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Journal of Chemistry and Chemical Sciences, Vol. 5(7), 414-423, July 2015 ISSN 2229-760X (Print) (An International Research Journal), www.chemistry-journal.org ISSN 2319-7625 (Online)

Kinetic and Mechanistic Study of Ru (III) Catalyzedoxidation of Galactitol by Chloramine-T: in Acidic medium.

Amrita Srivastava and Swarn Lata Bansal

Department of Chemistry, University of Lucknow, Lucknow, INDIA. email:[email protected], [email protected].

(Received on: July 29, 2015)

ABSTRACT

Kinetics and mechanismof Ru (III)-catalysed oxidation ofGalactitolby N- chlorosulphonamide (CAT) have been investigated in perchlorlc acid media in the presence of mercuric acetate as chloride ion scavenger. The results showed zero order kinetics with respect to galactitol and first order with respect to Ru (lll) and chloramine-T. There is no effect of sodium perchlorate, KCl and mercuric acetate show zero effect on reaction rate. Various activation parameters have been computed. Galatonic acid has been identified as the products, and a suitable mechanism consistent with observed kinetic results has been proposed.

Keywords: Kinetics, oxidation, galactitol, chloramine-T, Ru (III) catalyst, mercuric acetate.

INTRODUCTION

Sugar alcohols are obtained through hydrogenation of mono- and disaccharides. The most common sugar alcohols derived from monosaccharaides are sorbitol, mannitol, erythritol, xylitol and galactitol. Most of these sugar alcohols are chemically converted from corresponding sugars using a metal catalyst such as raney-nickel. Galactitol (Dulcitol) is a naturally occurring sugar alcohol with six carbon atoms, the reduction product of galactose. It is a saccharine substances (C 6H14 O6) and isomeric with mannitol. In the people with galactokinase deficiency is a form in the lens of eye leading to cataracts. It is produced from galactose in the reaction catalyzed by aldose reductose. Galactose itself comes from the metabolism of the disaccharide lactose into glucose and galactose. Some work has been done on the sugar alcohols such as sorbitol and xylitol dehydration catalysed by silicotungestic acid in water 1, oxidation of mannitol by cerium (IV) in aqueous sulphuric acid medium 2,

July, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org Amrita Srivastava, et al., J. Chem. & Cheml. Sci. Vol.5(7), 414-423(2015) 415 oxidation of mannitol by potassium bromate in aqueous acidic medium 3, oxidation of erythritol and dulcitol by N-Bromosuccinamide 4, oxidation of erythritol by chloramine T 5, oxidation of xylitol on Pt(III) in acidic medium 6, oxidationofdulcitol in N-chloroparatoluene- sulphonamide in alkaline medium 7. Ruthenium (III) is an efficient catalyst 8-10 in many redox reactions due to its ability to form intermediate complexes, free radicals, and multiple oxidation states. Chloramine-T11-13 is well known as analytical reagent for the determination of diverse substrates and mechanistic 14-15 aspects of reactions have been documented. They react with wide range of functional groups effecting an array of molecular transformations. The chemistry of sodium N-haloarenesulphonamines (N-haloamines) are the precursors of haloniumcations, hypohalite species and N anions capable of acting both as bases and as nucleophiles. It resemble hypohalites in their oxidative behavior. However there are no reports on the oxidation kinetics of galactitol with CAT. The kinetics of oxidation of sugars 16 has been subject of extensive research in recent years. This is attributed to the economic and biological importance of carbohydrates to living organisms. The oxidations have been carried out in both acidic 17 and alkaline 18 media using such oxidants as transition metal ions, inorganic acids, organometallic complexes and enzymes. Carbohydrates are a major sources of energy for living organisms and the understanding of the oxidation of sugars is therefore of immense importance. The oxidation of sugars especially the mono and disaccharides has been the subject of extensive research. Sugar oxidation occurs under different conditions of, temperature and ionic strength giving products that depend on the oxidants used. The kinetics and mechanism 19-20 of oxidation of monosaccharaides and disaccharides have been studied in both acidic and alkaline media, employing different transition metal ions, inorganic acids, complex ions and hydrogen peroxide as oxidants. The results showed that the mechanism may depend on the nature of the substrates, in some cases it involves the formation of intermediate complex, free radical or transition states. Therefore, it was imperative to study the kinetics of oxidation of polyhydroxy alcohols by CAT in the presence of acidic solution of ruthenium (III) chloride. In the present communication we report the results of oxidation of galactitol by chloramine-T in per chloric acidic medium with Ruthenium (III) as catalyst.

