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Kinetics and Mechanism of Oxidation of L-Proline by N-Bromosuccinimide in Aqueous Acidic Medium Alaa Eldin Mokhtar Abdel-Hady* Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Sinai University, Areesh City P.O. Box 45518, Cairo, Egypt ABSTRACT: Kinetics of oxidation of L-poline by N-bromosuccinimide (NBS) in aqueous acidic medium and wateralcohol solvent mixtures were performed. The reaction was first order dependent on both [proline] and [NBS]. The rate of reaction was inversely dependent upon [H+] through the pH range 2.63.3 and varied with the cosolvent according to the order MeOH > EtOH. The conjugate base of amino acid was considered as the main reactive species. An inner-sphere mechanism in which the conjugate base of amino acid was attached by NBS to form the precursor intermediate complex was proposed.
1. INTRODUCTION N-bromosuccinimide (NBS) has been used for the oxidation of some transition metal complexes.16 The kinetics of oxidation of (aqua-2-amino-methyl-pyridine)CrIII complex by (NBS) in aqueous solutions to yield CrVI has been studied spectrophotometrically.1 This reaction was first order with respect to both reactants and increased with pH over the range 6.87.6. Inner-sphere oxidation of 2-aminomethylpyridine cobalt(II) complex2 by NBS in aqueous solutions was also studied. The rate of oxidation increased by increasing [NBS], [complex] and was inversely proportional to [H+] over the range 4.35.5. The kinetics of oxidation of [CoII(EDTA)] (EDTA = ethylenediaminetetraacetate) and [CoII(HEDTA)] (HEDTA = N-2-hydroxyethyl-ethylenedimine N,N0 ,N0 -triacetate) by NBS was found to proceed via the initial formation of cobalt(III) products, which was consistent with an inner-sphere mechanism.3,4 The kinetics of oxidation of [Cr(EDTA)(H2O)] by NBS has also been investigated;5 it was reported that the electron transfer proceeded through an innersphere mechanism with the hydroxo-ligand of chromium(III) bridging the two reactants. Furthermore, the kinetics of oxidation of binary and ternary complexes of chromium(III) involving inosine and glycine by (NBS) was studied over the temperature range 2545 °C.6 These reactions exhibited first order dependence on both [NBS] and [CrIII] and increased with pH over the range 6.647.73 in both cases. The kinetics of oxidation of ferrocyanide by (NBS) has been studied spectrophotometrically in aqueous acidic medium over the temperature range 2035 °C, pH = 2.84.3, and ionic strength = 0.10.5 mol dm3 over a range of [Fe2+] and [NBS].7 The reaction showed a first order dependence on both reactants, and increased with pH, [NBS], and [Fe2+]. The rate of this oxidation obeyed the rate law: d[Fe3 + ]/dt = [Fe(CN)6]4[HNBS+]/(k2 + k3/[H+]). An outersphere mechanism has been proposed for the oxidation pathway of both protonated and deprotonated ferrocyanide species. Furthermore NBS has been extensively used for the oxidation of organic compounds,8 in this case the oxidation proceeded via the bromium ion (Br+)911 in polar media, or through a free radical path involving homolytic dissociation of (NBS).12,13 Kinetics of benzoin oxidation by [NBS] in 80% aqueous acetic acid has been investigated; the reaction followed first-order r 2011 American Chemical Society
kinetics with respect to both [NBS] and [benzoin]. The effect of varying ionic strength and the dielectric constant indicated that the reaction is a dipoledipole type.14 Moreover, the oxidation of 3-carboxy-3-hydroxy pentanedioic acid (citric acid) by (NBS) in an acetic acid/sodium acetate buffered medium (pH 3.65.2) was carried out at 308 K and constant ionic strength. The reaction rate was described as first order with respect to both 3-carboxy-3-hydroxy pentanedioic acid and NBS. The rate was inverse first order in succinimide; the reduction product of NBS, and decreased with an increase in pH.15 Kinetics of oxidation of maltose (mal) and D-galactose (gal) by protonated N-bromosuccinimide (NBSH+) using chloro-complex of Rh(III) in its nanoconcentration range as homogeneous catalyst have been studied at 40 °C for the first time. Almost constant values of the pseudo-first-order rate constant (k1) throughout the variation of (NBS) in the oxidation of both the reducing sugars had clearly demonstrated that the order of reaction with respect to [NBS] was unity. First order kinetics with respect to each [Rh(III)], [sugar], and [H+] was evident from the observed values of k1 which increased in the same proportion where the concentration of each reactant was increased.16 Kinetics of oxidation of Lproline by hexacyanoferrate (III) catalyzed by osmium (VIII) in alkaline medium was studied at 30 °C. A mechanism involving a free radical pathway was proposed,17 and the glutamic acid was identified as the oxidation product. Additionally the kinetics of ruthenium(III) catalyzed oxidation of L-proline by permanganate in alkaline medium at constant ionic strength was studied spectrophotometrically using a rapid kinetics accessory. The reaction exhibited 2:1 stoichiometry (permanganate/L-proline), first order dependence on [permanganate] and [ruthenium(III)], and an apparent less than unit-order dependence each in L-proline and alkali concentration;18 a mechanism involving the formation of a complex between catalyst and substrate was proposed. A systematic kinetics study on the oxidation of glycine by (NBS) in the presence of mercuric acetate in acetic acidwater Received: May 10, 2011 Accepted: October 4, 2011 Revised: September 20, 2011 Published: October 04, 2011 12421
