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Kinetics and Mechanism of Deamination and Decarboxylation of 2-Aminopentanedioic Acid by Quinolinium Dichromate (QDC) in Aqueous Perchloric Acid Medium Aftab Aslam Parwaz Khan, Ayaz Mohd, Shaista Bano, and K. S. Siddiqi* Department of Chemistry, Aligarh Muslim University, Aligarh, 202002, Uttar Pradesh, India ABSTRACT: Kinetic data for the oxidation of 2-aminopentanedioic acid/L-glutamic acid (Glu-e) by quinolinium dichromate (QDC) in perchloric acid medium are reported. The results of the reactions studied spectrophotometrically over a wide range of experimental conditions show that the rate of the reaction is increased by increases in perchloric acid, the ionic strength, and the concentration of Glu-e. The reactions are first order with respect to both QDC and Glu-e. Added products are found to have an insignificant effect on the reaction rate. The activation parameters and thermodynamics have been evaluated and lend further support to the proposed mechanism.
1. INTRODUCTION Oxidation of amino acids is of great importance both from a chemical viewpoint and from its bearing on the mechanism of amino acid metabolism. Amino acids not only act as building blocks in protein synthesis but also play a significant role in the metabolism. They are subjected to many reactions and can supply precursors for many endogenous substances such as hemoglobin in blood. They can undergo many kinds of reactions, depending on whether a particular amino acid contains a polar or nonpolar subtituent. 2-Aminopentanedioic acid/L-Glutamic acid (Glu-e) is one of the most abundant amino acids, especially high in cereal proteins, and can be oxidized by different oxidants.14 A salt of Glu-e is used in the production of monosodium glutamate and nutritional supplements. L-Glutamate itself can be used as a medicine and promotes the oxidation process. It combines with ammonia as a drug-free glutamine. It is mainly used for the treatment of hepatic coma and severe liver dysfunction, but the response is not satisfactory. Racemic glutamate is used for the production of drugs. Physiologically, it plays a role in the metabolism of amino groups and is the precursor of the neurotransmitter γ-aminobutyric acid, L-glutamic acid. Being acidic in nature, it is important in determining three-dimensional conformations of proteins. It is well established that the reduction of chromium(VI) to chromium(III) with a variety of organic and inorganic reductants can occur by a multiplicity of mechanisms which depend on the nature of the reducing agent.5 The existence of different species of chromium(VI) in acid solutions, unstable oxidation states [chromium(IV) and chromium(V)], and the tendency of chromium(III) to form a variety of complexes all combine to give systems of considerable complexity.6 Attempts have been made to confirm the intermediacy of chromium(IV) and chromium(V) by the use of competitive experiments.7,8 In acid solution, the reported reduction potential of the Cr(VI)/Cr(III) couple is 1.33 V.9 The reagent quinolinium dichromate (QDC) is a versatile oxidant that deserves further investigation, and some kinetic studies of the oxidation of inorganic substrates by QDC are available.10 A number of reports on the mechanism of oxidation of r 2011 American Chemical Society
several substrates by QDC are available. QDC is shown to oxidize primary and secondary alcohols to the corresponding aldehydes11,12 and cyclic alcohols to cyclic ketones,13 bicyclic alcohols,14 and benzyl alcohol.15 The R-hydroxyacids16 and Rketoacids17 are oxidized with QDC, and their reactions are studied kinetically. QDC oxidizes cinnamic and crotonic acids smoothly in N,N-dimethylformamide in the presence of an acid to give aldehydes.1827 Since there are no reports on the kinetics of oxidation of 2-aminopentanedioic acid by QDC, we are reporting the kinetics of its oxidation by QDC in perchloric acid, in order to identify the chromium(VI) intermediate and to propose a suitable mechanism. The activation parameters and thermodynamic quantities have been determined and discussed.
