Indian Journal of Chemistry Vol. 42A, August 2003, pp. 1871-1 875

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

alkaline medium

Ashok Kumar Singh*, Neena Gupta, Shahla Rahmani, Vinod Kumar Singh & Bharat Singh

Department of Chemistry, University of Allahabad, Allahabad 2 11 002, India

Received 7 December 2001 .. revised 17 March 2003

The ruthenium (TU) catalysis of periodate ox idation of D­ribose, D-sorbose and maltose in aqueous alkaline 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 (lIT). 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 kinetics with periodate ion, under the conditions

[IO~ ) > >[Ru(II1)h suggests the formation of a I : I complex

between Ru(II1) 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(III) catalysis in the oxidation of reducing sugars by periodate has yet not been probed. In the present study, we have carried out the oxidation of the monosaccharides D-ribose, D­sorbose and the disaccharide maltose by periodate ion in aqueous alkaline medium with ruthenium(III) chloride as a homogeneous catalyst. Periodate is a less potent oxidant in alkaline than in acidic medium and is widely employed as a diol cleaving reagent5.

Further, period ate is known to exist as different species6 in alkaline medium and it is necessary to know 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 Merck) was prepared by dissolving a weighed amount in doubly distilled water, standardized iodometrically and preserved in black coated vessels in order to avoid photochemical

deterioration . The solution of ruthenium(III) chloride (Ubichem limited) was prepared by dissolving the sample in hydrochloric acid of known strength. Aqueous solution of sugars were prepared fresh each day .

All the reactions were studied at constant temperature 45 DC (±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 [IO~ ]

over [sugar] with fixed concentrations of Ru(III) and

OH- were kept for 25 h at 318 K and then analysed. The unreacted oxidant was assayed iodometricall/

when [IO~ ] »[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,-C2 splitting.

The stoichiometric Eqs (1-3) accordingly may be written for the oxidation of ribose, sorbose and maltose respectively.

CsH IOOs + 2IO~ R~~" ) ) C4 HgOs + HCOOH + 2IO~

(Ribose) (Erythronic acid) .. . (1)

C H ° + 210- Ru(lII ) C H ° HCOOH 210-6 12 6 4 OW ) s 10 6 + + 3

(Sorbose) (Arabinonic acid) (2)

C1 2H22011 + 4IO~ R~~~I ) ) 2CsH I00 6 + 2HCOOH

+410~ (3)

(Maltose) (Arabinonic acid)

Formic acid and corresponding acids were confirmed by the help of spot tests, equivalence, kinetic studies and thin layer chromatography.

1872 INDIAN J CHEM, SEC A, AUGUST 2003

Results and discussion The kinetics of the oxidation of reducing sugars by

periodate ion in aqueous alkaline medium in the presence of ruthenium(III) were investigated at several initial concentrations of the reactants. A plot

of initial velocity, (-dc/dt) versus [IO~ ] shows that

first order kinetics with respect to periodate in lower concentrations tends towards zero-order in its higher concentration. The reaction shows zero-order kinetics

in sugars. The first order rate constant (k~) values are

practically constant showing zero order dependence on each sugar. The reaction, which follows first order

kinetics 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(lII)] plot is linear with a slope near unity (1.02, 0.98 and 1.01 in D-ribose, D­sorbose and maltose, respectively) which confirms first order dependence of the reactions on ruthenium(III) chloride. There is slight increase in the

first order rate constant ( k;) 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-I and 111' values (Table I). Thus on the basis of first order kinetics each III

lower concentration of OH- and IO~, zero order

kinetics in reducing sugar concentrations and first order dependence on Ru(III ) concentrations, a probable rate law can be given as:

_ d [lO~ ] = k [IO~ ][OH - ][Ru ( IJ] ))T df

... (4)

It would be more appropri ate at this stage to discuss the actual reacting species of ruthenium(JIO chloride in aqueous alkaline medium. It is reported7

that at the instant of preparation Ru(III ) exists in solution in the pH range 0.4-2.0 as four major species, [RuCl4(H20 hr ' , [RuCl3(H20)3], [RuCh(H20)4t and [RuCl(H20)5t2

. Out of these four species, [RuCl 2(H20 )4t is stabilized in its hydrolysed form, [RuCI 2(H20 h OH) according to the following equilibrium:

[RuCI2 (H 20 )4f + H2 0 ~ [RuCI2 (H2 0 )PH]+H)O+

... (5)

Table I- Rate constants and thermodynamic parameters at 45°C

Parameters Ribose Sorbose Maltose

105 k, (mor2)2s·l) 11.06 14. 12 18.66

1O.12xA (mor212s· l) 0.18 1.65 9.93

Ea (kJ mOrl ) 61.49 66.61 70.31

I'lW (kJ mOrl ) 58.84 63.96 67.66

Mt (1K I mOrl ) -38.09 -19.74 -05 .67

Since the experiments were performed with Ru(III) chloride dissolved in O.OIM HCI solution, hydrolysed

species of [RuCI 2 (H 2 0 )4 t i.e . [RuCI2 (H 2 0 )3 OH)

can be assumed as the starting species of Ru(III) chloride in the present investigation . Our assumption is also supported by the peak observed at 300 nm for hydrolysed species of Ru(III) 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 alkaline medium.

