Kinetics and mechanism of oxidation of chondroitin-4

Journal of Molecular Liquids 244 (2017) 353–359

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Kinetics and mechanism of oxidation of chondroitin-4-sulfate polysaccharide as a sulfated polysaccharide by hexacyanoferrate(III) in alkaline solutions with synthesis of novel coordination biopolymer chelating agent Samia M. Ibrahim a, Ismail Althagafi b, Hideo D. Takagi c, Refat M. Hassan d,⁎ a

Chemistry Department, Faculty of Science, Assiut University, New Valley Branch, El-Kharja 72511, New Valley, Egypt Chemistry Department, Faculty of Applied Sciences, Umm Al-Qura University, Makkah Al-Mukarramah 13401, Saudi Arabia c Chemistry Department, School of Science, Research Center of Materials Science, Nagoya 464-01, Japan d Chemistry Department, Faculty of Science, Assiut University, Assiut 71516, Egypt b

a r t i c l e

i n f o

Article history: Received 13 January 2017 Received in revised form 8 February 2017 Accepted 3 August 2017 Available online 10 August 2017 Keywords: Chondroitin-4-sulfate Hexacyanoferrate(III) Reduction Oxidation Kinetics Mechanisms

a b s t r a c t The kinetics and mechanism of reduction of hexacyanoferrate (III) by biodegradable chondroitin-4-sulfate (CS) in alkaline solutions at a constant ionic strength of 1.0 mol dm−3 have been investigated spectrophotometrically. The kinetic results indicated that the reaction was first-order dependence in the oxidant and fractional-order kinetics with respect to the [CS]. The influence of the base on the oxidation rates indicates that the reaction was base-catalyzed. A kinetic evidence for formation of 1:1 intermediate complexes was revealed. The kinetic parameters have been evaluated and a tentative reaction mechanism consistent with the kinetic results obtained is suggested and discussed. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Although, much work has been reported by numerous of investigators on the oxidation kinetics of sustainable natural polymers [1] such as polysaccharides by multi-equivalent oxidants such as chromic acid [2–4] and permanganate in either alkaline [5–11] or acidic solutions [12–15]. A little attention has been focused on the oxidation of such sustainable macromolecules by hexacyanoferrate(III) ion as oneequivalent oxidant. Indeed, Hassan and Coworkers investigated the reduction of this oxidant by methyl-cellulose [16] and alginate [17] as polysaccharides in alkaline media, a lack of information on the nature of electron-transfer and transition states in the rate-determining step still remains not completed and poor understand. Although, hexacyanoferrate (III) is a weak oxidant with redox potentials of 0.40 V and 0.36 V for [Fe (CN)6]3−/[Fe(CN)6]4− couple in alkaline and acidic media [18,19], respectively, it has been attracted many chemists owing to the possibility of elucidation of a variety of reaction mechanisms. Therefore, the reaction of various substrate reductants

⁎ Corresponding author. E-mail address: [email protected] (R.M. Hassan).

http://dx.doi.org/10.1016/j.molliq.2017.08.012 0167-7322/© 2017 Elsevier B.V. All rights reserved.

by hexacyanoferrate (III) was considered as a tool for inception of chemical kinetics. In view of the above aspects and our interest in the kinetics of redox reactions involving sustainable polysaccharides with various oxidants, the present work have been undertaken with the aims of shedding more light on the kinetics and mechanism of reduction of oneequivalent oxidant by sulfated polysaccharides as well as to compensate the lack of information on the nature of electron-transfer and transition states in the rate-determining steps.

2. Experimental section 2.1. Materials Potassium hexacyanoferrate (III) (Mallinckrodt chemical works) was used without further purification. The solution of K3[Fe(CN)6] was prepared by weighting the desired amount of the reagent, which was placed in ambercolored-bottle and kept in dark to prevent photodecomposition. The appropriate dilution was made before each run. Stock solutions of chondroitin-4-sulfate (CS) (ICN Biomedicals, Inc.) were prepared by stepwise addition of the reagent powder to doubly

