Kinetics and mechanism of oxidation of substituted 5-oxoacids by sodium perborate in aqueous acetic acid medium have been studied. The reaction exhibits first order both in [perborate] and [5-oxoacid] and second order in [H+]. Variation in ionic strength has no effect on the reaction rate, while the reaction rates are enhanced on lowering the dielectric constant of the reaction medium. Electron releasing substituents in the aromatic ring accelerate the reaction rate and electron withdrawing substituents retard the reaction. The order of reactivity among the studied 5-oxoacids is
Sodium perborate (NaBO3·4H2O) is a nontoxic cheap large scale industrial chemical primarily used as a source of “active oxygen” in detergents and as a mild antiseptic. This active oxygen has the oxidising properties of hydrogen peroxide. PMR spectral analysis [
5-Oxoacid is an attractive substrate in terms of its enolization. In strong acid medium the substrate undergoes enolization. The reactive species of the substrate has been reported in the literature to be the enol form [
In recent years, 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 oxoacid has been reported so far [
Sodium perborate, NaBO3·4H2O (Riedel), was used as received. Acetic acid (BDH) was refluxed for 6 h over chromium (VI) oxide and distilled through a column. Aqueous solutions of perborate were prepared as and when required and standardized iodometrically. Sodium metaborate and perchloric acid were prepared in double distilled water. Double distilled water (conductivity < 10
All absorption measurements were made with Shimadzu UV-visible spectrophotometer (MPS-5000) equipped with a temperature controller. Regression analysis of experimental data yielded the regression coefficient (
The reaction mixture, containing 5-oxoacid 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 in aqueous acetic acid at constant temperature by measuring the concentration of benzoic acid formed iodometrically under pseudo-first order conditions by keeping the substrate in excess over the oxidant. The pseudo-first-order rate constant (
The stoichiometry of the reaction was determined by equilibrating reaction mixture of various [perborate]/[5-oxoacid] ratios at 313 K for 12 h, keeping all other reagents constant. Estimation of unconsumed perborate (iodometrically) revealed that one mole of 5-oxoacid consumed one mole of perborate:
The products were extracted with ether, dried, and analyzed. Benzoic acid was identified by its melting point (121°C). Then it has been estimated quantitatively using UV-Vis spectrophotometry with a standard curve at
At fixed concentrations of acid and substrate, the decrease in the concentration of perborate is followed first order kinetics. The first order rate constants (
Typical first order plots in the perborate oxidation of 5-oxoacids.
The reactions of all the studied 5-oxoacids are first order in the [substrate] (Table
Effect of varying [substrate] on the rate of perborate oxidation of 5-oxoacids at 313 ± 0.1 K. {[Perborate] = 0.001 mol dm−3; [H2SO4] = 2.0 mol mol−3; HOAc-H2O = 1 : 1% (v/v)}.
[5-oxo acid] (mol dm−3) |
| ||||||
---|---|---|---|---|---|---|---|
|
|
|
−H |
|
|
|
|
0.005 | 95.91 | 13.44 | 9.43 | 6.36 | 2.14 | 1.77 | 0.77 |
0.01 | 191.9 | 26.85 | 21.67 | 12.78 | 4.25 | 2.15 | 1.52 |
0.015 | 287.9 | 40.33 | 29.98 | 19.20 | 6.40 | 4.43 | 2.34 |
0.02 | 383.7 | 53.71 | 36.71 | 25.59 | 8.51 | 5.74 | 3.05 |
0.03 | 575.8 | 80.59 | 60.76 | 38.14 | 12.78 | 8.92 | 4.56 |
0.04 | 767.7 | 107.5 | 124.91 | 51.13 | 17.15 | 12.53 | 6.05 |
0.06 | 1152 | 161.3 | 142.24 | 76.75 | 25.59 | 17.53 | 9.10 |
Effect of varying [H2SO4] on the rate of perborate oxidation of 5-oxoacids at 313 ± 0.1 K. {[Substrate] = 0.01 mol dm−3; [perborate] = 0.001 mol dm−3; HOAc-H2O = 1 : 1% (v/v)}.