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equilibrate in a water bath at the required temperature for ca. 20 min, then thoroughly mixed and quickly transferred to the absorption cell where the absorbance was measured for a definite period of time. Pseudo-first-order conditions were applied in all kinetic runs using a large (10-fold) excess of [L-proline] over [NBS]. The rate of reaction followed first order kinetics and further the rate constants kobs were calculated from the slopes of the first order plots, ln(A∞ At) versus time, where A∞ and At are the absorbance of the oxidation product at infinity and time t, respectively, according to lnðA∞ At Þ ¼ ln A ∞ kt
Figure 1. Absorption spectra of the oxidation products at different times. Peaks 1, 2, 3, 4, 5, 6, 7, 8, and 9 were measured at 0, 3, 5, 10, 15, 20, 30, 50, and 80 min from the time of initiation of reaction. [proline] = 4.0 102 mol dm3, [NBS] = 4.0 103 mol dm3, pH = 2.60 and T = 20 °C.
media was made. Near first-order dependence in NBS, glycine, near inverse-first-order dependence in hydrogen ion concentration, negligible ionic strength effect, and positive dielectric constant have all been observed.19 Kinetics of oxidation of L-proline by periodate were also studied at pH 1.408.83 and 30 °C. The reaction was first-order in each periodate and amino acid, and the overall reaction followed a second-order kinetics. No evidence for the formation of an appreciable amount of intermediate was detected, and the reaction exhibited the highest velocity at pH 47 and catalyzed by HPO42.20 Some reports about the oxidation of L-proline claimed that the ring cleavage took place between the N and C of NHCHCOOH by retaining the NH2 group with the main moiety without liberating ammonia, and the decarboxylation was proposed as a mechanism for the oxidation.21 In the present work, there was an intention to study the kinetics of oxidation of L-proline by (NBS), since the amino acid is an essential constituent of some proteins as collagen and has also found extensive applications in pharmaceuticals and medicines.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Proline was obtained from Sigma. All other reagents were of Merk analytical grade or Fluka. Doubly distilled water was used in all kinetic runs and preparations. Buffer solutions were prepared by mixing different volumes of 0.10 mol dm3 citric acid and 0.20 mol dm3 Na2HPO4 for the required pH values. An aqueous solution of NaCl was used to adjust the ionic strength in the different buffers used. Freshly prepared solutions of NBS and L-proline were prepared daily by dissolving accurately weighed amounts in buffer solution. The pH of the reaction mixture was measured using a 3505 Jenway pH-meter. 2.2. Measurement of Rate Constant. The progress of the oxidation reaction of L-proline by NBS in the required buffer at constant ionic strength, pH, and temperature was followed by measuring the increasing of the absorbance of the oxidation products with time at λ = 390 nm (Figure 1) using a Shimadzu 1700 UVvisible p.c. spectrophotometer. Separate solutions of L-proline and NBS in the required buffer were allowed to
ð1Þ
2.3. Stoichiometry. The stoichiometry of the reaction was ascertained by carrying out several sets of experiments with varying amounts of NBS concentration largely in excess over Lproline concentration in the required buffer, and the mixtures were allowed to stand until completion. A 5 mL aliquot of each of the reaction mixtures was withdrawn and transferred to a titrating flask containing 5 mL of 4% KI. The liberated iodine was estimated by standard Na2S2O3 solution using starch as indicator. The results showed that one mole of L-proline consumed two moles of NBS, and the reaction can be represented stoichiometrically as
The other products were expected to be succinimide, the wellknown product of organic NBS oxidation,8 in addition to carbon dioxide and water. 2.4. Product Analysis. Aqueous solutions of L-proline and (NBS) were mixed under the reaction condition and set aside for 24 h. The solution was then extracted with ether. The ether extract was concentrated by evaporation and mixed with concentrated hydrochloric acid (2 mL). The residue was then evaporated several times with water (ca. 5 mL portions) to remove the excess of hydrochloric acid, and finally with methanol (10 mL). The white needles produced were collected, dried, and analyzed for C, H, N, and Cl contents. The elemental analysis was consistent with that of pyrrolid-2-one hydrochloride (C8H15N2O2Cl). (Found: C, 45.8; H, 7.5; N, 13.0; Cl, 16.6. Calcd: C, 46.5; H, 7.3; N,13.6; Cl,17.2%.)