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S.M. Ibrahim et al. / Journal of Molecular Liquids 244 (2017) 353–359

distilled water whilst rapidly stirring the solution to avoid the formation of aggregates, which swell with difficulty. All other reagents were of analytical grade and their solutions were prepared by dissolving the requisite amount of the reagent in doubly distilled water. The ionic strength of the reaction mixtures was maintained constant at 1.0 mol dm−3 using NaClO4 as an inert electrolyte. Doubly distilled conductivity water was used in all preparations. The temperature was controlled within ±0.05 °C. 2.2. Kinetic measurements Preliminary experiments indicated that the present redox reaction is of such a rate to be measured by a conventional spectrophotometer. To maintain pseudo first-order conditions, a large excess of [CS] was present over that of hexacyanoferrate (III) concentration in all kinetic measurements. The reaction was initiated by mixing the thermostated solutions of reactants which containing the required concentrations of NaOH and NaClO4 into the reaction cell. The zero time was taken when half of [Fe(CN)6]3− solution had been added to CS solution into the reaction cell. The progress of the redox reaction was followed by at the wavelength monitoring the decrease in absorbance of Fe(CN)3− 6 of 420 nm, its absorption maximum, where other reagents of the reaction mixture do not absorb significantly at this wavelength, as a function of time. The applicability of Beer's law for [Fe(CN)6]3− at 420 nm has been verified giving ∈ = (1040 ± 20) dm3 mol−1 cm−1 in good agreement with the values reported elsewhere [20–22]. The absorbance measurements were made in a thermostated cell compartment at the desired temperature within ±0.05 °C on a Shimadzu UV – 2101/3101 PC automatic scanning double beam spectrophotometer fitted with a wavelength program controller using cells of a pathlength 1 cm. The spectral change during the progress of the oxidation reaction is shown in Fig. 1. 3. Results 3.1. Stoichiometry and product analysis The stoichiometry of the overall reaction was determined by mixing different initial concentrations of the reactants in 0.2 mol dm−3 NaOH and constant ionic strength of 1.0 mol dm−3 at room temperature, spectrophotometrically. The unreacted [Fe(CN)6]3− was estimated periodically until it reached a constant value. The results for various ratios of the equilibrated reactants indicated that 4.0 mol of [Fe(CN)6]3 −

consumed 1.0 mol of CS (±0.1). This result indicates that the stoichiometry of the oxidation reaction conforms to the following equation, 3‐ ðC14 H21 NO14 S‐ Þn þ 6 FeðCNÞ6 þ 6OH‐ 4‐ ¼ ðC14 H17 NO15 S‐ Þn þ 6 FeðCNÞ6 þ 5H2 O

ð1Þ

where C14H21NO14S− and C14H17NO15S− represent the chondroitin-4sulfate and its corresponding keto acid derivative, respectively. The reaction product was identified by the IR spectra and elemental analysis as described elsewhere [22–24].The recorded IR spectra of CS and its oxidation product showed a decay of the band at 3450 cm−1as well as an enhancement of the band at 1700 cm−1 of CS indicating the interconversion of secondary hydroxyl groups (OH) to the keto (C_O) groups. 3.2. Reaction time-curves Reaction time-curves were found to be of complexity where plots of ln (absorbance) against time gave straight lines up to 70%, and then it deviates from linearity. This deviation may be attributed to the interference one of the products, [Fe(CN)6]4−, species. The pseudo first – order rate constants, kobs, were evaluated from the slopes of the linear portions of such plots. Again, some results were calculated from the tangents of the initial rates of reaction to avoid a such complexity (from the plots of remaining [Fe (CN)6]3− - time curves at initial stages). The results were found to be reproducible to each other within experimental errors of about ±4%. Therefore, kobs values were used to interpret the results throughout the present study. These values were calculated by the method of least-squares. 3.3. Dependence of reaction rate on [Fe(CN)6]3− and [CS] The effect of hexacyanoferrate (III) on the reaction rates was examined by keeping all other concentrations fixed. It was found that the change in [Fe(CN)6]3 − in the range of (4–10) × 10− 4 mol dm−3, at [CS] = 1.5 × 10− 2, [OH−] = 0.2, I = 1.0 mol dm−3 at 40 °C did not alter the values of the oxidation rates. The independency of the rate constants on the oxidant concentration confirms that the reaction is firstorder in hexacyanoferrate (III) concentration. Again, the linearity obtained from plots of the initial rates against [Fe(CN)6]3− can be also indicate that the reaction is first-order in [Fe(CN)6]3−. The dependence of kobs on [CS] was deduced from the measurement of the observed first – order rate constants at various [CS]0 and fixed of all other reagents concentration. A fractional first-order dependence in [CS]0 was observed (log kobs – log [CS]0 plots). Again the double reciprocal plots of kobs - [CS] were found to be linear with positive intercepts on 1/kobs axis. This behavior seems to exhibit the Michaelis-Menten kinetics for formation of 1:1 intermediate complex. A typical plot is shown in Fig. 2. 3.4. Dependence of reaction rate on [OH−]

