kinetics of the oxidation of d-glucose and cellobiose by acidic solution of n-bromoacetamide using...

Chinese Journal of Chemistry, 2008, 26, 1057—1067 Full Paper

* E-mail: [email protected], [email protected]; Tel.: 0091-532-2462266 (O), 0091-532-2640434 (R) Received April 25, 2007; revised November 1, 2007; accepted December 13, 2007.

© 2008 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Kinetics of the Oxidation of D-Glucose and Cellobiose by Acidic Solution of N-Bromoacetamide Using Transition Metal

Complex Species, [RuCl3(H2O)2OH]－, as Catalyst

SINGH, Ashok Kumar* SRIVASTAVA, Jaya SRIVATAVA, Shalini RAHMANI, Shahla

Department of Chemistry, University of Allahabad, Allahabad-211002, India

The kinetics of Ru(III)-catalyzed and Hg(II)-co-catalyzed oxidation of D-glucose (Glc) and cellobiose (Cel) by N-bromoacetamide (NBA) in the presence of perchloric acid at 40 ℃ have been investigated. The reactions exhibit the first order kinetics with respect to NBA, but tend towards the zeroth order to higher NBA. The reactions are the first order with respect to Ru(III) and are fractional positive order with respect to [reducing sugar]. Positive effect of Cl－ and Hg(OAc)2 on the rate of reaction is also evident in the oxidation of both reducing sugars. A negative effect of variation of H＋ and acetamide was observed whereas the ionic strength (µ) of the medium had no influence on the oxidation rate. The rate of reaction decreased with the increase in dielectric constant and this enabled the com-putation of dAB, the size of the activated complex. Various activation parameters have been evaluated and suitable explanation for the formation of the most reactive activated complex has been given. The main products of the oxi-dation are the corresponding arabinonic acid and formic acid. HOBr and [RuCl3(H2O)2OH]－ were postulated as the reactive species of oxidant and catalyst respectively. A common mechanism, consistent with the kinetic data and supported by the observed effect of ionic strength, dielectric constant and multiple regression analysis, has been proposed. Formation of complex species such as [RuCl3•S•(H2O)OH]－ and RuCl3•S•OHgBr•OH during the course of reaction was fully supported by kinetic and spectral evidences.

Keywords kinetics, N-bromoacetamide, reducing sugar, ruthenium(III) chloride, perchloric acid, Hg(II)-co- catalysis

Introduction

The use of N-bromoacetamide (NBA) as an agent for allylic bromination was first reported by Wohl1 in 1919. Under different conditions, N-halo compounds have been used successfully not only as halogenating agents, but several of them have been found to be effective agents for oxidations and dehydrogenations. Recently, considerable attention has been focused on the diverse nature of the chemistry of N-halo compounds2-6 owing to their ability to act as source of halogenonium cations, hypohalide species and nitrogen anions which act both as bases and nucleophiles. N-halo compounds oxida-tions of organic substrates are complicated by parallel bromine oxidation which is obviated by Hg(II). Reports are available on oxidative capacity of N-halo com-pounds in catalyzed7-16 and uncatalyzed17-30 processes. Energy is generated by the oxidation of sugars and this whole phenomenon occurs in liver and is regulated by enzymes. Besides fulfilling specific nutritional or physiological roles, sugars may also have therapeutic or pharmacological actions.31 D-Glucose is the major fuel for most organisms and the building block of the most abundant polysaccharides, such as starch and cellulose.

Cellobiose is the repeating disaccharide unit of cellulose. Mechanistic studies of the oxidation of reducing sugars by NBA or NBS using transition metal ions such as Ru(VIII),32 Pd(II),8,10,12-14 Ir(III)9,15 and Pt(IV)7 as ho-mogeneous catalyst were reported. In such cases, me-dium of the reaction was maintained either acidic or alkaline. Very few reports11 are available in literature, where oxidation of the reducing sugars by NBA in the presence of Ru(III) chloride as a homogeneous catalyst has been studied. In recent years, ruthenium and its chloro complexes particularly in ＋3 oxidation state have been used as the homogeneous catalyst in various redox processes.33 On the basis of observed kinetic data,

36RuCl － has been reported as the reactive species of

Ru(III) chloride in the oxidation of galactose and ribose by NBA in an acidic medium.11(a) In view of very few reports available for NBA oxidation made in Ru(III)- catalyzed reactions, especially with biologically, bio-chemically and biotechnologically important substrates like sugars, an attempt has been made to study the ki-netics and mechanism of oxidation of D-glucose and cellobiose by NBA in an acidic medium using Ru(III) as a homogeneous catalyst, which may become helpful to understand various types of physiological problems in

1058 Chin. J. Chem., 2008, Vol. 26, No. 6 SINGH et al.


the living systems, and their solutions. In this paper, [RuCl3(H2O)2•OH]－ and HOBr have been proposed as the reactive species of Ru(III) chloride and NBA respec-tively.

Experimental

All the reagents used were of the highest purity available. Doubly distilled water was used throughout. A stock standard solution of NBA was prepared afresh daily by dissolving a known weight of NBA (E-Merck) in doubly distilled water and its concentration was as-certained iodometrically. In order to avoid photochemi-cal deterioration the NBA solution was preserved in a black coated flask. The solution of ruthenium(III) chlo-ride (Uchem limited) was prepared by dissolving the sample in hydrochloric acid of known strength. Aqueous solutions of sugars (E-Merck) were prepared freshly each day. A standard solution of mercuric acetate (E-Merck) was acidified with 20% acetic acid. Perchlo-ric acid (E-Merck) used as a source of H＋ ions, was diluted with doubly-distilled water and standardized via acid base titration. All other standard solutions of KCl, NaClO4 and acetamide (E-Merck) were prepared with doubly distilled water. All the reactions were studied at constant temperature (40±0.1) ℃. The reaction was initiated by adding the requisite volume of pre-equili- brated sugar solution to the reaction mixture and the progress of the reaction was monitored by estimating the amount of unreacted NBA at regular time intervals io-dometrically.