EXPERIMENTAL

Materials

Aqueous solution of galactitol (E. Merck), Chloramine-T (A.R grade), and mercuric acetate (E. Merck) were prepared by dissolving the weighed amount of sample in triple distilled water. Perchloric acid (60%) of E. Merck grade was used as a source of hydrogen ions. Ruthenium (lII) chloride (Johnson Matthey) was prepared by dissolving the sample in hydrochloric acid of known strength. All other reagents of analytical grade were available. Sodium perchlorate (E. Merck) was used to maintain the ionic strength of the medium. The reaction stills were blackened from outside to prevent photochemical effects.

July, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org 416 Amrita Srivastava, et al., J. Chem. & Cheml. Sci. Vol.5(7), 414-423(2015) Kinetics

A thermostated water bath was used to maintain the desired temperature within ±1.0 °C. Requisite volume of all reagents including substrate, were taken in reaction vessel and thermostated at 40°C for thermal equilibrium. A measured volume of chloramine-T solution, which was also maintained separately at the same temperature, was rapidly poured into the reaction vessel. The kinetics was followed by examining aliquot portion of reaction mixture for chloramine-T iodometrically using starch as an indicator, after suitable time intervals.

Stoichiometry and product analysis

To determine the stoichiometric ratio of the reaction, sets of reaction mixture containing varying ratios of CAT and galactitol were prepared and the reaction in each case allowed proceeding for 48 h. A blank was run under similar conditions for the same period of time. The excess CAT remaining in each set was estimated iodometrically. It has been found that one mole of galactitolconsumed two mole of CAT. Accordingly, the following stoichiometric equation for overall reaction could be formulated: RCH 2OH + 2CH 3C6H4SO 2NNaCl RCOOH + 2CH 3C6H4SO 2NH 2+ 2NaCl Where R=CH 2OH (CHOH) 4, whose oxidation product is Galactonic acid has been detected.

Spectra measurement

The absorption spectra of solutions of different concentrations of Ru (III) ion and CAT were recorded in the visible region: 200-600 nm respectively. The peak observed at 245 nm in the presence of acidic medium in the mixture of oxidant (CAT) and Ru(III) catalyst. The kinetic data were collected at 240-260 nm at different concentration of reaction mixture. The set of reaction mixture contain reducing Galactitol, Perchloric acid, Mercuric acetate Ruthenium chloride, excess of Chloramine-T were taken in the reaction vessel and kept for two day at room temperature and the spectral data was taken. The absorbance bands of these solutions were observed at 255nm.

Set of Reaction mixture A 1 Ru(N/10)+CAT(N/400) 0.7 B A Galactitol(N/400)+CAT(N/200) S 8 B 0.6 S O 0.5 R 6 O B R 0.4 A 4 B 0.3 N A C 2 N 0.2 C E 0.1 0 E 0.0 200 300 40 500 600 700 800 240 245 25 25 260 265 27 Wavelength (nm) Wavelength (nm)

Fig 1. An infrared spectrum has been used in order to identify the nature of reaction product (in solution) and to follow their stretching and bending modes. This spectrum shows

July, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org Amrita Srivastava, et al., J. Chem. & Cheml. Sci. Vol.5(7), 414-423(2015) 417 typical and broad O-H stretch which is superimposed upon C-H stretch. The sharp –OH absorption band around 2971 cm -1 can be observed in dilute solution. This shows C=O stretching at 1721 cm -1 in the solution.

