Kinetics of oxidation of Pentaamminecobalt (III) complexes of α-hydroxy acids By Mn(III)Perchlorate in Micellar Medium
T. Palanisamy1, P. Rajkumar2 and K. Subramani3
1PG Department of Chemistry, H.H. The Rajah’s College (Affiliated to Bharathidasan University, Tiruchirapalli), Pudukkotai- 622001, Tamilnadu, India.
2Department of Chemistry, Priyadarshini Engineering College (Affiliated to Anna University, Chennai)
Vaniyambadi-635751, Vellore District, Tamilnadu, India.
3PG & Research Department of Chemistry, Islamiah College (Affiliated to Thiruvalluvar University, Vellore),
Vaniyambadi-635752, Vellore District, Tamilnadu, India.
*Corresponding Author E-mail: drprajkumar2014@gmail.com
ABSTRACT:
The oxidation of pentaamminecobalt(III) complexes of α-hydroxy acids and by Manganese(III) Perchlorate (Mn(III)(ClO4)3) in micellar medium yielding nearly 100% of carbonyl compounds and 100% Co(II) are ultimate products. In this reaction the rate of oxidation shows first order kinetics each in [Co(III)] and [Mn(III)(ClO4)3]. The unbound α-hydroxy acids yield about 100% of carbonyl compounds in presence of micelles. The rate of oxidation of Co(III) complexes of both bound and unbound α- hydroxy acids are enhanced more in the presence of cationic micelle Cetyltrimethylammonium bromide (CTAB), when compared to the anionic micelle of Sodium laurylsulphate (NaLS). The 1 mole of cobalt(III) complexes of α-hydroxy acids reacts with nearly 0.5 mole of Manganese(III) Perchlorate, similarly 1 mole of α-hydroxy acids reacts with nearly 1 mole of Manganese(III) Perchlorate. The reaction goes by free radical mechanism was proved by acrylonitrile polymerization. The appropriate methodology has been inducted.
KEYWORDS: Manganese(III) Perchlorate (Mn(III)(ClO4)3), α-hydroxy acids, Stoichiometry, Sodium laurylsulphate (NaLS), Cetyltrimethylammonium bromide (CTAB).
The Kinetics studies1-3 employing Manganese(III) Perchlorate (Mn(III)(ClO4)3) is an efficient reagent for oxidation of primary and secondary alcohols to carbonyl compounds. Oxidation is an important process in organic chemistry. In recent years a variety of Mn(III) Perchlorate complexes have been prepared and tested to be effective oxidants4-6. Mn(III) Perchlorate is one of them. Introduction of Mn(III)(ClO4)3 is economic and effective reagents for oxidation under mild and anhydrous conditions constitute a standing challenge. It is strong oxidizing agent. It liberates Iodine almost instantaneously from KI. The little work has been done on Mn(III)(ClO4)3 as oxidant in micellar medium.
A large class of organic compounds was oxidized by Mn(III)(ClO4)3 has been reported. Since induced electron transfer in pentaamminecobalt(III) complexes of α-hydroxy acids with various oxidants have been studied7.
The oxidation of α-hydroxy acids and their pentaamminecobalt(III) complexes using Mn(III)(ClO4)3 as an oxidant in the presence of Micelles. Induced electron transfer reactions in pentaamminecobalt(III) complexes of α-hydroxy acids result in nearly 100% reduction at cobalt(III) centre with synchronous carbon-carbon bond fission and decarboxylation8-10. Such an electron transfer route seems to be unavailable for Mn(III) Perchlorate in its reaction with cobalt(III) bound and unbound α-hydroxy acids to respective keto acid cobalt(III) complexes in Sodium laurylsulphate (NaLS)11, Cetyl trimethylammonium bromide(CTAB)12 possibly the transition state is more electron deficient. Such a transition state can be envisaged only when the C-H bond fission occurs in the slow step with hydride ion transfer. The absence of formation of cobalt(II) rules out the synchronous C-C bond fission and electron transfer to cobalt(III). The rate of Mn(III) Perchlorate oxidation of cobalt(III) complexes of α-hydroxy acids depends on the first power of Mn(III) Perchlorate concentration. Similarly the reaction between Mn(III)(ClO4)3 and unbound α-hydroxy acids exhibits first order kinetics with respect to concentration of Mn(III)(ClO4)313,14. The 1 mole of Co(III) complexes of α-hydroxy acids consumes 0.5 mole of Mn(III)(ClO4)3, whereas 1 mole of unbound α-hydroxy acids consumes 1.0 mole of Mn(III)(ClO4)3.
