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Effect of Mn(II) and Ce(IV) Ions on the Oxidation of Lactic Acid by Chromic Acid
ACTA PHYSICO-CHIMICA SINICA Volume 23, Issue 7, July 2007 Online English edition of the Chinese language journal
Cite this article as: Acta Phys. -Chim. Sin., 2007, 23(7): 1013−1017.
ARTICLE
Effect of Mn(II) and Ce(IV) Ions on the Oxidation of Lactic Acid by Chromic Acid Maqsood Ahmad Malik,
Zaheer Khan*
Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India
Abstract:
The kinetics and mechanism of lactic acid oxidation in the presence of Mn(II) and Ce(IV) ions by chromic acid were
studied spectrophotometrically. The oxidation of lactic acid by Cr(VI) was found to proceed in two measurable steps, both of which gave pyruvic acid as the primary product in the absence of Mn(II) . 2Cr(VI)+2CH3CHOHCOOH→2CH3COCOOH+Cr(V)+Cr(III) Cr(V)+CH3CHOHCOOH→Cr(III)+CH3COCOOH The observed kinetics was explained due to the catalytic and inhibitory effects of Mn(II) and Ce(IV) on the lactic acid oxidation by Cr(VI). The reactivity of lactic acid depends upon the experimental conditions. It acts as a two- or three-equivalent reducing agent in the absence or presence of Mn(II). It was examined that Cr(III) products resulting from the direct reduction of Cr(VI) by three-equivalent reducing agents. The oxidation of lactic acid follows the complex order kinetics with respect to [lactic acid]. The activation parameters Ea, ∆H#, and ∆S# were calculated and discussed. Key Words:
Kinetics; Lactic acid; Cr(VI); Ce(IV); Mn(II); Oxidation
Cr(IV) is a reactive intermediate in most systems involving reduction of Cr(VI) to Cr(III). The oxidation of oxalic acid[1], cooxidation of alcohols with oxalic acid and α-hydroxy acid[2−4], and the oxidation of several α-hydroxy acids by acid chromate ion (HCrO4−)[5,6] are reported to involve initial three-electron reductions of HCrO4− by substrate (S), producing radicals (R•). The radicals reduce additional Cr(VI) to Cr(V), giving products (P): Cr(VI)+2S→CrS2→Cr(III)+P+R• (1) R•+Cr(VI)→Cr(V)+P (fast step) (2) Cr(V)+S→Cr(III)+P (3) Chromic acid has been long and successfully used as an oxidizing agent, for both preparative and analytical purposes. The procedures underlying the stoichiometric equations (4) and (5). HCrO4−+3Fe2++7H+→Cr3++3Fe3++4H2O (4) 2HCrO4−+6I−+14H+→2Cr3++3I2+8H2O (5) Eqs. (4) and (5) are among the classic methods of quantitative analysis.
As a preparative reagent[7] chromic acid is customarily used, in aqueous sulfuric acid or in acetic acid solution, to oxidize primary alcohols or aldehydes to acids and to oxidize secondary alcohols to ketones. Chromic acid in acetic acid oxidizes toluene to benzoic acid, ethylbenzene to acetophenone, triphenylmethane to triphenylcarbinol, fluorene to fluorenone, and chrysene to chrysoquinone. Chromic anhydride, dissolved in acetic anhydride and sulfuric acid, oxidizes o-xylene to the tetraacetate of o-phthalaldehyde. Etard′s reagent (chromyl chloride, CrO2Cl2) oxidizes substituted toluenes to the corresponding substituted benzaldehydes. The involvement of Cr(IV), formed as an intermediate in the redox reaction of Cr(VI) with different organic and inorganic reducants, has generated much debate in the past. As a result, it has been accepted that the one- and two-electron oxidation of different substrates by chromic acid must proceed through the formation of Cr(IV) and Cr(V) species[8]. The aqua complex of Cr(IV) has generally been considered to be highly reactive and unstable[9]. However, Espenson et al.[10]
Received: March 13, 2007; Revised: April 9, 2007. * Corresponding author. Email: [email protected]. Copyright © 2007, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Maqsood Ahmad Malik et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 1013−1017
have described the methods of preparation of aqueous Cr(IV) in the absence of stabilizing ligands. The reactivity of Cr(IV) in various possible reactions, i.e. disproportionation, oxidation, reduction, and coordination, have been discussed by Haight et al.[11]. Available information indicates that Cr(IV) is a strong oxidant, which can react rapidly with any available reducing agent[8−12]. Mn(II) plays an important role (catalysis[13] or inhibition[14,15]) in chromic acid oxidation, where it has been recognized as a frequently used tool to determine the involvement of Cr(IV) as an intermediate[16]. Recently, we examined the catalytic role of Mn(II) in the oxidation of organic acids by chromic acid[17]. This role of Mn(II), contrary to its general effects (i.e. inhibition), is to trap[11,14] the intermediate Cr(IV). Cr(IV)+Mn(II)→Cr(III)+Mn(III) (6) Ce(IV)+Cr(IV)→Ce(III)+Cr(V) (7) On the other hand, the pioneering work of Beattie and Haight[14] suggests that the direct oxidation of Mn(II) by Cr(VI) is thermodynamically unfavorable. The inhibition effect arises from competition between the organic substrates and Mn(II) for Cr(IV)[18]. The present study was undertaken to verify the role of Mn(II) and Ce(IV) on oxidation of lactic acid by Cr(VI).