Stoichiometry and product analysis

Different sets of reactions containing excess [NBA] over [sugar] with fixed concentrations of all other reac-tants were kept for 72 h at room temperature and then analyzed. Determination of unconsumed NBA revealed that oxidation of one mole of sugar would consume 2 mol of NBA for Glc and 4 mol of NBA for Cel. Ac-cordingly, the following stoichiometric equations could be formulated:

6 12 6 3 2

3 2

+Ru(III)/H /Hg(II)C H O 2CH CONHBr 2H O

( -glucose) (NBA)

HCOOH RCOOH 2HBr 2CH CONH

formic acid arabinonic acid (NHA)

D

⎯⎯⎯⎯⎯⎯→＋＋

＋＋＋

12 22 11 3 2

3 2

Ru(III)/H /Hg(II)C H O 4CH CONHBr 5H O

Cellobiose (NBA)

2HCOOH 2RCOOH 4HBr 4CH CONH

formic acid arabinonic acid (NHA)

⎯⎯⎯⎯⎯⎯→＋

＋＋

＋＋＋

where R＝(CHOH)3CH2OH. Formic acid and arabi-nonic acid were confirmed by the help of spot tests, thin layer chromatography, equivalence and kinetic studies.

Results and discussion

The kinetics of the oxidation of Glc and Cel was in-vestigated at several initial concentrations of the reac-tants. The reactions were carried out by varying the concentration of oxidant, reducing sugar, catalyst and acid in turn while keeping all other conditions constant. The initial rate i.e. (－dc/dt) for each kinetic run was calculated by the slope of the tangent of the plot be-tween remaining [NBA] and time drawn at fixed NBA except that in NBA variation where tangent was drawn at fixed time. The value of pseudo first-order rate con-stant, k1, was obtained by dividing the initial rate of the reaction with the concentration of NBA. First order ki-netics with respect to NBA in its low concentration range tends to zero order at its higher concentration for both the reducing sugars (Table 1). Since the study, for the effects of other reactants concentrations on the rate of reaction, has been made at low concentration of NBA, for the purpose of calculation of pseudo first-order rate constant (k1), the order of reaction with respect to NBA has been taken as unity. Order with respect to each sub-strate concentration is unity except that at high concen-tration, where there is a change from first to zero-order

Table 1 Pseudo first order rate constants (k1) observed for the variations of [NBA], [S] and [Ru(III)] in the oxidation of D-glucose and cellobiose at 40

℃

k1/(10－4 s－1) [NBA]/(10－4 mol•L－1)

[S]/(10－2 mol•L－1)

[Ru(III)]/(10－6

mol•L－1) Glucose Cellobiose

2.50a 5.00a 7.50a

10.00a 12.50a 15.00a 17.50a 22.50a 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b 10.00b

2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 1.00 2.00 4.00 5.00 6.00 8.00 9.00

10.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00

4.58 4.58 4.58 4.58 4.58 4.58 4.58 4.58 4.58 4.58 4.58 4.58 4.58 4.58 4.58 4.58 1.22 3.05 4.58 6.10 7.63 9.16

10.68 12.21

3.34 3.27 3.41. 3.25 3.32 2.92 2.79 2.33 1.25 2.70 4.35 5.25 6.38 8.32 9.35 10.05 0.80 2.02 3.05 4.04 5.25 6.16 7.27 8.75

3.88 3.60 —

3.75 —

3.65 3.17 2.60 1.85 3.34 5.25 —

7.65 10.45

— 12.05 1.11 2.45 3.37 5.95 —

8.85 —

12.65

[Hg(OAc)2]＝3.00×10－3 mol•L－1 ([NBA] variation) a and 1.25×10－3 mol•L－1 ([S] and [Ru(III)] variations) b, [HClO4]＝13.33×10－3 mol•L－1, [KCl]＝3.00×10－4 mol•L－1, [NHA]＝3.50×

10－3 mol•L－1 ([NBA] variation) a and 1.11×10－3 mol•L－1 (in [S] and [Ru(III)] variations) b, µ＝5.00×10－2 mol•L－1.

N-Bromoacetamide Chin. J. Chem., 2008 Vol. 26 No. 6 1059


kinetics (Table 1). The reaction follows first order ki-netics in Ru(III) in the case of each reducing sugar (Ta-ble 1). Inverse fractional order, i.e. decreasing effect of H＋ on the rate, was observed (Table 2). This result was further verified by the plots of －log k1 versus pH for the oxidation of D-glucose and cellobiose, where straight lines of the nature shown in Figure 1 clearly demonstrates the increase in pseudo first-order rate con-stant, k1, values with the increase in pH of the solution. Positive effect of Hg(OAc)2 and Cl－ was also recorded (Table 2). Variation of acetamide concentration exhibits decreasing effect on the rate of reaction (Table 2). Al-most no effect of the ionic strength of the medium on the rate of reaction was observed. The rate constants measured at 30, 35, 40 and 45 ℃ led to the values of different activation parameters in the oxidation of Glc and Cel (Table 3). Identical kinetic results obtained for the oxidation of Glc and Cel suggest that both reactions

follow a common mechanism.