3. RESULTS AND DISCUSSION Kinetics of oxidation of L-proline by (NBS) in citric acidsodium monohydrogen phosphate buffered medium was investigated under pseudo-first-order kinetics, that is, [L-proline] . [NBS] (at least 10-fold). The rate was measured at the commencement of the slow reaction by measuring the absorbance of the oxidation products at λ = 390 nm for a definite period of time at fixed ionic strength, pH, and temperature and for a range of NBS and L-proline concentrations. The constancy of kobs at different (NBS) concentration ranges (2.06.0) 103 mol dm3, shown in (Table 1) indicates that the reaction was firstorder dependent on [NBS] according to eq 3. To avoid any possible bromine oxidation, all kinetics studies were made with [mercuric acetate] greater than [NBS] which simply means that Br2 oxidation was completely suppressed. Mercuric acetate acted as a scavenger for any bromide (Br) formed in the reaction, 12422
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Table 1. Dependence of the Reaction Rate on [Prol.], [NBS], and Temperature at pH = 2.60 and I = 0.10 mol dm3 temperature
102 [prol.] 3
Table 2. Effect of pH on kobs (s1) at [NBS] = 4.0 103 mol dm3, I = 0.01 mol dm3, and T = 30 °C
103[NBS]
(°C)
(mol dm )
(mol dm3)
103kobs (s1)
25 25
4.0 5.0
4.0 4.0
1.48 1.63
25
6.0
4.0
2.18
25
7.0
4.0
2.53
25
8.0
4.0
2.92
30
4.0
4.0
3.30
30
5.0
4.0
4.09
30
6.0
4.0
5.12
30 30
7.0 8.0
4.0 4.0
6.43 7.33
30
8.0
2.0
7.42
30
8.0
3.0
7.31
30
8.0
5.0
7.52
30
8.0
6.0
7.64
35
4.0
4.0
5.68
35
5.0
4.0
7.16
35 35
6.0 7.0
4.0 4.0
8.67 10.10
35
8.0
4.0
11.70
40
4.0
4.0
7.92
40
5.0
4.0
9.88
40
6.0
4.0
11.70
40
7.0
4.0
13.60
40
8.0
4.0
15.60
45 45
4.0 5.0
4.0 4.0
9.93 12.40
45
6.0
4.0
14.90
45
7.0
4.0
17.60
45
8.0
4.0
20.10
102 kobs (s1) pH
102[prol.] (mol dm3):
4.0
5.0
6.0
7.0
8.0
2.60
0.330
0.409
0.512
0.643
0.730
2.90
0.569
0.733
0.858
0.989
1.190
3.30 3.60
1.690 2.621
2.160 3.270
2.55 3.990
2.940 4.680
3.400 5.500
Figure 2. Plot of kobs versus 1/[H+].
thus ensuring that oxidation took place purely through NBS itself.23,24 d½pyrrolid-2-one=dt ¼ kobs ½NBS
ð3Þ
3.1. Effect of [L-Proline] on kobs. The dependence of kobs on L-proline concentration was examined over concentrations in the 2 3
range (4.08.0) 10 mol dm at different temperatures in the range 2545 °C. The results in (Table 1) showed that the rate constant increased with the increase in [L-proline]. Further the plot of kobs versus [L-proline] was linear, passing through the origin according to eq 4. The zero intercept revealed that the selfdecomposition of NBS did not take place under the experimental conditions employed in this study. k obs ¼ k2 ½proline
ð4Þ
3.2. Effect of pH on kobs. The effect of pH on the rate constant was studied by varying the pH values over the range 2.603.60 and keeping other parameters constant. The rate constant kobs values increased with an increase in pH (Table 2) and supported the involvement of the deprotonated form of L-proline in the rate determining step. The plot of kobs versus [H+]1 was linear with positive intercept (Figure 2).