RESULT AND DISCUSSION

Kinetics of oxidation of galactitol by acidic solution of ChloramineTin presence of Ru (III) chloride as homogeneous catalyst has been studied at constant temperature 40 0C(Table 1). Order of reaction with respect to each reactant was determined by varying the concentrations of oxidant, substrate, Ru (III) chloride, H+ and mercuric acetate one by one in different sets keeping concentrations of all other reactants constant at constant temperature 40 0C (fig 1& 2). In each kinetic run, the initial rate (-dc/dt) of the reaction was determined by the slope of the tangent drawn at fixed time. The first order rate constant K1, for the variations of all reactants were calculated as Rate = (-dc/dt) [CAT]* Where [CAT]*denotes the [CAT] at which (-dc/dt) was determined.

First order kinetics was observed in case of RuCl 6, which was also confirmed by a plot of (-dc/dt) versus [RuCl 6] (Fig.-1). Mercuric acetate was found to have a limited role as chloride ion scavenger only as it’s showed zero effect on the rate of oxidation of galactitol. The effect of ionic strength ( µ) variation on rate of reaction was studied by carrying out investigation in the presence of different amounts of sodium perchlorate (Table 2). The results indicated negligible effect of µ on the reaction rate. The reactions were studied at four different temperatures i.e., 35 0, 40 0, 45 0, 50 0C (Table 3). The rate constant at their temperatures lead to compute Ea, ∆S*, ∆H*, and ∆G* in the oxidation of galactitol and there activation parameters are recorded(Table-4). Negligible effect of ionic strength and dielectric constant might be due to involvement of neutral substrate in the reaction. The moderate ∆H* and ∆S* values are favorable for electron transfer reaction. The value of ∆H* was due to energy of solution changes in the transition state. The negative value of ∆S* suggests that the intermediate complex is more ordered than the reactants. The observed enthalpy of activation

July, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org 418 Amrita Srivastava, et al., J. Chem. & Cheml. Sci. Vol.5(7), 414-423(2015) and a higher rate constant for the slow step indicates that the oxidation presumably occurs via an inner-sphere mechanism. This conclusion is supported by earlier observations. The activation parameters evaluated for the catalyzed reaction explain the catalytic effect on the reaction. Kinetics observations show that the reactions under investigation are complex reaction, which usually take place in more than one step. In such reactions as in the present investigation, there is every possibility for the formation of complex or complexes between different reactants of the reactions.

Table 1 Effect of variation of Oxidant, Ru (III) and Galactitol at Temp 40 0C

3 -3 6 3 7 5 -1 [CAT] X 10 Moldm Ru(III)X10 Galactitol X10 (-dc/dt)X10 K1 X 10 S Moldm -3s-1 Moldm -3 ML -1S-1 0.83 1.00 1.00 0.18 0.21 1.00 1.00 1.00 0.20 0.20 1.25 1.00 1.00 0.25 0.20 1.67 1.00 1.00 0.35 0.21 2.50 1.00 1.00 0.50 0.20 5.00 1.00 1.00 1.00 0.20 1.00 1.0 1.00 0.06 0.06 1.00 1.5 1.00 0.09 0.06 1.00 2.0 1.00 0.13 0.06 1.00 2.5 1.00 0.15 0.06 1.00 3.0 1.00 0.18 0.06 1.00 3.5 1.00 0.22 0.06 1.00 1.00 0.83 0.20 - 1.00 1.00 1.00 0.17 - 1.00 1.00 1.25 0.145 - 1.00 1.00 1.67 0.14 - 1.00 1.00 2.50 0.135 - 1.00 1.00 3.33 0.13 -