2. EXPERIMENTAL DETAILS 2.1. Chemicals and Solutions. Stock solutions of L-glutamic acid (Merck, Mumbai, India), QDC (Sigma-Aldrich), and chromium(III) potassium sulfate [Cr2(SO4)3 3 K2SO4 3 24H2O] (BDH, AnalaR) were prepared in double-distilled water and standardized iodometrically.12 HClO4 and NaClO4 were employed to maintain the required acidity and ionic strength, respectively. 2.2. Instruments Used. A Shimadzu UVvisible 1601 spectrophotometer, an Elico-LI 120 pH meter, and a water bath shaker NSW 133, India, were used. IR spectra were recorded with a Fourier transfer infrared (FT-IR; 5700-Nicolet) spectrometer. 2.3. Kinetic Studies. The reaction was initiated by mixing the previously thermostatted solutions of Glu-e and QDC, which also contained the required amounts of perchloric acid, sodium perchlorate, and double-distilled water. The reaction was followed spectrophotometrically at 360 nm. The spectral changes during the reaction under standard conditions at room Received: April 10, 2010 Accepted: July 17, 2011 Revised: May 15, 2011 Published: July 17, 2011 9883
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Table 1. Effect of Variation of QDC, Glu-e, and Perchloric Acid Concentrations on the Oxidation of Glu-e by QDC in Perchloric Acid Medium at I = 2.80 mol dm3 and at 25 °C (Scheme 2) kobs 103 (s1)
kcal 103 (s1)
3.00
6.24
6.35
3.00
6.31
6.35
2.0
3.00
6.19
6.35
10.0
2.0
3.00
5.98
6.35
16.0
2.0
3.00
6.36
6.35
20.0
2.0
3.00
6.34
6.35
8.0
0.5
3.00
2.41
2.43
8.0
1.0
3.00
3.96
3.56
8.0
2.0
3.00
6.87
7.36
8.0
3.0
3.00
8.15
8.18
8.0
4.0
3.00
8.96
8.93
8.0
5.0
3.00
9.30
9.31
8.0 8.0
2.0 2.0
1.50 2.50
3.71 5.36
3.71 5.36
8.0
2.0
3.00
6.78
6.78
8.0
2.0
3.50
7.47
7.47
8.0
2.0
4.00
8.16
8.16
8.0
2.0
4.50
8.73
8.73
[QDC] 104 (mol dm3)
[Glu-e] 102 (mol dm3)
2.0
2.0
6.0
2.0
8.0
[HClO4] (mol dm3)
Variation of [QDC]
Figure 1. Spectral scan of the reaction mixture of Glu-e and QDC in perchloric acid medium at 25 °C (scanning time interval 1.0 min).
Variation of [Glu-e]
Variation of [HClO4]
Figure 2. First order plot of the oxidation of Glu-e by QDC in aqueous perchloric acid medium at 25 °C. [Glu-e] = 2.0 102 mol dm3, [HClO4] = 3.0 mol dm3, I = 2.80 mol dm3, and [QDC] = (A) 2.0, (B) 6.0, (C) 8.0, and (D) 10.0 mol dm3.
temperature are given in Figure 1. Application of Beer’s law under the reaction conditions was verified between 1.0 104 and 1.0 103 mol dm3 QDC, and the extinction coefficient was found to be ε = 1246 ( 12 dm3 mol1 cm1. pH values ( 0.9985, σ < 0.0836). The effect of relative permittivity (D) on the rate constant was studied by varying the acetic acid content (volume/ volume). An attempt to measure the relative permittivity was 9884
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Figure 3. FT-IR spectrum of of the product of oxidation of Glu-e by QDC.
Scheme 1
Figure 4. Plot of log kobs versus I1/2.
unsuccessful. However, relative permittivities were computed from the values of pure liquids.31 The rate constant was found to increase with an increase in the dielectric constant of the medium. A plot of log kobs versus 1/D was linear with positive slope (Figure 5) (r > 0.9614, σ < 0.0791). 3.5. Effect of Added Product. The effect of initially added product, chromium(III), was studied in the 5.0 1055.0 104 mol dm3 concentration range, keeping the ionic strength, reactant concentrations, and other conditions constant. No significant effects on the reaction rate were observed. 3.6. Effect of Temperature. The rate of reaction was measured at different temperatures with varying concentration of perchloric acid and keeping other conditions constant. The rate constant, k, obtained from the intercept of the plots of 1/kobs
Figure 5. Plot of log kobs versus 1/D.
versus 1/[Glu-e], was found to increase with temperature .The values of the rate constants for Glu-e at 293, 298, 303, and 308 K were found to be 6.50, 7.36, 11.32, and 13.28 s1, respectively. The energy of activation corresponding to these constants was evaluated from a plot of log k versus 1/T (Table 2).