Figure 1 Plots between －log k1 and pH at 40 ℃, [NBA]＝1.00×10－3 mol•L－1, [sugar]＝2.00×10－2 mol•L－1, [Ru(III)]＝4.58×10－6 mol•L－1, [Hg(II)]＝1.25×10－3 mol•L－1, [Cl－]＝3.00×10－4 mol•L－1, [NHA]＝1.11×10－3 mol•L－1, µ＝5.00×

10－2 mol•L－1.

Table 2 Pseudo first order rate constants (k1) observed for the variations of [HClO4], [Hg(OAc)2], [KCl] and [NHA] in the oxidation of D-glucose and cellobiose at 40 ℃a

k1/(10－4 s－1) [HClO4]/(10－3 mol•L－1) [Hg(OAc)2]/(10－3 mol•L－1) [KCl]/(10－4 mol•L－1) [NHA]/(10－3 mol•L－1)

Glucose Cellobiose

1.00

2.00

4.00

6.00

8.00

10.00

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

13.33

1.25

1.25

1.25

1.25

1.25

1.25

1.11

2.00

3.00

4.00

5.00

6.00

8.00

10.00

1.25

1.25

1.25

1.25

1.25

1.25

1.25

1.25

1.25

1.25

1.25

1.25

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

1.60

2.60

3.60

4.60

5.60

6.60

3.00

3.00

3.00

3.00

3.00

3.00

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.11

1.00

2.00

4.00

6.00

8.00

10.00

15.14

8.91

5.89

4.47

3.80

3.05

2.63

3.98

5.12

6.30

7.24

8.32

8.95

9.35

2.25

2.65

3.15

3.45

3.85

4.05

3.66

2.05

1.65

1.26

1.15

0.87

17.38

11.49

7.76

4.79

4.07

3.39

3.31

5.55

—

6.85

—

9.05

10.25

10.97

2.45

2.75

3.85

4.05

4.45

4.85

4.75

2.95

2.10

1.68

1.38

1.23 a [NBA]＝10.00×10－4 mol•L－1, [S]＝2.00×10－2 mol•L－1, [Ru(III)]＝4.58×10－6 mol•L－1, µ＝5.00×10－2 mol•L－1.

Table 3 Values of activation parameters for the oxidation of D-glucose and cellobiose at 40 ℃

Reducing sugar Ea/(kJ•mol－1) k/(mol－2•dm6•s－1) S#/(J•K－1•mol－1) H#/(kJ•mol－1) G#/(kJ•mol－1) A/(mol－2•dm6•s－1)

D-Glucose

Cellobiose

62.20

57.39

2.41×105

1.96×105

36.32

19.31

59.56

63.12

48.19

48.74

5.07×1015

0.66×1015



Test for free radicals

In order to probe the presence of free radicals in the reaction, a reaction mixture containing acrylamide was placed for 24 h in an inert atmosphere. The reaction mixture was then diluted with the help of methanol and no precipitate was seen to be formed. This clearly sug-gests that free radicals are not present in the reaction under investigation.

Reactive species of NBA

It was reported23,24,34-36 that NBA existed in the fol-lowing equilibrium in an acidic medium:

3 2 3 2CH CONHBr H O CH CONH HOBr＋＋� (i)

2HOBr H H OBr＋＋＋ � (ii)

or

3 3 2CH CONHBr H (CH CO NH Br)＋＋＋ � (iii)

3 2 2 3 2 2(CH CONH Br) H O CH CONH H OBr＋＋＋＋� (iv)

From the above two sets of equilibria, it is clear that there are four possible reactive species of NBA in an acidic medium. These species are NBA itself, HOBr, protonated NBA, i.e. (CH3CONH2Br)＋ and cationic bromine i.e. (H2OBr)＋. If NBA or (CH3CONH2Br)＋ is assumed as the reactive species of NBA, the rate law obtained fails to explain the observed negative effect of acetamide. When (H2OBr)＋ is taken as the reactive species, it gives a rate law which shows first-order ki-netics with respect to H＋ ions, contrary to our observa-tion, although it fully explains the negative effect of acetamide. It is thus clear that out of the four possible species of NBA in the acidic medium, the species NBA itself, (CH3CONH2Br)＋ and (H2OBr)＋ can not be con-sidered as the reactive species of NBA in the oxidation of D-glucose and cellobiose by NBA in the acidic me-dium using Ru(III) chloride as a homogeneous catalyst. Now the only choice left is HOBr, which when taken as reactive species gives a rate law capable of explaining negative effect of both acetamide and H＋ ions and all other kinetic observations. Hence we proposed that HOBr should be the reactive species of NBA in the pre-sent investigation.

Reactive species of ruthenium(III) chloride

Taqui Khan and his co-workers37 have investigated the chloro complexes of Ru(III) in 0.1 mol•L－1 KCl at pH 0.4, 1.0 and 2.0 and at 25 ℃ with the use of elec-trochemical and spectrophotometric techniques. They have reported that at the instant of preparation Ru(III) exists in solution in the pH range 0.4—2.0 as four major species, [RuCl4(H2O)2]－, [RuCl3(H2O)3], [RuCl2-

(H2O)4]＋ and [RuCl(H2O)5]

2＋. They have also reported that the species [RuCl4(H2O)2]

－ , [RuCl3(H2O)3] and [RuCl(H2O)5]

2＋ are fairly stable at pH 0.4, moderately stable at pH 1.0 and highly unstable at pH 2.0. The spe-cies [RuCl2(H2O)4]

＋ was reported to be fairly stable at pH 2.0. According to them out of these four species, the species [RuCl2(H2O)4]

＋ is stabilized in its hydrolyzed form, [RuCl2(H2O)3OH] according to the following equilibrium (V).

