Induced Electron Transfer Reactions in Pentaammine Cobalt(III) Complexes of αlpha-Hydroxy Acids B

IJSRD - International Journal for Scientific Research & Development| Vol. 3, Issue 11, 2016 | ISSN (online): 2321-0613

Induced Electron Transfer Reactions in Pentaammine Cobalt(III) Complexes of α-Hydroxy Acids By Quinoxalinium Bromo Chromate in Aqueous Acetic Acid (50%v/v) Medium: A Kinetic Study S.Chandrasekaran1 Dr.K.Subramani2 1,2 Department of Chemistry 1,2 Islamiah College, Vaniyambadi, TN, India Abstract— The kinetics of induced electron transfer reactions for QxBC with Cobalt (III) complexes of αhydroxy acids and free α-hydroxy acids on micelles have been attempted. QxBC oxidize Pentaammine Cobalt (III) complexes of α-hydroxy acids and free α-hydroxy acids. It rules out the synchronous C-H bond fission and Hydride ion transfer. The reaction was followed by observing the decrease in the absorbance at 365nm for Cr (IV) in spectrophotometer. The increase in the rate is observed to increase in the concentration of the Surfactants. The added cationic micelle CTAB enhances the rate of oxidation of reaction much more than non-ionic micelle TRITON-X 100 and anionic micelles NaLS. As well as added non-ionic micelles TRITON enhances the rate of oxidation of reaction much more than anionic micelles NaLS. Key words: α-Hydroxy Acids, Pentaammine Co(III) Complexes, Quinoxalinium Bromochromate (QxBC), Micelles I. INTRODUCTION The oxidation of α-Hydroxy acids such as Mandelic acid, Lactic acid, Glycolic acid and their pentaammine cobalt (III) complexes using Quinoxalinium Bromo Chromate(QxBC) in the presence of various micelles have been attempted. In quinoxlinium bromo chromate induced electron transfer in the complex, the intermediate radical formed dissociates in a nearly synchronous manner with C-C bond cleavage and the rest of it proceeding with α-C-H fission yielding keto acid cobalt(III) complex. Chromium exists usually in both trivalent and hexavalent forms in sedimentary systems.

Nevertheless, these two oxidation states are characterized by markedly different physical/chemical behavior and toxicity.

A variety of compounds containing chromium (VI) have proved to be versatile reagents capable of oxidizing almost every oxidizable functional group [1]. A number of new Cr(VI) containing compounds, with heterocyclic bases, like pyridinium chlorochromate(PCC) [2], pyridinium bromochromate (PBC) [3], quinoliium chlorochromate (QCC) [4], benzimidazolium fluorochromate (BIFC) [5], quinolinium bromochromate (QBC) [6], imidazolium fluorochromate (IFC) [7], pyridinium fluorochromate (PFC) [8], tributylammonium chlorochromate (TriBACC) [9], tripropylammonium fluorochromate (TriPAFC) [10] quinaldinium fluorochromate (QnFC) [11] quinolinium fluorochromate (QFC) [12] and imidazoliun dichromate (IDC) [13] have been developed to improve the selectivity of oxidation of organic compounds. The kinetics and mechanism of oxidation of α-hydroxy acids by various oxidants have been reported. However, the kinetics of oxidation of α-hydroxy acid by QxBC, a Cr(VI) reagent has not yet been studied. This prompted us to undertake the present investigation. The present work reports the kinetics of oxidation of α-hydroxy acid by QxBC and evaluates the

rate constants. Mechanistic aspects are also discussed.

Literature survey reveals that no report is available on the kinetics of oxidation of cobalt (III) complexes of α-hydroxy acid and free α-Hydroxy acids by QxBC. In this article, the kinetics and mechanism of oxidation of α-hydroxy acid by QxBC is reported.

II. MATERIALS AND METHODS Lactic acid (99%, s.d. fine), CrO3 (99%, Merck, India), Hydrobromic acid (47%, Merck, India) Mandelic and Glycolic acid from (Karnataka Fine Chem, 97%,India), were used as supplied and their stock solutions were prepared in double distilled (first time from alkaline KMnO4), Deionized and CO2 free water. HClO4 (Merck, India 95%) was standardized using standard sodium carbonate (BDH.AR) solution with methyl orange as an indicator.

Acetic acid was purified by standard method and the fraction distilling at 1180C was collected. Quinoxalinium Bromochromate has been prepared. QxBC is a mustard yellow, non-hygroscopic, but moderately light-sensitive solid which should be protected from light during preparation and storage [14].The purity of QxBC was checked by the iodometric method.