Experimental15
Carbonatopentaamminecobalt(III) nitrate was prepared by dissolving 58 g of powdered ammonium carbonato in 60 mL of water and 100 mL of concentration aqueous ammonia, adding a solution of 30 g of cobalt (II) nitrate. 6 hydrate in 40 mL of water and then bubbling air very slowly through the mixture (20 bubbles/min.) for 20 days. The solution was cooled to 0o and 600 mL of methanol was added slowly with stirring. The preparation was kept at 0o for 3 days, and the precipitated carbonato nitrate was filtered off. This was purified by dissolving in twice its weight of water, adding LiCl (1 g of LiCl / 2 g of complex), filtering and then slowly adding an equal volume of methanol. The solution was kept 0o for 10 hr and the crystalline complex was filtered off and dried in vacuum.
Pentaaminecobalt (III) complexes of α-hydroxy acids
The monomeric cobalt(III) complexes of mandelic acid, lactic acid and glycolic acids were prepared as their perchlorates following the procedure of Fan and Gould16.
10mmol of the α-hydroxy acids was dissolved in 20ml of methanol taken in a 50ml of R.B.flask and a pellet (0.50 to 1.00g) of NaOH was added. About 0.40g of finely powdered carbanatopentaamminecobalt(III) nitrate was added and the mixture was refluxed at 70°C for 2 hours. It was then cooled under ice for 30 minutes; about 3ml of 70% perchloric acid was added drop wise while shaking the mixture was cooled again under ice for 1 hour. The cobalt(III) complex precipitated as perchlorate and was filtered through a sintered glass crucible, washed well with ethanol followed by diethyl ether, dried and preserved in a desicator.
Preparation of Mn (III) perchlorate
10 g of Manganese (III) perchlorate kept at 0ºC was added 15 mL of 12 M HClO4. The 20 mL of a saturated solution of KMnO4 in water was added slowly with stirring until it becomes decolorized. The solution was then filtered, and the manganese (III) perchlorate in the filtrate was estimated by an iodometric procedure.
Kinetic method
All the glass apparatus were made of pyrex glass and stoppers were well ground. The loss of solvent, tested in standard flask and in reaction bottles, was found to be negligible. Burettes, pipettes and standard flasks were standardized by usual produce.
Rate measurement17
For the Manganese(III) perchlorate oxidant of Co(III) complexes of α-hydroxy acids and unbound ligands, the rate of measurements were made at 29oC in 100% aqueous medium. The standard solution prepared and required amount solutions were pipette out into a 1 cm cell. The total volume of the reaction mixture in the spectrophotometer cell was kept as 2.5 mL in each kinetic run. A UV-Visible spectrophotometer was used to follow the rate of the reaction. Rates of these unbound ligand and Co (III) bound complexes were calculated from the observed decrease in absorbance at 350 nm. For all the kinetic experiments, conversion were followed at least for four half-lives and specific rates from successive half-lives agreed with + or 7% and the average values did not differ from a plot of logarithmic change in concentration verse time calculated using integrated rate equation.
Stoichiometric studies
The stoichiometric studies for the Mn(III) Perchlorate oxidation of pentaamminecobalt (III) complexes of α-hydroxy acids and unbound ligands in the presence of micelles (Table 1) were carried out with the oxidant in excess. The [H+] and ionic strength were maintained as in the corresponding rate measurements. The temperature was maintained at 29 ± 0.2oC. After 100 hours when the reaction was nearing completion, the concentration of unreacted Mn (III) Perchlorate was determined both Iodometrically and Spectrophotometrically from the change in absorbance measured at 350 nm.
TABLE - 1
Stoichiometric data for Mn(III)(ClO4)3 Oxidation of Co(III) bound and unbound α-hydroxy acids in the presence of NaLS and CTAB.
