A Kinetic and Mechanistic Study of Ir (III) Catalyzed Oxidation of Methionine by HCF (III) in aqueous alkaline medium medium

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A Kinetic and Mechanistic Study of Ir (III) Catalyzed Oxidation of Methionine by HCF (III) in aqueous alkaline medium medium

https://doi.org/10.21203/rs.3.rs-3751886/v1

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The kinetic Study of Ir (III) chloride catalyzed oxidation of Methionine by hexacyanoferrate (abbreviated as HCF) III ions in aqueous alkaline medium has been investigated spectrophotometrically at constant ionic strength of 0.5 mol dm^-3 and temperature 35 ± 0.1^●C. The reaction rate shows first order kinetics with respect to [HCF (III)], [OH^-] and [IrCl₃]. The order of reaction is also first with respect to [Substrate] at its lower concentration tending towards zero at higher concentration. The ionic strength of the reaction mixture shows positive salt effect on the reaction rate. Different activation parameters were also evaluated by studying the reaction at four different temperatures. The stoichiometry of the reaction corresponds to a reaction in which one mole of the amino acid reacts with four moles of HCF (III) ions. Methionine sulphone was found to be the main product of oxidation was identified using an organic solvent method and IR Spectroscopy. A suitable mechanism has been proposed to explain all the experimental results. It is assumed that reaction proceeds through complex formation between substrate (abbreviated) and iridium trichloride. A rate law has been derived which verifies the results.

CH₃ –S-(CH₂)₂-CH (NH₂) – COO⁻ + 4HCF (III) Ir(III),H₂O → CH₃-SO₂-(CH₂)₂-CH (NH₂)-COO⁻ + 4HCF (II)

Food Science & Technology

Iridium (III)

Hexacyanoferrate (III)

Methionine

Oxidation

Kinetics

Mechanism

Methionine is sulphur containing essential amino acid which acts as a powerful reductant in both acid and alkaline medium. It plays an important role in human metabolism and several in borne disease result from disturbances in its biochemistry. Individuals with methionine malabsorption syndrome are unable to absorb the compound from gut and its deficiency leads to the information of a fatty liver mainly due to accumulation of triacyl glycerol.^(1–3)

HCF (III) has been proven to be an efficient oxidant for a wide variety of organic substrate because the CN- ligands are resistant to substitution reaction and thereby outer –sphere electron transferred is the preferred oxidation pathway. Therefore oxidations are normally clean, devoid of side reactions which greatly facilitate the monitoring. ^(4–5). It is for this reason that a large number of transition metal ions such as Os(VIII)^(6–7),Cu(III)⁸, Ru(III)^(9–10). Ir (III) ^{(11 –13)}.

Since very fewer kinetics studies of the oxidation of methionine have been reported, the title reaction is one such reaction in which Ir (III) has been employed as a catalyst to understand the reactivity pattern of methionine.

The Kinetic experiment were carried out by mixing the required quantity of methionine solution maintained at constant temperature with solution of HCF(III), Sodium hydroxide, potassium Chloride and Iridium(III) Chloride kept at the same temperature. The solution of methionine was prepared by dissolving a weighed quantity in 0.25 M solution of NaOH. The concentration of HCF (III) was not more than 6 X 10^− 4 mol dm^-3. All experiments were carried out in thermostatic water bath at constant temperature 35 ± 0.1^●C and ionic strength 0.5mol dm^-3. Potassium chloride was used to keep the ionic strength constant. The progress of the reaction was measured spectrophotometrically at ƛmax 420 nm and there was no shift in ƛmax during the entire period of kinetic study. Plane mirror and Guggenheim methods were used for evaluation of initial rates (dA/dt) _i and pseudo –first order rate constant k₁ respectively. Spectroscopic and TLC methods were used to identify the oxidation product.

The Iridium (III) catalyzed oxidation of methionine has been studied in aqueous alkaline medium. Kinetic experiments were comes out at different concentrations of one reactant keeping the concentrations of other constant.

Stoichiometric and Product analysis

The stoichiometry of the reaction was studied by estimating the amount of HCF (II) ions produced after definite interval of time with standard solution of ceric (IV) sulphate using Ferroin indicator¹⁴. The result obtained indicated that four moles of HCF (III) was consumed by one mole of methionine and the product was identified as methionine sulphone^15–16. The stoichiometry of the reaction was found to correspond to the equation.

CH₃ –S-(CH₂)₂-CH (NH₂) – COO⁻ + 4HCF (III)\(\underrightarrow{\varvec{I}\varvec{r}\left(\varvec{I}\varvec{I}\varvec{I}\right),\varvec{H}2\varvec{O}}\) CH₃-SO₂-(CH₂)₂-CH (NH₂)-COO⁻ + 4HCF (II)

The reaction product was isolated by solvent extraction method using an organic solvent, n-butanol – glacial acetic acid-water and it was identified by its IR spectrum(Fig. 1) which showed the bands(v) at 1152.64 and 1394.32 cm^− 1 corresponding to O = S = O stretching, the band(v) at 1655cm^− 1 due to –COOH stretching and a broad band(v) at 3223cm^− 1 corresponds – NH stretching respectively. The positions of the absorption bands for sulphone were found to be in good agreement with the literature values of symmetric and asymmetric SO2 stretching frequency.

