Mechanistic study of oxidation of d-arabinose by N-bromophthalimide in presence of micro-amount of chloro-complex of Ru(III) as a homogeneous catalyst

2.1 Materials

Analytical grade chemicals and double distilled water were used throughout the investigation. N-bromophthalimide (Lancaster, 98%), was used as received. Solution of NBP was prepared in 80% acetic acid and stored in a black coated flask in order to prevent photochemical deterioration and prepared solution of NBP was standardized iodometrically. An aqueous solution of d-Ara (AR grade) was always freshly prepared. Ru(III) chloride solution was prepared by dissolving a known weight of RuCl₃ (Johnson-Matthey) in HCl of known concentration. The Ru(III) chloride solution was also stored in a black coated bottle. Standard solution of KCl, KNO₃, HClO₄ and phthalimide (all of Merck) were prepared and mercuric acetate (Loba chem., Mumbai, India) was acidified with 20% acetic acid. Acetic acid and acetonitrile (Both are SD Fine) were use for study of dielectric constant of the medium.

2.2 Kinetic procedure

All the kinetic measurement were carried out in a black-coated vessel at 308 K and performed under pseudo first-order condition with [d-Ara] ≫ [NBP]. The reaction was initiated by the rapid addition of known amounts of substrate to reaction mixture containing the required amount of the NBP, HClO₄, Ru(III) chloride, mercuric acetate, acetic acid and water in glass stoppered Pyrex boiling tubes; thermostated at 308 K. The progress of reaction was monitored by iodometric determination of unconsumed [NBP] in known aliquots of the reaction mixtures at different time intervals.

The rate constants of reaction in each kinetic run were determined by first order rate equation which is given below:

2.3 Stoichiometry and product analysis

Different ratios of NBP to d-Ara were equilibrated at 308 K in the presence of requisite amount of perchloric acid, mercuric acetate and acetic acid under the condition of [NBP] ≫ [d-Ara] for 48 h. Determination of unconsumed NBP revealed that two mole of NBP were required for the oxidation of each mole of d-Ara (Table 5). Accordingly, the following stoichiometric equation could be formulated as:

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Formic acid and erythronic acid were identified as the main oxidation products of the d-Ara by the help of kinetic studies, equivalence, TLC (Singh et al., 2007).

Finally, the formic acid was detected by conventional spot test method (Feigl, 1966). A drop of the test solution is mixed in a test tube with a drop of 2 M hydrochloric acid; magnesium powder is added until no more gas is liberated. Three milliliters of 1.2 M sulfuric acid and a little chromotropic acids (1,8-dihydroxy naphthalene-3,6-disulfonic acid) are then added; the tube is kept for 10 min at 60° in a water bath. A violet–pink color appeared. The positive response indicated that formic acid is present in the product.

2.4 Test for free radicals

The generation of free radicals during the course of the oxidation was confirmed by using acryl amide monomer. In a reaction mixture containing NBP, d-Ara and acetic acid, no precipitation was marked when a known amount of acryl amide was added. The negative response of addition of acryl amide indicated that no free radicals were formed during the reaction.

2.5 Kinetic measurements & results

The kinetics of the oxidation of d-Ara by NBP was investigated at several initial concentrations of the reactants, at constant temperature of 308 K. The initial rate of reaction in each run was determined by the slope of the tangent drawn at a fixed concentration of NBP, except for studies of NBP variation, where the slope of the tangent was drawn at fixed time. Considering NBP, d-Ara, hydrogen ions and Ru(III) as the main reactants, the general form of rate equation for the reaction can be written as

In order to determine the experimental rate law, a series of experiments with varying initial concentrations of NBP were performed at constant concentration of all other reactants and at constant temperature. Uniform pseudo-first-order rate constant k₁ values for the variation of [NBP] (from 0.40 × 10⁻⁴ mol dm⁻³ to 2.00 × 10⁻⁴ mol dm⁻³) clearly indicate that the order with respect to [NBP] is unity (Table 1 or Fig. 1). This is also obvious from the plots of remaining log [NBP] versus time were found to be straight lines, indicating that the order with respect to the oxidant was one and from the slope of the lines k₁, rate constant was calculated (Fig. 2).

