Ruthenium (III) catalysis of periodate oxidation of reducing sugars in aqueous alkaline medium

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1 ndian Journal of Chemistry Vol. 42A, August 2003, pp Ruthenium () catalysis of periodate oxidation of reducing sugars in aqueous alaline medium Asho Kumar Singh*, Neena Gupta, Shahla Rahmani, Vinod Kumar Singh & Bharat Singh Department of Chemistry, University of Allahabad, Allahabad , ndia Received 7 December revised 17 March 2003 The ruthenium (TU) catalysis of periodate ox idation of D ribose, D-sorbose and maltose in aqueous alaline medium has been investigated. The reactions have been found to be zero order with respect to each reducing sugar and first order with respect to ruthenium (lt). The linear dependence of the reaction rate at lower concentrations of periodate ion and hydroxide ion tends towards zero order at their higher concentrations. Positive effect of [Cn on the rate of reaction has also been observed. The observed inetics with periodate ion, under the conditions [O ) > >[Ru(1)h suggests the formation of a : complex between Ru(1) and periodate in the rate controlling step, which subsequentl y interacts with reactive species of reducing sugar to give the final products, through a series of fast steps. The rate law is derived. Several papers have been devoted to the elucidation of the mechanism of oxidation,-4 of reducing sugars by various oxidants. However, ruthenium() catalysis in the oxidation of reducing sugars by periodate has yet not been probed. n the present study, we have carried out the oxidation of the monosaccharides D-ribose, D sorbose and the disaccharide maltose by periodate ion in aqueous alaline medium with ruthenium() chloride as a homogeneous catalyst. Periodate is a less potent oxidant in alaline than in acidic medium and is widely employed as a diol cleaving reagent 5. Further, period ate is nown to exist as different species 6 in alaline medium and it is necessary to now the active form of the oxidant in the reaction. Experimental All the reagents used were of highest purity available. An aqueous solution of sodium metaperiodate (E Merc) was prepared by dissolving a weighed amount in doubly distilled water, standardized iodometrically and preserved in blac coated vessels in order to avoid photochemical deterioration. The solution of ruthenium() chloride (Ubichem limited) was prepared by dissolving the sample in hydrochloric acid of nown strength. Aqueous solution of sugars were prepared fresh each day. All the reactions were studied at constant temperature 45 D C (±O.l DC). The reaction was initiated by adding the requisite volume of preequilibrated sugar solution to the reaction mixture and the progress of the reaction was monitored by estimating the amount of unreacted periodate at regular time intervals iodometrically. Stoichiometry and product analysis Different sets of reactions containing excess [O ] over [sugar] with fixed concentrations of Ru() and OH- were ept for 25 h at 318 K and then analysed. The unreacted oxidant was assayed iodometricall/ when [O ]»[sugar]. The results showed a mole ratio of consumption of oxidant to reductant of 2: 1 in the case of ribose and sorbose while 4: 1 in case of maltose. The final oxidation products from sugars were characterized as formic acid and erythronic acid in oxidation of ribose whereas formic acid and arabinonic acid were identified in oxidation of sorbose and maltose. These products are indicative for C,-C 2 splitting. The stoichiometric Eqs (1-3) accordingly may be written for the oxidation of ribose, sorbose and maltose respectively. CsH OOs + 2O R " ) ) C 4 HgOs + HCOOH + 2O (Ribose) (Erythronic acid)... (1) C H Ru(l ) C H HCOOH OW ) s (Sorbose) (Arabinonic acid) (2) C1 2H O R ) ) 2C s H HCOOH (Maltose) (Arabinonic acid) +410 (3) Formic acid and corresponding acids were confirmed by the help of spot tests, equivalence, inetic studies and thin layer chromatography.

1872 NDAN J CHEM, SEC A, AUGUST 2003 Results and discussion The inetics of the oxidation of reducing sugars by periodate ion in aqueous alaline medium in the presence of ruthenium() were investigated

