In this work, kinetics and mechanism of Ru(III) catalyzed oxidation of cyclohexanone by acidified solution of potassium bromate has been studied. Present study employ mercuric acetate Hg (OAc) 2 as a scavenger for Br ¯ ion to prevent parallel oxidation by bromine. The kinetics and mechanism have also been studied in the temperature range of 30 °C - 45 °C. The reaction exhibits first order kinetics with respect to Ru (III), while zero order kinetics with respect to KB rO 3 and HClO 4. The influence of Hg(OAc) 2 and ionic strength on the rate of reaction was found to be insignificant. Positive effect in the reaction mixture was also observed upon addition of chloride ion; while as the negative effect was revealed with acetic acid. A suitable mechanism in conformity with the kinetic observations has been proposed and the rate law is derived on the basis of obtained data. The various activation parameters such as energy of activation (ΔE*), Arrhenius factor (A), entropy of activation (ΔS*) were calculated from the rate measurements at 30 °C, 35 °C - 40 °C and 45 °C.
Catalysis by transition metals plays a significant role in understanding the mechanism of redox reactions. Ruthenium(III) acts as an efficient catalyst in many redox reactions. Besides this, a number of oxidants like N-bromoacetimde (NBA) [
The solution of oxidant KBrO3 (CDH), Cyclohexanone (CDH), KCl (CDH) and perchloric acid (CDH) were prepared by dissolving its weighed sample in distilled water.
The solution of Ruthenium trichloride (Loba) was prepared in HCl of known strength.
Hg (OAc)2 (CDH) solution was prepared by dissolving it in 10 % CH3COOH solution in distilled water.
4% solution of KI (CDH) was prepared by dissolving its sample in distilled water.
5.1% starch (CDH) solution was prepared a fresh each day.
A thermo stated water bath was used to achieve and maintain the desired temperature within ±0.1˚C. Requisite volume of all reagents including substrate, were taken in a reaction vessel and temperature was maintained around 35˚C ± 0.1˚C for thermal equilibrium. Here measured volume of KBrO3 solution was poured rapidly into the reaction vessel which was also maintained separately at similar temperature. The kinetics was followed by examining desired portions of reaction mixture for KBrO3 iodometrically using starch as indicator after suitable time intervals. In all our titration experiments, micro burettes were used.
The stoichiometry of the reaction was determined by equilibrating varying ratios of [KBrO3] to cyclohexanone at 35˚C for 48 hrs. Estimation of unconsumed KBrO3 revealed that one mole of the substrate consumes two moles of the oxidant. The product analysis by conventional method [
In order to propose a probable reaction mechanism for Ru(III) catalyzed
Scheme 1. Oxidation of cyclohexanone to 1, 2-cyclohexanedione.
oxidation of cyclohexanone by acidic bromate, it is necessary to study the effect of concentration of different reactants on the rate of reaction. The kinetics of the Ru (III) catalyzed oxidation of cyclohexanone by acidic bromate was investigated at several initial reactant concentrations (
Kinetic results obtained on varying concentration of hydrogen ions indicate negligible effect of [H+] ions. Besides, an insignificant effect was observed upon variation of ionic strength of the medium. Moreover, effect on the reaction rate determined by varying the mercuric acetate concentration is also clear from kinetic data (
The above observations lead us to suggest the following reaction mechanism in the title reaction.
Now on the basis of above proposed reaction steps, and further applying steady state approximation, it yields rate law in terms of loss of concentration of potassium bromated:
The rate law is in agreement with all observed kinetics. The proposed mechanism is in consistent with the activation parameters given in
The experimental results obtained in this work revealed that the reaction rate doubles upon doubling the concentration of the catalyst [Ru(III)]. The rate law is in conformity with all kinetic observations and the proposed mechanistic steps are supported by the negligible effect of ionic strength. Negative effect of acetic acid addition signifies a positive dielectric effect. From these investigations, it is concluded that HBrO3 [
Y. Arafat highly acknowledges faculty members of department of chemistry LPU and GDC, for their support during and kind suggestions. M. A. Kaloo gratefully acknowledges the DST, New Delhi for DST-INSPIRE Faculty Award (DST/ INSPIRE/04/2016/000098).
