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)
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 [
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 Scheme
Structure of polyethylene glycol (PEG).
Polyethylene glycol (PEG) is a condensation polymer of ethylene oxide and water with the general formula
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 Aliquots of the reaction mixture were withdrawn into a cuvette and placed in the cell compartment of the laboratory visible spectrophotometer.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.
Reactions were conducted under two different conditions. Under pseudo first order conditions [CAF] ≫ [CAN], plots of This reaction is also conducted under second order conditions with equal concentrations of 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 (Tables 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 as 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.
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.
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.
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 (Figure
Absorption of spectra of CAN in presence of PEG.
The equilibrium constant
In the above equations,
Binding constants of [CAN-PEG] at 303°K using Benesi-Hildebrand method.
S. N. | PEG | Benesi-Hildebrand Equation |
|
|
|
---|---|---|---|---|---|
1 | PEG-200 |
|
743 | 19.23 | 16.7 |
2 | PEG-300 |
|
611 | 18.18 | 16.2 |
3 | PEG-400 |
|
1420 | 14.08 | 18.3 |
Activation parameters of caffeine in different PEG media.
Type of PEG | PEG % (V/V) |
|
Equation obtained for plot of |
|
|
|
|
---|---|---|---|---|---|---|---|
kJ/mol | |||||||
PEG-200 | 0.5 | 0.4 |
|
0.985 | 34.1 | 75.2 | 138 |
1.0 | 0.5 |
|
0.963 | 32.4 | 74.7 | 141 | |
2.0 | 0.6 |
|
0.968 | 28.3 | 74.5 | 154 | |
3.0 | 0.6 |
|
0.989 | 43.5 | 74.7 | 103 | |
4.0 | 0.7 |
|
0.999 | 37.2 | 74.2 | 123 | |
5.0 | 0.9 |
|
0.999 | 33.1 | 73.6 | 135 | |
| |||||||
PEG-300 | 0.5 | 0.2 |
|
0.997 | 47.5 | 76.6 | 99.7 |
1.0 | 0.3 |
|
1.00 | 36.5 | 76.1 | 132 | |
2.0 | 0.4 |
|
0.986 | 34.1 | 75.5 | 138 | |
3.0 | 0.5 |
|
0.998 | 25.0 | 75.1 | 167 | |
4.0 | 0.6 |
|
0.995 | 17.8 | 74.5 | 189 | |
5.0 | 0.7 |
|
0.996 | 15.4 | 73.5 | 196 | |
| |||||||
PEG-400 | 0.5 | 0.2 |
|
0.992 | 33.9 | 75.6 | 139 |
1.0 | 0.4 |
|
0.974 | 13.6 | 70.0 | 188 | |
2.0 | 0.5 |
|
0.998 | 10.8 | 64.8 | 180 | |
3.0 | 0.6 |
|
0.999 | 8.90 | 61.4 | 175 | |
4.0 | 0.7 |
|
0.971 | 25.1 | 74.0 | 163 | |
5.0 | 0.8 |
|
0.992 | 25.0 | 73.6 | 162 | |
| |||||||
PEG-600 | 0.5 | 0.2 |
|
0.980 | 41.3 | 77.0 | 119 |
1.0 | 0.3 |
|
0.988 | 31.3 | 76.3 | 150 | |
2.0 | 0.4 |
|
0.992 | 25.1 | 75.5 | 168 | |
3.0 | 0.6 |
|
0.966 | 13.5 | 70.8 | 191 | |
4.0 | 0.7 |
|
0.963 | 11.6 | 67.1 | 185 | |
5.0 | 0.9 |
|
0.999 | 5.42 | 55.5 | 167 |
Activation parameters of Xanthine in different PEG media.
