Mechanistic study of osmium(viii) promoted oxidation of crotonic acid by aqueous alkaline solution of potassium iodate

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1 Indian Journal of Chemistry Vol. 54A, November 2015, pp Mechanistic study of osmium(viii) promoted oxidation of crotonic acid by aqueous alkaline solution of potassium iodate Bharat Singh*, Sandeep Kumar Mishra, Shipra Tripathi & Deepmala Gupta Department of Chemistry, University of Allahabad, Allahabad , India Received 8 September 2015; revised and accepted 30 October 2015 Kinetics and mechanism of Os(VIII) promoted oxidation of crotonic acid (CA) by alkaline solution of potassium iodate (KIO 3 ) have been investigated at 35 C. Zero order kinetics with respect to potassium iodate is observed. The reaction exhibits linear dependence of reaction rate at low [CA], tending towards zero order at its higher concentrations. First order kinetics with respect to catalyst osmium tetroxide (OsO 4 ) has been observed. Accelerating effect of [OH ] ion on reaction rate is also observed. The reaction shows negligible effect of variation of ionic strength of the medium, while a marked - effect of rise in temperature on the rate is observed. IO 3 and [OsO 4 (OH) 2 ] -2 are postulated as the reactive species of potassium iodate and Os(VIII), respectively. The reaction has been studied at four different temperatures (30, 35, 40 and 45 0 C) and the overall activation parameters have been computed. A plausible mechanism conforming to the observed kinetics has been suggested. The main oxidation products have been identified as acetic acid and glyoxylic acid. A suitable mechanism involving attack of potassium iodate species on complex formed between catalytic species of osmium tetroxide and crotonic acid in the slow and rate determining step is suggested. The rate law, Rate = [ ] = [] [ ][()], is consistent with observed kinetic results. [][ ] Keywords: Kinetics, Reaction mechanisms, Oxidation, Potassium iodate, Crotonic acid, Osmium tetroxide Potassium iodate is used as a source for dietary iodine in many countries. Potassium iodate may also be used to protect against accumulation of radioactive iodine in the thyroid by saturating the body with a stable source of iodine prior to exposure 1. Approved by the World Health Organization for radiation protection, potassium iodate (KIO 3 ) is an alternative to potassium iodide (KI), which has poor shelf life in hot and humid climates 2. During nuclear emergency, nuclear disaster or nuclear power plant reactor meltdown, potassium iodate is used as nuclear protector agent for all kinds of life. The oxidative capacity of potassium iodate 3,4 has earlier, been reported with and without the presence of metals ions as catalyst. Literature on Rh(III)-catalysed and Hg(II) co-catalysed oxidation of diols is also reported 5,6. Household uses of crotonic acid include toilet, metal and drain cleaners, rust remover, in batteries, and as a primer for artificial nails. Industrial uses include metal refining, plumbing, bleaching, engraving, plating, photography, disinfection, munitions, fertilizer manufacture, metal cleaning, and rust removal. Crotonic acid is also used in the copolymerization of crotonic acid hydrogel systems by using gamma-rays. The crosslinked copolymeric crotonic acid hydrogels release fertilizers and drugs that help prevent environmental pollution 7. Bio-based production of crotonic acid by pyrolysis of poly(3-hydroxybutyrate) inclusions is also reported 8. Crotonic acid undergoes most of the additions and condensation peculiar to ethylenic bond and the carboxylic group. The oxidation of crotonic acid by hexacyanoferrate(iii) 9, chloramine-t 10, periodate 11, potassium bromate 12 and N-chlorosuccinimide 13 have been shown to bring about cleavage of the carboncarbon bond. Earlier reports 14 show that the double bond of unsaturated acids is reactive but its oxidation is restricted towards one-electron oxidants and hence the need for their catalysed oxidation is prompted. Osmium tetroxide has been widely used as a homogeneous catalyst in various redox reactions, particularly in alkaline medium 15,16. It is known to react across olefinic double bonds, as it is a well-known agent for cis-hydroxylation. Earlier, osmium(viii) catalysis in potassium bromate 12 oxidation of crotonic acid (CA) in alkaline medium, Pd(II) and Os(VIII)

