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Kinetics and Mechanism of Chromic Acid Oxidation of Oxalic Acid in Absence and Presence of Different Acid Media. A Kinetic Study ZAHEER KHAN, ATHAR A. HASHMI, LATEEF AHMED, M. M. HAQ Department of Chemistry, Jamia Millia Islamia, Jamia Nagar, New Delhi-110 025, India Accepted 8 August 1997
ABSTRACT: Kinetics and mechanism of the reaction of Cr(VI) with oxalic acid have been studied in presence and absence of H2SO4 , HClO4 , and CH3COOH by monitoring the formation of Cr(III)-oxalic acid complex at 560 nm. The effect of total [oxalic acid], [Cr(VI)], [H2SO4], [HClO4], and [CH3COOH] on the reaction rate was determined at 30⬚C. Formation of carbon dioxide was also confirmed. The oxidation rate increases with [oxalic acid] and [CH3COOH] while it decreases with [H2SO4], [HClO4], and pH. The rate law governing the oxidation of oxalic acid over a wide range of conditions is rate ⫽ k1 Kes1 [oxalic acid]T [Cr(VI)]T1 ⫹ Kes1 [oxalic acid]T, where only undissociated oxalic acid is kinetically active. Kinetic evidence for the formation of a Cr(VI)9 oxalic acid 1 : 1 complex has been obtained and the equilibrium constant for their formation has been determined. The 1 : 1 complex exists most likely in an open chain form. The rate-limiting step of the oxidation reaction involves the breaking of the C9 C bond in the 1 : 2 complex. Oxidizing ability of Cr(VI) species have been discussed. Mechanism with the associated reaction kinetics is assigned. 䉷 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 335– 340, 1998
INTRODUCTION Chromium(VI) is a powerful oxidant [1] having a redox potential of ⫹ 0.52 V for the couple Cr(VI)/ Cr(III). Its aqueous solution (pH ⫽ 2.0 – 7.0) is stable and has found extensive use as a catalyst in the oxidation of organic and inorganic compounds [2]. The common mechanism of the catalyzed reactions is the formation of a chromium(VI) substrate complex which decomposes in the rate-limiting step to chromium(IV) and oxidation products of the substrate. On the other hand, oxalic acid is also a strong reductant [3] redox Correspondence to: Z. Khan
䉷 1998 John Wiley & Sons, Inc.
CCC 0538-8066/98/050335-06
potential ⫽ ⫺ 0.90 V. Therefore, kinetics of oxidation and cooxidation of oxalic acid by chromium(VI) have attracted the attention of a large number of workers [4 – 7]. A cyclic intermediate (oxalyl chromate) was postulated [4] in the chromium(VI)-oxalic acid reaction mechanism, it had an equilibrium constant of 9.5 dm6 mol⫺2 in aqueous solution at 25⬚C. Raju et al. [7] studied the cooxidation of oxalic acid and arsenic(III) by chloro chromate in aqueous acetic acid and reported that the reaction proceeds through the formation of a cyclic intermediate. Though the mechanism had been given, neither pH had been mentioned at which the oxalic acid coordinate with chromium(VI) nor the effect of CH3COOH and HCl on the oxidation had been discussed.
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Various species of chromium(VI) have been reported in presence of different acids [8 – 13]. Since such environmental influences appear to effect the rate and perhaps even the mechanism of chromium(VI) oxidations. Therefore, it was worth while to carry out a systematic investigation of these effects. The present report describes the kinetics of oxidation of oxalic acid by chromium(VI) and changes that occur in the properties of the chromium(VI) species as its environment is changed by varying the identity of the acid present with it in aqueous solution. The acids were used CH3COOH, H2SO4 , and HClO4 . In addition, the oxidizing ability of these acids have been discussed.
EXPERIMENTAL Oxalic acid (Sigma), potassium dichromate (Merck), glacial acetic acid (99.5%, Merck), sulphuric acid (ca. 98%, Merck), perchloric acid (70% reagent, Merck), and acrylonitrile (Sigma) were used without further purification. Other chemicals used were of reagent grade. Water was redistilled from alkaline KMnO4 in an all-glass still. Reaction vessel containing required volumes of reactants was kept in a thermostate maintained at the desired temperature. Reaction rates were determined spectrophotometrically by monitoring oxalic acidchromium(III) complex at 560 nm on a spectronic 21D spectrophotometer. Pseudo- first-order conditions were maintained in all runs by using a large excess of oxalic acid (ⱖ ten-fold, except in some runs where [chromium(VI)] ⬎ [oxalic acid]. Rate constant, kobs, obtained from multiple determinations were within ⫾ 5% of each other. Formation of CO2 was estimated qualitatively as described elsewhere [14].