4. DISCUSSION QDC reacts with 2 mol of HClO4 to give H2CrO4.32,33 H2CrO4 reacts with Glu-e forming a complex, which is reduced to an intermediate chromium(IV) and a product of Glu-e. Subsequently, another molecule of acid chromate reacts with one molecule of Glu-e, producing another chromium(IV) intermediate and a product of Glu-e. In a further fast step, one 9885
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Table 2. Activation Parameters and Thermodynamic Quantities of the Oxidation of Glu-e by QDC in Aqueous Perchloric Acid Medium Effect of Temperature with Respect to the Slow Step of Scheme 1 temp (K) k 103 (s1) log k 1/T 103 293
5.21
0.716
3.413
298 303
7.36 11.32
0.866 1.053
3.356 3.300
308
13.2
1.120
3.246
Activation Parameters parameter
value
1
Ea (kJ mol ) ΔHq (kJ mol1) ΔSq (J K1 mol1)
62.3
ΔGq (kJ mol1)
59.1
temp (K)
Figure 6. Verification of rate law 7 in the form of eq 9.
58.2 51.7
The total concentration of QDC is given by ½QDCt ¼ ½QDCf þ ½H2 CrO4 þ ½complexC¼ ½QDCf
Variation of K1 and K2 with Temperature K1 102 (dm6 mol2) K2 102 (dm3 mol1)
þ K1 ½QDCf ½Hþ f þ K1 K2 ½Glu-ef ½QDCf ½Hþ f ¼ ½QDCf 2
þ f1 þ K1 ½Hþ f þ K1 K2 ½Glu-ef ½Hþ f g 2
293 298
6.34 4.66
1.32 1.45
Therefore
303
3.12
1.63
308
1.97
2.34
½QDCf ¼
parameter
Thermodynamic Parameters using K1 values
using K2 values
ΔHq (kJ mol1) ΔSq (J K1 mol1)
71.2 251
21.3 119
ΔGq (kJ mol1)
1.70
14.1
2
K1 ½Hþ f 2
1 þ
2
½QDCt þ K1 K2 ½Glu-ef ½Hþ f 2
ð2Þ
where “t” and “f” stand for total and free concentration of QDC. ½Glu-et ¼ ½Glu-ef þ ½complexC ¼ ½Glu-ef þ K1 K2 ½Glu-ef ½QDCf ½Hþ f
2
¼ ½Glu-ef f1 þ K1 K2 ½QDCf ½Hþ f g 2
Therefore molecule of chromium(IV) reacts with one more molecule of Glu-e giving, chromium(III) and the final product of succinic semialdehyde (Scheme 2). The results indicate the formation of a complex between Glu-e and chromium(VI) in the presence of perchloric acid. The formation of this complex was proved kinetically by MichaelisMenten plot, i.e., a nonzero intercept of the plot of 1/kobs versus 1/[Glu-e] (Figure 6). The color indicates the formation of Cr(VI) species and the appearance of Cr(III) species with the isosbestic point at λ = 530 nm. The appearance of the isosbestic point indicates very low concentration of the probable intermediates such as Cr(V) and Cr(IV)34 and their gradual decay to Cr(III). The characteristic of the electronic spectrum of Cr(III) species lies in the range 320600 nm. The original absorption maxima (560 and 430 nm) was replaced by a single peak at 360 nm due to the formation of chromium(VI). Similar results have also been obtained by the oxidation of 2-propanol from chromium(VI) in aqueous acetic acid and oxidation of thallium(I) by QDC in aqueous acetic acidchloride media.35,36
½Glu-ef 1 þ K1 K2 ½QDCf ½Hþ f 2
½Glu-et ¼ ½Glu-ef
ð1Þ
ð4Þ
and ½Hþ t ¼ ½Hþ f þ ½H2 CrO4 þ ½complexC ¼ ½Hþ f 2
2
2
þ K1 ½QDCf ½Hþ f þ K1 K2 ½Glu-ef ½QDCf ½Hþ f 2
2
¼ ½Hþ f f1 þ K1 ½QDCf þ K1 K2 ½Glu-ef ½QDCf g 2
½Hþ f ¼ 2
½Hþ t 1 þ K1 ½QDCf þ K1 K2 ½Glu-ef ½QDCf 2
ð5Þ
Similarly 2
2
ð3Þ
In view of the low concentration of QDC used in the experiment (eq 3), the term K1K2 [QDC]f [H+ ]f 2 can be neglected in comparison to unity. Hence
½Hþ t ¼ ½Hþ f
d½QDC rate ¼ k 3 complexC ¼ dt ¼ kK1 K2 ½Glu-ef ½QDCf ½Hþ f
½Glu-et ¼
2
ð6Þ
Substituting eqs 2, 4, and 6 in eq 1, we get eq 7, which explains all the experimentally observed orders with respect to different 9886
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Scheme 2
From (intercept)1, k = 1.328 102 s1. Hence
species of reaction.