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Figure 1

Effect of variation of [NBP] on the observed rate constants at 308 K. [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [H⁺] = 4.00 × 10⁻³ mol dm⁻³, [Ru(III)] = 1.15 × 10⁻⁵ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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Figure 2

Sample individual time plots for remaining log [NBP] for various concentrations at 308 K. [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [NBP] = 2.00 × 10⁻⁴ mol dm⁻³, [Ru(III)] = 1.15 × 10⁻⁵ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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The pseudo-first-order rate constant (k₁) for the oxidation of d-Ara by NBP for each kinetic run was calculated as:

The values of k₁ under different conditions of the experiments are given in Table 1.The values of k₁ slightly vary with the concentration of d-Ara (from 2.00 × 10⁻³ mol dm⁻³ to 20.00 × 10⁻³ mol dm⁻³), suggesting that the reaction rate is positive fractional order with respect to d-Ara (Table 1 or Fig. 3). A linear increase in the apparent first-order rate constant was observed with increase in Ru(III) chloride concentration (from 0.38 × 10⁻⁵ mol dm⁻³ to 3.85 × 10⁻⁵ mol dm⁻³) throughout its variation (Fig. 4). This result was further verified by the plot that gives unity order 0.99 in Ru(III) from the slope of log k₁ versus log[Ru(III)] (Table 1 or Fig. 5). On increasing [HClO₄] the values of reaction rate increased without following any relationship. This showed positive effect of [H⁺] on the rate of reaction (Fig. 6). The order of reaction with respect to hydrogen ion (obtained from perchloric acid) was determined as 0.52 from the slope of the plot of log k₁ versus log [H⁺] (Table 1). Negative effect of [Cl⁻] (from 0.20 × 10⁻⁵ mol dm⁻³ to 4.00 × 10⁻⁵ mol dm⁻³) was found (Table 2 or Fig. 7). When variations in ionic strength of the medium was made, it was observed that there is almost no change in pseudo-first-order rate constant, k₁ values with the change in ionic strength of the medium (Table 1). Mercuric acetate and phthalimide did not bring about any significant change in k₁ values under the constant experimental conditions (Table 3). It has been reported earlier (Singh et al., 2004) that Hg(II) can act as a homogeneous catalyst, co-catalyst and oxidant. In order to ascertain the real role of Hg(OAc)₂, as a Br⁻ scavenger, several experiments were performed with different initial concentrations of Hg(OAc)₂ with and without the presence of NBP under similar experimental conditions. The kinetic observations showed that the reaction rate was almost constant with the increase in concentration of Hg(OAc)₂, which negates its role as a catalyst and co-catalyst in the reaction. Also, the reaction did not proceed under similar conditions with Hg(OAc)₂ without using NBP; indicating Hg(OAc)₂ is not involved as an oxidant. Such kinetic observations suggest that Hg(OAc)₂ acted only as a Br⁻ scavenger forming [HgBr₄]²⁻ (Bailar, 1956). Rate studies were carried out with acetonitrile and acetic acid in different composition (5–30% by volume and 15–40% by volume respectively) in reaction mixture, thereby varying the dielectric constant of the medium. It was found that there is negative effect of dielectric constant of the medium on the rate of reaction. The value of d_AB was evaluated using slopes of acetic acid is 3.01 Ǻ. It was observed that acetonitrile and acetic acid were oxidized by NBP under the experimental conditions. Finally, the effect of temperature (from 303 to 323) on the reaction rate was determined by keeping constant concentrations of other constituents of the solution. Activation energy was calculated from the linear Arrhenius plot of log k₁ versus 1/T, which give straight line equation (y = −3692 × +12.62, Fig. 8). The values of the activation energy (ΔEa), enthalpy of activation (ΔH^#), entropy of activation (ΔS^#), and Gibb’s free energy of activation (ΔG^#) were determined from the effects of temperature on the rate. The calculated values are given in Table 4.

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Figure 3

Pseudo first order rate constants against substrate concentration at 308 K. [NBP] = 2.00 × 10⁻⁴ mol dm⁻³, [H⁺] = 4.00 × 10⁻³ mol dm⁻³, [Ru(III)] = 1.15 × 10⁻⁵ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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Figure 4

Pseudo first order rate constants against catalyst concentration at 308 K, [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [NBP] = 2.00 × 10⁻⁴ mol dm⁻³, [H⁺] = 4.00 × 10⁻³ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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Figure 5

Plot of log k₁ versus log [NBP] for the oxidation of d-Ara at 308 K, [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [NBP] = 2.00 × 10⁻⁴ mol dm⁻³, [H⁺] = 4.00 × 10⁻³ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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Figure 6

Effect of variation of [H⁺] on the observed rate constants at 308 K. [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [NBP] = 2.00 × 10⁻⁴ mol dm⁻³, [Ru(III)] = 1.15 × 10⁻⁵ mol dm⁻³ [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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Figure 7

Effect of variation of [KCl] on the observed rate constants at 308 K. [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [NBP] = 2.00 × 10⁻⁴ mol dm⁻³, [Ru(III)] = 1.15 × 10⁻⁵ mol dm⁻³, [H⁺] = 4.00 × 10⁻³ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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Figure 8

Plot of log k₁ versus 1/T for the oxidation of d-Ara. [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [NBP] = 2.00 × 10⁻⁴ mol dm⁻³,[Ru(III)] = 1.15 × 10⁻⁵ mol dm⁻³, [H⁺] = 4.00 × 10⁻³ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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3.1 Reactive species of NBP