RESULT AND DISCUSSION Over the whole range of pH used in the kinetic measurement UV-VIS studies showed that the potassium dichromate solution consist a shoulder at 450 nm, characteristic of the Cr2O72⫺ ion [15]. Measurement of pH (employing a digital pH meter, Elico model LI120) showed that no significant variation in acidity occurred in the reaction mixture containing varying amounts of oxalic acid. It was also observed that pH of the working reaction mixture (just after mixing) and after the completion of the reaction remains the same as initial pH. In the presence of added acrylonitrile (20 ml v/v) reaction mixture containing 2.8 ⫻ 10⫺3 and 4.3 ⫻ 10⫺2 mol dm⫺3 of chromium(VI) and oxalic acid, respectively, shows polymer formation (white precipi-
tate) after 50 min. This indicates the free radical intervention in the reaction [16]. The stoichiometry of the reaction of oxalic acid with chromium(VI), as determined by Job’s method, was four oxalic acid per chromium(VI). This is consistent with three oxalic acid molecules providing three electrons to reduce chromium(VI) to chromium(III), it indicate that only one carboxyl group of oxalic acid participates in the redox reaction, i.e., only one-COOH group coordinates with chromium(VI). After the formation of chromium(III), one molecule of oxalic acid associated with chromium(III). Therefore, the observed stoichiometry of four is a composite value resulting from consumption of oxalic acid not only by oxidation but also by coordination with chromium(III). To identify the nature of the colored product formed in the reaction, the absorption spectra of the reaction mixture after 24 h were measured. The spectra show peaks at 425 and 560 nm. Hamm et al. [17] has reported that oxalate-chromium(III) exhibits peaks at 417 and 557 nm, provides supporting evidence for the formation of reaction products, i.e., oxalate-chromium(III) complex.
Kinetics in Absence of CH3COOH, H2SO4 , and HClO4 The observed rate constants were calculated at different [Cr(VI)]T but at constant [oxalic acid]T , [H⫹], and temperature. The rate was found to be constant with increasing amounts of [chromium(VI)]T (T ⫽ total), i.e., kobs (⫻ 105) were 4.2, 4.3, 4.1, 4.2, s⫺1 at [chromium(VI)]T values 0.0023, 0.0025, 0.0027, 0.0029, respectively. The independence of kobs at 30⬚C under the conditions [oxalic acid]T ⫽ 0.04 mol dm⫺3 is in agreement with a first-order dependence on [chromium(VI)]T. The rate law is therefore, as in eq. (1). d[complex] d[Cr(VI)]T ⫽⫺ ⫽ kobs[Cr(VI)]T dt dt
(1)
The effect of [oxalic acid]T on the reaction rate is shown in Table I. The plot of rate constants vs. [oxalic acid]T was a curve passing through the origin (Fig. 1(a)). Further, when log rate constants were plotted against log [oxalic acid]T a straight line with slope (0.85) was obtained indicating the order in oxalic acid to be fractional. The chromium(VI) oxidation of oxalic acid involves the formation of a cyclic-ester intermediate. Therefore, the presence of dianionic species of oxalic acid is essential. In order to test whether the presence of 9COOH group in the molecule of oxalic acid is
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CHROMIC ACID OXIDATION OF OXALIC ACID Table I Effect of [Oxalic Acid]T and [CH3COOH]T on the Observed Rate Constants (kobs) for the Redox Reaction between Cr(VI) and Oxalic Acid; [Cr(VI)]T ⫽ 1.0 ⫻ 10⫺3 mol dm⫺3, t ⫽ 30 oC, and [H⫹] ⫽ 1.0 ⫻ 10⫺2 mol dm⫺3 [Oxalic Acid]T/ mol dm⫺3 0.01 0.02 0.03 0.04 0.05 0.07 0.09 [oxalic acid]T/ mol dm⫺3 0.04
105kobs/s⫺1
(kobs-kcal)/(kobs)
1.2 2.2 3.0 4.2 5.1 6.4 7.4 [CH3COOH]T/ mol dm⫺3 0.0 0.5 1.0 2.5 3.7 5.0
0.00 ⫺0.04 ⫺0.10 0.00 ⫹0.01 ⫺0.04 ⫺0.04 105kobs/s⫺1 4.2 4.2 4.3 5.8 7.3 9.5
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was not observable at lower acidity (1.0 ⫻ 10⫺6 – 1.0 ⫻ 10⫺4 mol dm⫺3), even after prolonged incubation at 30⬚C. Since we found no significant reaction at short reaction times at [H⫹] ⫽ 1.0 ⫻ 10⫺3 mol dm⫺3. These results confirms that the reductant reacts with chromium(VI) at a significant rate only when it is protonated or must supply chromium(VI) with a proton [3]. Since the ka’s of oxalic acid are 5.9 ⫻ 10⫺2 and 6.4 ⫻ 10⫺5, under our experimental conditions (pH ⬍ 2.0), unionized form of oxalic acid, O O i.e., H 9 O 9 C 9 C 9 OH, is responsible for the proton supply to chromium(VI) and is kinetically active. A possible mechanism which would account for the observed kinetics involves the oxidation of oxalic acid with chromium(VI) to form a chromium(VI)-oxalic acid complex which then undergoes a unimolecular redox reaction given in Scheme I.