91:37 ¼ 1=ð1:328 102 ÞK2 ð2:0 102 Þ þ 1=ð1:328 102 Þ
d½QDC rate ¼ dt ¼
kK1 K2 ½Glu-e½QDC½Hþ 2 1 þ K1 ½Hþ 2 þ K1 K2 ½Glu-e½Hþ 2
1 þ K2 ð2:0 102 Þ ¼ 91:37ð1:328 102 ÞK2 ð2:0 102 Þ
ð7Þ
1 þ 0:02K2 ¼ 0:0242678K2
or rate ¼ Kobs ½QDC ¼
kK1 K2 ½Glu-e½Hþ 2 1 þ K1 ½Hþ 2 þ K1 K2 ½Glu-e½Hþ 2
K2 ¼ 234:31 dm3 mol1 ðslopeÞ2 ¼ 812:2 ¼ 1=kK1 K2 ½Glu-e
ð8Þ
812:2 ¼ 1=ð1:328 102 Þð234:31 dm3 mol1 Þð2:0 102 ÞK1
Rate law 8 can be rearranged to eq 9, which is suitable for verification. 1 1 1 1 þ ¼ 2 þ þ Kobs K1 K2 ½Glu-e k kK1 K2 ½Glu-e½H
According to eq 8, plots of 1/kobs versus 1/[Glu-e] and 1/kobs versus 1/[H+]2 should be linear (Figure 6). The slopes and intercepts of such plots determine the values of K1, K2, k, and the thermodynamic quantities for the first and second equilibrium steps of Scheme 2 (Table 2). For example, at 35 °C, the following are calculated: (i) From the plot of 1/kobs versus 1/[Glu-e] ðinterceptÞ1 ¼ 1=k Therefore k ¼ 1=ðinterceptÞ1 ¼ 1:328 102 s1 ðslopeÞ1 ¼ 1=kK1 K2 ½Hþ 2 þ 1=k (ii) From the plot of 1/kobs versus 1/[H+]2 ðinterceptÞ2 ¼ 91:37 ¼ 1=kK1 K2 ½Glu-e þ 1=k
K1 ¼ 1:978 102 dm6 mol2
ð9Þ
Therefore, at 35 °C K1 ¼ 1:978 102 dm6 mol2 K2 ¼ 234:31 dm3 mol1 k ¼ 1:328 102 s1 Similarly k, K1, and K2 were calculated at different temperatures (Table 2). van’t Hoff plots were made (log K1 versus 1/T and log K2 versus 1/T). The ΔH, ΔS, and ΔG values were calculated for the first and second equilibrium steps (Scheme 2). A comparison of these values (Table 2) with those obtained for the slow step refer to the rate-limiting step, supporting the fact that the reaction before the rate-determining step is fairly fast and involves a high activation energy.3739 An increase in the volume of acetic acid leads to an increase in the reaction rate. A plot of log kobs versus 1/D was linear with positive slope, which is contrary to convention. Perhaps the effect is countered substantially by 9887
dx.doi.org/10.1021/ie100853p |Ind. Eng. Chem. Res. 2011, 50, 9883–9889
Industrial & Engineering Chemistry Research the formation of active species to a greater extent in a low dielectric constant medium leading to the net increase in the rate.40 The mechanism is also supported by moderate values of ΔH and ΔS . The negative ΔS value indicates that complex C is more ordered than the reactants.41 The observed modest enthalpy of activation and a higher rate constant for the slow step indicate that the oxidation presumably occurs via an inner-sphere mechanism. This conclusion is supported by observations4245 made earlier.
5. CONCLUSION The reaction between Glu-e and QDC is very slow in low perchloric acid concentration at room temperature. The oxidant, chromium(VI), exists in acid medium as H2CrO4, which takes part in the chemical reaction. The rate constant of the slow step and other equilibrium constants involved in the mechanism are evaluated, and activation parameters with respect to the slow step of reaction were computed. The overall mechanistic sequence described here is consistent with the products formed. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: 0091- 571-2401664.
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