2 1388 INDIAN J CHEM, SEC A, NOVEMBER 2015 catalysis in oxidation of CA by chloramine-t 10 in acid and Pd(II) catalysis in N-chlorosuccinimide 13 and periodate 11 oxidation of CA in acidic medium have been reported. However, to date no information is available regarding Os(VIII) catalysis in oxidation of CA by potassium iodate in alkaline medium. In view of potential applications of unsaturated compounds in general and industrially important CA in particular, there seems to be most exciting chemistry in further probing the oxidative capacity of potassium iodate in oxidation of CA, particularly in the presence of alkaline solution of osmium tetroxide. This prompted us to study the kinetics and mechanism of Os(VIII) catalysis in oxidation of CA by potassium iodate in alkaline medium with the primary aim to ascertain (i) the reactive species of potassium iodate in alkaline medium, (ii) the actual catalytic species of Os(VIII) in alkaline medium, (iii) the reaction products, and finally, (iv) the mechanistic steps and to discuss the rate law consistent with the observed kinetic results. Materials and Methods The reagents employed, i. e., crotonic acid (Koch-light), OsO 4 (Johnson and Matthey), NaClO 4 (E. Merck), KIO 3 (BDH Laboratory), and all other reagents used were of AR grade. All solutions were prepared in doubly distilled water. The stock solution of OsO 4 was prepared in dilute NaOH solution ( mol dm -3 ). The solution of KIO 3 was prepared by direct weighing and was standardized iodometrically. The reaction vessels were coated on the outside with black varnish to avoid any photochemical effect. NaClO 4 was used to maintain the required ionic strength of the medium. Appropriate volumes of the solutions of CA, NaOH, NaClO 4, OsO 4 and the requisite volume of doubly distilled water were taken in the reaction vessel, which was placed in an electrically operated thermostatic water bath maintained at the desired temperature within the ±0.1 C range. When the mixture attained the bath temperature, the reaction was initiated by adding the required volume of KIO 3 solution, which was also kept separately in the same bath in another vessel. The kinetics of the reaction was followed by estimating the quantity of unconsumed KIO 3. An aliquot (5 ml) of the reaction mixture was withdrawn at regular time intervals and was monitored by iodometric determination of the remaining KIO 3 up to two half lives of the reaction. The rate of reaction (-dc/dt) in each kinetic run was determined by the slope of the tangent drawn at fixed concentrations of KIO 3 (i.e. [KIO 3 ] * ) in the plots of unconsumed [KIO 3 ] versus time. The order of the reaction with respect to each reactant was determined with the help of (-dc/dt) values calculated for various concentrations of each reactant. Stoichiometry and product analysis Varying [CA]: [KIO 3 ] ratios were equilibrated at 35 C for 72 hours under the experimental condition [CA] << [KIO 3 ]. Estimation of remaining [KIO 3 ] in different sets indicated that one mole of CA consumes one mole of KIO 3. Accordingly, the following stoichiometric equation can be formulated: Acetic acid was identified as the product by thin layer chromatography 17 using n-butanol-dimethylamine-water in the ratio of 85:1:14 as the developing solvent to verify the presence of acetic acid in oxidation of CA. The R f value was found to be 0.44, which is very close to the reported R f value of 0.45 for acetic acid in the aforesaid solvent system. These products were analyzed by NUCON gas chromatography using Porapak-Q 101column and programmed oven temperature with an FID detector 18. The major products were identified as glyoxylic acid and acetic acid in the Os(VIII) promoted oxidation of CA by KIO 3 in alkaline medium, by comparing the retention times of the identified products and their standard solutions. Results and Discussion The kinetic investigations were carried out at several initial concentrations of reactants. The value of the initial rate in each kinetic run was calculated from the slope of the tangent of plot of unconsumed [KIO 3 ] versus time at fixed [KIO 3 ]* except in [KIO 3 ] variation when a tangent is drawn at a fixed time. The initial reaction rate (-dc/dt) remains almost constant with increase in concentration of KIO 3, showing thus zero order kinetics with respect to [KIO 3 ] (Table 1). The initial reaction rate (-dc/dt) increases linearly with increase in [Os(VIII)], indicating first order kinetics with respect to [Os(VIII)] (Table 1). Furthermore, the first order rate constant k 1 = (-dc/dt) / [Os(VIII)] values are nearly the same for different [Os(VIII)], establishing a first order dependence on the [Os(VIII)] (values not shown in table). First order kinetics with respect to Os(VIII)