[H2SO4] (mol dm−3) | −Ho |
| ||||||
---|---|---|---|---|---|---|---|---|
|
|
|
−H |
|
|
|
||
0.51 | −0.13 | 11.89 | 2.31 | 1.87 | 1.58 | — | — | — |
0.75 | 0.07 | 29.93 | 4.38 | 3.28 | 2.88 | — | — | — |
1.01 | 0.26 | 47.98 | 7.68 | 5.19 | 4.52 | 1.27 | 1.21 | 1.18 |
1.49 | 0.56 | 95.96 | 15.98 | 12.67 | 8.84 | 2.32 | 1.98 | 1.26 |
2.01 | 0.84 | 191.9 | 26.87 | 19.28 | 12.78 | 4.26 | 3.09 | 1.52 |
2.49 | 1.12 | 287.9 | 47.98 | 38.69 | 25.59 | 7.19 | 5.16 | 2.57 |
3.00 | 1.38 | 479.8 | 134.3 | 79.81 | 34.85 | 12.79 | 7.28 | 3.97 |
4.01 | 1.85 | 1151 | 287.8 | 140.63 | 57.58 | 25.58 | 16.89 | 7.69 |
|
2.01 | 2.01 | 2.0 | 2.00 | 2.01 | 2.0 | 2.00 | |
|
1.3 | 1.0 | 1.0 | 1.0 | 1.1 | 1.1 | 1.0 |
The effect of dielectric constant (
Effect of varying solvent composition on the rate of perborate oxidation of 5-oxoacids at 313 ± 0.1 K. {[Substrate] = 0.01 mol dm−3; [perborate] = 0.001 mol dm−3; [H2SO4] = 2.0 mol mol−3}.
ACOH-H2O (v/v |
| ||||||
---|---|---|---|---|---|---|---|
|
|
|
−H |
|
|
|
|
30 : 70 (53.18) | 63.8 | 9.50 | 8.67 | 7.60 | 2.38 | 1.97 | 1.18 |
40 : 60 (46.48) | 115.2 | 13.10 | 11.86 | 9.60 | 3.10 | 2.12 | 1.29 |
50 : 50 (39.78) | 191.8 | 26.87 | 18.73 | 12.80 | 4.27 | 3.62 | 1.50 |
60 : 40 (33.08) | 586 | 44.70 | 31.01 | 15.90 | 7.20 | 4.89 | 1.75 |
70 : 30 (26.38) | 1751 | 162.9 | 71.23 | 38.30 | 12.80 | 7.61 | 2.69 |
There was no induced polymerization of acrylonitrile monomer, ruling out the possibility of free radical formation during the course of the reaction.
In the temperature range of 298–323 K in 2.0 mol dm−3 sulphuric acid and acetic acid-water medium (50%
Arrhenius plots of
Plot of
The effect of substituents on the rate of oxidation was studied using different phenyl substituted 5-oxoacids (ArCOCH2CH2CH2COOH, where Ar = 4′-methoxy, 4′-methyl, 4′-phenyl, 4′-chloro, 4′-bromo, 3′-nitro substituted phenyl) at different temperatures. In all these cases, the reaction orders are the same, namely, first order with respect to [oxidant] and [substrate] and second order with respect to [acid]. Electron releasing substituents in the phenyl ring enhance the rate of oxidation and electron withdrawing substituents decrease it (Table
Effect of substituent on the rate of oxidation of 5-oxoacids by perborate at 313 ± 0.1 K. {[Substrate] = 0.01 mol dm−3; [perborate] = 0.001 mol dm−3; [H2SO4] = 2.0 mol mol−3; HOAC-H2O = 1 : 1% v/v}.
Substituent |
|
|
|
5 + log |
---|---|---|---|---|
|
−0.27 | −0.78 | 191.8 | 2.2829 |
|
−0.17 | −0.31 | 26.9 | 1.4293 |
p-C6H5 | −0.11 | −0.22 | 18.8 | 1.2623 |
−H | 0.00 | 0.00 | 12.8 | 1.1070 |
|
0.23 | 0.11 | 4.3 | 0.6299 |
|
0.43 | 0.38 | 2.7 | 0.3470 |
|
0.71 | 0.67 | 1.5 | 0.1802 |
Hammett plots of
A reaction series which exhibits a common point of intersection in the Hammett plot (Figure
In aqueous acetic acid the possible oxidizing species of perborate are perborate anion, perboric acid, peracetic acid, and H2O2. Even though isolation of free perboric acid has been proved exceedingly difficult, it has demonstrable existence in solution [
The oxoacid is a weak acid (
In oxidation reactions, the keto group of the substrate can react either directly or through the enol form. Oxidation rates faster than the rates of enolization have been observed with ceric ion [
The rates of oxidation and enolization were found to be equal in the oxidation reactions by manganic pyrophosphate [
In the present study, the rate of enolization (measured by the bromination method [
Formations of benzoic acid and succinic acid were the final products of oxidation.
The proposed mechanism is also in accordance with the observed stoichiometry. The rate equation in consonance with the mechanism proposed is as given in the following equation:
The above study shows that the
The authors declare that there is no conflict of interests regarding the publication of this paper.