2 2 4 2 2 2 3 3[RuCl (H O) ] H O [RuCl (H O) OH] H O＋＋＋＋� (v)

In the present investigation, since the experiments were performed with Ru(III) chloride dissolved in 0.01 mol/L HCl and throughout the study the pH of the solu-tion was maintained at about 2.0, with nil effect of [Cl－] on the rate of reaction, the lone species, i.e. [RuCl2- (H2O)3OH], present in the solution, can be considered as the reactive species of Ru(III) chloride in the acidic me-dium. Since throughout the study, [Cl－] ions have been added to the reaction mixture as one of the reactants in the oxidation of both D-glucose and cellobiose, hence to observe the effect of [Cl－] on the rate of reaction when its concentration has been varied at constant concentra-tion of all other reactants and at constant temperature, it has been found that with the increase in [Cl－] there is an increase in the rate or pseudo first-order rate constant, k1, of the reaction. Considering the positive effect of [Cl－] on the rate of reaction, the following equilibrium can be assumed to exist in the reactions under investigation.

2 2 3 3 2 2 2[RuCl (H O) OH] Cl [RuCl (H O) OH] H O－－

＋＋� (vi)

Above equilibrium, with positive effect of [Cl－] on the rate of reaction, clearly indicates that it is the species [RuCl3(H2O)2OH]－ that can be taken as the reactive species of Ru(III) chloride in the present investigation.

Spectral evidence for the formation of various com-plexes

It was reported11,38 that Ru(III) formed a complex with the sugar molecule. In order to verify the existence of [Ru(III)-sugar] complex and to probe the possibility of formation of other complexes in the reaction, first of all UV spectra of Ru(III) chloride (Peak No. 1), sugar solution (Peak No. 2), NBA solution (Peak No. 3) and Hg(II) solution (Peak No. 4) have been collected (Figure 2). After collecting the spectra for above reactants sepa-rately, efforts were made to collect spectra for Hg(II) with NBA solution (Peak No. 5), Ru(III) chloride solu-tion with two different concentrations of the sugar solu-tion (Peak Nos. 6 & 7), Ru(III) chloride and sugar solu-tion with two different concentrations of the NBA solu-tion (Peak Nos. 8 & 9) and Ru(III) chloride, sugar and NBA solution with two different concentrations of the Hg(II) solution (Peak Nos. 10 & 11) at room tempera-ture (Figure 2). From the spectra of Ru(III) chloride so-



Figure 2 UV spectra of (1) [Ru(III)]＝7.63×10－4, (2) [sugar]＝50×10－2, (3) [NBA]＝1.00×10－3, (4) [Hg(II)]＝1.00×

10－3, (5) [NBA]＝1.00×10－3, [Hg(II)]＝1.00×10－3, (6) [Ru(III)]＝7.63×10－4, [sugar]＝5.00×10－2, (7) [Ru(III)]＝7.63×10－4,

[sugar]＝50×10－2, (8) [Ru(III)]＝7.63×10－4, [sugar]＝50× 10－2, [NBA]＝1.00×10－3, (9) [Ru(III)]＝7.63×10－4, [sugar]＝50×10－2, [NBA]＝2.00×10－3, (10) [Ru(III)]＝7.63×10－4, [sugar]＝50×10－2, [NBA]＝1.00×10－3, [Hg(II)]＝1.00×10－3, (11) [Ru(III)]＝7.63×10－4, [sugar]＝50×10－2, [NBA]＝1.00×10－3, [Hg(II)]＝2.00×10－3 mol•L－1.

lution and Ru(III) chloride solution with two different concentrations of sugar solution, it is clear that with the addition of sugar solution of concentrations 5×10－2 and 50×10－2 mol•L－1, there is an increase in absorbance from 2.56 to 2.74 and 3.02 (Figure 2, peak Nos. 1, 6 and 7). This increase in absorbance with the increase in sugar concentration, clearly shows that the reactive spe-cies of Ru(III) chloride forms a complex with the sugar molecule according to the following equilibrium:

3 2 2 3 2 2[RuCl (H O) OH] S [RuCl S (H O)OH] H Oi i

－－

＋＋� (vii)

Since with the increase in sugar concentration, the Eq. (vii) will shift towards right hand side with more and more formation of the complex [RuCl3•S•(H2O)- OH]－, it is reasonable to assume that the increase in absorbance from 2.56 to 2.74 and 3.02 is definitely an evidence for the formation of a complex between reac-tive species of Ru(III) chloride and sugar molecule. When the spectrum of Hg(II) solution (Peak No. 4) was compared with the spectrum of Hg(II) and NBA solu-tion (Peak No. 5), it was found that there was an in-crease in absorbance from 2.44 to 2.88, indicating the formation of a complex between Hg(II) and the reactive species of NBA, i.e. HOBr according to the equilibrium (viii).

2 + +Hg HOBr [Hg OBr] + H←＋＋ � (viii)

Similarly, when the spectrum of NBA solution (Peak No. 3) was compared with that of Hg(II) and NBA solu-tion (Peak No. 5), again an increase in absorbance from 2.1 to 2.88 was noted (Figure 2). This further proves the existence of equilibrium (viii) in the reaction, showing the formation of the complex [Hg OBr]← ＋ . At last, when an effort was made to draw the inference from the spectra of Ru(III) chloride, sugar and NBA solution (Peak No. 8) and Ru(III) chloride, sugar and NBA solu-tion with two different concentrations of Hg(II) solution (Peak No.10 and 11), it was concluded that the increase in absorbance from 3.26 to 3.7 and 3.84 was due to more and more formation of the complex RuCl3•S•OBr(Hg)•OH according to the equilibrium indicated below:

3 2

3 2

[Hg OBr] [RuCl S (H O) OH]

[RuCl S OBr (Hg) OH] H O

← i i i

i i i

＋－＋

＋

� (ix)

The observed increase in absorbance with a batho-chromic shift in λmax value towards longer wavelength not only supports the formation of the most reactive ac-tivated complex RuCl3•S•OBr(Hg)•OH but it also the fact that when a chromophore (Hg(II)) combines with an auxochrome (Cl－ ion), it gives rise to another chromo-phore RuCl3•S•OBr(Hg)•OH.

