CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.10 No.7, pp 341-349, 2017
Kinetics of Oxidation of few organic substrates by chromium
Abstract : Chemical Kinetics is the study of time as a prime factor that adds interest and difficulty to this branch of chemistry. The testing of rate theories, the measurment of equilibrium constants, the analysis of solution, including the solutes and solvents and their properties depend upon the rate of the reaction.The highly toxic chromium (VI) compounds are converted into environmental friendly non-toxic chromium (III) compounds by using several oxidants like Nicotinum dichromate, Piperidinium chlorochromate, Quinolinium fluorochromate, Quinolinium dichromate. The reaction kinetics and rate constant for various chemical reactions are studied and the activation parameters have been evaluated. The key to the application of kinetics is also to determine the quality loss of deterioration of food. Key Words: Chemical kinetics, Rate constant, Nicotinium dichromate, Piperidinium chlorochromate, Quinolinium fluorochromate, Quinolinium dichromate.
The metal chromium has both beneficial and detrimental properties. Cr (III) is essential in human
1,2, 3, 4
nutrition in glucose metabolism. But most of the hexavalent compounds are toxic, and cause lung cancer. Economically the advantages of chromium have wide applications in industries, such as the production of stainless steel, in electroplating, tanning, pigment and chemical industry, etc.,. The soil, groundwater and surface water are contaminated with anthropogenic Cr (VI)5,6 . To understand the mechanistic aspect of reduction of Cr (VI) to Cr (III), several kinetics studies of chromic acid oxidation of different types of organic substrates have been carried out by different workers7, 8 . The source of Cr (VI) is being chromium trioxide, sodium or potassium dichromate. The structure of chromium trioxide has been determined by X-ray analysis9 to be a linear polymer of chromium and oxygen atoms, with two additional oxygen atoms linked to each chromium atom. It is polymerised in water and forms strong acid10
Oxidation States of Chromium Metal
+2+3+6 -+4+5
Cr, Cr, Crare the stabe states and in nitrosyls, carbonyls group 2,-1 state is seen. Cr, Crare
+3+6+6
unstable in water and they form Cr, Cr. Cris used as an oxidant for various reactions and the source is mentioned above. Organic compounds are oxidised in presence of acids like H2SO4, CH3COOH, acetone, DMSO and ethers if the compounds are water soluble. Chromium (VI) exists as aceto chromate ion in presence of 97% acetic acid38. New chromium species are formed on adding anions like Cl -, Br -, F-, and SO42-anions.
Experimental and Discussion
1. Nicotinium Dichromate -Oxidation of Few Amino Acids by NDC:
Nicotinum Dichromate (NDC) is a mild, non-hygroscopic, stable and selective oxidizing reagent in
11, 12, 13
synthetic organic chemistry . The author has used NDC as oxidant to oxidise essential amino acids in aqueous medium in the presence of perchloric acid which resulted in the formation of a complex giving corresponding aldehyde14. The effect of Temperature was studied by the author at 303K, 313K, 323K and 333K. He observed the rate constant of the reaction increased with increasing temperature and the reaction followed pseudo-first order kinetics as shown in the Table 1. The negative sign15 of the entropy change ΔS#, suggests that the transition state is more orderly when compared with the reactants.
Table 1.Rate constant for the oxidation of few amino acids by NDC at 313 K in perchloric acid 14
[NDC]10 3 mol dm -3 | [Amino acid]10 2 mol dm -3 | [H +]10 2 mol dm -3 | Valine k 1 10 4 s -1 | Leucine k 1 10 4 s -1 | Histidine k 1 10 4 s -1 |
---|---|---|---|---|---|
6.0 | 2.0 | 5.0 | 4.34 | 4.46 | 9.25 |
8.0 | 2.0 | 5.0 | 4.30 | 4.43 | 9.20 |
10.0 | 2.0 | 5.0 | 4.37 | 4.40 | 9.29 |
12.0 | 2.0 | 5.0 | 4.29 | 4.49 | 9.23 |
14.0 | 2.0 | 5.0 | 4.30 | 4.46 | 9.27 |
6.0 | 2.0 | 5.0 | 4.34 | 4.46 | 9.25 |
6.0 | 3.0 | 5.0 | 5.32 | 5.45 | 11.20 |
6.0 | 4.0 | 5.0 | 6.14 | 6.28 | 12.80 |
6.0 | 5.0 | 5.0 | 6.88 | 7.04 | 14.70 |
6.0 | 6.0 | 5.0 | 7.60 | 7.80 | 15.90 |
6.0 | 2.0 | 5.0 | 4.34 | 4.46 | 9.25 |
6.0 | 2.0 | 7.5 | 8.23 | 10.30 | 17.60 |
6.0 | 2.0 | 10.0 | 15.16 | 17.80 | 30.00 |
6.0 | 2.0 | 12.5 | 23.23 | 27.80 | 49.30 |
6.0 | 2.0 | 15.0 | 30.99 | 38.70 | 87.00 |
The kinetics of oxidation of aniline16 and para-(Me, OMe, COMe, NHCOMe, NO2, Br, Cl, F) and meta(Me, COOH, NO2, Et, OMe, COMe) substituted anilines were carried by the author under pseudo-first-order condition and from the least squares method, the rate constants were found and the linear plots, r ≥ 0.96 of log
# -1# -1-1
[NDC] vs time and ΔGis 8 kJ mol, ΔSvaries from 0.1305 to 0.8599 JKmol. The product azobenzene was identified by its melting point 66 0C, IR and UV spectra. The Thermodynamic parameters of few substituted aniline is shown in the Table 2.