2 1872 NDAN J CHEM, SEC A, AUGUST 2003 Results and discussion The inetics of the oxidation of reducing sugars by periodate ion in aqueous alaline medium in the presence of ruthenium() were investigated at several initial concentrations of the reactants. A plot of initial velocity, (-dc/dt) versus [O ] shows that first order inetics with respect to periodate in lower concentrations tends towards zero-order in its higher concentration. The reaction shows zero-order inetics in sugars. The first order rate constant () values are practically constant showing zero order dependence on each sugar. The reaction, which follows first order inetics at lower [OH- ], tends to zero order at its higher concentrations (optimum ph 13.20) as evidenced by a (-dc/dt) versus [NaOH] plot which confirms positive effect of [OH- ] on reaction nite. The log (-dc/dt) versus log [Ru(l)] plot is linear with a slope near unity (1.02, 0.98 and 1.01 in D-ribose, D sorbose and maltose, respectively) which confirms first order dependence of the reactions on ruthenium() chloride. There is slight increase in the first order rate constant ( ;) values with the increase in [Cl- ], indicating positive effect of [Cn on the rate of reaction. A negligible effect of variations of sodium perchlorate on the reacti on rate is observed. Experiments at 30, 35, 40, 45 and 50 C led to compute E"!j.J- and 111' values (Table ). Thus on the basis of first order inetics each lower concentration of OH- and O, zero order inetics in reducing sugar concentrations and first order dependence on Ru() concentrations, a probable rate law can be given as: _ d [lo ] = [O ][OH - ][Ru ( J] ))T df... (4) t would be more appropriate at this stage to discuss the actual reacting species of ruthenium(jo chloride in aqueous alaline medium. t is reported 7 that at the instant of preparation Ru() exists in solution in the ph range as four major species, [RuCl4 0 hr', [RuCl3 0)3], [RuCh 0)4t and [RuCl 0)5t 2. Out of these four species, [RuCl 2 0 )4t is stabilized in its hydrolysed form, [RuC 2 0 h OH) according to the following equilibrium: [RuC 2 0 )4f + H 2 0 [RuC 2 0 )PH]+H)O+... (5) Table - Rate constants and thermodynamic parameters at 45 C Parameters Ribose Sorbose Maltose 10 5, (mor 2 )2s l) O. 12 xa (mor s l) Ea (J morl) 'lw (J morl ) Mt (1K morl) Since the experiments were performed with Ru() chloride dissolved in O.OM HC solution, hydrolysed species of [RuC 2 0 )4 t i.e. [RuC 2 0 )3 OH) can be assumed as the starting species of Ru() chloride in the present investigation. Our assumption is also supported by the pea observed at 300 nm for hydrolysed species of Ru() chloride as reported earlier. Considering the observed positive effects of [Cl- ] and [OH- ] on the rate of reaction, the following equilibria can be assumed to exists in aqueous alaline medium. [RuC 2 (HzO)pHf +cr => [RuCl) (H zo)ohrl+hzo [RuCl) (H zo)ohr l +OH- =>... (6) [RuC) (H z O)(OH)z 12 + H (7) Since the observed rate of reaction increases with the increase in [OH- ], on the basis of equilibrium (7) the species [RuC 3 0 )(OHhr 2 can be taen as the main reactive species of Ru(l) in the present in vestigation. This assumption is fully supported by the observed spectra of Ru(l) at two different concentrations of [OH- ] (.00x0-2 and 5.00xlO- 2 M) at 45 C (Fig. ) where increase in absorbance with increase in [OH- ] is due to shift in equilibrium (7) to the right side. The two different species of Ru() i.e. [RuC3(H20)20Hrl and [RuC) 0)(OH)2r 2 alali ne medium are observed at the wavelength 215 nm and 385 nm, respectively from the spectra recorded by Hitache 220V spectrophotometer. t is well-nown 8 that, in the presence of alali, reducing sugars undergo a tautomeri change through the formation of an intermediate enediol. The base catalysed formation of enediol might be written as: in