Reaction path

On the basis of observed effects of [NBA], [reducing sugar], [Ru(III)], [H＋], [Hg(II)], [Cl－], [NHA], ionic strength and dielectric constant of the medium on the rate of reaction and also on the basis of spectral evi-dence collected for the possible formation of various complex species in the reaction and the discussion made above for the reactive species of NBA and Ru(III) chlo-ride, a reaction mechanism which is common for the oxidation of both D-glucose and cellobiose and is sup-ported by entropy of activation, isokinetic relationship and multiple regression analysis, has been proposed.

2NBA H O NHA HOBr＋　＋� (I)

22 2 3

1

3 2 2 2

2

[RuCl (H O) OH] Cl

(C )

[RuCl (H O) OH] H O

(C )

k��⇀↽��

－

－

＋　

＋

(II)

32

3

Hg HOBr [Hg OBr] H

(C )

k←��⇀

↽��＋＋＋

＋＋ (III)

43 2 2

2 2

[RuCl (H O) OH] S

[RuCl S (H O)OH] H O3

k��⇀↽��

i i

－

－

＋

＋

(IV)



where S stands for D-glucose only as one molecule of cellobiose also contains two units of D-glucose .

53 2

3 2

[Hg OBr] [RuCl S (H O) OH]

RuCl SOBr(Hg)OH H O

k← ��⇀i i i ↽��

i

＋－＋

－＋

(V)

63 2 2

23 2 2

2

Slow and rate deteming step

[RuCl S (H O) OH] 2H O

[RuCl (H O) OH] OH Hg

RCH OHCH(OBr)OH

k⎯⎯⎯⎯⎯→i i i

i

－

－－＋

＋

＋＋＋ (VI)

where R stands for C4H8O4.

3 2 2 2

2 3

fast[RuCl (H O) OH] H O

[RuCl (H O) OH] Cl2

⎯⎯⎯→－

－

＋

＋

(VII)

On applying the law of chemical equilibrium to steps I, II, III, IV and V and taking the total concentration of Ru(III), i.e. [Ru(III)]T as

[Ru(III)]T＝ [C1]＋[C2]＋[C4]＋[C5],

the rate law for the loss of NBA can be expressed as Eq. (1), Eq. (1) is the final rate law which is in close con-formity with the experimental findings. Eq. (1) can also be written as Eq. (2).

According to Eq. (2), if a plot is made between

T[Ru(III)]

rate and [NHA] or [H＋] or 1/[NBA] or

1/[Hg(II)] or 1/[S] or 1/[Cl－], a straight line having an

in tercept on y -axis wi l l be obta ined. When

T[Ru(III)]

rate values were plotted against [NHA], [H＋],

1/[NBA], 1/[Hg(II)], 1/[S] and 1/[Cl－], straight lines with positive intercepts on y-axis were obtained, which proves the validity of the rate law (1) and hence the proposed mechanism (Figures 3—8). From the values of the intercepts and slopes of the plots, the values of k6, K1K3K5, K4 and K2 have been calculated and found as 1.85×10－1 s－1, 1.07×102, 2.64 and 2.13×103 mol－1•L respectively for the oxidation of D-glucose and 0.94×10－1 s－1, 1.05×102, 3.8 and 1.55×103 mol－1•L, respectively for the oxidation of cellobiose. When these values of the constants were used to calculate rates on the basis of rate law (1) for the variations of [Ru(III)], [H＋], [Hg(II)], [Cl－] and [NHA], it was found that the calculated rates were very close to the rates obtained experimentally (Tables 4—8). The close resemblance between the two rates i.e. the rate calculated and the rate observed, further proves the validity of the rate law (1)

and hence the proposed reaction Eqs. (I—IX).

Figure 3 Plots between ([Ru(III)]T/rate) and [NHA] at 40 ℃.

[NBA]＝1.00×10－3 mol•L－1, [sugar]＝2.00×10－2 mol•L－1, [Ru(III)]＝4.58×10－6 mol•L－1, [H＋]＝13.33×10－3 mol•L－1, [Hg(II)]＝1.25×10－3 mol•L－1, [Cl－]＝3.00×10－4 mol•L－1, µ＝5.00×10－2 mol•L－1.

Figure 4 Plots between ([Ru(III)]T/rate) and [H＋] at 40 ℃. [NBA]＝1.00×10－3 mol•L－1, [sugar]＝2.00×10－2 mol•L－1, [Ru(III)]＝4.58×10－6 mol•L－1, [Hg(II)]＝1.25×10－3 mol•L－1, [Cl－]＝3.00×10－4 mol•L－1, [NHA]＝1.11×10－3 mol•L－1, µ＝5.00×10－2 mol•L－1

Figure 5 Plots between ([Ru(III)]T/rate) and 1/[NBA] at 40 ℃. [sugar]＝2.00×10－2 mol•L－1, [Ru(III)]＝4.58×10－6 mol•L－1, [H＋]＝13.33×10－3 mol•L－1, [Hg(II)]＝1.25×10－3 mol•L－1, [Cl－]＝3.00×10－4 mol•L－1, [NHA]＝3.50×10－3 mol•L－1, µ＝5.00×10－2 mol•L－1.