Table 2. Oxidation of aniline by NDC in benzene/2-methylpropan-2-ol mixture 16 -Thermodynamic parameters
Mole fraction of benzene in 2-methylpropan-2-ol | |||||||||
---|---|---|---|---|---|---|---|---|---|
Substituent in aniline moiety | TP a | 0.1305 | 0.1465 | 0.1766 | 0.317 | 0.4192 | 0.5199 | 0.6679 | 0.8599 |
H | Ea | 13.6 | 15.1 | 9.6 | 27.8 | 23.4 | 32.7 | 45.3 | 50.8 |
ΔH# | 11.5 | 13 | 7.6 | 25.8 | 21.4 | 30.7 | 43.3 | 48.8 | |
ΔS# | -28 | -283 | -299 | -231 | -243 | -211 | -167 | -148 | |
ΔG# | 98.1 | 97.9 | 97.2 | 94.9 | 94.3 | 94 | 93.4 | 93.1 | |
p-Me | Ea | 9 | 10 | 14.1 | 14.6 | 19.9 | 21.5 | 24.8 | 32.9 |
ΔH# | 7.9 | 7.9 | 12 | 12.6 | 17.8 | 1.4 | 22.7 | 30.9 | |
ΔS# | -300 | -299 | -285 | -282 | -263 | -255 | -245 | -216 | |
ΔG# | 97.7 | 97.6 | 97.4 | 97 | 96.6 | 95.9 | 95.9 | 95.6 | |
p-OMe | Ea | 8.8 | 9 | 11.7 | 10.7 | 21.5 | 31.9 | 31.1 | 40 |
ΔH# | 6.7 | 7 | 9.7 | 8.6 | 19.5 | 29.8 | 29 | 37.8 | |
ΔS# | -295 | -294 | -284 | -285 | -247 | -211 | -211 | -178 | |
ΔG# | 95.1 | 94.9 | 94.7 | 93.8 | 93.6 | 93.1 | 92.4 | 91.1 |
The author investigated the kinetics and mechanism of oxidation of para-and meta-substituted benzaldehydes with NDC in an acidic medium 17. The author maintained the substrate concentration in excess over NDC, first order conditions shown in Table 3. The high solvation of transition state over the reactants showed the entropies of activation ∆S#, are negative. Also the rate data for oxidation of the substituted benzaldehydes gave a good correlation, r = 0.996, ρ = +0.55 with the Hammett σ value at 300K.
Table 3. Effect of concentration of reactants on reaction rates at 300K [H+] = 1.50 mol dm –3, 70% acetic acid – water (v/v) 17
102 [Benzaldehyde] /mol/dm 3 | 104 [NDC] /mol/dm 3 | 104 kobs/ s -1 |
---|---|---|
2.0 | 7.5 | 3.43 |
2.0 | 10.0 | 3.60 |
2.0 | 12.5 | 3.38 |
2.0 | 15.0 | 3.27 |
2.0 | 20.0 | 3.51 |
1.5 | 10.0 | 2.75 |
2.5 | 10.0 | 4.26 |
3.0 | 10.0 | 5.13 |
3.5 | 10.0 | 6.02 |
4.0 | 10.0 | 6.76 |
2. Piperidinium chlorochromate -Oxidation of α-hydroxy acids by PipCC
The kinetics of oxidation of α-hydroxy acids by piperidinium chlorochromate was investigated by the author at several concentrations of the reactants. The author found at 313K glycolic acid was oxidised in 50% acetic acid-water medium and he states that the reaction was found to be first order as evidenced by a linear plot of log absorbance versus time 18 shown in Fig 1. The plot of [PipCC] versus 1/k1 is linear and as the concentration of substrate increased the author states, the rate also increased and the reaction was found to be first order with respect to substrate evidenced by the unit slope, r = 0.998 of the plot of log k1 versus log [substrate].The rate constants are shown in Table 4.