3 NOTES u c o.0 ::; "0 \.0 <{ (b) Keto sugars H H -C- OH H-C-OH +OW..,--- + H 2 0 C=O C- O- enediol anion H-C - OH H -C- OH + H 20..,--- + OH - C- 0 - C-OH R (\,2 enediol) Waveoleongth (nm ) Fig. -Spectra of sugar solution and Ru() solutions, Ru(l) and O solutions, Ru(l), O and sugar solutions in alaline medium recorded at 45 C. () [Sugar] = 5.00xlO-' M, (2) [Ru(l)] = 2.30x0-4 M, [OW]=1.00x lo- 2 M, (3) [Ru(l)] = 2.30xlO- 4 M, [OW] =5.00xlO- 2 M, (4) [Ru(T)] = 2.30x 10-4 M, [OW] =5.00x lo 2 M, [O ]=1.00xlO- 3 M, (5) [Ru()]= 2.30xlO-4 M, [OWJ=5.00x lo- 2 M, [O ]=1.33x O- 3 M, (6) [Ru()J = 2.30xlO-4 M, [OWJ=5.00xlO- 2 M, [O ]=1.00x O- 3 M, [Sugar] = 1.00x 10-2 M, (7) [Ru()] = 2.30x 10-4 M, [OW]=5.00xlO- 2 M, [O J= 1.00x M, [Sugar]=5.00xlO 2 M. (a) Aldehyde sugars H - C=O H -C OW..,--- + H 2 O H-C-OH C - OH H-C H 2O C-OH R..,--- enediol anion - H -C-OH +OW C-OH (1,2 enediol) Oxidation of reducing sugars will tae place through enediol anion or enediol. Although in the present investigation, the order of the reaction with respect to reducing sugars is zero even then under the experimental conditions, it can be safely assumed that it is the enediol form of sugar which is taing part in the reaction. t is reported 9 that under the alaline condition, the main species of O are expected to be H 3 O - and H 2 O-. t has also been reported on the basis of the observed fractional order in [OH- ] that H 2 O - the main reactive species of 104, When the spectrum of 104 in the presence of OH- was taen it was found that there is only one pea at 237 nm, which clearly indicates that it is 104 which can be taen as the reactive species of sodium metaperiodate in alaline medium. The inetic data collected in the present investigation suggest that the reactive species of the catalyst i.e. [RuCl 3 0)(OH)2 r2 and oxidant i.e. 10:; form a complex (C 4 ) in a slow and rate determining step which interacts with reactive species of reducing sugar i.e. enediol to form an intermediate ex) and other products along with regeneration of catalyst in a fast process. The intermediate (X) is finally converted into the reaction products along with the catalyst. The complex formation between oxidant and catalyst has also been reported earliero,, n order to ascertain the possible formation of complex between [RuCl 3 0)(OH)2 r 2 and [104], is

4 1874 lndan J CHEM, SEC A, AUGUST 2003 the spectra for the solution of Ru() and OH- well as for the solution of Ru() and OH- with two different concentrations of 104 were obtained, where it has been observed that with the addition of 10:; solution, there is an additional pea in existence at 318 nm along with two peas already observed at 215 nm and 385 nm. The existence of an additional pea at 318nm clearly indicates the formation of Ru() - 10:;- complex i.e. C 4. The formation of complex C 4 is also supported by the decrease in absorbance of the species [RuCl 3 0)(OH)2r 2 at 385 nm. When the reducing sugar solutions of two different concentrations (l.oox 10-2 and 5.00x 10-2 M) were added to the solution of Ru(), 10:;- and OH-, it was observed that with the addition of sugar solution, peas observed at 318 nm completely disappear, indicating that C 4 has reacted with the sugar immediately. However, two new peas appear at 330 nm, showing no change in absorbance with time. On the basis of this, it can be concluded that the complex responsible for 330 nm is inetically inert and might be formed by the sugar indicating that enediol is the reactive species. The complex Cs at 330 nm can be considered as the complex formed between [RuCl) 0)(OH)2r 2 and sugar. The formation of such type of complex between Ru(l) and sugar is also reported 12 earlier. Now, on the basis of the above spectral evidences and observed inetic data, the following scheme for the oxidation of reducing sugar is described. K, [RuC 2 0)3 OH] + C- <==> [RuC) 0)2 ORr' as +H (i) C 4 + E + OH- fasl) ntermediate (X) + 10; +H [RuCl) 0)(OH)2r 2 (v) "/OH- / N ( 11J" RC eoh )CH -----) HCOOH ex) + RCOOH [RuCl) 0)(OHh r 2 (vi) where Ru(J)* stands for [RuCl 3 0)(OH)zr2 The structure of C 4 might be of the form o olo O Ruel 3 (OR) 2 Considering the stoichiometric data and above mechanistic steps, the rate of oxidation of the reducing sugars may be expressed in terms of decreasing concentration of periodate as: (8) where n is 2 for ribose and sorbose and n is 4 for maltose On applying the law of chemical equilibrium to steps (i) and (iv), we have K = [C 2 ], [e, ][C- ]... (9), [Ruel) (R 2 0)2 ORr' + OH- <==>. _, (C 2 ) [RuC1) 0)(OH)2 r 2 + H 2 0 (C 3 ) [RuCl) 0)(OH)2 r , ) C + H 0 slow and rate 4 2 oel cr min ing step (ii) K = [C5 ] 4 [e 3 ][S]... (10) On applying steady state approximation to the concentrations of C 3, we obtain [e ] = 2 [C 2 ][OH- ] ) _2 + ) [O ]... (11) K, <==> [RuC)(S)(OH)2 r 2 + H20 (iv) (Cs) According to the mechanism, the total concentration of Ru(H) at any time can be expressed as

NOTES 1875... (2) On substituting the values of C 2, C 3 and Cs from Eqs (9), (0) and (11) in Eq. (12), we have Eq.