0.21 1.0

1 - 1

0.18 - S 1 S - 1 - 0.8 0.15 ML 7 ML - -7

7 0.6 0.12 0.4 0.09 -dc/dt)X10 (

dc/dt)X10 0.2 - 0.0 ( 6 0 1.0 1.5 2.0 2.5 3.0 3.5 1 2 3 4 5 6 3 Ru(III)X10 M [CAT]X10 M 6 -7 1 Plot between [Ru(III)]X10 6 M and (-dc/dt)X10 7 Plot between [CAT]X10 3 M and (-dc/dt)X10 7 ML -1S- ML -1S-1

July, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org Amrita Srivastava, et al., J. Chem. & Cheml. Sci. Vol.5(7), 414-423(2015) 419

0 Table -2 Effect of variation HClO 4, KCl, Hg(OAc) 2, NaClO 4 at 40 C

3 3 3 3 7 ML -1S -1 [HClO 4] X 10 Mol [KCl] x 10 Mol [Hg(OAc) 2] X 10 Mol [NaClO 4] x 10 Mol -(dc/dt)X 10 0.83 1.00 1.00 1.00 0.22 1.00 1.00 1.00 1.00 0.21 1.25 1.00 1.00 1.00 0.22 1.67 1.00 1.00 1.00 0.21 2.50 1.00 1.00 1.00 0.18 5.00 1.00 1.00 1.00 0.16 1.00 0.83 1.00 1.00 0.16 1.00 1.00 1.00 1.00 0.16 1.00 1.25 1.00 1.00 0.16 1.00 1.67 1.00 1.00 0.15 1.00 2.50 1.00 1.00 0.13 1.00 5.0 1.00 1.00 0.15 1.00 1.00 0.83 1.00 0.15 1.00 1.00 1.00 1.00 0.15 1.00 1.00 1.25 1.00 0.12 1.00 1.00 1.67 1.00 0.15 1.00 1.00 2.5 1.00 0.13 1.00 1.00 5.0 1.00 0.12 1.00 1.00 1.00 0.83 0.15 1.00 1.00 1.00 1.00 0.15 1.00 1.00 1.00 1.25 0.14 1.00 1.00 1.00 1.67 0.14 1.00 1.00 1.00 2.5 0.13 1.00 1.00 1.00 5.0 0.12

Table -3: Effect of the temperature on the Reaction Rate

T0C (-dc/dt) 30 0C 1.2 35 0C 1.8 40 0C 2.6 45 0C 3.5 50 0C 4.2

Table-4: Values of Activation Parameters

Substrate Ea(KJmol -1) log A ∆S*(JK -1mol -1) ∆F*(KJmol -1) ∆H*(KJmol -1) ∆G*(KJmol -1)

Galactitol 86.13 13.78 -283.57 174.88 83.53 88.84

July, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org 420 Amrita Srivastava, et al., J. Chem. & Cheml. Sci. Vol.5(7), 414-423(2015)

4.5

4.0

3.5

3.0

2.5 4+log K 4+log K 2.0

1.5

1.0

0.5 29 30 31 32 33 1/T X10 4 K-1 Arrhenius plot for oxidation of Galactitol

REACTION MECHANISM

A common mechanism for oxidation of galactitol obtained from free energy value (∆G*). The zero order rate constants against the alcohol concentrations were linear. The linear plot of 1/kobs against 1/[S] is also an indication that the rate of the oxidation of this alcohol is related to the substrate concentration by the mechanism.