3 SINGH et al.: Os(VIII) OXIDATION OF CROTONIC ACID BY AQUEOUS ALKALINE KIO Table 1 Effect of variation of [KIO 3 ], [Os(VIII)], [CA], [NaOH] and temperature on the rate constants [KIO 3] 10 4 [Os(VIII)] 10 5 [CA] 10 2 [NaOH] 10 3 (-dc/dt) 10 7 (mol dm -3 s -1 ) a b c d a 30 C; b 35 C; c 40 C; d 45 C. concentration is further confirmed by plotting a graph between (-dc/dt) and [Os(VIII)], where a straight line passing through origin is obtained (Fig. 1). The value of (-dc/dt) increases linearly with increase in [CA] at its lower range but tends to become nearly constant at its higher concentration range, showing first order in lower range of [CA] with tendency of shifting to zero order in its higher concentration range (Table 1). The plot of initial rate ( dc/dt) versus [CA] was linear in its low concentration range but deviates from linearity with the rate becomes nearly constant on further increasing Fig. 1 Plot of (-dc/dt) versus [Os(VIII)] at 35 C under the conditions of Table 1. Fig. 2 Plot of (-dc/dt) versus [CA] at 35 C under the conditions of Table 1. its concentration in its higher range, indicating first order at low [CA] tending to zero order at its higher concentrations (Fig. 2). The rate of the reaction increased with increase in [OH - ] and plot of log (-dc/dt) versus log [OH - ] was linear with fractional slope (0.61), showing a fractional order dependence of the reaction on [OH - ] (Fig. 3). The effect of ionic strength of the medium was studied in the range of to mol dm -3 (adjusted with NaClO 4 solution) at constant experimental conditions. It was found that ionic strength variation had negligible effect on the reaction rate (data not given in the table) indicating involvement of a dipole in the rate determining slow step. The reaction was studied in temperature range C, keeping other experimental conditions constant and various activation parameters have been

4 1390 INDIAN J CHEM, SEC A, NOVEMBER 2015 Fig. 3 Plot of log (-dc/dt) versus log [OH - ] at 35 C under the conditions of Table 1. computed from the rate measurements at these temperatures (Table 2). Addition of the reaction mixture to aqueous acrylamide solution does not initiate polymerization, ruling out the possibility of a free radical mechanism. Reactive species of potassium iodate in alkaline medium The oxidative power of potassium iodate has been earlier used in oxidation of compound in acidic as well as in alkaline medium 19-24, sometimes in the presence of catalyst 25,26. In acidic and alkaline media, potassium iodate is ionized as + - KIO3 K + IO3. The IO 3 - species is expected to act as an oxidizing species in acidic as well as in alkaline media. The iodide ions formed by the reduction of IO 3 -, may react with IO 3 - to give I 2 which being an oxidant in alkaline medium may trigger a parallel oxidation of crotonic acid (CA) and thus oxidation of CA by KIO 3 in alkaline medium may be complicated. Results in the presence of Hg(OAc) 2 as well as in its absence were practically the same for the oxidation of CA by alkaline potassium iodate. This experimental finding ensured pure KIO 3 oxidation of CA in absence of Hg(OAc) 2, hence all experiments were carried out without using mercuric acetate. Thus, assuming IO 3 - species as oxidising species Scheme 1 is suggested. Catalytic species of osmium(viii) in alkaline medium In alkaline medium osmium tetroxide is reported to exist in equilibria (1) and (2) at different concentration of OH ions, where equilibrium constant K 1 and K 2 have values of 24 and 6.8 respectively. Table 2 Effect of temperature variation on k 0 (-dc/dt) and values of activation parameters for the oxidation of crotonic acid by KIO 3 in the presence of Os(VIII) catalyst in alkaline medium. {Solution cond.: [KIO 3 ] = mol dm 3 ; [CA] = mol dm 3 ; [OH ] = mol dm 3 ; [Os(VIII)] = mol dm 3 } Temp. (K) k (mol dm -3 s -1 ) Activation parameters log k r 0.34 E a (kcal mol -1 ) H # (kcal mol -1 ) G # (kj mol -1 ) S # (J K 1 ) log A 9.75 K 1 [OsO 3 (OH) 3 ] + (OH) [OsO 4 (OH) 2 ] 2 + H 2 O K 2 [OsO 4 (OH) 2 ] 2 + OH [OsO 5 (OH)] 3 + H 2 O (1) (2) Equilibrium (1) is reported 18 to be operative at lower [OH - ] and equilibrium (2) exists at higher [OH - ] (>0.15 mol dm -3 ). Since the rate of the reaction increases with increase in [OH - ] in its lower concentration range employed in the present investigation, it is quite reasonable and convincing to assume equilibrium (1) to be operative. According to equilibrium (1), with the increase in [OH - ] the equilibrium shifts towards the right resulting in increased formation of the [OsO 4 (OH) 2 ] 2- species, which acts as the effective catalytic species in the present investigation. This contention also finds support 12 in the UV spectra of Os(VIII) with different concentration of alkali. It may thus be concluded that only one species, i. e., [OsO 4 (OH) 2 ] 2-, is present in the alkali concentration range studied in the present investigation. It is also reported that Os(VIII) forms complexes with unsaturated compounds. UV spectra of Os(VIII) with different [CA] clearly shows increase in the absorbance of pure Os(VIII) solution from 2.60 to 3.00 and 3.20 with [CA] increasing from 0 to to mol dm -3 (Fig. 4), indicating shifting of equilibrium (4) in Scheme 1 towards the right and thus facilitating the formation of [Os(VIII)-CA] complex which becomes the sole factor for the increase in absorbance. The observed shift in λ max value to the higher side with addition of increasing