Lone, Y.A., Kaloo, M.A. and Khaleel, F.D. (2018) Mechanistic Study of Ruthenium(III) Catalyzed Oxidation of Cyclohexanone by Acidic Bromate. Journal of Surface Engineered Materials and Advanced Technology, 8, 27-36. https://doi.org/10.4236/jsemat.2018.82003
[Substrate] × 102 mol∙dm−3 | [KBrO3] × 103 mol∙dm−3 | Ru(III) × 10−6 mol∙dm−3 | [HClO4] × 103 mol∙dm−3 | (?dc/dt) × 107 mol∙dm−3∙s−1 | k1 × 104 s−1 |
---|---|---|---|---|---|
0.33 | 1 | 96 | 1 | 1.2 | 1.29 |
0.4 | 1 | 96 | 1 | 1.38 | 1.5 |
0.5 | 1 | 96 | 1 | 1.75 | 1.94 |
0.66 | 1 | 96 | 1 | 2.28 | 2.62 |
1 | 1 | 96 | 1 | 3.4 | 4.14 |
2 | 1 | 96 | 1 | 6.66 | 10.24 |
1 | 0.83 | 96 | 1 | 3.5 | 5.14 |
1 | 1 | 96 | 1 | 3.4 | 4.14 |
1 | 1.25 | 96 | 1 | 3.5 | 3.39 |
1 | 1.66 | 96 | 1 | 3.43 | 2.48 |
1 | 2.5 | 96 | 1 | 3.33 | 1.58 |
1 | 3.33 | 96 | 1 | 3.34 | 1.19 |
1 | 1 | 9.6 | 1 | 0.4 | 0.41 |
1 | 1 | 24 | 1 | 0.92 | 0.99 |
1 | 1 | 48 | 1 | 1.75 | 1.92 |
1 | 1 | 72 | 1 | 2.6 | 2.98 |
1 | 1 | 96 | 1 | 3.4 | 4.14 |
1 | 1 | 120 | 1 | 4.3 | 5.73 |
1 | 1 | 96 | 0.83 | 3.5 | 4.26 |
1 | 1 | 96 | 1 | 3.4 | 4.14 |
1 | 1 | 96 | 1.25 | 3.42 | 4.17 |
1 | 1 | 96 | 1.66 | 3.45 | 4.2 |
1 | 1 | 96 | 2.5 | 3.55 | 4.32 |
1 | 1 | 96 | 5 | 3.33 | 4.06 |
[Ru(III)] = 96.00 × 10−6 M, [KCl] = 1.00 × 10−3 M [Hg(OAc)2] = 1.25 × 10−3 M.
[KCl] × 10−3 mol∙dm−3 | [Hg(OAc)2 | [NaClO4] | (?dc/dt) × 107 mol∙dm−3∙s−1 | k1 × 104 s−1 |
---|---|---|---|---|
× 103 mol∙dm−3 | × 103 mol∙dm−3 | |||
0.83 | 1.25 | - | 3.25 | 3.91 |
1 | 1.25 | - | 3.4 | 4.14 |
1.25 | 1.25 | - | 3.5 | 4.26 |
1.66 | 1.25 | - | 3.63 | 4.48 |
2.5 | 1.25 | - | 3.8 | 4.75 |
---|---|---|---|---|
5 | 1.25 | - | 4 | 5.06 |
1 | 0.83 | - | 3.5 | 4.26 |
1 | 1 | - | 3.33 | 4.01 |
1 | 1.25 | - | 3.4 | 4.14 |
1 | 1.66 | - | 3.6 | 4.33 |
1 | 2.5 | - | 3.4 | 4.09 |
1 | 5 | - | 3.5 | 4.21 |
1 | 1.25 | 1 | 3.5 | - |
1 | 1.25 | 2 | 3.43 | - |
1 | 1.25 | 2.5 | 3.33 | - |
1 | 1.25 | 5 | 3.5 | - |
1 | 1.25 | 10 | 3.42 | - |
1 | 1.25 | 12.5 | 3.52 | - |
[Ru(III)] = 96.00 × 10−6 M, [cyclohexanone] = 2.00 × 10−2 M [HClO4] = 1.00 × 10−3 M, [KBrO3] = 1.00 × 10−3 M.
Activation parameters | Temp/˚C | Cyclohexanone |
---|---|---|
kr × 104 s−1 | 30 | 2.81 |
kr × 104 s−1 | 35 | 4.14 |
kr × 104 s−1 | 40 | 6.59 |
kr × 104 s−1 | 45 | 9.88 |
log A | --- | 8.8 |
ΔE*(kj∙mol−1) | --- | 54.77 |
ΔG*(kj∙mol−1) | 35 | 79.62 |
ΔH*(kj∙mol−1) | 35 | 73.7 |
ΔS*(JK−1∙mol−1) | 35 | −19.21 |