[H2SO4] = 0.25 mol dm-3
[NaLS] = 1.00 x 10-3 mol dm-3
[CTAB] = 1.00 x 10-3 mol dm-3
Temperature = 29 ± 0.2°C
|
103[Compound] mol dm-3 |
102[Mn(III) ClO4)3]initial mol dm-3 |
102[Mn(III)(ClO4)3]final mol dm-3 |
∆103[Mn(III)(ClO4)3] mol dm-3 |
[Compound]: ∆[ Mn(III)(ClO4)3] |
|
Mandelic acid 1.0 2.0 4.0 Lactic acid 1.0 2.0 4.0 Glycolic acid 1.0 2.0 4.0 |
1.0 2.0 2.0
1.0 2.0 2.0
1.0 2.0 2.0 |
0.89 1.80 1.60
0.90 1.81 1.60
0.88 1.78 1.57 |
1.10 2.00 4.00
1.00 1.90 4.00
1.20 2.20 4.30 |
1.00 : 1.10 1.00 : 1.00 1.00 : 1.00
1.00 : 1.00 1.00 : 0.95 1.00 : 1.00
1.00 : 1.20 1.00 : 1.10 1.00 : 1.07 |
RESULTS AND DISCUSSION:
Dependence of rate on α-hydroxy acids in micellar medium:
The rate of Mn(III) Perchlorate oxidation of α-hydroxy acids had been followed under pseudo first order condition by keeping excess of the α-hydroxy acids concentration than the reagent. The rate constants were calculated by the integrated rate equation. The graph of logarithm of concentration versus time was linear and the rate constants calculated from the slope of the graph agreed with the experimental value, which shows first order dependence on [α-hydroxy acids] (Table - 2), (Figure - 1). This was further substantiated from the study of changing the concentration of α-hydroxy acids from [0.5 to 2.5] X 102 mol dm-3 at a fixed concentration in micellar medium. The rate constants obtained for the different concentration of α-hydroxy acids were nearly a constant. Hence the rate of disappearance of α-hydroxy acids in this concentration range studied is given as (Table - 3), (Figure - 2).
-d[α-hydroxy acids] / dt = k1[α-hydroxy acids] …...(1)
All the kinetic runs were repeated and the rate constants were reproducible within ± 2% range.
TABLE - 2
[Mn(III)(ClO4)3] = 0.08 mol dm-3
[H2SO4] = 0.25 mol dm-3
Temperature = 29 ± 0.2°C
L = Mandelic acid
|
Time (s) |
log(a-x) mol dm-3 |
104 k1 (s-1) |
|
300 600 900 1200 1500 1800 2100 2400 2700 3000 |
0.662 0.632 0.603 0.574 0.545 0.516 0.487 0.458 0.429 0.401 |
2.461 2.464 2.463 2.471 2.464 2.459 2.457 2.461 2.463 2.461 |
Fig - 1 First order dependence plot
TABLE – 3
[Mn(III)(ClO4)3] = 0.08 mol dm-3
[H2SO4] = 0.25 mol dm-3
[Micelles] = 1.00 x 10-3 mol dm-3
Temperature = 29 ± 0.2°C
|
102[α-hydroxy acids] mol dm-3 |
104 k1 (s-1) |
102k2dm3 mol-1s-1 |
NaLS |
CTAB |
||
|
104 k1 (s-1) |
102k2dm3 mol-1s-1 |
104 k1 (s-1) |
102k2dm3 mol-1s-1 |
|||
|
Mandelic acid 0.5 1.0 1.5 2.0 2.5 Lactic acid 0.5 1.0 1.5 2.0 2.5 Glycolic acid 0.5 1.0 1.5 2.0 2.5 |
0.802 1.712 2.601 3.625 4.623
1.025 2.236 3.336 4.469 5.589
0.723 1.604 2.502 3.402 4.302
|
0.856 1.769 2.706 3.513 4.562
1.149 2.365 3.487 4.503 4.637
0.869 1.568 2.601 3.488 4.416
|
0.974 1.980 2.881 3.855 4.929
1.233 2.483 3.680 4.920 6.241
0.796 1.763 2.647 3.554 4.601
|
0.998 1.888 2.923 3.984 4.821
1.380 2.599 3.726 4.892 6.344
0.923 1.836 2.726 3.526 4.498
|
1.088 2.033 3.054 4.099 5.001
1.400 2.798 4.254 5.659 7.005
0.956 1.927 2.848 3.820 4.751
|
1.136 2.148 3.196 4.024 5.026
1.500 2.990 4.402 5.568 7.193
0.942 1.956 2.938 3.986 4.882
|
Fig-2 Dependence of rate on [α-hydroxy acid] in NaLS
Dependence of rate on cobalt(III) complexes of α-hydroxy acids in micellar medium
The rate of Mn(III) Perchlorate oxidation of pentaamminecobalt(III) complexes of α-hydroxy acids had been followed under pseudo first order condition by keeping excess of the complex concentration than the reagent. The rate constants were calculated by the integrated rate equation. The graph of logarithm of concentration versus time was linear and the rate constants calculated from the slope of the graph agreed with the experimental value, which shows first order dependence on [(NH3)5Co(III)-L]2+]. This was further substantiated from the study of changing the concentration of pentaamminecobalt(III) complexes of α-hydroxy acids from [0.5 to 2.5] X 102 mol dm-3 at a fixed concentration in micellar medium. The rate constants obtained for the different concentration of [(NH3)5Co(III)-L]2+] complexes of α-hydroxy acids were nearly a constant. Hence the rate of disappearance of complexes in this concentration range studied is given as (Table - 4), (Figure - 3).