6 2 3 4 5 T

2 2 4 2 3 4 5

d[NBA]Rate

d[S][NBA][Hg(II)][Cl ][Ru(III)]

[NHA][H ] [Cl ][NHA][H ] S][Cl ][NHA][H ] [S][NBA][Hg(II)][Cl ]

tn k K K K K K

K K K K K K K K−

－

１

＋＋－＋－

１

＝－＝

＋＋＋

(1)



T

6 3 4 56 2 3 4 5

6 3 5 6

[Ru(III)] [NHA][H ] [NHA] [H ]Rate [S][NBA][Hg(II)][S][NBA][Hg(II)][Cl ]

[NHA][H ] 1[NBA][Hg(II)]

nk K K K Knk K K K K K

nk K K K nk

+

＋＋

－

１１

１

＝＋　＋　

　　　　　　＋

(2)

Figure 6 Plots between ([Ru(III)]T/rate) and 1/[Hg(II)] at 40

℃. [NBA]＝1.00×10－3 mol•L－1, [sugar]＝2.00×10－2 mol•L－1, [Ru(III)]＝4.58×10－6 mol•L－1, [H＋]＝13.33×10－3 mol•L－1, [Cl－]＝3.00×10－4 mol•L－1, [NHA]＝3.50×10－3 mol•L－1, µ＝5.00×10－2 mol•L－1.

Figure 7 Plots between ([Ru(III)]T/rate) and 1/[S] at 40 ℃.

[NBA]＝1.00×10－3 mol•L－1, [Ru(III)]＝4.58×10－6 mol•L－1, [H＋]＝13.33×10－3 mol•L－1, [Hg(II)]＝1.25×10－3 mol•L－1, [Cl－]＝3.00×10－4 mol•L－1, [NHA]＝3.50×10－3 mol•L－1, µ＝5.00×10－2 mol•L－1.

Figure 8 Plots between ([Ru(III)]T/rate) and 1/[Cl－] at 40 ℃.

[NBA]＝1.00×10－3 mol•L－1, [sugar]＝2.00×10－2 mol•L－1, [Ru(III)]＝4.58×10－6 mol•L－1, [H＋]＝13.33×10－3 mol•L－1, [Hg(II)]＝1.25×10－3 mol•L－1, [NHA]＝3.50×10－3 mol•L－1, µ＝5.00×10－2 mol•L－1.

Figure 9 Plots between (4＋log k1) and 1/D at 40 ℃. [NBA]＝1.00×10－3 mol•L－1, [sugar]＝2.00×10－2 mol•L－1, [Ru(III)]＝4.58×10－6 mol•L－1, [H＋]＝13.33×10－3 mol•L－1, [Hg(II)]＝1.25×10－3 mol•L－1, [NHA]＝3.50×10－3 mol•L－1, [Cl－]＝3.00×10－4 mol•L－1, µ＝5.00×10－2 mol•L－1.

Table 4 Comparison of observed rates with the rates calculateda on the basis of rate law (1) and the rates calculatedb by the help of mul-tiple regression analysis under the conditions of Table 1

(－dc/dt)×107/(mol•L－1•s－1)

D-Glucose Cellobiose [Ru(III)]×106/(mol•L－1)

Experimental Calculateda Calculatedb Experimental Calculateda Calculatedb

1.22 3.05 4.58 6.10 7.63 9.16

10.68 12.21

0.80 2.02 3.05 4.04 5.25 6.16 7.27 8.75

0.70 1.74 2.87 3.77 4.85 5.82 6.98 8.65

0.66 1.75 2.90 4.06 5.05 5.98 7.06 8.57

1.11 2.45 3.37 5.95 —

8.85 —

12.65

0.96 2.39 3.04 5.83 —

8.70 —

12.47

1.19 2.26 3.51 5.79 —

8.74 —

12.14




(－dc/dt)×107/(mol•L－1•s－1)

D-Glucose Cellobiose [H＋]×103/(mol•L－1)


1.00 2.00 4.00 6.00 8.00

10.00

15.14 8.91 5.39 4.47 3.80 3.05

15.05 9.29 6.04 4.68 3.94 3.21

15.08 9.05 5.90 4.56 3.82 2.95

17.38 11.49 7.76 4.79 4.07 3.39

17.06 10.95 7.67 5.21 4.13 3.56

17.25 11.38 7.92 4.86 4.02 3.41


(－dc/dt)×107/(mol•L－1•s－1)

D-Glucose Cellobiose [Hg(II)]×103/(mol•L－1)


1.11 2.00 3.00 4.00 5.00 6.00 8.00

10.00

2.63 3.98 5.12 6.30 7.24 8.32 8.95 9.35

2.35 3.83 5.15 6.24 7.14 7.91 9.02

10.04

2.58 4.02 5.65 6.48 7.58 8.66 9.07 9.43

3.31 5.55 —

6.85 —

9.05 10.25 10.97

2.86 5.11 —

6.58 —

8.63 9.92

10.87

3.17 5.86 —

7.02 —

9.02 10.04 11.07


(－dc/dt)×107/(mol•L－1•s－1)

D-Glucose Cellobiose [Cl–]×104/(mol•L－1)


1.6 2.6 3.6 4.6 5.6 6.6

2.25 2.65 3.15 3.45 3.85 4.05

1.82 2.42 2.95 3.37 3.75 3.98

2.06 2.57 3.02 3.68 4.02 4.32

2.45 2.75 3.85 4.05 4.45 4.85

2.16 2.61 3.56 3.96 4.25 4.86

2.62 2.72 3.88 4.07 4.62 4.92


(－dc/dt)×107)/(mol•L－1•s－1)