Type of PEG | PEG % (V/V) |
|
Equation |
|
|
|
|
---|---|---|---|---|---|---|---|
kJ/mol | |||||||
PEG-200 | 0.5 | 0.09 |
|
0.987 | 56.0 | 79.2 | 77.5 |
1.0 | 0.11 |
|
0.989 | 63.9 | 78.7 | 49.6 | |
2.0 | 0.14 |
|
0.999 | 60.0 | 78.3 | 61.3 | |
3.0 | 0.16 |
|
0.986 | 60.2 | 77.7 | 58.6 | |
4.0 | 0.18 |
|
0.977 | 59.9 | 77.4 | 58.5 | |
5.0 | 0.21 |
|
0.971 | 57.0 | 77.1 | 67.1 | |
| |||||||
PEG-300 | 0.5 | 0.07 |
|
0.961 | 52.9 | 79.8 | 89.8 |
1.0 | 0.09 |
|
0.983 | 51.7 | 79.0 | 91.9 | |
2.0 | 0.11 |
|
0.987 | 58.8 | 79.2 | 68.2 | |
3.0 | 0.14 |
|
0.951 | 55.7 | 78.1 | 74.7 | |
4.0 | 0.14 |
|
0.966 | 55.6 | 78.1 | 75.0 | |
5.0 | 0.16 |
|
0.958 | 53.5 | 77.7 | 80.8 | |
| |||||||
PEG-400 | 0.5 | 0.11 |
|
0.999 | 17.0 | 73.4 | 188 |
1.0 | 0.14 |
|
0.980 | 13.6 | 67.3 | 179 | |
2.0 | 0.14 |
|
0.999 | 41.2 | 78.1 | 123 | |
3.0 | 0.21 |
|
0.999 | 36.5 | 77.0 | 135 | |
4.0 | 0.28 |
|
0.991 | 31.0 | 76.3 | 151 | |
5.0 | 0.30 |
|
0.985 | 29.5 | 76.3 | 156 | |
| |||||||
PEG-600 | 0.5 | 0.16 |
|
0.957 | 28.5 | 78.0 | 165 |
1.0 | 0.18 |
|
0.995 | 28.3 | 77.5 | 164 | |
2.0 | 0.18 |
|
1.00 | 37.3 | 77.5 | 134 | |
3.0 | 0.21 |
|
0.994 | 34.3 | 77.2 | 143 | |
4.0 | 0.30 |
|
0.999 | 22.3 | 76.3 | 180 | |
5.0 | 0.39 |
|
0.999 | 19.0 | 75.7 | 189 |
Activation parameters of hypoxanthine in different PEG media.
Type of PEG | PEG % (V/V) |
|
Equation |
|
|
|
|
---|---|---|---|---|---|---|---|
kJ/mol | |||||||
PEG-200 | 0.5 | 0.04 |
|
0.976 | 75.2 | 103 | 94.7 |
1.0 | 0.09 |
|
0.978 | 48.3 | 79.2 | 103 | |
2.0 | 0.11 |
|
0.962 | 48.1 | 78.7 | 102 | |
3.0 | 0.18 |
|
0.998 | 34.8 | 77.4 | 142 | |
4.0 | 0.21 |
|
0.965 | 34.4 | 77.3 | 143 | |
5.0 | 0.02 |
|
0.981 | 35.7 | 76.8 | 137 | |
| |||||||
PEG-300 | 0.5 | 0.14 |
|
1.00 | 36.1 | 78.1 | 140 |
1.0 | 0.16 |
|
0.972 | 36.0 | 77.1 | 139 | |
2.0 | 0.18 |
|
0.998 | 34.9 | 77.5 | 142 | |
3.0 | 0.21 |
|
0.988 | 35.4 | 77.1 | 139 | |
4.0 | 0.23 |
|
0.982 | 33.0 | 76.8 | 146 | |
5.0 | 0.25 |
|
0.978 | 34.4 | 76.7 | 141 | |
| |||||||
PEG-400 | 0.5 | 0.16 |
|
0.971 | 35.8 | 77.8 | 140 |
1.0 | 0.18 |
|
0.997 | 34.8 | 77.7 | 143 | |
2.0 | 0.21 |
|
0.997 | 32.8 | 77.2 | 148 | |
3.0 | 0.23 |
|
0.999 | 32.9 | 77.0 | 147 | |
4.0 | 0.25 |
|
0.998 | 35.5 | 76.6 | 137 | |
5.0 | 0.28 |
|
0.998 | 32.7 | 75.0 | 141 | |
| |||||||
PEG-600 | 0.5 | 0.14 |
|
0.963 | 24.9 | 78.3 | 178 |
1.0 | 0.21 |
|
1.00 | 25.0 | 77.2 | 174 | |
2.0 | 0.25 |
|
0.997 | 21.8 | 76.7 | 183 | |
3.0 | 0.30 |
|
0.995 | 20.0 | 76.4 | 188 | |
4.0 | 0.35 |
|
0.974 | 20.8 | 76.0 | 184 | |
5.0 | 0.42 |
|
0.999 | 18.9 | 75.3 | 188 |
Activation parameters of theophylline in different PEG media.