A. Kinetic Measurements All kinetic measurements were carried out on a Spectrophotometer. The progress of the reaction was followed at 365 nm by monitoring the changes in absorbance of remaining Cr(VI) and 502nm by monitoring the changes in absorbance of remaining Co(III). The required α-hydroxy acid, HClO4 and QxBC were premixed in a reaction vessel, thermostated in an oil bath, and QxBC solution (thermally equilibrated) was then added prior to the absorbance measurements. Under pseudo-first-order conditions of α-hydroxy acid, the plots of log A versus time (A is absorbance intensity) were linear up to 80% completion of the reaction with an average of linear regression coefficients, r ≥ 0.996 (Table-1 and Figure-1).

The monomeric Cobalt(III) complexes of α-hydroxy acids were prepared as perchlorates using the method of Fan and Gould. Kinetic measurements of the oxidation of cobalt(III) complexes of α-hydroxy acids and free ligands were carried out under pseudo first order conditions in 50% acetic acid50% water (v/v) medium at 323K. The stiochiometric studies for the oxidation of pentaamminecobalt (III) complexes of α- hydroxy acids and free ligands by QxBC in the presence of micelles were carried out with the oxidant in excess. The [H+] and ionic strength were maintained as in the corresponding rate measurements. After nine half lives when the reaction was nearing completion, the concentration of unreacted was determined spectrophotometrically from the change in absorbance measured at 365 nm for Cr(IV) in free ligands and Co(III) complexes of α-Hydroxy acids. The

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Induced Electron Transfer Reactions in Pentaammine Cobalt(III) Complexes of α-Hydroxy Acids By Quinoxalinium Bromo Chromate in Aqueous Acetic Acid (50%v/v) Medium: A Kinetic Study (IJSRD/Vol. 3/Issue 11/2016/023)

stoichiometry was calculated from the ratio between reacting oxidant and substrate from the decrease in the absorbance measured for the cobalt (III) complex, the amount of Cr(VI) in QxBC reduced was calculated.

Time(s) log(Absorbance) 104 k1 (s-1) 0 -0.208 0 305 -0.242 2.052 600 -0.261 1.649 906 -0.281 1.720 1205 -0.306 1.751 1507 -0.332 1.787 1810 -0.356 1.793 2109 -0.380 1.804 2406 -0.405 1.817 2706 -0.429 1.828 3005 -0.465 1.919 Table 1: Calculation [QxBC] = 10-3 mol dm-3 [HClO4] = 0.1 mol dm-3 Temperature = 323K Mandellic Acid = 1.00 x10-2mol dm-3 [NaLS] = 1.00x10-4 mol dm-3

glycolato ( Table-3) in the presence of micelles were varied in the range of 1.0 x 10-2 to 3.0 x 10-2 mol dm-3 at 323 K and keeping all other reactant concentrations as constant and the rates were measured. Both the free hydroxy acids and their Co (III) complexes of rate constants were calculated by the integrated rate equation. The graph of logarithms was concentration versus time linear and the rate constants calculated from the slope of the graph agreed with the experimental value. Which shows a first order dependence plot on α-hydroxy acids and their complexes. The rate of oxidation increased progressively with increasing the concentration of α-hydroxy acids in the presence of NaLS (Fig 2) and their complexes in presence of NaLS (Fig 3).

The plot of log k1 versus log [α-hydroxy acids] gave the slope of ≥ 0.998.

B. Effect of Varying Perchloric Acid Concentration Perchloric acid has been used as a source of H+ in the reaction medium. The concentration of perchloric acid was varied in the range 0.06 to 0.14 mol dm-3 keeping all other reactant concentration as constant at 323 K as well as Cobalt (III) complexes and the rates were measured. The acid catalyzed nature of this oxidation is confirmed by an increase in the rate with the addition of Perchloric acid. The plot of log k1 versus log [H+] is a straight line with a slope of ≥0.996.

C. Effect of Acidity The reaction is catalyzed by hydrogen ions. The acid– catalysis may well be attributed to a protonation of QxBC to give a stronger oxidant and electrophile.

O2CrBrO-QxH+ + H+ ↔ (OH)OCr+BrO-QxH+

Fig. 1: Dependence of first order plot B. Product Analysis Product analysis was carried out under kinetic conditions i.e., with excess of the reductant over QxBC. In a typical experiment, Lactic acid (0.l0 mol), perchloric acid (0.10mol) and QxBC (0.01 mol) were dissolved in acetic acid – water mixture (50%- 50%) and the solution was allows to stand in the dark for about 24 h to ensure completion of the reaction.

