Ruthenium(III) Catalysis in Perborate Oxidation of 5-Oxoacids

Ruthenium(III) catalyzes perborate oxidation of substituted 5-oxoacids in acidic solution. The catalyzed oxidation is first order with respect to the oxidant and catalyst. The rate of ruthenium(III) catalyzed oxidation displays the Michaelis-Menten kinetics on the reductant and is independent of [H] of the medium. Hydrogen peroxide is the reactive species of perborate and the kinetic results reveal formation of ruthenium(III) peroxo species-5-oxoacid complex. Electron-releasing substituents accelerate the reaction rate and electron-withdrawing substituents retard it. The order of reactivity among the studied 5-oxoacids is p-methoxy ≫ p-methyl > p-phenyl > −H > p-chloro > p-bromo >m-nitro. Activation parameters are evaluated using Arrhenius and Eyring’s plots. A mechanism consistent with the observed kinetic data is proposed and discussed. A suitable rate law is derived based on the mechanism.


Introduction
Sodium perborate (NaBO 3 ⋅4H 2 O) is a cheap and environment friendly large-scale industrial chemical used extensively in detergents as a bleaching agent. It is a mild oxidant, and the search for suitable catalysts for perborate oxidation is of interest. It is a convenient source of hydrogen peroxide [1], commercially, industrially, and also in the laboratory. Proton magnetic resonance (PMR) spectral analysis [2] and X-ray diffraction studies [3] conclude perborate as a true peroxysalt with water of crystallisation. It is an effective reagent in organic synthesis, and acetic acid is the solvent of choice [4]. Perborate in aqueous solution yields hydrogen peroxide and the kinetic studies in aqueous and partly aqueous acidic media confirm perborate oxidation as hydrogen peroxide oxidation [5,6]. This stable and easily handled crystalline substance oxidizes organic sulphides [7], anilines [8], and indole [9]. This communication reports ruthenium(III) catalysis in perborate oxidation of 5-oxoacids. Vanadium(V) also catalyzes perborate oxidation of 5-oxoacids [10].
In 5-oxoacids and their phenyl substituted compounds, two carbon atoms separate the carbonyl and the carboxyl groups, and so they behave both as oxocompounds and as acids without the direct influence of the other group. In acidic medium, 5-oxoacids undergo enolization, and on selective oxidation, they result in malonic and substituted benzoic acids, which find widespread applications in synthetic organic chemistry. Hence, 5-oxoacids are attractive substrates in terms of their mechanistic aspects. 5-oxocid is an attractive substrate in terms of its enolization. In strong acid medium the substrate undergoes enolization. The reactive species of the substrate is reported in the literature to be the enol form [11,12]. Studies of the oxidation of various organic compounds by perborate have attracted considerable attention. A thorough literature survey reveals that relatively little work on the oxidation of oxoacids have been reported so far [13,14]. Although the perborate oxidations of organic compounds have been studied, there seems to be no report on a systematic kinetic study of the oxidation of 5-oxoacids by perborate, and we report here for the first time the kinetics and mechanism of ruthenium(III) catalysis in perborate oxidation of substituted and unsubstituted 5-oxoacids. The various unsubstituted and substituted 5-oxoacids (S1-S7) employed in the present study are listed in Scheme 1.

Experimental
2.1. Materials. Sodium perborate, NaBO 3 ⋅4H 2 O (Riedel), and ferrous sulphate (Merck) were used as received. Other chemicals were of AR grade. Analytical grade acetic acid (BDH) was refluxed for 6 h over chromium(VI) oxide and distilled through a column. Solutions of perborate were prepared freshly and standardized iodometrically. Kinetics of the oxidation in aqueous sulphuric acid at constant temperature was studied iodometrically under pseudo-first order conditions with a very large excess of 5-oxoacids. Doubledistilled water (conductivity < 10 S⋅cm −1 ) was employed in all kinetic runs. All the chemicals used were 99.8% pure. The parent 5-oxoacid, namely, 5-oxo-5-phenylpentanoic acid (S1), and the phenyl-substituted 5-oxoacids (S2-S7) were prepared by Friedel-Crafts acylation of the substituted benzene with glutaric anhydride [15][16][17][18][19]. All the 5-oxoacids were crystallized twice from water and their purity was checked by their melting points and UV, IR, and NMR spectra. All absorption measurements were made with Shimadzu UV-visible spectrophotometer (MPS-5000) equipped with a temperature controller.

Kinetic
Measurements. The reaction mixture, containing 5-oxoacid, catalyst, and sulphuric acid solutions, was thermally equilibrated and the reaction was initiated by the addition of temperature-equilibrated perborate solution of requisite concentration. The oxidation kinetics was followed iodometrically in aqueous acetic acid at constant temperature under pseudo-first order conditions by keeping the substrate in excess over the oxidant. The pseudo-first order rate constant was calculated from the slope of the linear plot of log [perborate] against time by the method of least squares. The error quoted in * is the 95% confidence limit of a student's t-test. The progress of the oxidations was followed by iodometric determination of the oxidant. Freshly prepared solutions of oxoacids in purified acetic acid were used to avoid any possible side reactions. The products were extracted with ether, dried, and analyzed. Benzoic acid was identified by its melting point (121 ∘ C). Then it was estimated quantitatively using UV-visible spectrophotometry with a standard curve at max = 235 nm. Succinic acid was identified by its melting point (185 ∘ C) and also tested with its characteristic spot test [20]. Identification of the products, namely, benzoic and succinic acids, was also made by comparing the values of the authentic samples.

Effect of Catalyst.
The oxidation is catalyzed by ruthenium(III) as well. The specific rate of oxidation in perborate increases with the increasing [Ru(III)] ( Table 2) ) is the specific rate of ruthenium(III) catalyzed oxidation, and the ruthenium(III) catalyzed oxidation is first order with respect to the catalyst.