Figure 3. Plots of kobs versus [proline] at different temperatures.
3.3. Effect of Temperature on kobs. The effect of temperatures on the reaction rate were studied over the range 2545 °C and keeping all the other parameters at constant values. The kobs values increased with increase in the temperature. Plots of kobs versus [L-proline] at different temperatures were linear passing through the origin (Figure 3). Values of k2K1 were calculated from the slopes of the plots at different temperatures (Table 3). Thermodynamic activation parameters, associated with k2K1, were calculated using a least-squares fit to the transition state theory equation as, ΔHq = 43.65 kJ mol1 and, ΔSq = 166.52 J K1 mol1. Both ΔHq and ΔSq are composite values that include formation of the precursor intermediate complex and the intramolecular electron transfer step. The reaction was endothermic as indicated from the positive value of ΔHq, and the intermediate was rigid as indicated from the negative value of the entropy of activation (ΔSq). 12423
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Table 3. Values of K1k2 at Different Temperatures temperature (°C)
104 K1k2 (s1, mol1 dm3)
25
0.929
30
2.61
35
3.75
40
4.79
45
6.41
Table 4. Effect of Dielectric Constant of the Reaction Medium on kobs at T = 40 °C, pH = 2.60, I = 0.1 mol dm3, [NBS] = 4 103 mol dm3, and [Proline] = 5 102 mol dm3 MeOH wt %
D
1/D
10 20 30 40
68.90 64.13 59.53 54.82
0.0145 0.0156 0.0168 0.0182
EtOH 3
10 kobs log kobs wt % 8.56 7.23 5.80 4.50
2.06 2.14 2.23 2.34
10 20 30 40
D
1/D
67.86 62.41 56.73 51.08
0.0147 0.016 0.0176 0.0195
103 kobs log kobs 8.12 4.89 2.69 1.20
2.09 2.31 2.57 2.92
Figure 4. Plots of log kobs versus 1/D.
Thus in acidic media, NBS itself, Br+, or protonated NBSH+ may be the possible oxidizing species.9,10,26 Amino acid exists as zwitterions in aqueous solution.27 The equilibrium involved in an acid medium is represented as
3.4. Effect of Ionic Strength on kobs. The effect of ionic
strength on the oxidation reaction was studied by varying the ionic strength of the medium using an aqueous solution of NaCl of known concentration and maintaining other parameters constant. The experimental results indicated that there is no significant effect of the ionic strength on the values of kobs and supported that the reaction took place between charged and noncharged species. The effect of ionic strength on the reaction rates has been considered according to Bronsted and Bjerrum theory, through the formation of an activated complex. 3.5. Effect of Dielectric Constant on kobs. The effect of dielectric constant (D) of the reaction medium on the reaction rate was investigated by using different MeOHwater and EtOH water solvent mixtures over 040 wt % alcohol range at T = 40 °C, and at constant pH, ionic strength, [proline], and [NBS]. The values of the dielectric constant for various weight percentage compositions of MeOH and EtOHwater solvent mixtures were abstracted as reported.25 Values of the rate constant (kobs) were decreased when alcohol percentage increased (Table 4). In case of EtOHwater mixtures, the reaction was decreased much more as compared to MeOHwater mixture. This phenomenon has been attributed to the large solvation of L-proline in the case of EtOH water than MeOHwater solvent mixtures. The electron densities on the alcohol function decrease in the order EtOH > MeOH, providing a possible explanation of the variation of the reaction rate with solvent. A plot of log kobs versus 1/D was linear with negative slope (Figure 4) and gave further confirmation for the effect of solvent on the reaction rate. 3.6. Test for Free Radical Intermediates. When acrylonitrile was added to the reaction mixture as a separate experiment and at the reaction conditions; no polymerization was observed and the absence of free radicals was supported. The addition of AgNO3 solution to the reaction mixture led to slow formation of a pale yellow precipitate of AgBr. The addition of succinimide to the reaction mixture has no significant effect on the reaction rate. NBS is known to exist in acidic media in the following equilibria. NBS þ Hþ f NBSHþ