-d[(NH3)5Co(III)-L]2+] / dt = k1[(NH3)5Co(III)-L]2+] ..(2)
All the kinetic runs were repeated and the rate constants were reproducible within ± 2% range.
TABLE - 4
[Mn(III)(ClO4)3] = 0.08 mol dm-3
[H2SO4] = 0.25 mol dm-3
[Micelles] = 1.00 x10-3 mol dm-3
Temperature = 29 ± 0.2°C
|
102[(NH3)5Co(III) - L] mol dm-3 |
104 k1 (s-1) |
102k2dm3 mol-1s-1 |
NaLS |
CTAB |
||
|
104 k1 (s-1) |
102k2dm3 mol-1s-1 |
104 k1 (s-1) |
102k2dm3 mol-1s-1 |
|||
|
Mandelato 0.5 1.0 1.5 2.0 2.5
Lactato 0.5 1.0 1.5 2.0 2.5
Glycolato 0.5 1.0 1.5 2.0 2.5 |
1.026 2.123 3.236 4.356 5.402
1.431 2.632 3.896 5.021 6.321
0.869 1.900 2.963 4.102 5.102
|
1.123 2.016 3.360 4.469 5.506
1.526 2.763 3.990 5.147 6.178
0.926 1.963 2.999 4.206 5.214
|
1.204 2.498 3.631 4.834 5.990
1.783 3.378 4.890 6.368 7.891
1.126 2.298 3.357 4.567 5.641
|
1.368 2.567 3.563 4.987 5.891
1.670 3.276 4.906 6.287 7.704
1.283 2.325 3.206 4.987 5.418
|
1.567 3.089 4.567 6.024 7.498
1.906 3.859 5.794 7.637 9.521
1.367 2.599 3.814 5.203 6.408
|
1.678 3.189 4.609 6.193 7.590
1.897 3.894 5.367 6.736 8.209
1.490 2.678 3.908 5.199 6.000
|
Fig-3 Dependence of rate on [Co(III)] in CTAB
MECHANISM:
Mechanism of Mn(III)(ClO4)3 oxidation of pentaamminecobalt(III) complexes of both bound and unbound α-hydroxy acids in micellar medium.
Mn(III)(ClO4)3 oxidizes OH centre of the α-hydroxy acids at a rate of comparable to that of the free ligand. There is 100% reduction at the Proton centre, forms a Mn(III) Perchlorate ester which can decompose in a slow step, proceeds through C-C bond fission leading to the formation of carbonyl compounds with the evolution of carbon dioxide and H2 gas.
Considering these facts and findings with these results, the following reaction schemes has been proposed for the Mn(III)(ClO4)3 oxidation of pentaamminecobalt(III) complexes of both bound and unbound α-hydroxy acids.
CONCLUSION:
An induced electron transfer reaction has been attempted with Mn(III) Perchlorate and pentaamminecobalt(III) complexes of α-hydroxy acids in the presence of NaLS and CTAB. The reaction exhibits second order kinetics. In these reaction the rate of oxidation shows first order kinetics each in [cobalt(III)] and [Mn(III)(ClO4)3]. Product and Stoichiometric analysis were carried out for the oxidation of complexes and free ligands in two different (Anionic and Cationic) micellar medium with increasing micellar concentration an increase in the rate is observed. Mn(III)(ClO4)3 oxidizes cobalt(III) bound and unbound α-hydroxy acids through free radical. It explains the synchronous C-C bond fission, decarboxylation and electron transfer to cobalt(III) centre. The added CTAB enhances the rate of oxidation of a reaction much more than NaLS. Among two different micelles CTAB is react faster than NaLS.
A mechanism involving the one electron transfer for the complex and two electron transfer for the ligand was proposed i.e., the 1 mole of Co(III) complexes of α-hydroxy acids consumes 0.5 mole of Mn(III)(ClO4)3, whereas 1 mole of unbound α-hydroxy acids consumes 1.0 mole of Mn(III)(ClO4)3. The reaction goes by free radical mechanism was proved by acrylonitrile polymerization. The appropriate methodology has been inducted.
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Received on 06.08.2016 Modified on 19.08.2016
Accepted on 23.08.2016 © AJRC All right reserved
Asian J. Research Chem. 2016; 9(11): 553-560.
DOI: 10.5958/0974-4150.2016.00075.4