Table 4: Rate Constants for the Oxidation of Glycolic Acids by PipCC at 313 K18
[PipCC] 103 (mol dm -3) | [Glycolic Acid] 102 (mol dm -3) | k1 104 (s -1) | k2 (s -1 mol -1 dm3) |
---|---|---|---|
3.3 | 7.5 | 11.87 | - |
4.4 | 7.5 | 10.62 | - |
5.5 | 7.5 | 9.38 | - |
6.6 | 7.5 | 8.44 | - |
7.7 | 7.5 | 7.39 | - |
4.4 | 5 | 7.01 | 0.14 |
4.4 | 7.5 | 10.62 | 0.141 |
4.4 | 10 | 14.32 | 0.143 |
4.4 | 12.5 | 17.78 | 0.142 |
4.4 | 15 | 21.25 | 0.141 |
[H+] = 6.8 x 10 -2mol dm -3 AcOH :H2O = 50 :50 (v/v)
Fig 1: Plot of 1/k1 versus [PipCC] 19
The author prepared piperidinium chlorochromate and S-phenyl mercapto acetic acid and its purity was checked iodometrically by finding the Cr (VI) concentration20. UV-Visible spectro-photometer at 470 nm was used to determine the kinetic runs and the temperature was maintained at desired value to an accuracy of ±0.1 0C. From the slope of log absorbance versus time plots the pseudo-first order rate constants were calculated by the investigator. At constant temperature, constant concentration of (oxidant) perchloric acid, the rate of reaction increased with increase in the concentration of the substrate as shown by the author in the Table 5. By using Eyring’s plot of log k2/T versus 1/T, the thermodynamic and activation parameters were calculated and
# -1 # -1-1 # -1 20
reported by the author, ΔH= 4.78 kJmol, ΔS= -184.57 JKmol, ΔG= 62.55 kJmol.
Table 5. Oxidation S-phenyl mercaptoaceticacid by piperidinium chlorochromate at 313 K and the Rate Constants20
[PipCC] 103 (mol dm -3) | [PMAA] 103 (mol dm -3) | [H+] 102 (mol dm -3) | [NaClO4]102 (mol dm -3) | AcOH : H2O v/v | [MnSO4]102 (mol dm -3) | k1 104(s -1) |
---|---|---|---|---|---|---|
3.3 -7.7 | 7.5 | 0.68 | 50:50 | - | 9.11 -9.24 | |
4.4 | 5.0 -17.5 | 0.68 | 50:50 | - | 6.34 -19.91 | |
4.4 | 7.5 | 0.68 -3.41 | 50:50 | - | 7.43 -15.98 | |
4.4 | 7.5 | 0.68 | 0.00-20.04 | 50:50 | - | 9.05 -9.06 |
4.4 | 7.5 | 0.68 | 55:45-30:70 | - | 6.82-16.61 | |
---|---|---|---|---|---|---|
4.4 | 7.5 | 0.68 | 50:50 | 0.00-20.04 | 9.05-6.41 |
3. Quinolinium Dichromate -Oxidation of Diols using QDC
Oxidation of Ethylene Glycol to yield an R-hydroxy carbonyl compound such as
-
HOCH2CH2OH + HCrO4 + H + → HOCH2CH2OCrO3H + H2O
HOCH2CH2OCrO3H → HOCH2CHO + Cr (IV)
The investigator reported the kinetics of trans-1, 2-cyclohexanediol21 into R-hydroxy carbonyl compound in acid medium with QDC. The author conducted the reaction at temperature 0.1 K. He also studied the kinetic runs with UV-vis absorption band at 440 nm. The sharp band at 1687 cm-1, shows the hydrogen bonded C=O group and absorption band at 3050 cm-1 showed the presence of the OH group from the Infra Red Spectroscopy. The product was confirmed by the author as 2-hydroxycyclohexanone and with respect to QDC the reaction follows first order kinetics and the rate constants are shown in the Table 6.