Fig. 1. Spectral changes (200–550 nm) during the formation of intermediate complexes in the oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate (III) [Fe(CN)6]3− = 7 × 10−4, [CS] = 1.5 × 10−2, [OH−] = 0.2 and I = 1.0 mol dm−3 at 40 °C (scanning time intervals = 1 min).

In order to clarify the influence of [OH−] on the rate of reaction and to elucidate a reaction mechanism, kinetic measurements were conducted at various [OH−] keeping all other reagents concentration fixed. The values of kobs were found to increase with increasing [OH−] up to 0.5 mol dm−3 after that turbidity in the tested solution has been appeared which made any spectrophotometric measurements very difficult. The results are summarized in Table 1. Therefore, all the experimental measurements were performed at [OH−] b 0.5 mol dm−3 to avoid such difficulty. This behavior is similar to that observed in the oxidation of methyl cellulose by this oxidant [16]. Fractional first-order dependence with respect to [OH−] (log kobs-log [OH−] plots) was observed.


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Table 2 Influence of [Fe(CN)6]4− on the observed first-order rate constants in the oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate (III). [Fe(CN)6]3− = 7 × 10−4, [CS] = 1.5 × 10−2, [OH−] = 0.2 and I = 1.0 mol dm−3 at 40 °C. −3 103[Fe(CN)4− 6 ], mol dm 4

−1

10 ks, s

0.0

0.7

1.4

2.8

10.3

8.34

7.66

6.56

evaluated from the slopes and intercepts of such plots using the leastsquares method. 3.8. Polymerization test

Fig. 2. A reciprocal plot of Michaelis-Menten kinetics in the oxidation of chondroitin-4sulfate by alkaline hexacyanoferrate (III) [Fe(CN)6]3− = 7 × 10−4, [OH−] = 0.2 and I = 1.0 mol dm−3 at 40 °C at various [CS].

3.5. Dependence of reaction rate on ionic strength To shed some light on the reactive species in the rate determining step, kinetic runs were performed at constant [OH−] = 0.2 mol dm−3 as NaClO4 concentration was increased to 1.0 mol dm−3. The values of kobs were found to increase with increasing the ionic strength. A plot of ln kobs against I0.5 according to Debye – Hückel equation was found to be linear with positive slope as shown in Fig. 3. However the present measurements lie far outside the Debye – Hückel region covering a range over which the activity coefficients of many electrolytes are known to be fairly dependent on ionic strength. The ionic strength dependence of the rate constants is qualitatively as expected when considering the charges involved [25].

3.6. Dependence of Reaction Rate on [Fe(CN)6]4− The effect of the initially added [Fe(CN)6]4− product on the rate of reaction was also examined in the range (0.7–2.8) × 10−3 mol dm−3 at constant of all other reagents concentration and 40 °C. The added [Fe(CN)6]4 − was found to decrease the reaction rates, i.e. it retarded the oxidation process (Table 2). A similar behavior has been observed in some redox reactions involving the formation of intermediates [1, 23,26,27].