D-Glucose Cellobiose [NHA]×103/(mol•L－1)


1.00 2.00 4.00 6.00 8.00 10.00

3.66 2.05 1.65 1.26 1.15 0.87

3.85 1.95 1.60 1.25 1.04 0.83

3.78 2.32 1.79 1.38 1.06 0.92

4.75 2.95 2.10 1.68 1.38 1.23

4.87 2.75 1.92 1.61 1.36 1.04

5.05 3.02 2.15 1.73 1.42 1.09

Multiple regression analysis

The use of multiple regression analysis has also been made to calculate the rates on the basis of equations of the fitted model for D-glucose and cellobiose for the variation of each [Ru(III)], [H＋], [Hg(II)], [Cl－] and

[NHA]. In each case it has been found that the rates calculated in this manner are close to the rates observed experimentally and the rates calculated on the basis of rate law (1) (Tables 4—8). This clearly supports the rate law (1) and hence the proposed reaction Scheme 1. Fit-



ted model equations which were used to calculate the rates for the variation of each [Ru(III)], [H＋], [Hg(II)], [Cl－] and [NHA] in the oxidation of D-glucose and cel-lobiose are as follows Eqs. (3), (4).

0.82 1.07 0.651

0.66 0.47 0.43

[Glc] [Ru(III)] [H ]

[Hg(II)] [NHA] [Cl ]

k •＋－

－－

＝

　　

(3)

0.85 1.08 0.611

0.58 0.46 0.45

[Cel] [Ru(III)] [H ]

[Hg(II)] [NHA] [Cl ]

k •＋－

－－

＝

　　　

(4)

where 1[NBA]/

[NBA]

d dtk＝－ .

Effect of dielectric constant of the medium on the rate of oxidation of D-glucose and cellobiose

In order to find out the effect of dielectric constant of the medium on the rate of reaction, a series of experi-ments were performed with varying percentage concen-trations of ethanol at constant concentrations of all other reactants and at a constant temperature of 40 ℃. The values of pseudo first order rate constant thus obtained for each set, clearly show that there is an increase in the rate of reaction with the decrease in dielectric constant of the medium. It was reported39 that although primary alcohols were oxidized by N-chlorosuccinimide, which is treated as strong oxidant, NBA or NBS failed to oxi-dize aliphatic primary alcohols. Later on, a report23 came into existence which indicates that in the catalyzed range of [H＋] between 0.2 and 0.75 mol•L－1, primary alcohols were oxidized by NBA. Thus, it was necessary for us to ascertain whether under our experimental con-ditions, ethanol can be oxidized by NBA or not? When experiments were performed by taking ethanol as an organic substrate instead of reducing sugars in the usual manner, it was found that ethanol under the conditions of our experiments (at or below 13.33×10－3 mol•L－1 H＋ ion concentration) was not at all oxidized by NBA in the presence of Ru(III) as a homogeneous catalyst. The only conclusion is therefore that the observed in-crease in the rate of reaction with the addition of ethanol to the reaction mixture is due to the change in dielectric constant of the medium. It is a known fact that in the case of a reaction between two ions, the effect of dielec-tric constant of the medium on the rate constant of the reaction can be shown by the help of the following equation

2A B

00 AB

1log log

2.303 (4π )

Z Z e Nk k

d RT D∈×

�

＝－ (5)

where k0 is the rate constant in a medium of infinite di-electric constant, ZA and ZB are the changes of reacting ions, dAB refers to the size of activated complex, T is absolute temperature and D is dielectric constant of the medium.

Here in the present paper, a common reaction mechanism in the name of Eqs. (I—IX) has been pro-posed for the oxidation of D-glucose and cellobiose, which indicates that the most reactive activated complex, [RuCl3•S•OBr(Hg)•OH] is formed by the interaction of two ionic species i.e. +[Hg OBr]← and [RuCl3•S•(H2O)OH]–. Thus for the purpose of calcula-tion of dAB, we can very easily take the help of Eq. (5), where ZA and ZB will be taken as (＋1) and (－1) re-spectively. According to Eq. (5), when log k1 values obtained for the oxidation of glucose and cellobiose were plotted against 1/D, straight lines with positive intercept on log k1 axis were obtained (Figure 9). From the slopes of the straight lines, the values of dAB have been calculated and found as 4.20 and 3.72 Å for glu-cose and cellobiose respectively. These values of dAB are in close agreement with reactions of similar nature.40 Positive slopes obtained for the oxidation of glucose and cellobiose support the existence of an activated complex, [RuCl3•S•OBr(Hg)•OH], in the reaction which is formed by the interaction of two oppositely charged species i.e. [Hg←OBr]＋ and [RuCl3•S•(H2O)OH]– (step VI). This activated complex further decomposes into reaction products via a rate determining step.