Type of PEG | PEG % (V/V) |
|
Equation |
|
|
|
|
---|---|---|---|---|---|---|---|
kJ/mol | |||||||
PEG-200 | 0.5 | 0.39 |
|
0.993 | 11.8 | 66.1 | 181 |
1.0 | 0.49 |
|
0.993 | 21.9 | 75.0 | 177 | |
2.0 | 0.51 |
|
0.984 | 23.4 | 75.0 | 172 | |
3.0 | 0.53 |
|
0.964 | 27.8 | 74.9 | 157 | |
4.0 | 0.56 |
|
0.995 | 29.8 | 74.8 | 150 | |
5.0 | 0.65 |
|
0.984 | 31.7 | 74.3 | 142 | |
| |||||||
PEG-300 | 0.5 | 0.23 |
|
0.952 | 43.1 | 76.7 | 112 |
1.0 | 0.30 |
|
0.992 | 36.0 | 76.2 | 134 | |
2.0 | 0.37 |
|
0.979 | 31.1 | 75.5 | 148 | |
3.0 | 0.39 |
|
0.970 | 30.4 | 75.4 | 150 | |
4.0 | 0.44 |
|
0.969 | 27.3 | 75.3 | 160 | |
5.0 | 0.51 |
|
0.998 | 25.5 | 75.0 | 165 | |
| |||||||
PEG-400 | 0.5 | 0.16 |
|
0.995 | 43.6 | 77.8 | 114 |
1.0 | 0.25 |
|
0.999 | 32.3 | 76.7 | 148 | |
2.0 | 0.28 |
|
0.986 | 34.0 | 76.3 | 141 | |
3.0 | 0.30 |
|
0.988 | 35.1 | 76.2 | 137 | |
4.0 | 0.32 |
|
0.978 | 36.9 | 75.9 | 130 | |
5.0 | 0.44 |
|
0.993 | 26.4 | 75.3 | 163 | |
| |||||||
PEG-600 | 0.5 | 0.11 |
|
0.956 | 40.2 | 78.6 | 128 |
1.0 | 0.21 |
|
0.953 | 27.0 | 77.1 | 167 | |
2.0 | 0.25 |
|
0.981 | 25.9 | 76.6 | 169 | |
3.0 | 0.35 |
|
0.979 | 17.6 | 75.8 | 194 | |
4.0 | 0.44 |
|
0.980 | 13.0 | 68.8 | 186 | |
5.0 | 0.60 |
|
0.958 | 6.33 | 56.4 | 167 |
Activation parameters of theobromine in different PEG media.
Type of PEG | PEG % (V/V) |
|
Equation |
|
|
|
|
---|---|---|---|---|---|---|---|
kJ/mol | |||||||
PEG-200 | 0.5 | 0.21 |
|
0.999 | 43.7 | 80.0 | 111 |
1.0 | 0.23 |
|
0.999 | 49.1 | 77.0 | 93.3 | |
2.0 | 0.30 |
|
0.996 | 39.3 | 76.2 | 123 | |
3.0 | 0.32 |
|
0.999 | 38.2 | 76.0 | 126 | |
4.0 | 0.35 |
|
0.979 | 36.5 | 75.8 | 131 | |
5.0 | 0.44 |
|
0.981 | 29.4 | 75.3 | 153 | |
| |||||||
PEG-300 | 0.5 | 0.16 |
|
0.973 | 48.9 | 77.7 | 96.3 |
1.0 | 0.21 |
|
0.980 | 41.3 | 77.0 | 119 | |
2.0 | 0.23 |
|
0.987 | 43.0 | 76.6 | 112 | |
3.0 | 0.30 |
|
0.972 | 43.4 | 76.7 | 111 | |
4.0 | 0.35 |
|
0.956 | 38.4 | 75.6 | 124 | |
5.0 | 0.42 |
|
0.962 | 38.4 | 75.3 | 123 | |
| |||||||
PEG-400 | 0.5 | 0.14 |
|
0.987 | 57.5 | 78.1 | 68.8 |
1.0 | 0.21 |
|
0.953 | 50.4 | 77.0 | 88.8 | |
2.0 | 0.30 |
|
0.971 | 37.6 | 76.0 | 128 | |
3.0 | 0.35 |
|
0.972 | 43.9 | 75.7 | 106 | |
4.0 | 0.35 |
|
0.978 | 52.3 | 108 | 186 | |
5.0 | 0.42 |
|
0.997 | 47.4 | 76.1 | 93.7 | |
| |||||||
PEG-600 | 0.5 | 0.18 |
|
0.995 | 28.3 | 77.5 | 164 |
1.0 | 0.21 |
|
1.00 | 25.0 | 77.2 | 174 | |
2.0 | 0.28 |
|
0.996 | 17.2 | 76.3 | 197 | |
3.0 | 0.32 |
|
0.998 | 19.7 | 79.4 | 188 | |
4.0 | 0.42 |
|
0.963 | 14.8 | 72.4 | 192 | |
5.0 | 0.51 |
|
0.995 | 24.3 | 75.0 | 169 |
Benesi-Hildebrand plot of CAN-PEG-200.
Earlier reports on CAN oxidation studies from our laboratory and elsewhere show that a variety of CAN species such as
CAN oxidation of xanthine alkaloids in ACN medium.
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 Figure
Eyring’s plot: PEG-300 catalysed oxidation of caffeine by CAN.
The plots of
CAN oxidation mechanism in presence of PEG.
The enthalpy and entropy of activation (
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.