Influence of Salt Effect and Solvent
At low and high acidities, the ruthenium(III) catalyzed oxidation rate increases with the decreasing dielectric constant of the reaction medium; the dielectric constant was decreased by the addition of acetic acid (Table 6).

Effect of Hydrogen Peroxide.
Under identical conditions, the rates of ruthenium(III) catalyzed oxidation of 5-oxoacids by perborate and hydrogen peroxide are almost the same (Table 7).

Influence of Metaborate and Orthoboric Acid.
Boric acid or borate neither retards nor enhances the oxidation. Initial addition of metaborate or boric acid to the reaction solution has insignificant effect on the rates of oxidation by perborate and hydrogen peroxide ( Table 8). The reaction solution, when the oxidation is in progress, does not induce polymerization of vinyl monomer acrylonitrile. Also, initial addition of acrylonitrile to the reaction mixture fails to suppress the oxidation rate (Table 8). During the course of oxidation, the reaction solution does not show an ESR signal (Bruker Xband) for any radical.  Table 9.

Isokinetic Relationships.
The plot between ΔH # and ΔS # is linear ( Figure 5; ≥ 0.97; ≤ 0.029) and the isokinetic temperature ( ) obtained is 355.9 K. The calculated from Exner's plot ( Figure 6; ≥ 0.987; ≤ 0.04) of log 318 K against log 308 K is 353.7 K, which is in good agreement with the value obtained from the ΔH # against ΔS # plot. The isokinetic relationship in the present study implies that all the 5-oxoacids undergo oxidation by the same mechanism [21].

Active Species of Reactants.
The oxoacid is a weak acid ( = 5.77 at 40 ∘ C in aqueous solution) [22], and the undissociated form of the substrate can be taken as the only form in acidic media. In acid solutions, 5-oxoacid undergoes keto-enol tautomerism (Scheme 3).
In crystalline state sodium perborate exists as a dimer with anionic formula: B 2 (O 2 ) 2 (OH) 4 2− . But in aqueous solution it affords hydrogen peroxide [1,23]. Although perboric acid is reported to exist in equilibrium with hydrogen peroxide, the equilibrium constant reveals that, even in the presence of large excess of boric acid, the concentration of perboric acid is insignificant [24]. ruthenium(III) followed by the reduction of ruthenium(III) by 5-oxoacids. Chemical tests confirm the presence of ruthenium(III) and the absence of ruthenium(III) in the reaction solution when the oxidation is in progress. Formation of the Michaelis-Menten type complex between ruthenium(III) and 5-oxoacid is also unlikely.
Another possible mechanism is of the Fenton type. But this mechanism is also ruled out as the oxidation is not through radical pathway. ESR spectral study of the reaction solution, while the oxidation is in progress, does not show the presence of any radical. The ruthenium(III) catalyzed oxidation is insensitive to the addition of vinyl monomer and the reaction solution fails to initiate polymerization of acrylonitrile. Although Ru 2 O 2 are suggested as intermediates in ruthenium(III) catalyzed decomposition of hydrogen peroxide they are not considered as the active oxidizing species in the present work [25]. If they were to be the oxidizing species, contrary to the experimental findings, the oxidation should be second order with respect to the catalyst. In acidic solution hydrogen peroxide affords ruthenium(III) peroxo species [26,27]. This species contains ruthenium and peroxide in the ratio 1 : 1 and is formulated as Ru(O 2 H) 2+ . The following mechanism accounts for the observed kinetic results (Scheme 4).

Rate Law. The rate law for the catalyzed oxidation is
(1 + 2 [5-oxoacid]) . (1) The specific rate in perborate of the catalyzed oxidation is 8 Advances in Physical Chemistry

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Advances in Physical Chemistry Table 9: Values of rate constant at different temperatures and activation parameters a .

5-Oxoacid
The rate law is compatible with the experimental results, namely, first order with respect to [perborate], zero order in [H + ], the Michaelis-Menten type dependence on [5oxoacid], almost the same rates of oxidation by perborate and hydrogen peroxide under identical conditions, the absence of enhancement or inhibition of perborate, hydrogen peroxide oxidations by borate or boric acid, and so forth.

Conclusions
Ruthenium(III) catalyzed perborate oxidation of substituted 5-oxoacids in acidic solution is associated with isokinetic relationship. [perborate] 0 ≪ [5-oxoacid] 0 , the oxidation is first order with respect to the oxidant and catalyst. The rate of catalyzed oxidation displays the Michaelis-Menten kinetics on the reductant and is independent of [H + ] of the medium. Hydrogen peroxide is the reactive species of perborate, and the kinetic results reveal formation of ruthenium(III) peroxo species−5-oxoacid complex. The variation of oxidation rate with ionic strength of the medium is quite small at low [H + ] but appreciable at high [H + ]. The oxidation rate increases with decreasing dielectric constant of the medium at low and high acidities. The rates of ruthenium(III) catalyzed perborate and hydrogen peroxide oxidations are almost the same. Borate and boric acid do not influence the oxidations. Electron-releasing substituents accelerate the reaction rate and electron-withdrawing substituents retard the reaction. The order of reactivity among the studied 5-oxoacids is pmethoxy ≫ p-methyl > p-phenyl > -H > p-chloro > pbromo > m-nitro. Activation parameters have been evaluated using Arrhenius and Eyring's plots. A mechanism consistent with the observed kinetic data is proposed and discussed. A suitable rate law is derived based on the mechanism. The experimental protocol suggests that this reaction could find utility as a regioselective route for the synthesis of carboxylic acids, specially substituted benzoic acids.