activity was found to be in the decreasing order: PEG-200 > PEG-300 > PEG-400 > PEG-600. UV-Visible Spectroscopic results presented in Figure5 clearly indicated a bathochromic/hypsochromic shift from 459 nm to around 442 nm, followed by hypochromic shift clearly indicate CAN and PEG interactions to afford “PEG bound CAN” [PEG-CAN] according to the following equilibrium  (see Figure 6).Figure 5
Eyring’s plot: PEG-300 catalysed oxidation of caffeine by CAN.Figure 6The plots ofkm (rate constant of PEG reaction) versus CPEG (concentration of PEG) indicated a rate maxima nearly in the vicinity of 1.50 mol dm−3 PEG-200, 0.99 mol dm−3PEG-300, 0.75 mol dm−3, PEG-400, 0.500 mol dm−3, and PEG-600. Mechanism of PEG mediated CAN-xanthine alkaloids reactions was explained in the lines of micellar catalysis because PEG resembles the structure of non-ionic micelles such as Triton-X. Menger and Portnoy model is used to explain PEG effects, which closely resemble that of an enzymatic catalysis [44–48]. According to this model, formation of PEG bound reagent (PEG-Ce(IV)) could occur in the preequilibrium step due to the interaction of Ce(IV) with PEG. The complex thus formed may possess higher or lower reactivity to give products. A general mechanism is proposed by considering the bulk phase and micellar phase reactions as shown in Scheme 3, where km and k0 or (kw) represent rate constants for PEG and bulk phases, respectively, and K is the [PEG-Ce(IV)] binding constant. For the above mechanism, rate law could be derived according to the following sequence of steps in the lines of micellar catalyzed reactions. From Scheme 3 rate (V) of the reaction comes out as
(7)V={k0[CAN]+km[PEG_CAN]}CS,VCS=k′=k0[CAN]+km[PEG_CAN].
Considering the total concentration of (CS) as the algebraic sum of free species and PEG bound CAN complex (PEG-CAN) species,
(8)CCAN=[CAN]+[PEG-CAN].
From PEG-CAN binding equilibrium,
(9)K=[PEG-CAN][PEG][CAN]or[CAN]=[PEG-CAN]K[PEG].
Substitution of [CAN] in (7) gives
(10)CCAN=[PEG-CAN]K[PEG]+[PEG-CAN]=[PEG-CAN]+K[PEG][PEG-CAN]K[PEG]or[PEG-CAN]=K[PEG]CCAN1+K[PEG].
Similarly free substrate [CAN] is written as, [CAN]=CCAN-[PEG-CAN],
(11)[CAN]=CCAN-K[PEG]CCAN1+K[PEG].
After simplification, the above equation reduces to
(12)[CAN]=CCAN1+K[PEG].
Substitution of [PEG-CAN] and [CAN] in rate equation (7) gives
(13)k′=k0CCAN1+K[PEG]+kmK[PEG]CCAN1+K[PEG],(14)orkφ=k0+kmK[PEG]1+K[PEG],(15)kφ=k0+kmK[PEG]1+K[PEG],
where kφ=(k′/[CAN]), the second order rate constant in PEG media. Subtracting k0 from both the sides of equation and rearranging,
(16)kφ-k0=(km-kw)K[PEG]1+K[PEG].
However, since the reactions are too sluggish in the absence of [PEG], the rate constant (k0) would be much smaller than (kmK[PEG]), that is, (k0⋘kmK[PEG]). Therefore the (k0) term could be neglected in the above equation. On the basis of the foregoing discussion, the most plausible mechanism for PEG catalysed reaction could be given as in Scheme 4. The rate law for Scheme 4, could then be considered as
(17)kφ=kmK[PEG]1+K[PEG].
This rate law resembles Michaelis-Menten type rate law that is used for enzyme kinetics. Interestingly the plots of rate constant (kφ), that is, second order rate constant of PEG mediated reaction versus [PEG], indicated Hill type curves (i.e., a gradual increase with an increase in [PEG] passing through a maximum point in the profile). This observation points out that beyond certain concentration, PEG bound [CAN] inhibits the reaction rates. This could be attributed to the fact that [CAN] is tightly bound to PEG and surrounded by PEG environment, giving less scope for rate accelerations. In view of this reaction kinetics are studied in detail at various PEG concentrations in order to have an insight into the variation in the enthalpies and entropies of activation with [PEG].Scheme 3
CAN oxidation mechanism in presence of PEG.Scheme 4


## 3.3. Effect of Structure on Enthalpy and Entropy Changes

The enthalpy and entropy of activation (ΔH# and ΔS#) are the two parameters typically obtained from the slope and intercepts of Eyring’s plot of ln(k′′/T) versus (1/T) as shown in Figure 5. The positive values for ΔS# suggest a dissociative mechanism, while negative ΔS# values indicate an associative mechanism. Values near zero are difficult to interpret [26, 49, 50]. Almost similar magnitude of ΔG# in a series of closely related reactions generally indicates a similar type of mechanism operative for closely related reactions under study. Overall free energy of reaction (ΔG) may be considered to be the driving force of a chemical reaction. When ΔG<0 the reaction is spontaneous; when ΔG=0 the system is at equilibrium and no net change occurs; and when ΔG>0 the reaction is not spontaneous. Entropies of activation data compiled in Tables 1 to 6 of the present study are highly negative, which are in accordance with an associative mechanism leading to a well-organized transition state. These results probably support the association of PEG with CAN, which brings about changes in the transition state and cause simultaneous association and dissociation of species causing disorderness in the transition state leading to a chemical reaction. Similar type of trends is recorded in all the PEGs used in this study.


## 4. Conclusions

We have studied oxidation of Xanthine alkaloids such as Xanthine (XAN), hypoxanthine (HXAN), caffeine (CAF), theophylline (TPL), and theobromine (TBR), by a common laboratory desktop reagent CAN in catalytic amounts. Oxidation of xanthine derivatives afforded uric acid derivatives. Even though the reaction is too sluggish in acetonitrile media even at reflux temperatures, it underwent smoothly in presence of Poly ethylene glycols (PEG). Reaction kinetics indicated first order in both [CAN] and [Xanthine alkaloid]. Rate of oxidation is accelerated with an increase in [PEG] linearly. Mechanism of oxidation in PEG media has been explained by Menger-Portnoy enzymatic model with the oxidation of PEG bound oxidant (PEG-CAN) as more reactive species than (CAN) itself.



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*Source: 835610-2013-06-10.xml*