It has been reported earlier by several workers (Krishnakumar et al., 2005; Khazaei and Manesh, 2005; Kirsch and Luning, 1998; Ramachandrappa et al., 1998; Kirsch et al., 1999; Day et al., 1986; Das and Indrasenan, 1986, 1987; Joshi et al., 2006; Katre et al., 2008) that NBP is good oxidizing and brominating agent because of large polarity of >N-Br bond. NBP, like other similar N-halo imides, may exist in various forms in acidic medium, i.e., free NBP, protonated NBP, Br⁺, HOBr, (H₂OBr)⁺, as per the following equilibria:

When (H₂OBr)⁺ was taken as reactive species, the rate law obtained showed first-order kinetics with respect to hydrogen ion concentrations, contrary to our observed negative fractional order in [HClO₄]. Therefore, the possibilities of involvement of (H₂OBr)⁺ and cationic bromine (Br)⁺ as reactive species were ruled out. When HOBr was assumed as the reactive species, the derived rate law failed to explain negligible effect of phthalimide. Hence neither of these species could be considered as the reactive species. Thus, only choice left was NBP, which when considered as the reactive species, led to a rate law explaining all the kinetic observations and other effects. Hence in the light of kinetic observation, NBP could be assumed to be the main reactive species for the present reaction.

3.2 Reactive species of Ru(III) chloride

Electronic spectral studies by Cady and Connick (1958) and by Connick and Fine (1960) reveal that species such as [RuCl₅(H₂O)]²⁻, [RuCl₄(H₂O)₂]⁻, [RuCl₃(H₂O)₃], [RuCl₂(H₂O)₄]⁺ and [RuCl(H₂O)₅]²⁺ do not exist in an aqueous solution of RuCl₃. A study on the oxidation states of ruthenium has shown that Ru(III) exists (Backhours et al., 1950; Davfokratova, 1963; Griffith, 1967) in the acid medium as:

Several researchers (Singh et al., 1984, 1980) have employed the above equilibrium in Ru(III)-catalyzed oxidation of various substrates in acid medium. However, in the present study, addition of Cl⁻ ion in the form of KCl at fixed [H⁺] had negative effect on the rate, indicating that equilibrium (I) play an important role in the reaction. Therefore, the [RuCl₅(H₂O)]²⁻ may most likely the reactive species in the reaction of d-Ara with NBP.

Considering the reactive species of Ru(III) chloride and NBP and with the help of above experimental findings, the probable reaction mechanism is proposed and considering the fact that 1 mol of d-Ara is oxidized by 2 mol of NBP.

On the basis of the reaction Scheme 1, the rate in term of decrease in concentration of NBP can be expressed as:

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Scheme 1

Reaction path f or the oxidation of d-Ara by NBP in the presence of Ru(III) chloride.

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Putting the value of [C₂] in above equation, we have

Putting the value of [C₃] and [C₄] in above equation, we have

Substitution of Eq. (16) into Eq. (10) gives

At any time of the reaction, the total concentration of NBP, i.e. [NBP]_T can be shown as

On substituting the value of [C₃] & [C₅] in Eq. (18) we get Eq. (19)

Putting the value of [C₁], [C₂], [C₄] and [C₅] in above equation, we have

Eq. (25), suggests first order dependence of rate with respect to [NBP] as well as [Ru(III)], and positive fractional order with respect to [d-Ara] and [H⁺],whereas it shows negative effect with respect to [Cl⁻], which are consistent with the experimentally observed results. Overall, the derived rate law is in agreement with the kinetics obtained in the Ru(III) chlorocomplex catalyzed oxidation of d-Ara by NBP in the acidic medium.

The rate expression obtained in Eq. (25) can be rewritten as

According to Eq. (26), graph is plotted between 1/rate versus [Cl⁻], 1/[Ru(III)], 1/[H⁺] and 1/[d-Ara] should be straight line having an intercept on the y-axis (Figs. 9–12). The value of kK₁K₂K₃K₄, kK₂K₃K₄, kK₂K₄ and kK₁K₃K₄, K₁, K₂, K₃, kK₄ calculated from the slope and intercept of the plot were 1.08 × 10² mol⁻¹ dm³, 3.57 × 10⁻¹ mol⁻¹ dm³, 7.71 × 10³ mol⁻¹ dm³, 6.25 × 10⁶ mol⁻¹ dm³ 0.30 × 10³ mol⁻¹ dm³, 1.70 × 10⁻⁵ mol⁻¹ dm³, 4.60 × 10⁻⁵ mol⁻¹ dm³ and 4.50 × 10⁸ mol⁻¹ dm³, respectively.