an equally important requirement in the oxidation reaction, we measured the oxidation of oxalic acid at different [H⫹] between 1.0 ⫻ 10⫺6 – 1.0 ⫻ 10⫺3 mol dm⫺3 at fixed [oxalic acid]T and [chromium(VI)]T at 30⬚C. The oxidation of oxalic acid
Figure 1 Effect of variation of (a) [oxalic acid]T, (b) [CH3COOH]T and acidity function of (c) H2SO4 , and (d) HClO4 on the oxidation of oxalic acid by Cr(VI); [Cr(VI)]T ⫽ 1.0 ⫻ 10⫺3, t ⫽ 30⬚C.
Scheme I
As soon as one molecule of oxalic acid enters the inner coordination sphere an open chain ester is
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formed. The second molecule of oxalic acid would make the central chromium ion more positive and thereby increase the tendency of the chromium(VI) to oxidize the coordinated substrate [18]. As a result, the subsequent coordination of oxalic acid is rapid. The interaction of second oxalic acid molecule with ester complex is not a rate-determining step. The rate law corresponding to this mechanism may be expressed as in eq. (2), using the mass balance for chromium(VI), the rate law (3) was obtained which, on Rate ⫽
d[complex] ⫽ k1ecs dt
(2)
comparison with eq. (1) given (4) d[complex] k1Kes1[oxalic acid]T[CrVI ]T ⫽ dt 1 ⫹ Kes[oxalic acid]T kobs ⫽
k1Kes1[oxalic acid]T 1 ⫹ Kes1[oxalic acid]T
(3) (4)
equation (4) can be rewritten as (5) 1 1 1 ⫽ ⫹ kobs k1Kes1[oxalic acid]T k1
(5)
Plot of 1/kobs against 1/[oxalic acid]T show the expected linear relationship (Fig. 2). The Burk – Lineweaver-type double reciprocal plot (i.e., 1/kobs vs. 1/[oxalic acid]T is linear with positive intercept, indicating the association of oxalic acid and chromium(VI) in some preequilibrium steps before the electron transfer step. A convenient way of treating
Figure 2 Plot of 1/kobs vs. 1/[oxalic acid]T. Conditions were the same as in Figure 1(a).
Figure 3 Absorption spectra of [Cr(VI)] in different [CH3COOH]; [Cr(VI)]T ⫽ (1) 1.0 ⫻ 10⫺3 mol dm⫺3; and [CH3COOH] ⫽ (2) 3.4, (3) 8.7, (4) 15.6, and (5) 19.3 mol dm⫺3.
the experiment of rate data is to calculate the values of k1 first. The constant Kesl can then easily be determined. k1 and Kesl were determined from the intercept and slope of Figure 2 and are found to be 2.0 ⫻ 10⫺4 s⫺1 and 6.66 dm⫺3 mol⫺1, respectively. These values indicate that the redox reaction is a rate-determining step.
Kinetics in Presence of CH3COOH, H2SO4 , and HClO4 The reaction was studied as a function of [CH3COOH], [H2SO4], and [HClO4] at fixed [oxalic acid]T and constant [chromium(VI)]T at 30⬚C. It is evident that the rate constant increases with [CH3COOH]T (Fig. 1(b)) and decreases with acidity functions of H2SO4 (Fig. 1(c)) and HClO4 (Fig. 1(d)). It is observed that the plot of kobs vs. [CH3COOH]T is nonlinear with a positive intercept suggesting that oxidation involves both acetic acid-dependent and acetic acid-independent paths. From Figure 1(b) results, the reaction is considered to proceed via two different acetic acid-dependent pathways. The dependence of the rate constant on [CH3COOH]T is linear at lower [CH3COOH]T. The experimental results obtained at higher [CH3COOH]T are not in agreement with this. In order to interpret the data, it was necessary to determine the active form of chromium(VI) in presence of CH3COOH, H2SO4 , and HClO4 solutions. The
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absorption spectra of mixtures containing the same chromium(VI) and CH3COOH in different molar ratios exhibited different absorption at the same max(⫽ 450 nm, Fig. 3). The absorbance decrease with increasing the [CH3COOH]T indicating that Cr2O72⫺ is O converted into HOCrO2O 9 C 9 CH3 which is a reactive and major existing species [8]. The same absorption spectra were obtained in presence of H2SO4 and HClO4 . These results are consistent with the Scheme II in the presence of CH3COOH. After the slow step, fast steps are the same as those of Scheme I.