The residue was treated with an excess (200 mL) of a saturated solution of 2,4-dinitro phenylhydrazine in 1mol HCl and kept overnight in a refrigerator. The precipitated 2,4-dinitro phenyl hydrozone (DNP) was filtered off, dried and recrystallised from ethanol. The product was identical with melting point and mixed melting point to an authentic sample of the DNP of pyruvic acid.

III. RESULTS AND DISCUSSION A. Effect of Varying Substrate Concentration The concentration of the substrates, Mandelic acid, Lactic acid, Glycolic acid ( Table-2) in the presence of micelles were varied in the range of 0.5 x 10-2 to 2.5 x 10-2 mol dm-3 at 323 K and keeping all other reactant concentrations as constant and the rates were measured. The concentration of the substrates, Co(III) Mandelato, Co(III) lactate and Co(III)

D. Effect of Varying Micelles Concentration The concentration of micelles, sodium laryl sulfate, cetyl trimethyl ammonium bromide and Triton X 100 were variedin the range of 1.0x10-4 to 1.0x 10-2 mol dm-3 at 323K from the mixture of both free ligands and Cobalt(III) complexes and keeping all other reactant concentrations as constant and the rates were measured. The rate of oxidation increased with increasing the concentration of micelles.

When formation of micelles on the substrates the rate of reaction was observed constantly. A plot of specific rate constant versus micellar concentration is sigmoidal in shape both free ligand and cobalt (III) complexes. The increase in the rate is observed to increase in the concentration of the Surfactants. The added CTAB enhances the rate of oxidation of reaction much more than TRITON-X 100 and NaLS. The micelles of cationic, anionic and non-ionic are acting as a positive catalyst E. Comparison of Rate of Oxidation The specific rate of the lactic acid is more when compared to the rate of both glycolic acid and Mandelic acid is due to the ligation of lactic acid to cobalt (III) center has probably increased its reactivity towards QxBC similarly the rate of the lactate complex is more compared to the rate of both glycolato and mandelato complex. This effect seems to be more specific for ligands only. In an NMR spectrum of lactate complex the alpha methine proton has undergone a considerable downfield shift compared to the alpha C-H proton of the free ligand [δ C-H is 1.73 ppm. in lactic acid

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Induced Electron Transfer Reactions in Pentaammine Cobalt(III) Complexes of α-Hydroxy Acids By Quinoxalinium Bromo Chromate in Aqueous Acetic Acid (50%v/v) Medium: A Kinetic Study (IJSRD/Vol. 3/Issue 11/2016/023)

and δ C-H is 2.30 ppm. in lactate complex whereas δ C-H is 4.75 ppm. in Mandelic acid, δ C-H is 3.85 ppm. in the mediator complex][15]. Suggesting an increase in the acidic nature of methine proton of lactic acid is due to ligation to a metal center. If the reaction proceeds through a performed protonated QxBC, then the rate of alpha C-H will be enhanced, resulting in an increased rate of oxidation of lactato complex such a precursor complex may be sterically hindered in the case of mandelato and glycolato complexes as well as free mandelic acid and glycolic acid.

NaLS CTAB TRITON 102 [α-hydroxy acid] 104k1 104k1 104k1 mol dm-3 (s-1) (s-1) (s-1) Mandelic acid 0.5 0.956 1.093 0.996 1.0 1.905 2.188 1.994 1.5 2.861 3.273 2.990 2.0 3.808 4.400 3.980 2.5 4.775 5.485 4.955 Lactic acid 0.5 1.074 1.267 1.168 1.0 2.145 2.536 2.356 1.5 3.222 3.807 3.522 2.0 4.300 5.080 4.672 2.5 5.368 6.343 5.845 Glycolic acid 0.5 0.781 1.024 0.852 1.0 1.562 2.046 1.713 1.5 2.331 3.072 2.559 2.0 3.122 4.152 3.430 2.5 3.908 5.163 4.283 NaLS CTAB TRITON 102 [α-hydroxy acid] 104k1 104k1 104k1 -3 -1 -1 mol dm (s ) (s ) (s-1) Mandelic acid 0.5 0.956 1.093 0.996 1.0 1.905 2.188 1.994 1.5 2.861 3.273 2.990 2.0 3.808 4.400 3.980 2.5 4.775 5.485 4.955 Lactic acid 0.5 1.074 1.267 1.168 1.0 2.145 2.536 2.356 1.5 3.222 3.807 3.522 2.0 4.300 5.080 4.672 2.5 5.368 6.343 5.845 Glycolic acid 0.5 0.781 1.024 0.852 1.0 1.562 2.046 1.713 1.5 2.331 3.072 2.559 2.0 3.122 4.152 3.430 2.5 3.908 5.163 4.283 Table 2: Calculation Kinetic data for the oxidation of α-Hydroxyacids by QxBC [QxBC] = 10-3moldm-3 [HClO4] = 0.1moldm-3 Temperature = 323K [Micelles] = 1.00x10-2 moldm-3