1 1.1
Experimental Materials
Lactic acid (99%, s.d. fine), HgCl2 (99%, Merck, India), K2Cr2O7 (99%, Merck, India) and Mn(II) (97%, Loba) were used as supplied and their stock solutions were prepared in double distilled (first time from alkaline KMnO4), deionized and CO2 free water. K2Cr2O7 was finely ground and dried for 2 h at 383 K and its solution was restandardized frequently. 1.2
Kinetic measurements
All kinetic measurements were carried out on a Bausch & Lomb Spectronic-20D Spectrophotometer. The progress of the reaction was followed at 350 nm by monitoring the changes in absorbance of remaining Cr(VI). All measurements were made at 306 K at an ionic strength of 0.96 mol·L−1 (NaClO4). The required [lactic acid], [HClO4], [NaClO4], and [complexing agents] were premixed in a reaction vessel, thermostated in an oil bath, and K2Cr2O7 solution (thermally equilibrated) was then added prior to the absorbance measurements. Under pseudo-first-order conditions of lactic acid, the plots of lgA versus time (A is absorbance intensity) were linear up to 80% completion of the reaction with an average of linear regression coefficients, r≥0.994. 1.3
Product identification
In a typical experiment, lactic acid, Cr(VI) and/or Mn(II) were mixed at temperature of 306 K in a reaction flask. The
solution was kept in dark until completion of the reaction. The reaction mixture was then treated with an excess of a freshly saturated solution of 2,4-dinitrophenylhydrazine in 2 mol·L−1 HCl. The precipitate was filtered, washed, and dried. The procedure was repeated with known concentrations of pyruvic acid and acetaldehyde under the same conditions. The most useful method to identify 2,4-dinitrophenylhydrazone was by infra red spectrum comparison with authentic samples[12]. Pyruvic acid and acetaldehyde were found as the oxidation products of lactic acid in the absence and presence of Mn(II), respectively.
2
Results and discussion
2.1
[oxidant] dependence
The values of rate constant (kobs) were calculated at various [Cr(VI)] (0.20−1.00 mmol·L−1), constant [lactic acid] (0.96 mol·L−1), and temperature (306 K). The kobs (Table 1) was found to be independent of the initial [Cr(VI)] and the reaction follows a pseudo-first order rate law, i.e. v=−d[Cr(VI)]/dt =kobs[Cr(VI)]T (8) The order with respect to lactic acid lies between one and two (the plot of lgkobs versus lg[lactic acid] is linear with a slope of 1.50) (Fig.1). The catalytic effects of Mn(II) clearly reflect the involvement of this species somewhere before the rate determining step. 2.2
Oxidation of lactic acid
This study using lactic acid as substrate was originally undertaken to fill in the series oxalic acid ((COOH)2), glycolic acid (CH2OHCOOH), lactic acid (CH3CHOHCOOH), and 2-hydroxy-2-methylpropionic acid ((CH3)2COHCOOH). Studies of the chromic acid oxidation of oxalic, lactic, and malic acids have shown that the kinetics of these reactions is extremely complex. First and foremost, there are large differences in reaction products. Oxalic acid, as expected, is oxidized to carbon dioxide, but lactic acid is oxidized not at the Table 1 Effect of varying [Cr(VI)] and [lactic acid] on the pseudofirst-order rate constants for the oxidation of lactic acid by Cr(VI) in the absence (presence) of Mn(II) at 306 K 104[Cr(VI)]/(mol·L−1) 2.0 4.0 6.0 8.0 10.0 10.0
[lactic acid]/(mol·L−1) 0.96
0.24 0.48 0.72 0.11 0.12 0.14
104kobs/s−1 9.7(17.3) 9.6(17.4) 9.7(17.5) 9.8(17.3) 9.7(17.6) 0.5(0.5) 1.5(4.6) 3.0(10.1) 15.2(24.1) 19.1(26.4) 21.4(32.0)
The values obtained in the presence of Mn(II) (1.00 mmol·L−1) are given in parentheses.
Maqsood Ahmad Malik et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 1013−1017
Fig.2
Fig.1 Plot of lgkobs versus lg[lactic acid]
Arrhenius plots for the oxidation of lactic acid by Cr(VI) in the presence of Mn(II) (A), Ce(IV) (B) and in the
−1
reaction conditions: [Cr(VI)]=1.00 mmol·L ; T=306 K
absence of complexing agents (C) [lactic acid]=0.96 mol·L−1; [Cr(VI)]=1.00 mmol·L−1;
[19]
hydroxyl but at the carboxyl group : 8H++2HCrO−4 +3CH3CHOHCOOH→ 3CH3CHO+3CO2+2Cr3++8H2O (9) Reaction (9) probably does not involve an oxidation of lactic acid to pyruvic acid, followed by a decarboxylation; for pyruvic acid, under the experimental conditions in question, undergoes further oxidation much more rapidly than it undergoes decarboxylation. Presumably the lactic acid is attacked at the carboxyl group. Although the nature of the products obtained by chromic acid oxidation from other hydroxy acids has not been fully determined, reactions analogous to reaction (9) are probably not infrequent. The rate constant for the oxidation in sulfuric acid solution is much greater than that for the thermal decomposition in the same solution[20,21]. Hence, the oxidation must clearly involve a direct attack of the oxidizing agent upon the organic acid (or ion). Furthermore, the rates of oxidation of both oxalic and lactic acids are considerably increased by the addition of small amounts of manganous ion[22]. Lactic acid shows inhibitory character with Ce(IV) (reaction rate decreases with increasing [Ce(IV)]).
represent a single elementary kinetic step; it is a function of true rate, binding and ionization constants.