[RuCI 2 (HzO)pHf +cr ¢=> [RuCl) (H zO)OHrl+H zO

. . . (6)

[RuCl) (H zO)OHrl +OH- ¢=>

[RuCI) (HzO)(OH)z 12 + H20 . . . (7)

Since the observed rate of reaction inc reases with the

increase in [OH- ], on the basis of equilibrium (7) the

species [RuCI3 (H 20 )(OHhr2 can be taken as the

main reactive species of Ru(IIl) in the present investigation . This assumption is fully supported by the observed spectra of Ru(IIl) at two different

concentrations of [OH- ] (I.00xI0-2 and 5.00xlO-2M) at 45°C (Fig. I ) where increase in absorbance with

increase in [OH- ] is due to shi ft in equilibrium (7) to the right side. The two different species of Ru(III) i.e.

[RuCI3(H20)20Hrl and [RuCI) (H 20)(OH)2r 2 in

alkaline medium are observed at the wavelength 215 nm and 385 nm, respectively from the spectra recorded by Hitache 220V spectrophotometer.

It is well-known8 that, in the presence of alkali, reducing sugars undergo a tautomeri change through the formation of an intermediate enediol. The base catalysed formation of enediol might be written as:

NOTES 1873

3·2

2·8

2·4

~ 2·0 u c o

.0

::; "0 \II .0 <{

0·4 -200 240 280 320 350 400 440 480 500

Waveoleongth (nm )

Fig. I-Spectra of sugar solution and Ru(III) solutions, Ru(lII)

and IO~ solutions, Ru(lII) , IO~ and sugar solutions in alkaline

medium recorded at 45°C. (I) [Sugar] = 5.00xlO-' M, (2)

[Ru(IlI)] = 2.30xI0-4 M, [OW]=1.00x lO-2 M , (3) [Ru(lII)] =

2.30xlO-4 M, [OW] =5.00xlO-2 M, (4) [Ru(TII )] = 2.30x 10-4 M,

[OW] =5 .00x lO·2 M, [IO~ ]=1.00xlO-3 M, (5) [Ru(III)]=

2.30xlO-4 M, [OWJ=5.00x lO-2 M, [IO~ ]=1.33x IO-3 M , (6)

[Ru(III)J = 2.30xlO-4 M, [OWJ=5.00xlO-2 M, [IO~ ]=1.00x IO-3

M, [Sugar] = 1.00x 10-2 M, (7) [Ru(III)] = 2.30x 10-4 M,

[OW]=5.00xlO-2 M, [IO~ J= 1.00x 10.3 M, [Sugar]=5.00xlO·2 M.

(a) Aldehyde sugars

H - C=O H -C - 0 -I +OW ~ II + H2O ..,---

H-C-OH C - OH

k k enediol anion

H-C - 0 - H -C-OH II + H2O ~ II +OW ..,---C-OH C-OH

I k R

(1,2 enediol)

(b) Keto sugars

H I

H -C- OH H-C-OH I +OW ~ II + H20 ..,---

C=O C- O-

k k enediol anion

H-C - OH H -C- OH II + H20 ~ II + OH -..,---

C- 0 - C-OH

k I R

( \ ,2 enediol)

Oxidation of reducing sugars will take 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 taking part in the reaction .

It is reported9 that under the alkaline condition, the

main species of IO~ are expected to be H3IO~- and

H2IO~- . It has also been reported on the basis of the

observed fractional order in [OH- ] that H2IO~- is

the main reactive species of 104, When the spectrum

of 104 in the presence of OH- was taken it was

found that there is only one peak at 237 nm, which

clearly indicates that it is 104 which can be taken as

the reactive species of sodium metaperiodate in alkaline medium.

The kinetic data collected in the present investigation suggest that the reactive species of the

catalyst i.e. [RuCl 3 (H 20)(OH)2 r2 and oxidant i.e.