ð5Þ
NBS þ Hþ f RNH þ Brþ
ð6Þ
Ka
H2 Nþ -R-COOH s F H2 Nþ -R-COO þ Hþ s R
ð7Þ
On the basis of the experimental results obtained, the rate of the oxidation reaction can be represented by eq 8. d½pyrrolid-2-one=dt ¼ k2 K 1 ½prol:=½Hþ ½NBS
ð8Þ
and kobs ¼ k2 K 1 ½prol:=½Hþ
ð9Þ
Where k2 is the overall second-order rate constant and K1 is the deprotonation equilibrium constant of L-proline. NBS is capable of coordinating the substrate through the carbonyl group31 and Δ1-pyrroline has been reported to be formed during the oxidation of proline.22 All the possibilities of reactions between the reducing species (proline, prol.H+, and prol.) and the oxidizing species (NBS, Br+, and NBSH+) have been explored, and the following scheme has been found in agreement with the observed kinetics data, wherein it is proposed that the NBS molecule attached the conjugate base of L-proline (prol.) in a slow step forming an intermediate complex (X) which subsequently decomposed to give Δ1-pyrroline. Δ1-Pyrroline reacted with another molecule of NBS in a fast step leading to the final products. So, the mechanistic pathway for the oxidation of proline by NBS may be represented as K1
prol: s F prol: þ Hþ s R k2
prol: þ NBS sf X
ð10Þ slow and rate determining step ð11Þ
K3
X sf Δ1 -pyrroline þ CO2 þ Br þ H2 O þ succinimide fast
ð12Þ
K4
Δ1 -pyrroline þ NBS sf pyrrolid-2-one þ succinimide þ Br 12424
fast
ð13Þ
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4. CONCLUSION Thus, in the oxidation of L-proline by NBS in acidic media, it has been found that NBS itself is the main oxidizing species, and the conjugate base of L-proline is the main reducing species and the reaction was first-order dependent on both [proline] and [NBS]. Pyrrolid-2-one was identified as the final oxidation product. An inner-sphere mechanism was proposed for the oxidation pathway owing to the following: (i) The reaction seems to be a stepwise reaction, but an outer-sphere reaction usually takes place instantaneously giving the final reaction products directly. (ii) Since NBS is capable of coordinating the substrate through the carbonyl group,28 it is clearly expected that the formation of an intermediate between the two reactants is likely. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) Al-Shihri, A. S. M.; Abdel-Hady, A. E. M. Kinetics and mechanism of oxidation of (aqua-2-amino-methyl-pyridine) chromium (III) by N-bromosuccinimide. Transition Met. Chem. 1996, 21, 406. (2) Abdel-Hady, A. E. M.; El-Shihri, A. S. M. Inner-sphere oxidation of 2-aminomethylpyridine cobalt(II) complex by N bromosuccinimide. Transition Met. Chem. 2001, 26, 417. (3) Abdel-Khalek, A. A.; Sayyah, S. M.; Khaled, E. S. H. Inner-sphere oxidation of ethylenediamine-tetraacetatocobaltate(II) by N-bromosucc-inimide. Transition Met. Chem. 1993, 18, 555. (4) Abdel-Khalek, A. A; Khalil, M. M.; Khaled, E. S. H. Kinetics of the oxidation of [N-(2-hydroxyethyl)-ethylene-diamine-N,N0 ,N0 -triacetato] cobalt(II) by N-bromosuccin. Transition Met. Chem. 1993, 18 (2), 153. (5) Abdel-Khalek, A. A.; Abdel-Hady, A. M.; El-Shahat, M. F. Kinetics and mechanism of electron transfer in the aquaethylenediaminetetra-acetato chromium(III)/N-bromo-succinimide reaction. Transition Met. Chem. 1993, 18, 283. (6) Ewais, H. A.; Nagdy, M. A.; Abdel-Khalek, A. A. Kinetics and mechanism of oxidation of the binary and ternary