Table 6. Rate Constants on Oxidant Concentration Diols (0.01 M), H2SO4 (1.0 M) T = 323K and rate constants on substrate concentration QDC (0.001 M), H2SO4 (1.0 M), T=323 K21
104[QDC] (M) | 104k1 (s -1) for 1,2 cyclohexanediol | 102[diol] (M) | 104k1 (s -1) for 1,2 cyclohexanediol |
---|---|---|---|
1.0 | 7.75 | 1.0 | 7.67 |
5.0 | 7.52 | 5.0 | 38.5 |
7.5 | 7.36 | 7.5 | 58.0 |
10.0 | 7.67 | 10.0 | 77.1 |
20.0 | 7.62 | 20.0 | 155 |
The investigator prepared quinolinium dichromate and its purity was checked by spectral analysis, where IR spectrum exhibited bands at 930, 875, 765 and 730 cm-1 and reactions were carried at temperature
0.1 K and the UV-visible absorption band was recorded at 440 nm. From the linear plots, r > 0.996, of log [QDC] against time the rate constants were found by the author. The oxidized products 2-furancarboxylic acids were obtained with yields 85– 90%. The thermodynamic and activation parameters were calculated and
#-1 #-1-1
reported by the author, ΔH± 2 kJmol ΔS± 5 Jmol K. The rate of oxidation of substituted 2-furaldehydes at 313 K is shown in the Table 7.
Table 7: Rate data for the oxidation of substituted 2-furaldehydes at 313 K23 4. Quinolinium fluorochromate – Oxidation of benzaldehyde by QFC
[Substrate] (102 M) | [QDC](103 M) | [H2SO4](M) | 2Furaldehyde 104 k (s−1) | 5-Bromo-2furaldehyde 104 k (s−1) | 5-Methyl-2furaldehyde 104 k (s−1) |
---|---|---|---|---|---|
1 | 1 | 0.5 | 1.25 | 1.1 | 1.5 |
2.5 | 1 | 0.5 | 3.12 | 2.6 | 3.8 |
5 | 1 | 0.5 | 6.21 | 5.6 | 7.8 |
7.5 | 1 | 0.5 | 9.32 | 8.3 | 12.1 |
10 | 1 | 0.5 | 12.5 | 11 | 15.6 |
1 | 0.75 | 0.5 | 1.22 | 1.12 | 1.52 |
1 | 0.5 | 0.5 | 1.25 | 1.09 | 1.5 |
1 | 0.25 | 0.5 | 1.24 | 1.1 | 1.45 |
1 | 0.1 | 0.5 | 1.27 | 1.15 | 1.43 |
1 | 1 | 0.75 | 1.88 | 1.7 | 2.3 |
1 | 1 | 1 | 2.5 | 2.15 | 3.1 |
1 | 1 | 1.25 | 3.2 | 2.7 | 3.7 |
1 | 1 | 1.5 | 3.8 | 3.4 | 4.6 |
QFC of molecular formula C9H7NH[CrO2F] 41 was prepared by the author and its purity was checked by iodometric method and oxidation of mono substituted benzaldehyde by QFC in Dimethyl sulphoxide 22 was reported by the investigator. The reactions were carried at temperature ±0.1K and spectrphotometrically monitoring the decrease in concentration of QFC at 354 nm, and pseudo-first order rate constant (kobs) was
=
--
Fig 2. Decomposition of a component for a reaction of varying order n having same initial concentration and rate constant 24
Toxicity of Chromium-(VI) and (III)
Chromium (Cr) was discovered in the year 1798 by the French chemist Vauquelin33.
It is a transition element located in the group VI-B of the periodic table with a ground-state electronic
51 -1
configuration of Ar 3d4s. Naturally occurring in soil, Cr ranges from 10 to 50 mg. kgdepending on the parental material. In fresh water, Cr concentrations generally range from 0.1 to 117 µg L-1, whereas values for seawater range from 0.2 to 50 µg L-1. Nutrient solution with 9.6 µM Cr (VI) decreased the uptake of K, Mg, P, Fe and Mn in the roots of few plants like soybean34. Chromium (III) is cationic and can easily adsorbs on the
35,36
clay particles, organic matter, and other negatively charged particles and Cr(VI) adsorb more tightly to oxide than anions like Cl -, NO-3, and sulfate. Also Cr(VI) and (III) are causing lethal effects to Earthworms37 .
6+27, 29
Hexavalent chromium (Cr) is mobile, can penetrate the cell wall and cause various cancer diseases. Also short term exposure causes Ulcer, skin irritation and Long-term exposure causes damage to internal organs like
28, 2929, 30, 31, 32
liver, kidney, blood circulation, nerve tissue and death . Cr (III) accumulates in the cell membrane .
The study of chemical kinetics deals with the limit of extent to which a reaction occur, the sequence of steps by which the reaction proceeds, rates of the steps, factors, nature of reactants, concentration and temperature which affect the rate. The oxidation of various chromium compounds studied by different investigators and the kinetics, rate constants, toxicity and applications are reported above. The more active oxidants are made passive by using suitable oxidants and their kinetics studied are discussed above.
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