Effect of Oxidant

The effect of Oxidant was studying by varying the concentration of HCF(III) from 2 x 10^− 4 to 6 x 10^− 4 mol dm^− 3 were carried out keeping fixed concentrations of the other reagent. The initial rates were calculated by plotting absorbance vs time plots. (Fig. 2) Pseudo first order plots were made and the evaluated first order rate constant were found to be independent of the initial concentration of the oxidant, thereby confirming the order with respect to the oxidant to be one.(Fig. 3)

Effect of Substrate

To study the effect of substrate concentration on reaction rate, the substrate concentration has been varied from 0.1 x 10^− 4 to 1 x 10^− 4 mol dm^− 3. First order dependence of rate on lower concentration of substrate tends to be zero order at its higher concentration. The linear plots of rate^− 1 vs [S]⁻¹ suggests Michaeils – Menten type of kinetics.(Fig. 4)

Effect of Hydroxide ion

Increase in rate with hydroxide ion concentration reveals first order kinetics with respect to hydroxide ion concentration. The concentration has been varied from 0.1–0.4 mol dm-3.

Effect of Catalyst

The reaction rate also increases with the catalyst concentration indicating a direct dependence of rate on catalyst concentration. The concentration of iridium was varied from 0.335 x 10^− 5 to 3.35 X 10^− 5 mol dm^− 3.(Fig. 5)

Effect of Ionic Strength

Effect of ionic strength was studied by using KCl. The linear increase in log(-dc/dt) with √µ suggest the positive salt effect i.e.the involvement of two similarly charged reacting species in the reaction.(Fig. 6)

Activation Parameter

Thermodynamic parameters of the reaction were studied at four different temperature 35, 40, 45 and 50°C. Linear plots of log K2 vs 1/T indicate that the reaction obeys Arrhenius equation. The value of energy of activation Ea, enthalpy of activation ∆H#, entropy of activation∆S # and free energy of activation ∆F # were evaluated as ginen in table 2.(Fig. 7) The graphical value of energy of activation (Ea) is 13.07 Kcal mol^− 1.

Table 1. Effect of [HCF(III)],[Methionine],[NaOH],and [Ir(III)] on the reaction rate at temperature 35°C of reaction mixture

[HCF(III)] X 10^− 4 mol dm^− 3	[Methionine) ] X 10^− 4 ⁻ mol dm^− 3	[Ir(III)] X 10^− 5 ⁻ mol dm^− 3	[NaOH] X 10^− 4 ⁻ mol dm^− 3	(dA/dt)_i X 10³(min^− 1)
2.0	3.0	3.35	0.4	1.4
3.0	3.0	3.35	0.4	2.2
4.0	3.0	3.35	0.4	3.0
5.0	3.0	3.35	0.4	3.9
6.0	3.0	3.35	0.4	4.8
3.0	1.0	3.35	0.4	0.6
3.0	2.0	3.35	0.4	1.4
3.0	3.0	3.35	0.4	2.0
3.0	4.0	3.35	0.4	2.6
3.0	5.0	3.35	0.4	3.4
3.0	6.0	3.35	0.4	4.0
3.0	7.0	3.35	0.4	4.6
3.0	8.0	3.35	0.4	5.0
3.0	9.0	3.35	0.4	5.0
3.0	10.0	3.35	0.4	5.0
3.0	3.0	0.335	0.4	0.4
3.0	3.0	0.67	0.4	0.9
3.0	3.0	1.01	0.4	1.3
3.0	3.0	1.34	0.4	1.7
3.0	3.0	1.67	0.4	2.0
3.0	3.0	2.01	0.4	2.4
3.0	3.0	2.34	0.4	2.8
3.0	3.0	2.68	0.4	3.1
3.0	3.0	3.01	0.4	3.6
3.0	3.0	3.35	0.4	4.0
3.0	3.0	3.35	0.1	1.2
3.0	3.0	3.35	0.2	2.0
3.0	3.0	3.35	0.3	2.8
3.0	3.0	3.35	0.4	3.6

Table 2 Temperature Parameter

[HCF(III) = 3.00 X 10^− 4 mol dm^− 3, Methionine = 3.00 X 10^− 4mol dm^− 3,[NaOH] = 0.4mol dm^− 3, [Ir(III)] = 3.35 X 10-5mol dm^− 3,ƛmax = 420nm,µ = 0.5 mol dm^− 3.

Temp.

[●C]

Initial rate

(dA/dt)i x 10³

(min^− 1)

K2 x 102

(l mol^− 1sec^− 1)

Temp. Coefficient

Energy of Activation

(Ea)

(kcal mol^− 1)

Frequency Factor A X10-8(lmol^− 1 sec^− 1)

Energy of Activation

-∆S#

(E.U.)