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Figure 9

Verification of rate law for 1/[Cl⁻] oxidation of d-Ara by NBP in acidic medium at 308 K. [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [NBP] = 2.00 × 10⁻⁴ mol dm⁻³,[Ru(III)] = 1.15 × 10⁻⁵ mol dm⁻³, [H⁺] = 4.00 × 10⁻³ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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Figure 10

Verification of rate law for 1/[Ru(III)] oxidation of d-Ara by NBP in acidic medium at 308 K. [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [NBP] = 2.00 × 10⁻⁴ mol dm⁻³, [H⁺] = 4.00 × 10⁻³ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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Figure 11

Verification of rate law for 1/[H⁺] oxidation of d-Ara by NBP in acidic medium at 308 K. [d-Ara] = 2.00 × 10⁻³ mol dm⁻³, [NBP] = 2.00 × 10⁻⁴ mol dm⁻³, [Ru(III)] = 1.15 × 10⁻⁵ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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Figure 12

Verification of rate law for 1/[d-Ara] oxidation of d-Ara by NBP in acidic medium at 308 K. [NBP] = 2.00 × 10⁻⁴ mol dm⁻³, [Ru(III)] = 1.15 × 10⁻⁵ mol dm⁻³, [H⁺] = 4.00 × 10⁻³ mol dm⁻³, [KCl] = 1.00 × 10⁻⁵ mol dm⁻³, [Hg(OAc)₂] = 3.00 × 10⁻⁴ mol dm⁻³ and CH₃COOH = 20%.

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3.3 Activation parameters

The reaction was studied at five different temperatures (303–323 K), for the Ru(III) catalyzed reaction. From the plot of log k₁versus 1/T (Arrhenius plot), the activation Energy (Ea) was evaluated. The proposed mechanism is also supported by the moderate values energy of activation and other thermodynamic parameters. The positive values of free energy of activation and of the enthalpy of activation suggest that the transition state is highly solvated, while the negative entropy of activation indicates that transition state is highly associated. The order of frequency factor (A) and values of free energy of activation (ΔG^∗) for the redox system also supports a reaction scheme proposed for the oxidation of d-Ara by NBP in the presence of Ru(III) under acidic conditions.

3.4 Multiple regression analysis

With the help of multivariate regression analysis, a relationship between observed pseudo first-order rate constant (k₁) and concentrations of all the reactants of the reaction of the reaction was found to be:

With the help of Eq. (18), the reaction rate predicted for the hydrogen, chloride ions, Ru(III) chloride and d-Ara concentrations in the oxidation of d-Ara were found to be very close to the calculated and observed rates (Table 6). The close similarity among the two rates, i.e., the calculated and predicted rates, clearly proves the validity of the rate law (Eq. (17)), and hence the proposed mechanism.

3.5 Comparative study

When an effort was made to compare the findings of Ru(III) catalyzed oxidation of d-Ara with the results reported for thenovel and facile oxidation of d-glucose by NBP in the presence of chloro-complexes of Ru(III) (Singh et al., 2010) and Ir(III) (Singh et al., 2012), it was found that NBP itself is the reactive species in each case. Zero order kinetics was observed with respect to [d-Glu] in presence of Ru(III) (Singh et al., 2010) as well as Ir(III) (Singh et al., 2012), but in the present investigation with respect to [d-Ara], the reaction exhibited positive fractional order. With respect to increasing [Cl⁻], Ru(III) catalyzed oxidation reactions of [d-Glu] as well as [d-Ara] showed negative influences whereas [Ir(III)] catalyzed oxidation of d-Glu showed positive influence. Similarly, Ru(III) catalyzed oxidation of [d-Glu] and [d-Ara] showed no effect with respect to ionic strength but Ir(III) catalyzed oxidation of [d-Glu] showed positive effect. Negative effect with respect to dielectric constant was observed in both the cases of Ir(III) catalyzed oxidation of d-Glu and Ru(III) catalyzed oxidation of d-Ara, but no solvent effect was observed in case of Ru(III) catalyzed oxidation of d-Glu. The present study entirely differs from other two studies as for as effect of [H⁺] on the rate of reaction is concerned. Negative effect of [H⁺] was observed in Ru(III) (Singh et al., 2010) and Ir(III) (Singh et al., 2012) catalyzed oxidation of reducing sugars, whereas positive effect of [H⁺] on the rate of reaction was observed in the present study. No effect of [NHP] was observed in all the three catalyzed reactions. On the basis of observed kinetic data and observed negative entropy of activation, the species,[RuCl₅(H₂O)]²⁻ is proposed as the most reactive species in the present study whereas the species,[RuCl₂(H₂O)₃OH and the species, [IrCl₃(H₂O)₂(OH)₂]₂ have been reported as the most reactive species for Ru(III) (Singh et al., 2010) and Ir(III) (Singh et al., 2012) catalyzed oxidation of reducing sugars, respectively.