2.3
3
Effect of temperature
The effect of temperature on the reaction rate was studied in the presence and absence of complexing agents. The Arrhenius plots of lgkobs versus 1/T were linear (Fig.2). Table 2 shows the temperature dependence of the catalyzed and uncatalyzed oxidations of lactic acid; a comparison of Ea values of the uncatalyzed with that in the catalyzed (effect of Mn(II)) indicates that Mn(II) acts as catalysts providing a new reaction path with lower value of Ea. The large decrease in ∆S# shows that the transition state is well structured. However, the magnitude of ∆S# is not significantly affected, showing that the same mechanism is followed in the absence and presence of complexing agents. The more negative ∆S# supports the view that the rate-limiting step consists in the formation of an intermediate complex and does not involve the breaking of a bond. A meaningful mechanistic explanation of the apparent values of ∆S# and ∆H# is not possible because the kobs does not
[Mn(II)]=1.00 mmol·L−1; [Ce(IV)]=0.20 mmol·L−1
Table 2 Value of pseudo-first-order rate constants and activation parameters for the oxidation of lactic acid by Cr(VI) in the absence and presence of complexing agents and metal ions T/K 104kobs/s−1
Absence
with Mn(II)
with Ce(IV)
306
9.7
17.3
7.5
313
19.5
29.9
12.5
318
26.2
32.4
14.4
323
30.7
35.3
22.2
∆Ea/(kJ·mol−1)
48
37
45
∆H#/(kJ·mol−1)
45
34
42
∆S#/(J·mol−1·K−1)
−286
−285
−289
∆G#/(kJ· mol−1)
134
120
132
[lactic acid]=0.96 mol·L−1; [Cr(VI)]=1.00 mmol·L−1; [Mn(II)]=1.00 mmol·L−1; [Ce(IV)]=0.20 mmol·L−1
Mechanism
Consistent with all the experimental data obtained for the oxidation of lactic acid by Cr(VI), we assume that the reaction proceeds by two routes: (i) A one step two-electron oxidation (C−H bond cleavage); (ii) A direct one-step three-electron oxidation (C−C bond cleavage). A probable mechanism (Scheme 1) is considered to explain the observed kinetic results.
Scheme 1 A probable mechanism
In order to conform the path (i), i.e. involvement of Cr(IV) as an intermediate, the effects of Ce(IV) and Mn(II) were evaluated under the same kinetic conditions of lactic acid and
Maqsood Ahmad Malik et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 1013−1017
Cr(VI) (Tables 3 and 4). The reaction rate shows a decrease with increasing the [Ce(IV)] (Fig.3). A similar effect of Ce(IV) on chromic acid oxidation of isopropyl alcohol was observed[23]. On the other hand, the rate increased markedly with increasing [Mn(II)] (Fig.4). The positive catalytic effect is due to one-step three-electron oxidation of lactic acid which is in conformity with the cleavage of C−C bond and also for the reduction of Cr(VI) to Cr(III) as a final product[24]. This effect is contrary to the general effect of Mn(II), i.e. inhibition[25]. On the basis of the above results, the following mechanisms are proposed. Fig.4
Dependence of kobs on [Mn(II)]
Table 3 Values of pseudo-first-order rate constant and role of Ce(IV)
reaction conditions: [Cr(VI)]=1.00 mmol·L−1;
and Mn(II) on the oxidation of lactic acid by Cr(VI) at 306 K
[lactic acid]=0.96 mol·L−1; T=306 K
104kobs/s−1 Role of complexing agent/metal ions
Reaction conditions Cr(VI)+lactic acid
a
In the presence of Ce(IV):
9.7
Cr(VI)+metal ionb
no reaction
Ce(IV)+lactic acid
no reaction
Ce(IV)+Mn(II)
no reaction
Cr(VI)+lactic acid+Mn(II)
17.3
catalysis
Cr(VI)+lactic acid+Ce(IV)
6.1
inhibition
[Cr(VI)]=1.00 mmol·L−1, [lactic acid]= 0.96 mol·L−1; b[Ce(IV)]=1.60 mol·L−1,
a
[Mn(II)]=1.00 mmol·L−1; The same [Cr(VI)], [lactic acid], [Mn(II)], and [Ce(IV)] were used in all cases.