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[CH3COOH]T at lower acetic acid concentration show the linear relationship (Fig. 1(b)) with interk K [oxalic acid]T cept A ⫽ 1 es1 . At higher 1 ⫹ Kes1 [oxalic acid]T [CH3COOH]T, of ESC1 species to the oxidation kinetics of oxalic acid may be neglected. Therefore, eq. (6) becomes
冋
kobs ⫽ A ⫹
册
k3Kes3[CH3COOH]T2[oxalic acid]T 1 ⫹ Kes2[oxalic acid]T
(7)
Under these conditions too a linear straight line will be obtained on plotting kobs vs. [CH3COOH]T2 at a constant [oxalic acid]T. This is found to be so (Fig. 4). Intercept of Figure 4 indicate that the self-oxidation (in absence of catalyst) of oxalic acid by chromium(VI) cannot be completely ruled out. Intercept (4.4 ⫻ 10⫺5) was found to be the same as those of kobs (4.2 ⫻ 10⫺5 s⫺1) at constant [oxalic acid]T (4.0 ⫻ 10⫺2 mol dm⫺3). A partial explanation of the enhanced rates of oxidation observed in acetic acid may be found in this suggestion that acetyl group will increase electron accepting power of chromium(VI) but decrease the case of protonation of the species. The spectra of chromium(VI) in CH3COOH, H2SO4 , and HClO4 are all similar [13] but the rate of oxidation of oxalic acid depend on identify and nature of these acids (Table II). The expected oxidizing ability of the chromate-ester, decreases in the order of HCrCl7 ⬎ H2CrSO7 ⬎ AcOCrO3⫺ in the presence of HClO4 , H2SO4 , and
Scheme II
Since the oxidation in absence of [CH3COOH]T may not be neglected (Fig. 1(a)), the total rate may be taken to be the sum of the rate of the two reactions, one due to the direct oxidation of oxalic acid by chromium (VI) and the other due to the faster acetyl chromate interaction with oxalic acid as shown in eq. (6).
kobs At
(k2Kes2 ⫹ k3Kes3)[CH3COOH]T [oxalic acid]T[CH3COOH]T ⫽A⫹ 1 ⫹ Kes2[oxalic acid]T
constant
[oxalic acid]T,
plot
of
kobs
(6) vs.
Figure 4 Plot showing the dependence of kobs on [CH3COOH]T2 for the oxidation of oxalic acid by Cr(VI); [Cr(VI)T ⫽ 1.0 ⫻ 10⫺3 mol dm⫺3, [oxalic acid]T ⫽ 4.0 ⫻ 10⫺2 mol dm⫺3, and t ⫽ 30⬚C.
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Table II Effect of Acidity Function of H2SO4 and HClO4 on the Observed Rate Constants for the Redox Reaction Between Cr(VI) with Oxalic Acid [Cr(VI)]T ⫽ 1.0 ⫻ 10⫺3 mol dm⫺3, [oxalic acid]T ⫽ 0.04 mol dm⫺3; t ⫽ 30⬚C Acidity Function H2SO4 ⫹0.73 ⫹0.44 ⫺0.21 ⫺2.76
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105kobs/s⫺1
⫹0.75 ⫹0.33 ⫺0.38 ⫺2.93
4.2 4.5 2.0 1.8 4.0 0.63 0.40 0.38
CH3COOH. It has also been observed by Roess Stewar et al. [13]. In the oxidation of oxalic acid, it was observed that the oxidizing ability of chromate-ester, decreases in the order AcOCrO3⫺ ⬎ H2CrSO7 ⬎ HCrClO7 . The oxidizing strength of these species may be dependent upon the nature of the reactant, i.e., oxalic acid. H2SO4 catalyzes the chromic acid oxidation of DL-serine [19] as well as inhibits the oxidation of oxalic acid (present study). These two different roles of H2SO4 , clearly indicate that the nature of the reactant (electron donating or electron withdrawing) can not be completely ruled out in determining the oxidizing ability of HCrClO7 , H2CrSO7 , and AcOCrO3⫺. The mechanism of Scheme I differs from that of other workers (who proposed the cyclic-ester as an intermediate in presence of 0.097 and 2.43 mol dm⫺3 HClO4 and 60% CH3COOH in 0.60 mol dm⫺3 chloride, pH below the pKa of oxalic acid) from the view point of formation of cyclic-ester. Under the experimental conditions of these workers, only open chainester can be an intermediate. As pointed out earlier that the coordination of both the carboxyl groups takes place only at pH ⬎ 6.0, only hydrogen of carboxyl groups are responsible for the oxidation of oxalic acid by chromium(VI).
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