( fast) RNClNa - + RNCl + Na (l)

(fast) - + - RNCl + H RNHCl (ll)

K1 2- RNHCl + RuCl5(H2O) RuCl5(H2O)RNHCl (lll) [Y] K-1 [X]

2- CH2OH CH2-O-Cl 2 RuCl (H O)RNHCl K2 2- 5 2 + + 2RNH + 2 slow (CHOH) 2 (RuCl)5H2O [X] (CHOH)4 4 (IV)

CH2OH CH2OH [S]

CH2-O-Cl COOH 2Na+ (CHOH)4 (CH OH) + 2NaCl (V) -H+ (Fast) 2 4

CH2OH CH2OH

July, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org Amrita Srivastava, et al., J. Chem. & Cheml. Sci. Vol.5(7), 414-423(2015) 421 Now considering the above slow step and applying steady state treatment with a reasonable approximation, the Rate law may be written in terms of [RNCl -] as equation.

-d[RNCl] == K2 [X] [S] (1) d t d[X] == K [RNHCl] [Y] -- K - [X] -- K2 [X] [S] == 0 d t 1 -1 K1[RNHCl][Y] == [X] K +K [S] -1 2 K [RNHCl][Y] [X] == 1 (2) K-1 + K2[S] On substituting the value of [X] from (2) to (1), we get

-d[RNHCl] K2 K1[RNHCl][Y][S] == (3) d t K -1 + K2[S] The total concentration of Ru (lll) chloride, i.e. [Ru(lll)] T may be witten by equation (4)

[Y] [Ru(III)]T == + [X] (4) Putting value from equation (2) to (4), we get

K1[RNHCl][Y] [Ru(III)]T == [Y] + K-1 + K2 [S] == [Y] K [RNHCl] 1+ 1 K-1+K2[S] K-1 + K [S] + K RNHCl == [Y] 2 1 K-1 + K [S] 2 K-1 + K2[S] [Y] == [Ru (III)]T (5) K + [RNHCl] -1 K2[S] + K1 On comparing ion equation (3) and (4)

K2K1[RNHCl] [Ru (lll)]T K-1+ K2 [S] [S] -d [RNCl] == d t K +K [S] K + K [RNHCl] +K [S] -1 2 -1 1 2 -d[RNCl] K K [RNHCl] [Ru (lll)] [S] == 2 1 T d t K + K -1 1[RNHCl]+ K2[S] On assuming, K 2 [S] >> K -1+K 1[RNHCl] and on neglecting the second term in the denominator of equation (6), we get

-d[RNCl] K2K1[RNHCl] [Ru (lll)]T [S] d t == (6) K2[S] == K [RNHCl] 1 [Ru (lll)]T The rate law (6) is supported by the following experimental observations: 1. The experimental stoichiometry is in good agreement. 2. The rate law, derived above, is in accordance with the experimental observations.

July, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org 422 Amrita Srivastava, et al., J. Chem. & Cheml. Sci. Vol.5(7), 414-423(2015) 3. The negative effect of Chloramine T is supported by the mechanism. First-order kinetics with respect to [CAT] and [Ru(III)] is also supported by the derived rate law.

CONCLUSION

On the basis of observed kinetic data for Ru(III) catalyzed oxidation of galactitol by Chloramine T in acidic medium, it has been concluded that: - 1. CH 3C6H4SO 2NCl is the reactive species of CH 3C6H4SO2NHCl in acidic medium. + 2. [RuCl 2(H 2O) 4] is the reactive species of Ru(III) chloride in acidic medium. 3. There is formation of most reactive activated complex [RuCl 2 (H 2O) 3H] between reactive species of Ru(III) chloride and reactive species of chloramine T in acidic medium. 4. Mercuric acetate as one of the reactants plays the role of inhibitor in addition to its role as Cl - ion scavenger. 5. The rate of the oxidation of galctitol did not depend on the ionic strength of the medium of the reaction, indicating that a neutral molecule is involved at the transition state. 6. The high positive values of free energy of activation ( ∆G*)indicates highly solvated transition state, while fairly high negative values of entropy of activation ( ∆S*)suggest the formation of an activation complex with reduction in the degree of freedom. 7. The rate law is in conformity with all kinetic observations and proposed mechanistic steps are supported by the negligible effect of ionic strength.

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