Table 4 Effect of [Mn(II)] and [Ce(IV)] on the pseudo-first-order rate constants for the oxidation of lactic acid by Cr(VI)a at 306 K 4
−1
4
−1
10 [Mn(II)]/(mol·L ) 10 kobs/s
4
−1
4
In the presence of Mn(II):
−1
10 [Ce(IV)]/(mol·L ) 10 kobs/s
0.0
9.7
0.0
5.0
15.0
0.4
9.7 9.7
10.0
17.3
1.2
8.0
20.0
20.0
2.0
7.5
30.0
23.0
4.0
6.1
40.0
25.5
6.0
6.1
8.0
6.1
[lactic acid]=0.96 mol·L−1; [Cr(VI)]=1.00 mmol·L−1
a
Reactions (13) and (14) present the actual role of Ce(IV) and Mn(II), i.e. trapping of Cr(IV) intermediate and involvement of Mn(II) before the rate determining step, respectively. The most important aspect of reactions (14) to (16) is a C−C cleavage in lactic acid oxidation. Thus, we may safely conclude that both types of bond cleavage (C−H and C−C) are possible in a single organic substrate molecule if there functional groups are present at appropriate positions[26]. Fig.3
Dependence of kobs on [Ce(IV)]
reaction conditions: [Cr(VI)]=1.00 mmol·L−1; [lactic acid]=0.96 mol·L−1; T=306 K
4
Rate law
According to reactions (10) to (16), the following rate equations have been derived in the absence (Eq.(17)) and in the
Maqsood Ahmad Malik et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 1013−1017
presence (Eq.(18)) of Mn(II). kobs=
k1K es1[lactic acid] 1 + K p [H + ] + K es1[lactic acid]
5 Hasan, F.; Rocek, J. J. Am. Chem. Soc., 1975, 97: 1444
(17)
k2 K es2 K c2[lactic acid][Mn(II)] (18) 1 + K es2 [lactic acid] + K p [H + ] It is apparent that 1>>Kp[H+], therefore, equations (17) and (18) reduce to the final rate equations (19) and (20). k K [lactic acid] kobs= 1 es1 (19) 1 + K es1[lactic acid] kobsi=
kobsi=
5
k2 K es2 K c2[lactic acid][Mn(II)] 1 + K es2 [lactic acid]
(20)
Conclusions
The catalytic effect of Mn(II) on the oxidation of lactic acid by Cr(VI) was examined. It is shown that lactic acid acts as a two- or three-equivalent reducing agent. Cr(VI)- and Mn(II)lactic acid complexes, formed in situ, are considered to be the active catalytic species of oxidant and reductant, respectively. Ce(IV) inhibits the reaction rate.
6 Krumpolc, M.; Rocek, J. J. Am. Chem. Soc., 1977, 99: 137 7 Stoermer, R. In: Houben-Weyl′s arbeitsmethoden der organischen chemie. Vol.2. Leipaig: Georg Thieme, 1922: 3 8 Beattie, J. K.; Haight, Jr. G. P. Progr. Inorg. Chem., 1972, 17: 93 9 Kemp, T. J. In: Bonford, C. H.; Tipper. C. F. H. Eds. Comprehensive chemical kinetics. New York: Elsevier, 1972: 274 10 Scott, S. L.; Bakac, A.; Espenson, J. H. J. Am. Chem. Soc., 1991, 113: 7787; 1992, 114: 4205 11 Haight, Jr. G. P.; Huang, T. J.; Shakhashiri, B. Z. J. Inorg. Nucl. Chem., 1971, 33: 2169 12 Rahman, M.; Rocek, J. J. Am. Chem. Soc., 1971, 93: 5462 13 Beckwith, F. B.; Waters, W. A. J. Chem. Soc. (B), 1969: 929 14 Beattie, J. K.; Haight, Jr. G. P. In: Edwards, I. O. Ed. Inorganic reaction mechanism. Part 11. New York: Willy, 1972: 93−145 15 Perez-Benito, J. F.; Arias, C. Can. J. Chem., 1993, 71: 649 16 Srinivasan, C.; Chellamani, A.; Rajagopal, S. J. Org. Chem., 1985, 50: 1201 17 Kabir-ud-Din; Hartani, K.; Khan, Z. Transition Metal Chem., 2000, 25: 478 18 Arias, C.; Perez-Benito, J. F. Coll. Czech. Chem. Commun., 1992,
Acknowledgement The authors are thankful to the department of chemistry, Jamia Millia Islamia, New Delhi for providing all the facilities.
57: 1821 19 Chapmane, T.; Smithm, H. J. Chem. Soc., 1867, 20:173 20 Bredig, G.; Lichty, D. M. Z. Elektrochem., 1906, 12: 459 21 Lichty, D. M. J. Phys. Chem., 1907, 11: 225
References 1 (a) Hasan, F.; Rocek, J. J. Am. Chem. Soc., 1972, 94: 9073 (b) Hasan, F.; Rocek, J. Tetrahedron, 1974, 30: 21 2 Hasan, F.; Rocek, J. J. Am. Chem. Soc., 1972, 94: 8946 3 Hasan, F.; Rocek, J. J. Am. Chem. Soc., 1973, 95: 5421 4 Hasan, F.; Rocek, J. J. Am. Chem. Soc., 1972, 94: 3181
22 Dey, A. N.; Dhar, N. R. Z. Elektrochem., 1926, 32: 586. 23 Doyle, M.; Swedo, R. J.; Rocek, J. J. Am. Chem. Soc., 1973, 95: 8352 24 Huber, C. F.; Haight, Jr. G. P. J. Am. Chem. Soc., 1976, 98: 4128 25 Srinivasan, K. G.; Rocek, J. J. Am. Chem. Soc., 1978, 100: 2789 26 Dominic, Ip.; Rocek, J. J. Am. Chem. Soc., 1979, 101: 6311
Cite this article as: Acta Phys. -Chim. Sin., 2007, 23(7): 1013−1017.