Entropy of activation and other activation parame-ters

It is a well known fact that solvation, in general, in-creases with the charge on the ion. In the case of a reac-tion between two ions of opposite charge, their union will result in a lowering of the net charge and due to this some frozen solvent molecules will be released with an increase of entropy. But, on the other hand when a reac-tion takes place between two similarly charged ions, the transition state will be more highly charged ion and due to this more solvent molecules will be required than for the separate ions. This would lead to a decrease in en-tropy. In the present study of the oxidation of D-glucose and cellobiose by NBA in the presence of perchloric acid using Ru(III) as a homogeneous catalyst, the most reactive activated complex, i.e. [RuCl3•S•OBr(Hg)•OH] is being formed by the interaction of two oppositely charged species i.e. [Hg←OBr]＋ and [RuCl3•S•(H2O)• OH]－ and as a result of which the net charge on the activated complex becomes zero. Due to lowering of the net charge on the activated complex, some frozen sol-vent molecules will be released with an increase in en-tropy. Positive entropy of activation (ΔS#) observed in the oxidation of D-glucose and cellobiose clearly sup-ports the reaction Eqs. (I—IX), where the activated state would be less solvated than the reactants (Table 3). The order of frequency factor (A) being the same for the oxidation of D-glucose and cellobiose supports the op-eration of a common mechanism proposed for the aforesaid redox reactions (Table 3). Almost the same values of free energy of activation (ΔG#) in the oxida-tion of both reducing sugars also support the operation of a single mechanism for both the redox systems (Table 3).



Comparative studies

Efforts have also been made to compare the findings of this paper with the results already reported for Ir(III)15- and Ru(VIII)32-catalyzed oxidations of reduc-ing sugars by NBA in acidic and alkaline media respec-tively. During comparison special attention has been given to the point that how the transition metal catalysts, i.e. Ru(III), Ir(III) and Ru(VIII) in the acidic or alkaline media affect the role of NBA, reducing sugar molecule and other reactants of the reaction in the formation of the most reactive activated complex which ultimately converts into the final reaction products? The present study is similar to the other two ones,15,32 as far as the order with respect to NBA is concerned, but it differs from Ru(VIII)-catalyzed32 oxidation of glucose, man-nose and galactose, where in place of HOBr, OBr－ has been assumed as the reactive species of NBA in an alka-line medium. Observed first to zero-order kinetics in [sugar] is contrary to the reported zero-order kinetics for Ir(III)-15 and Ru(VIII)-catalyzed32 oxidations of the re-ducing sugars by NBA. The involvement of reducing sugar molecule before the rate determining step in the present study of Ru(III)-catalyzed oxidation distin-guishes the study from the other two reported studies15,32 where the reducing sugar molecule participates in the reaction after the rate determining step. In the reported Ir(III)-15 and Ru(VIII)-catalyzed32 oxidation of the re-ducing sugars, the order with respect to the catalyst was found to be first at low concentration which tended to zero-order at its higher concentration. This result is also not similar to the present study where first-order kinetics with respect to [Ru(III)] has been observed throughout its variation. On the basis of the observed kinetic data and the reported literature, the species [RuCl3•(H2O)2- OH]－ has been assumed as the reactive species of Ru(III) chloride in the present investigation. In the other two reported15,32 studies, the species [IrCl5(H2O)]2－ and HRuO5 were taken as the reactive species of Ir(III) chlo-ride and RuO4 in the acidic and alkaline media respec-tively. In the present study, the formation of a complex between the reactive species of Ru(III) chloride i.e. [RuCl3(H2O)2OH]－, and a sugar molecule finds support from spectral evidence as well as from the observed ki-netic data, but in the other two studies15,32 no such a complex formation was reported. The present study with respect to order in [Hg(II)] is similar to the reported Ru(VIII)-catalyzed32 oxidation of the reducing sugars but differs from Ir(III)-catalyzed15 oxidation, where a second-order kinetics tending towards first-order kinet-ics was observed. In each case, the role of Hg(II) was observed both as a Br－ ion scavenger and as a co- catalyst. In Ru(III)-, Ir(III)-15 and Ru(VIII)-catalyzed32 oxidations of the reducing sugars, a negative effect of NHA on the rate of reaction was observed. On the basis of observed positive entropy of activation in the present case as well as in Ir(III)-catalyzed15 oxidation of the reducing sugars, the species [RuCl3•S•OBr(Hg)•OH] and [BrO(Hg)IrCl4(ClHg)] have been assumed as the

most reactive species for both the redox systems. The negative entropy of activation observed in the Ru(VIII)- catalyzed32 oxidation of the reducing sugars led the au-thors to assume that the species 2

4[BrOLRuO LHg] ＋ is the most reactive species in the reaction, which via a rate determining step converts into final products along with regeneration of the reactive species of RuO4, i.e.

5HRuO－ . The facts mentioned above forced us to conclude that

the present study is entirely different with the other two reported studies15,32 as far as orders with respect to [sugar] and [Ru(III)] and formation of a reactive com-plex between reactive species of Ru(III) and a sugar molecule are concerned.

Conclusion

On the basis of observed kinetic data and spectral evidence collected for Ru(III)-catalyzed oxidation of D-glucose and cellobiose, it can be concluded that:

(1) HOBr is the reactive species of NBA in the aforesaid redox systems.

(2) Reducing sugar molecule forms a complex, [RuCl3•S•(H2O)•OH] － , with the reactive species of Ru(III) chloride i.e. [RuCl3•(H2O)2OH]－.

(3) Hg(II) acts both as a Br－ ion scavenger and as co-catalyst. It also forms a complex with the reactive species of NBA, i.e. HOBr of the type, [Hg←OBr]＋ in the oxidation of D-glucose and cellobiose.

(4) Positive effect of Cl－ ion played a role in decid-ing the reactive species of Ru(III) chloride whereas the negative effect of [NHA] and [H＋] forced us to assume HOBr as the reactive species of NBA in the aforesaid oxidation processes.

(5) The most reactive activated complex i.e. [RuCl3•S•OBr(Hg)•OH] is formed by the interaction of two oppositely charged species, i.e. [RuCl3•S•(H2O)- OH]– and [Hg←OBr]＋.

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(E0704251 LU, Y. J.)

kinetics of the oxidation of d-glucose and cellobiose by acidic solution of n-bromoacetamide using...

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