Fig. 2: Dependence of rate on α-Hydroxy acids with NaLS NaLS CTAB TRITON 102 [(NH3)5Co(III)-L] 104k1 104k1 104k1 -3 mol dm -1 -1 (s ) (s ) (s-1) Mandelato 1.0 2.018 2.560 2.347 1.5 3.015 3.849 3.540 2.0 4.042 5.116 4.712 2.5 5.055 6.410 5.905 3.0 6.048 7.704 7.074 Lactato 1.0 2.420 2.884 2.668 1.5 3.633 4.335 4.005 2.0 4.856 5.782 5.344 2.5 6.045 7.215 6.660 3.0 7.290 8.682 7.980 Glycolato 1.0 1.560 2.267 2.002 1.5 2.339 3.390 3.006 2.0 3.124 4.528 4.000 2.5 3.918 5.670 5.005 3.0 4.716 6.810 6.012 Table 3: Calculation Kinetic data for the oxidation of pentammine Co(III) of αHydroxy acids by QxBC [QxBC] = 10-3mol dm-3 [HClO4] = 0.1 mol dm-3 Temperature = 323K [Micelles] = 1.00 x10-2 mol dm-3

Fig. 3: Dependence of rate on Co(III) of α-Hydroxy acids with NaLS

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Induced Electron Transfer Reactions in Pentaammine Cobalt(III) Complexes of α-Hydroxy Acids By Quinoxalinium Bromo Chromate in Aqueous Acetic Acid (50%v/v) Medium: A Kinetic Study (IJSRD/Vol. 3/Issue 11/2016/023)

F. Mechanism The oxidation of α-hydroxy acids and their complexes by QxBC in an atmosphere of nitrogen failed to induce the polymerization of acrylonitrile which discounts the possibility of any radical formation. The oxidation of αhydroxy acids and their complexes was catalysed by perchloric acid, which may well be attributed to protonation of QxBC to give a stronger oxidant and electrophile. The formation of a protonated Cr(VI) species has been postulated earlier. The kinetic and presence of Co(III) of puruvate as a product leads to a mechanism involving one electron transfer to Cr(VI) [Scheme-1] the mechanism for the oxidation of Lactic acid, mandelic acid and glycolic acid will be similar to the mechanism for the oxidation of lactic acid. As well as the mechanism of glycolato and mandelato pentaammine cobalt(III) perchlorates are similar.

N CrO3Br-

+ H+

N+

N

kf

O-

Br

Cr O

kr

OH

N+

H

H

Protonated QxBC

QxBC H

O-QxH+

HO

CH3 C

+

OH

COOCoIII Co(III) Lactato

Cr Br

O

OH H

O-QxH+

CH3 C

Cr

O

Br

III

COOCo

OH

k Slow

H H

O-QxH+

O

Cr CH3 C

Br

O

COOCoIII

OH

fast COOCoIII CO

+ H2O +

O-QxH+

Cr Br

CH3

O

CrIV Co (III) Pyruvato Fig. 4: Sheme-1

IV. CONCLUSION The kinetics of oxidation of pentaammine cobalt(III) complexes of α-hydroxy acids has been investigated in aqueous acetic acid medium(50% v/v) in the presence of perchloric acid with various Micellar by spectrometrically at 323K. The rates of oxidation of reaction were increased on increasing [HA] and hydrogen ion. The reaction does not show the polymerization, which indicates the absence of free radical intermediate in the oxidation. The suitable mechanism is proposed involving the hydride ion transfer in the rate determining step. The rate of oxidation of reaction enhances while added the CTAB much more than TRITON and NaLS. ACKNOWLEDGEMENT The Authors express their sincere thanks to the Management of Vaniyambadi Muslim Educational Society and PG & Research Department of Chemistry, Islamiah College Vaniyambadi for providing the necessary facilities for the successful completion of this research work.

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