ARTICLE
Effect of Mn(II) and Ce(IV) Ions on the Oxidation of Lactic Acid by Chromic Acid Maqsood Ahmad Malik,
Zaheer Khan*
Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India
Abstract:
The kinetics and mechanism of lactic acid oxidation in the presence of Mn(II) and Ce(IV) ions by chromic acid were
studied spectrophotometrically. The oxidation of lactic acid by Cr(VI) was found to proceed in two measurable steps, both of which gave pyruvic acid as the primary product in the absence of Mn(II) . 2Cr(VI)+2CH3CHOHCOOH→2CH3COCOOH+Cr(V)+Cr(III) Cr(V)+CH3CHOHCOOH→Cr(III)+CH3COCOOH The observed kinetics was explained due to the catalytic and inhibitory effects of Mn(II) and Ce(IV) on the lactic acid oxidation by Cr(VI). The reactivity of lactic acid depends upon the experimental conditions. It acts as a two- or three-equivalent reducing agent in the absence or presence of Mn(II). It was examined that Cr(III) products resulting from the direct reduction of Cr(VI) by three-equivalent reducing agents. The oxidation of lactic acid follows the complex order kinetics with respect to [lactic acid]. The activation parameters Ea, ∆H#, and ∆S# were calculated and discussed. Key Words:
Kinetics; Lactic acid; Cr(VI); Ce(IV); Mn(II); Oxidation
Cr(IV) is a reactive intermediate in most systems involving reduction of Cr(VI) to Cr(III). The oxidation of oxalic acid[1], cooxidation of alcohols with oxalic acid and α-hydroxy acid[2−4], and the oxidation of several α-hydroxy acids by acid chromate ion (HCrO4−)[5,6] are reported to involve initial three-electron reductions of HCrO4− by substrate (S), producing radicals (R•). The radicals reduce additional Cr(VI) to Cr(V), giving products (P): Cr(VI)+2S→CrS2→Cr(III)+P+R• (1) R•+Cr(VI)→Cr(V)+P (fast step) (2) Cr(V)+S→Cr(III)+P (3) Chromic acid has been long and successfully used as an oxidizing agent, for both preparative and analytical purposes. The procedures underlying the stoichiometric equations (4) and (5). HCrO4−+3Fe2++7H+→Cr3++3Fe3++4H2O (4) 2HCrO4−+6I−+14H+→2Cr3++3I2+8H2O (5) Eqs. (4) and (5) are among the classic methods of quantitative analysis.
As a preparative reagent[7] chromic acid is customarily used, in aqueous sulfuric acid or in acetic acid solution, to oxidize primary alcohols or aldehydes to acids and to oxidize secondary alcohols to ketones. Chromic acid in acetic acid oxidizes toluene to benzoic acid, ethylbenzene to acetophenone, triphenylmethane to triphenylcarbinol, fluorene to fluorenone, and chrysene to chrysoquinone. Chromic anhydride, dissolved in acetic anhydride and sulfuric acid, oxidizes o-xylene to the tetraacetate of o-phthalaldehyde. Etard′s reagent (chromyl chloride, CrO2Cl2) oxidizes substituted toluenes to the corresponding substituted benzaldehydes. The involvement of Cr(IV), formed as an intermediate in the redox reaction of Cr(VI) with different organic and inorganic reducants, has generated much debate in the past. As a result, it has been accepted that the one- and two-electron oxidation of different substrates by chromic acid must proceed through the formation of Cr(IV) and Cr(V) species[8]. The aqua complex of Cr(IV) has generally been considered to be highly reactive and unstable[9]. However, Espenson et al.[10]
Received: March 13, 2007; Revised: April 9, 2007. * Corresponding author. Email: [email protected]. Copyright © 2007, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Maqsood Ahmad Malik et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 1013−1017
have described the methods of preparation of aqueous Cr(IV) in the absence of stabilizing ligands. The reactivity of Cr(IV) in various possible reactions, i.e. disproportionation, oxidation, reduction, and coordination, have been discussed by Haight et al.[11]. Available information indicates that Cr(IV) is a strong oxidant, which can react rapidly with any available reducing agent[8−12]. Mn(II) plays an important role (catalysis[13] or inhibition[14,15]) in chromic acid oxidation, where it has been recognized as a frequently used tool to determine the involvement of Cr(IV) as an intermediate[16]. Recently, we examined the catalytic role of Mn(II) in the oxidation of organic acids by chromic acid[17]. This role of Mn(II), contrary to its general effects (i.e. inhibition), is to trap[11,14] the intermediate Cr(IV). Cr(IV)+Mn(II)→Cr(III)+Mn(III) (6) Ce(IV)+Cr(IV)→Ce(III)+Cr(V) (7) On the other hand, the pioneering work of Beattie and Haight[14] suggests that the direct oxidation of Mn(II) by Cr(VI) is thermodynamically unfavorable. The inhibition effect arises from competition between the organic substrates and Mn(II) for Cr(IV)[18]. The present study was undertaken to verify the role of Mn(II) and Ce(IV) on oxidation of lactic acid by Cr(VI).