complexes of chromium(III) involving inosine and glycine by N-bromosuccinimide. J. Coord. Chem. 2007, 60 (22), 2471. (7) Abdel-Hady, A. M. Kinetics and mechanism of oxidation of ferrocyanide by N-bromosuccinimide in aqueous acidic medium. Transition Met. Chem. 2008, 33, 887. (8) Filler, R. Oxidations and dehydrogenations with N-bromosuccinimide and related N-halomides. Chem. Rev. 1963, 63, 21. (9) Kruse, P. F.; Grist, K. L.; McCoy, T. A. Studies with N-halo reagents. Anal. Chem. 1954, 26 (8), 1319. (10) Lecomte, J.; Gault, H. Oxidation of aromatic alcohols with N-bromosuccinimide. Comp. Rend. 1954, 238, 2538. (11) Mathur, N. K.; Narang, C. K. The Determination of Organic Compounds with N-Bromosuccinimide; Academic Press: New York, 1975. (12) Mushran, S. P.; Pandy, L.; Singh, K. Mechanism of the oxidation of some substituted acetophenones by N-bromosuccinimide in acidic media. Monatsh. Chem. 1980, 111 (5), 1135. (13) Singh, B.; Pandy, L.; Sharma, J.; Pandt, S. M. Mechanism of oxidation of some aliphatic ketones by N-bromosuccinimide in acidic media. Tetrahedron 1982, 38, 169. (14) Sridharan, R.; Mathiyalagan, N. J. M. Kinetics of oxidation of benzoin by N-bromosuccinimide in aqueous acetic acid medium. J. Sci. 2005, 4 (1), 55. (15) Sumithra, V.; Wilson, Crystal, Y.; Easwaramoorthy, D. A kinetic and mechanistic study on the oxidation of 3-carboxy-3-hydroxy pentanedioic acid in buffered medium. Ind. Eng. Chem. Res. 2010, 49 (19), 9077. (16) Singh, A.; Rashmi, S.; Shalini, S.; Jaya, S.; Shahla, R.; Bharat, S. N-Bromosuccinimide oxidation of maltose and D-galactose using
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chloro-complex of Rh(III) in its nano-concentration range as homogeneous catalyst: A kinetic and mechanistic study. J. Mol. Catal. 2009, 310, 64. (17) Sharanabasamma, K.; Angadi, M. A.; Salunke, M. S.; Tuwav, S. M. Osmium(VIII) catalyzed oxidative cleavage of pyrrolidine ring in L-proline by hexacyano-ferrate(III) in alkaline media. Ind. Eng. Res. 2009, 48 (23), p10381. (18) Shettar, R. S.; Hiremath, M. I.; Nandibewoor, S. T. Kinetics of ruthenium (III) catalyzed oxidation of L-proline by permanganate in alkaline medium. E-J. Chem. 2005, 2 (1), p91. (19) Tiwari, J. N.; Bose, A. K.; Mushran, S. P. Kinetics and mechanism of the oxidation of glycine by N-bromosuccinimide. Monatsh. Chem. 1977, 108 (6), 1471. (20) Pascual, R.; Herraez, M. A.; Calle, E. Kinetics and mechanism of the oxidation of proline by periodate. Can. J. Chem. 1989, 67 (4), 634. (21) Seregar, V. C; Hiremath, C. V; Nandibewoor, S. T. Mechanism of oxidation of L-proline by aqueous alkaline diperiodatoargentate (III): Decarboxylation and dehydration. Z . Phys. Chem. 2006, 220 (5), 615. (22) Bragg, P. D.; Hough, L. The oxidation of proline, hydroxyproline, and N-methyl glycine with periodate. J. Chem. Soc. 1958, 4050. (23) Bailer, J. C. The Chemistry of Coordination Compounds; Reinhold: New York, 1947; p 4. (24) Venkatasubramanian, N.; Thiagarajan, V. Mechanism of oxidation of alcohols with N-bromosuccinimide. Can. J. Chem. 1969, 47, 694. (25) Akerlof, G. Dielectric constants of some organic solventwater mixtures at various temperatures. J. Am. Chem. Soc. 1932, 54 (11), 4125. (26) Venkatasubramanian, N.; Srinivasan, N. S. A kinetic study of the oxidation of steroid alcohols by N-bromosuccinimide. Indian J. Chem. 1971, 9, 726. (27) Greenstein, J.; Winitz, M. Chemistry of Amino Acid; John Wiley Sons. Inc.: New York, 1961. (28) Varaprasad, D.V. P. R.; Mahadevan., V. Aqueous redox polymerization of acrylonitrile initiated by systems based on tervalent and tetravalent vanadium in combination with N-bromosuccinimide as the oxidant. J. Macromol. Sci. Chem. 1983, A19 (5), 781.
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