10:; form a complex (C4) 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 earlierIO,II ,

In order to ascertain the possible formation of

complex between [RuCl3 (H 20)(OH)2 r 2 and [104],

1874 lNDIAN J CHEM, SEC A, AUGUST 2003

the spectra for the solution of Ru(III) and OH - as

well as for the solution of Ru(III) 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 peak in existence at 318 nm along with two peaks already observed at 215 nm and 385 nm. The existence of an additional peak at 318nm clearly indicates the formation of

Ru(III) - 10:;- complex i.e. C4 . The formation of

complex C4 is also supported by the decrease in

absorbance of the species [RuCl 3 (H 20)(OH)2r2 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(III) , 10:;- and OH-,

it was observed that with the addition of sugar solution, peaks observed at 318 nm completely disappear, indicating that C4 has reacted with the sugar immediately . However, two new peaks 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 kinetically 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) (H 20)(OH)2r 2 and sugar. The

formation of such type of complex between Ru(IlI) and sugar is also reported 12 earlier.

Now, on the basi s of the above spectral evidences and observed kinetic data, the following scheme for the oxidation of reducing sugar is described .

K,

[RuCI2 (H 20)3 OH] + CI- <==> [RuCI) (H 20)2 ORr'

+H20 ... (i)

k,

[Ruel) (R 20)2 ORr' + OH- <==> . k_,

(C2)

[RuC1) (H20)(OH)2 r 2 + H20 (ii)

(C 3)

[RuCl) (H20)(OH)2 r 2 + 104- k., ) C + H 0 slow and rate 4 2

oel cr min ing step

K,

<==> [RuCI)(S)(OH)2 r2 + H20 (iv)

(Cs)

C4

+ E + OH- fasl) Intermediate (X) + 10;

+H20 + [RuCl)(H20)(OH)2r2 (v)

0 - 0

I II I04"/OH- / NII ( 1I1J"

RC eOH )C H -----~) HCOOH

eX)

+ RCOOH + 10~ +[RuCl) (H20)(OHh r 2 (vi)

where Ru(IJI)* stands for [RuCl 3 (H 20)(OH)zr2

The structure of C4 might be of the form

o II I

o~l~o O~ Ruel 3 (OR) 2

3-

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, ][CI- ] ... (9)

K = [C 5 ]

4 [e3 ][S]

... (10)

On applying steady state approximation to the concentrations of C3, we obtain

[e ] = k2 [C 2 ][OH- ] ) k_2 + k) [IO~ ]

... (11)

According to the mechanism, the total concentration of Ru(HI) at any time can be expressed as

NOTES 1875

. .. (2)

On substituting the values of C2, C3 and Cs from Eqs (9), (0) and (11) in Eq. (12), we have Eq. (13)

[C ] = [Ru(ill)h (k_2 + k) [IO~ ])

) (1 + K)[Cr](k_2 + k) [IO~ ]) + k2K)[Cr][OH- ]

+k2K)K

4[S][OH- ][Cl- ]

... (13)

From equations (9) , (11) and (13), we obtain the final rate law as Eq. (14):

d[IO~ ] nk2k)K) [Ru(III)h[OH- l[IO~ ][Cr] --=-----"c.=. = __ --=-=--=---__ --'-___ ---'-___ _ dt (1 + KJCr])(k_2 + k) [IO~ ])

+K)k2 [OH- ][Cr] + k2K)K4 [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 k2KJK4 [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, k; 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[IO~ ] --,---..::c.=. =

dt

nk2k)K) [Ru(III)]T[OH- ][IO~ ] [Cl- ]

(15)

Rate law (15) is in agreement with the observed zero order in [sugar], first order in [Ru(III)h and

fractional orders in [OH - ], [Cl - ] and [IO~].

Eq. (15) can also be written as:

. .. (16)

where rate = _ d [IO~ ] dt

According to Eq. (16) if a plot is made between

[Ru(IIl)h / rate and 1/ [104] orl/ [OH- ], linear plot

having positive intercept on [Ru(IIl)h Irate 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).

In the present study of oxidation of reducing sugars

by 104 in the presence of Ru(III) as homogeneous

catalyst, the activated state will be more highly charged ion and would be strongly solvated due to the

reactants [RuCl 3 (H 20)(OH)z r 2 and IO~. The

observed decrease in entropy is due to more solvation of the activated state than the reactants.

From the aforesaid kinetic studies the following conclusions can be drawn : (a) the main reactive species of ruthenium (III) chloride in aqueous alkaline

medium is [RuCl3 (H 20)(OH)z r 2 (b) the main

oxidising species of periodate in aqueous alkaline

medium is 104 and (c) oxidation of sugar takes place

via enediol form of sugar as explained earlier.

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