1 1.1
Experimental Materials
Lactic acid (99%, s.d. fine), HgCl2 (99%, Merck, India), K2Cr2O7 (99%, Merck, India) and Mn(II) (97%, Loba) were used as supplied and their stock solutions were prepared in double distilled (first time from alkaline KMnO4), deionized and CO2 free water. K2Cr2O7 was finely ground and dried for 2 h at 383 K and its solution was restandardized frequently. 1.2
Kinetic measurements
All kinetic measurements were carried out on a Bausch & Lomb Spectronic-20D Spectrophotometer. The progress of the reaction was followed at 350 nm by monitoring the changes in absorbance of remaining Cr(VI). All measurements were made at 306 K at an ionic strength of 0.96 mol·L−1 (NaClO4). The required [lactic acid], [HClO4], [NaClO4], and [complexing agents] were premixed in a reaction vessel, thermostated in an oil bath, and K2Cr2O7 solution (thermally equilibrated) was then added prior to the absorbance measurements. Under pseudo-first-order conditions of lactic acid, the plots of lgA versus time (A is absorbance intensity) were linear up to 80% completion of the reaction with an average of linear regression coefficients, r≥0.994. 1.3
Product identification
In a typical experiment, lactic acid, Cr(VI) and/or Mn(II) were mixed at temperature of 306 K in a reaction flask. The
solution was kept in dark until completion of the reaction. The reaction mixture was then treated with an excess of a freshly saturated solution of 2,4-dinitrophenylhydrazine in 2 mol·L−1 HCl. The precipitate was filtered, washed, and dried. The procedure was repeated with known concentrations of pyruvic acid and acetaldehyde under the same conditions. The most useful method to identify 2,4-dinitrophenylhydrazone was by infra red spectrum comparison with authentic samples[12]. Pyruvic acid and acetaldehyde were found as the oxidation products of lactic acid in the absence and presence of Mn(II), respectively.
2
Results and discussion
2.1
[oxidant] dependence
The values of rate constant (kobs) were calculated at various [Cr(VI)] (0.20−1.00 mmol·L−1), constant [lactic acid] (0.96 mol·L−1), and temperature (306 K). The kobs (Table 1) was found to be independent of the initial [Cr(VI)] and the reaction follows a pseudo-first order rate law, i.e. v=−d[Cr(VI)]/dt =kobs[Cr(VI)]T (8) The order with respect to lactic acid lies between one and two (the plot of lgkobs versus lg[lactic acid] is linear with a slope of 1.50) (Fig.1). The catalytic effects of Mn(II) clearly reflect the involvement of this species somewhere before the rate determining step. 2.2
Oxidation of lactic acid
This study using lactic acid as substrate was originally undertaken to fill in the series oxalic acid ((COOH)2), glycolic acid (CH2OHCOOH), lactic acid (CH3CHOHCOOH), and 2-hydroxy-2-methylpropionic acid ((CH3)2COHCOOH). Studies of the chromic acid oxidation of oxalic, lactic, and malic acids have shown that the kinetics of these reactions is extremely complex. First and foremost, there are large differences in reaction products. Oxalic acid, as expected, is oxidized to carbon dioxide, but lactic acid is oxidized not at the Table 1 Effect of varying [Cr(VI)] and [lactic acid] on the pseudofirst-order rate constants for the oxidation of lactic acid by Cr(VI) in the absence (presence) of Mn(II) at 306 K 104[Cr(VI)]/(mol·L−1) 2.0 4.0 6.0 8.0 10.0 10.0
[lactic acid]/(mol·L−1) 0.96
0.24 0.48 0.72 0.11 0.12 0.14
104kobs/s−1 9.7(17.3) 9.6(17.4) 9.7(17.5) 9.8(17.3) 9.7(17.6) 0.5(0.5) 1.5(4.6) 3.0(10.1) 15.2(24.1) 19.1(26.4) 21.4(32.0)
The values obtained in the presence of Mn(II) (1.00 mmol·L−1) are given in parentheses.
