Mechanistic Study of Ruthenium(III) Catalyzed Oxidation of Cyclohexanone by Acidic Bromate

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 KBrO3 and HClO4. 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.


Introduction
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) [1] [2], N-bromosuccinimide (NBS) [3] [4], Sodium periodiate (NaIO 4 ) [5] [6] have been used incorporation with transition metal ions like Osmium(VIII), Iridium(III), Ruthenium(VIII), etc. [7] [8] [9] for oxidation of var-ious compounds.The kinetics of redox reactions involving such catalysts and oxidants has been extremely investigated.However, a scant attention has been paid towards use of KBrO 3 as an oxidant in various metal catalyzed reactions [10] [11].The utility of ruthenium(III) chloride as a homogeneous catalyst has been reported by several workers [12] [13], but scant attention has been paid to explore catalytic role of ruthenium(III) chloride with potassium bromate as an oxidant.This fact prompted us to undertake the present investigation which consists of Ru(III) catalyzed oxidation of cyclohexanone by bromate in acidic medium.

Solutions and Reagents
The solution of oxidant KBrO 5.1% starch (CDH) solution was prepared a fresh each day.

Methodology
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 KBrO 3 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 KBrO 3 iodometrically using starch as indicator after suitable time intervals.In all our titration experiments, micro burettes were used.

Stoichiometry
The stoichiometry of the reaction was determined by equilibrating varying ratios of [KBrO 3 ] to cyclohexanone at 35˚C for 48 hrs.Estimation of unconsumed KBrO 3 revealed that one mole of the substrate consumes two moles of the oxidant.The product analysis by conventional method [12] shows the formation of diketone after the reaction.The stoichimetric determination indicated the overall reaction (Scheme 1).

Results and Discussions
In order to propose a probable reaction mechanism for Ru(III) catalyzed J. Surface Engineered Materials and Advanced Technology  A1, Supplementary Information).Here, first order kinetics was observed with respect to the catalyst, Ru (III).A plot of (-dc/dt) versus Ru (III) (Figure A1, Supplementary Information) confirms its first order kinetics with respect to the catalyst.It is also confirmed by plotting a graph between 8+ log (-dc/dt) and 6+ log [Ru (III)] for oxidation of cyclohexanone at 35˚C (Figure A2, Supplementary Information).It is clear from the Table A1, (Supplementary Information) that increase in concentration of substrate resulted in the increase of (-dc/dt) values.In addition to these, graph plotted between (-dc/dt) values against (cyclohexanone) (Figure A3, Supplementary Information) gives a straight line which confirms a unity order of reaction with respect to substrate (cyclohexanone).It is also clear from the Table A1, (Supplementary Information) that upon varying the concentration of KBrO 3 , constant value of (-dc/dt) is achieved.Hence it is a zero order of reaction with respect to KBrO 3 .

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 (Table A2, Supplementary Information).As negligible effect of mercuric acetate was observed, hence it excludes the possibility of its involvement either as catalyst or as an oxidant.Thus it acts as a scavenger [14] [15] for any bromide ions formed in the reaction.Similarly, addition of chloride ions to the reaction mixture influences the velocity of this reaction, and also possesses positive effect.Apart from Cl − , addition of acetic acid shows negative effect on the rate of reaction.With the help of rate measurements around 30˚C -45˚C, specific rate constants were used to plot log (-dc/dt) versus 1/T (Figure A4, Supplementary Information), which came to be linear.The values of energy of activation (∆E*) and free energy of activation (∆G*), were calculated from the rate measurements at 30˚, 35˚, 40˚ and 45˚C, and the corresponding values have been mentioned in the Table A3 (Supplementary Information).From the literature, it is well established that Ru(III) chloride gives a number of possible chloro species, and their existence is totally dependent on pH of the medium.
And it also well reported that Ru(III) exists in an equilibrium under the experimental pH range 10 -12 [16].Besides these, the data obtained data under the mentioned conditions indicate that addition of chloride ion has a probable effect on the reaction velocity.

Conclusion
The experimental results obtained in this work revealed that the reaction rate J. Surface Engineered Materials and Advanced Technology 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 HBrO 3 [13] and [RuCl 6 ] −3 are the reactive species of KBrO 3 and Ru(III) chloride respectively in acidic medium.

Supplementary Information
3 (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 % CH 3 COOH solution in distilled water.4% solution of KI (CDH) was prepared by dissolving its sample in distilled water.
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 TableA1(Supplementary Information).The high positive values of change in free energy of activation (∆G*) indicates highly solvated transition state, while fairly high negative values of change in entropy of activation (∆S*) suggest the formation of an activated complex with reduction in the degree of freedom of molecules.

Table A1 .
Effect of variation of reactants on the reaction rate.

Table A2 .
Effect of variation of chloride ion, mercury (II) acetate and sodium perchlorate at 35˚C.

Table A3 .
Activation parameters for Ru(III) catalyzed oxidation of cyclohexanone by acidic bromated.