Enthalpy of Activation

∆H#

(kcal mol^− 1)

Free Energy of Activation

∆F#

(kcal mol^− 1)

1.6

2.7

3.4

5.2

3.84

5.63

7.67

10.74

2.0

1.90

14.66

11.85

13.74

1.276

1.318

1.279

1.289

23.502

23.469

23.560

23.575

12.808

12.798

12.788

12.778

20.039

20.157

20.275

20.392

Mean value

1.95 ± 0.05

13.42 ± 1.57

1.291 ± 0.015

23.527 ± 0.058

12.793 ± 0.015

20.216 ± 0.177

Reacting Species of Iridium

Methionine exists as Zwitter ion in aqueous solution. In case of 3rd transition series, element of higher states are more stable. It is reported ^(17–21) that in hydrochloric acid medium IrCl₆³⁻ is the stable species. It does not give divalent complex. The stable species are Ir³⁺and Ir¹⁺. The aquation of IrCl₆³⁻ gives IrCl₅ (H₂O) ₂, IrCl₄ (H₂O) ₂¹⁻ species as shown as following equation -

[IrCl₆]³⁻ + nH₂O → [IrCl_6 − n (H₂O) _n]⁻³⁽⁻ⁿ⁾

Since no retarding effect of Cl⁻ ions was observed, the hydrated species cannot be the reactive species. Only [IrCl₆]³⁻ have been considered as the only reactive species. Sheila et al.²² also reported IrCl₆³⁻ as the reactive species in oxidation reaction.

Mechanism:-

In the mechanism it is assumed that organic substrate form a loosely bonded complex(C₁) with Ir³⁺ which slowly reacts with HCF(III) ions resulting into Ir⁺,HCF(II) and intermediate product (IP). Ir1 + is deoxidized to Ir³⁺ by two moles of HCF (III) via one electron transfer process and the IP disproportionate to final product.

The rate of reaction can be given as

Rate = \(\frac{-d\left[HCF\left(III\right)\right]}{dt}\) = K₂ [C₁][HCF (III)]

According to above mechanism the rate of disappearance of HCF(III) can be given as-

\(\frac{-d\left(HCF\right(III)}{dt}\) = \(\frac{k1k2K\left[S\right]\left[OH-\right]\left[HCF\left(III\right)\right]\left[Ir3+\right]T}{k-1+k2\left[HCF\left(III\right)\right]+ k1K\left[S\right][OH-]}\) -----(1)

At very low concentration of HCF (III)ions and organic substrate the value of k2[HCF(III)] and k1K[S][OH-] will be quite small. Hence neglecting these factors so Eq. 1 reduces to

r = \(\frac{k1k2K\left[S\right]\left[OH-\right]\left[HCF\left(III\right)\right]\left[Ir3+\right]T}{k-1}\) --- (2)

r = kk₂K[S][OH⁻][HCF(III)][Ir³⁺]T

where k =\(\frac{k1}{k-1}\)

The rate law Eq. (2) clearly accounts for the first order kinetics with respect to HCF(III), organic substrate, hydroxide ion and the catalyst at their lower concentration.

The validity of rate law might be ensured by rewriting the Eq. (1) as follows—

\(\frac{1}{r}\) = \(\frac{\text{k}-1}{\text{k}1\text{k}2\text{K}\left[\text{S}\right]\left[\text{O}\text{H}-1\right]\left[\text{H}\text{C}\text{F}\left(\text{I}\text{I}\text{I}\right)\right][\text{I}\text{r}3+]}\) + \(\frac{1}{\text{k}1\text{K}\left[\text{S}\right]\left[\text{O}\text{H}-\right]\left[\text{I}\text{r}3+\right]\text{T} }\) + \(\frac{1}{\text{k}2\left[\text{H}\text{C}\text{F}\left(\text{I}\text{I}\text{I}\right)\right]\left[\text{I}\text{r}3+\right]\text{T}}\)

The straight line plots between 1/rate vs 1/HCF (III) and 1/rate vs 1/Methionie proves the validity of above rate law. The value of k2 and k1Khas been calculated from the intercept of 1/rate vs 1/HCF (III) and 1/rate vs 1/methionine plots(2.551 X 10^− 6 and 4.0788 X 10⁴ mol^− 1S^− 1 respectively). The constancy in k1K and k2 clearly shows the validity of derived rate law equation on the basis of proposed mechanism.

Acknowledgements:- I am very grateful to Professor A.K.Shukla and Dr. Sreenivas Rao, Department of chemistry, East Point college of Engineering and Technology for their encouragement ,It’s really inspired me a lots. Author wants to dedicate this work to Prof. Anjali Goel, Department of chemistry, Kanya Gurukul Mahavidhyaya, Uttarnchal (U.A.) India.

Conflict of Interest:- The author declared no potential conflicts of interest with respect to the research, authorship and publication of this article.

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The authors declare no competing interests.

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A Kinetic and Mechanistic Study of Ir (III) Catalyzed Oxidation of Methionine by HCF (III) in aqueous alkaline medium medium

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Abstract

Figures

Introduction

Experimental

Result and Discussion

Stoichiometric and Product analysis

Effect of Oxidant

Effect of Substrate

Effect of Hydroxide ion

Effect of Catalyst

Effect of Ionic Strength

Activation Parameter

Reacting Species of Iridium

Mechanism:-

Declarations

References

Additional Declarations

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