Maqsood Ahmad Malik et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 1013−1017
Fig.2
Fig.1 Plot of lgkobs versus lg[lactic acid]
Arrhenius plots for the oxidation of lactic acid by Cr(VI) in the presence of Mn(II) (A), Ce(IV) (B) and in the
−1
reaction conditions: [Cr(VI)]=1.00 mmol·L ; T=306 K
absence of complexing agents (C) [lactic acid]=0.96 mol·L−1; [Cr(VI)]=1.00 mmol·L−1;
[19]
hydroxyl but at the carboxyl group : 8H++2HCrO−4 +3CH3CHOHCOOH→ 3CH3CHO+3CO2+2Cr3++8H2O (9) Reaction (9) probably does not involve an oxidation of lactic acid to pyruvic acid, followed by a decarboxylation; for pyruvic acid, under the experimental conditions in question, undergoes further oxidation much more rapidly than it undergoes decarboxylation. Presumably the lactic acid is attacked at the carboxyl group. Although the nature of the products obtained by chromic acid oxidation from other hydroxy acids has not been fully determined, reactions analogous to reaction (9) are probably not infrequent. The rate constant for the oxidation in sulfuric acid solution is much greater than that for the thermal decomposition in the same solution[20,21]. Hence, the oxidation must clearly involve a direct attack of the oxidizing agent upon the organic acid (or ion). Furthermore, the rates of oxidation of both oxalic and lactic acids are considerably increased by the addition of small amounts of manganous ion[22]. Lactic acid shows inhibitory character with Ce(IV) (reaction rate decreases with increasing [Ce(IV)]).
represent a single elementary kinetic step; it is a function of true rate, binding and ionization constants.
2.3
3
Effect of temperature
The effect of temperature on the reaction rate was studied in the presence and absence of complexing agents. The Arrhenius plots of lgkobs versus 1/T were linear (Fig.2). Table 2 shows the temperature dependence of the catalyzed and uncatalyzed oxidations of lactic acid; a comparison of Ea values of the uncatalyzed with that in the catalyzed (effect of Mn(II)) indicates that Mn(II) acts as catalysts providing a new reaction path with lower value of Ea. The large decrease in ∆S# shows that the transition state is well structured. However, the magnitude of ∆S# is not significantly affected, showing that the same mechanism is followed in the absence and presence of complexing agents. The more negative ∆S# supports the view that the rate-limiting step consists in the formation of an intermediate complex and does not involve the breaking of a bond. A meaningful mechanistic explanation of the apparent values of ∆S# and ∆H# is not possible because the kobs does not
[Mn(II)]=1.00 mmol·L−1; [Ce(IV)]=0.20 mmol·L−1
Table 2 Value of pseudo-first-order rate constants and activation parameters for the oxidation of lactic acid by Cr(VI) in the absence and presence of complexing agents and metal ions T/K 104kobs/s−1
Absence
with Mn(II)
with Ce(IV)
306
9.7
17.3
7.5
313
19.5
29.9
12.5
318
26.2
32.4
14.4
323
30.7
35.3
22.2
∆Ea/(kJ·mol−1)
48
37
45
∆H#/(kJ·mol−1)
45
34
42
∆S#/(J·mol−1·K−1)
−286
−285
−289
∆G#/(kJ· mol−1)
134
120
132
[lactic acid]=0.96 mol·L−1; [Cr(VI)]=1.00 mmol·L−1; [Mn(II)]=1.00 mmol·L−1; [Ce(IV)]=0.20 mmol·L−1
Mechanism
Consistent with all the experimental data obtained for the oxidation of lactic acid by Cr(VI), we assume that the reaction proceeds by two routes: (i) A one step two-electron oxidation (C−H bond cleavage); (ii) A direct one-step three-electron oxidation (C−C bond cleavage). A probable mechanism (Scheme 1) is considered to explain the observed kinetic results.
Scheme 1 A probable mechanism
In order to conform the path (i), i.e. involvement of Cr(IV) as an intermediate, the effects of Ce(IV) and Mn(II) were evaluated under the same kinetic conditions of lactic acid and
Maqsood Ahmad Malik et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 1013−1017
Cr(VI) (Tables 3 and 4). The reaction rate shows a decrease with increasing the [Ce(IV)] (Fig.3). A similar effect of Ce(IV) on chromic acid oxidation of isopropyl alcohol was observed[23]. On the other hand, the rate increased markedly with increasing [Mn(II)] (Fig.4). The positive catalytic effect is due to one-step three-electron oxidation of lactic acid which is in conformity with the cleavage of C−C bond and also for the reduction of Cr(VI) to Cr(III) as a final product[24]. This effect is contrary to the general effect of Mn(II), i.e. inhibition[25]. On the basis of the above results, the following mechanisms are proposed. Fig.4
Dependence of kobs on [Mn(II)]
Table 3 Values of pseudo-first-order rate constant and role of Ce(IV)
reaction conditions: [Cr(VI)]=1.00 mmol·L−1;
and Mn(II) on the oxidation of lactic acid by Cr(VI) at 306 K
[lactic acid]=0.96 mol·L−1; T=306 K
104kobs/s−1 Role of complexing agent/metal ions
Reaction conditions Cr(VI)+lactic acid
a
In the presence of Ce(IV):
9.7
Cr(VI)+metal ionb
no reaction
Ce(IV)+lactic acid
no reaction
Ce(IV)+Mn(II)
no reaction
Cr(VI)+lactic acid+Mn(II)
17.3
catalysis
Cr(VI)+lactic acid+Ce(IV)
6.1
inhibition
[Cr(VI)]=1.00 mmol·L−1, [lactic acid]= 0.96 mol·L−1; b[Ce(IV)]=1.60 mol·L−1,
a
[Mn(II)]=1.00 mmol·L−1; The same [Cr(VI)], [lactic acid], [Mn(II)], and [Ce(IV)] were used in all cases.