5 NOTES (2) On substituting the values of C 2, C 3 and Cs from Eqs (9), (0) and (11) in Eq. (12), we have Eq. (13) [C ] = [Ru(ill)h (_2 + ) [O ]) ) (1 + K)[Cr](_2 + ) [O ]) + 2 K)[Cr][OH- ] + 2 K)K 4 [S][OH- ][Cl- ]... (13) From equations (9), (11) and (13), we obtain the final rate law as Eq. (14): d[o ] n2)k ) [Ru()h[OH - l[o ][Cr] --=-----"c.=. = --=-=--=--- --'- ---'- dt (1 + KJCr])(_ 2 + ) [O ]) +K) 2 [OH- ][Cr] + 2 K)K 4 [S][OH - ][Cn... (4) Since observed order with respect to sugar is zero throughout its variation under our experimental condition, the role of the term 2KJK4 [S][OH- ] [Cl - ] in the denominator of rate law (14) can be assumed as insignificant. However, in the higher concentration range of [S], [OH- ] and [Cr], the decrease in uniform first order rate constant, ; with the increase in sugar concentration, as predicted by the rate law (14) may be obtained. The condition in which experiments were performed to study the effect of sugar concentration on the rate of reaction, the rate law (14) can be replaced by the rate law (15): d[o ] --,---..::c.=. = dt n2)k) [Ru()]T[OH- ][O ] [Cl- ] (15) Rate law (15) is in agreement with the observed zero order in [sugar], first order in [Ru()h and fractional orders in [OH- ], [Cl - ] and [O]. Eq. (15) can also be written as:... (16) where rate = _ d [O ] dt According to Eq. (16) if a plot is made between [Ru(l)h / rate and 1/ [104] orl/ [OH- ], linear plot having positive intercept on [Ru(l)h rate axis should be obtained and it was found to be so. This proves the validity of the rate law (15) and hence the proposed mechanism. The common mechanism for all the three reactions are obvious from the order of frequency factor which is the same for all the three reducing sugars (ribose, sorbose and maltose). n the present study of oxidation of reducing sugars by 104 in the presence of Ru() as homogeneous catalyst, the activated state will be more highly charged ion and would be strongly solvated due to the reactants [RuCl 3 0)(OH)z r 2 and O. The observed decrease in entropy is due to more solvation of the activated state than the reactants. From the aforesaid inetic studies the following conclusions can be drawn: (a) the main reactive species of ruthenium () chloride in aqueous alaline medium is [RuCl 3 0)(OH)z r 2 (b) the main oxidising species of periodate in aqueous alaline medium is 104 and (c) oxidation of sugar taes place via enediol form of sugar as explained earlier. References Shilov E A & Yamsiov A A, Urain Khim Zhur, 5 (1967) Singh A K, Singh A, Gupta R, Saxena M & Singh B, Trans met Ch ern, 17 (1992) sbell H S & Frush H L, Carbohydr Res, 28 (1973) Singh A K, Chopra D, Rahmani S, Singh B, Carbohydr Res, 314 (1998) Wiberg K B, Oxidation in organic chemistry (Academic Press, New Yor). ( 1965) Cotton FA & Wilinson G, Advanced inorganic chemistry A comprehellsive text (Wiley, nterscience, New Yor) 5th Ed (1988) Taqui Khan M M, Rama Chandraiah G & Rao A P, n org Chem, 25 ( 1986) Singh M P, Kri shna B & Ghosh S, Z phys Chern, 204 (1955) l; 205 (1956) 285; 208 (1958) Tuwar S M, Nandibewoor S T & Raju J R, J ndiall chern Soc, 69 (1992) Nandibewoor S T & Morab Y A, J chem Soc, Dalton Trans, (1995) 483. Agrawal M C, Singhal R K & Mushran S P, Z phys Chern, 62 ( 1963) Singh A K, Singh Y, Singh A K, Gupta N & Singh B, Carbohydr Res, 337 (2002) 345.

RESULTS AND DISCUSSION

RESULTS AND DISCUSSION 6.1 Reactive Species of Catalyst and Oxidant 6.2 Mechanism and Rate Law 6.3 Multiple Regression Analysis 6.4 Comparative Study 6.5 Conclusion 6.6 Future Prospect In this chapter,