5 SINGH et al.: Os(VIII) OXIDATION OF CROTONIC ACID BY AQUEOUS ALKALINE KIO H 3 C H H C C COOH 2- HO O Os O O O OH Fig. 4 UV spectra of Os(VIII) solution with different concentrations of CA solution. {Solution cond.: (1) [Os(VIII)] = mol dm -3 ; (2) [Os(VIII)] = mol dm -3, [CA] = mol dm -3 ; (3) [Os(VIII)] = mol dm -3, [CA] = mol dm -3 }. [CA] solution is ascribed to the increased presence of chromophore in the CA molecule. Reaction mechanism and derivation of rate law The following reaction steps in the Scheme 1 are suggested on the basis of reactive species of Os(VIII) and KIO 3 in alkaline medium and other kinetic results (I) obtained in the oxidation of crotonic acid by alkaline potassium iodate. The probable structure of the complex (C 3 ) may be given as (I) The diol formed as (X) in the slow step (6), further rapidly reacts with iodate ion to form iodate ester, which in the subsequent fast step gives acetic acid, glyoxylic acid and iodide ions as final products and regenerates the catalytic species for further catalysis (Steps 7-9). Step (7) is shown to occur as (7a), (7b) and finally (7c). Considering the steps (3-4) and (6-9) in Scheme 1, the rate of oxidation of crotonic acid (CA) may be written in terms of loss of concentration of KIO 3 with time as Eq. (10).

6 1392 INDIAN J CHEM, SEC A, NOVEMBER 2015 [Os(VIII)] = [C ] + K [C ][OH ] + k K [C ][CA][OH ] [Os(VIII)] = [C ] ( 1 + K [OH ] + k K [CA][OH ]) Or [C ] = [()] [ ] [][ ] (16) On substituting the value of [C 1 ] from Eq. (16) in Eq. (14) we have d [KIO ] = kd K K [CA] [OH ][Os(VIII)] dt 1 + K [OH ] + k K [CA][OH ] Or Rate = [ ] = kd[c ] (10) Considering the law of equilibrium for step (3) and (4) we have from step (3). [C ] K = [C ][OH ] Therefore [C ] = K [C ][OH ] (11) And from step (4) we have [C ] K = [C ][CA] Or [C ] = K [C ][CA] (12) Considering Eqs (11) and (12) we have [C ] = K K [C ][CA] [OH ] (13) On substituting the value of [C 3 ] from Eq. (13) in Eq. (10) we have [ ] = kd K K [C ][CA] [OH ] (14) Total concentration of osmium tetroxide, i.e., [Os(VIII)] T may be written as Eq. (15) from steps (3) and (4) [Os(VIII)] = [C ] + [C ] + [C ] (15) Considering Eqs (11), (13) and (15) we have [ ] = [] [ ][()] [ ] ( [] ) (17) Further on assuming the inequality K 2 [CA] >> 1, Eq. (17) can be written as Eq. (18). Rate = [ ] = [] [ ][()] [][ ] (18) The rate law (18) clearly explains the observed kinetics with respect to [CA] and [OH ]. Absence of [KIO 3 ] on the right side of Eq. (18) clearly indicates zero order with respect to potassium iodate. The observed decrease in rate at higher [Os (VIII)] is probably due to minor formation of the complex, [Os(VIII).IO 3 ] (Step 5) which seems to be involved either in the fast step or is inactive. The slow step (6) involves a dipole (H 2 O) indicating negligible effect of ionic strength of the medium on the rate of the reaction, validating the proposed scheme. Acknowledgement The authors wish to thank the University Grants Commission, New Delhi, India, for financial assistance to one of the authors (BS) under Project No. F /2010(SR) dated References 1 Astbury J, Horsley S & Gent N, J Public Health, 21 (4) (1999) Pahuja D N, Rajan M G, Borkar A V & Samuel A M, Health Phy, 65(5) (2008) Singh A K, Srivastava S, Srivastava J, Srivastava R & Singh P, J Mol Catal A: Chem, 72 (2007) Muthakia G K & Jonnalagadda S B, Int J Chem Kinet, 21 (1995) Singh B, Singh A & Singh A K, Indian J Chem, 50A (2011) Singh B, Singh A K & Singh A, Int J Pure Appl Chem, 3A (2011) 6.

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