Table 4 Effect of [Mn(II)] and [Ce(IV)] on the pseudo-first-order rate constants for the oxidation of lactic acid by Cr(VI)a at 306 K 4
−1
4
−1
10 [Mn(II)]/(mol·L ) 10 kobs/s
4
−1
4
In the presence of Mn(II):
−1
10 [Ce(IV)]/(mol·L ) 10 kobs/s
0.0
9.7
0.0
5.0
15.0
0.4
9.7 9.7
10.0
17.3
1.2
8.0
20.0
20.0
2.0
7.5
30.0
23.0
4.0
6.1
40.0
25.5
6.0
6.1
8.0
6.1
[lactic acid]=0.96 mol·L−1; [Cr(VI)]=1.00 mmol·L−1
a
Reactions (13) and (14) present the actual role of Ce(IV) and Mn(II), i.e. trapping of Cr(IV) intermediate and involvement of Mn(II) before the rate determining step, respectively. The most important aspect of reactions (14) to (16) is a C−C cleavage in lactic acid oxidation. Thus, we may safely conclude that both types of bond cleavage (C−H and C−C) are possible in a single organic substrate molecule if there functional groups are present at appropriate positions[26]. Fig.3
Dependence of kobs on [Ce(IV)]
reaction conditions: [Cr(VI)]=1.00 mmol·L−1; [lactic acid]=0.96 mol·L−1; T=306 K
4
Rate law
According to reactions (10) to (16), the following rate equations have been derived in the absence (Eq.(17)) and in the
Maqsood Ahmad Malik et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 1013−1017
presence (Eq.(18)) of Mn(II). kobs=
k1K es1[lactic acid] 1 + K p [H + ] + K es1[lactic acid]
5 Hasan, F.; Rocek, J. J. Am. Chem. Soc., 1975, 97: 1444
(17)
k2 K es2 K c2[lactic acid][Mn(II)] (18) 1 + K es2 [lactic acid] + K p [H + ] It is apparent that 1>>Kp[H+], therefore, equations (17) and (18) reduce to the final rate equations (19) and (20). k K [lactic acid] kobs= 1 es1 (19) 1 + K es1[lactic acid] kobsi=
kobsi=
5
k2 K es2 K c2[lactic acid][Mn(II)] 1 + K es2 [lactic acid]
(20)
Conclusions
The catalytic effect of Mn(II) on the oxidation of lactic acid by Cr(VI) was examined. It is shown that lactic acid acts as a two- or three-equivalent reducing agent. Cr(VI)- and Mn(II)lactic acid complexes, formed in situ, are considered to be the active catalytic species of oxidant and reductant, respectively. Ce(IV) inhibits the reaction rate.
6 Krumpolc, M.; Rocek, J. J. Am. Chem. Soc., 1977, 99: 137 7 Stoermer, R. In: Houben-Weyl′s arbeitsmethoden der organischen chemie. Vol.2. Leipaig: Georg Thieme, 1922: 3 8 Beattie, J. K.; Haight, Jr. G. P. Progr. Inorg. Chem., 1972, 17: 93 9 Kemp, T. J. In: Bonford, C. H.; Tipper. C. F. H. Eds. Comprehensive chemical kinetics. New York: Elsevier, 1972: 274 10 Scott, S. L.; Bakac, A.; Espenson, J. H. J. Am. Chem. Soc., 1991, 113: 7787; 1992, 114: 4205 11 Haight, Jr. G. P.; Huang, T. J.; Shakhashiri, B. Z. J. Inorg. Nucl. Chem., 1971, 33: 2169 12 Rahman, M.; Rocek, J. J. Am. Chem. Soc., 1971, 93: 5462 13 Beckwith, F. B.; Waters, W. A. J. Chem. Soc. (B), 1969: 929 14 Beattie, J. K.; Haight, Jr. G. P. In: Edwards, I. O. Ed. Inorganic reaction mechanism. Part 11. New York: Willy, 1972: 93−145 15 Perez-Benito, J. F.; Arias, C. Can. J. Chem., 1993, 71: 649 16 Srinivasan, C.; Chellamani, A.; Rajagopal, S. J. Org. Chem., 1985, 50: 1201 17 Kabir-ud-Din; Hartani, K.; Khan, Z. Transition Metal Chem., 2000, 25: 478 18 Arias, C.; Perez-Benito, J. F. Coll. Czech. Chem. Commun., 1992,
Acknowledgement The authors are thankful to the department of chemistry, Jamia Millia Islamia, New Delhi for providing all the facilities.
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22 Dey, A. N.; Dhar, N. R. Z. Elektrochem., 1926, 32: 586. 23 Doyle, M.; Swedo, R. J.; Rocek, J. J. Am. Chem. Soc., 1973, 95: 8352 24 Huber, C. F.; Haight, Jr. G. P. J. Am. Chem. Soc., 1976, 98: 4128 25 Srinivasan, K. G.; Rocek, J. J. Am. Chem. Soc., 1978, 100: 2789 26 Dominic, Ip.; Rocek, J. J. Am. Chem. Soc., 1979, 101: 6311