Since the present reaction is of non-complementary type, the intervention of free-radicals during the entire course of the reaction progression is highly expected. Therefore, the possibility of formation of freeradical has been tested by adding 10% (v/v) acrylonitrile monomer to the partially oxidized reaction mixtures. No precipitate was observed during the time of reaction progression. This result was found to be on contrary to the general expectation [21,28] and this result was discussed later. 4. Discussion The experimental observations indicated that the reaction kinetics of oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate was of complexity nature. The deviation of log (absorbance)-t plots from linearity after 70% linearity may attributed to the intervention of one of the oxidation products, [Fe(CN)6]4−, [29], the association of the some ions with [Fe(CN)6]4− or to a secondary salt effect [30,31]. The retardation effect observed in the rate constants on adding [Fe(CN)6]4− to the reaction mixture (Table 2) may support this suggestion [5,12,16,32]. Again obedience of the [CS]-dependence of the rate constants to the Michaelis-Menten kinetics suggests the formation of 1:1 intermediate complex between the reactants prior to the ratedetermining step. The formation of such intermediate is not only confirmed by the reciprocal plots of Michaelis-Menten kinetics but also by the increase in the initial absorbance of the reaction mixture observed on mixing in particularly at lower of temperature and reactant concentration. Again, the appearance of an isobestic point in the UV range at 285 nm may suggest the existence of equilibrium state between the oxidant and the intermediates which in term may confirm our suggested mechanism. When the progress of oxidation was investigated using reference cell containing the same concentrations of the oxidant and the

3.7. Dependence of reaction rate on temperature In order to evaluate the kinetic parameters, kinetics runs were performed at various temperatures. The experimental results were found to fit the Arrhenius and Eyring equations. The kinetic parameters were Table 1 The observed pseudo-first-order rate constants (kobs) in the oxidation of chondroitin-4sulfate by alkaline hexacyanoferrate (III). [Fe(CN)6]3− = 7 × 10−4 and I = 1.0 mol dm−3 at 40 °C.

a

[OH−]a

103 kobs, s−1

[S]b, mol dm−3

103 kobs, s−1

0.2 0.4

1.02 1.71

0.015 0.03

1.02 1.75

[S] = 1.5 × 10−2 mol dm−3; b- [OH−] = 0.2 mol dm−3.

Fig. 3. A typical plot of Debye-Huckel equation in the oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate (III) in the slow stage [Fe(CN)6]3− = 7 × 10−4, [CS] = 1.5 × 10−2, [OH−] = 0.2 at 40 °C.

356


alkali of the tested solution, a new peak was appeared at 350 nm as shown in Fig. 3. The lower absorptivity of such peak may prevent its detection during the oxidation process and, hence, its absence in the spectra changes in Fig.1. Furthermore, the positive slope obtained from ln kobs - I0.5 plot refers to the fact that the oxidation process occurs between two similar charges of reactant species. In view of these interpretations and the experimental observations, the most likely reaction mechanism which may be suggested involves a fast deprotonation of the substrate by alkali to form the more reactive alkoxide, as shown in Scheme I K1

S þ OH‐ ⇌ S‐ þ H2 O alkoxide

ð2Þ

followed by formation of an intermediate complex (C1) between the oxidant (Ox) and the formed alkoxide (S−): K2

S‐ þ Ox ⇌ C1

ð3Þ

Then, the formed complex (C1) is slowly decomposed in the ratedetermining step to give rise to the substrate radical (S•) and [Fe(CN)6]4− (Red) as initial oxidation products k

C1 → S þ Red þ H3 Oþ þ Initial oxidation products H2 O

Fast

S þ Ox → Products þ Red þ H3 Oþ H2 O

ð4Þ

Fig. 5. Plots of [CS]/kobs against [OH−]−1 in the oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate (III) [Fe(CN)6]3− = 7 × 10−4, [CS] = 1.5 × 10−2 and I = 1.0 mol dm−3 at various of both temperatures and [OH−].

The change of the rate constant with the change in the hydroxide ion and substrate concentrations can be expressed by the following ratelaw equation:

ð5Þ Rate ¼ −

Since the oxidant is of one-equivalent nature, it needs to accept only one -electron to give its corresponding reduced form. This means that the rate-determining step should involve the formation of a radical substrate. Hence, the delay of formation of such free-radical during the progress of this oxidation reaction may be attributed to the rapid oxidation of the formed free-radical that could mask polymerization of acrylonitrile. Sometimes, the vinyl compounds themselves are oxidized under the experimental conditions used and, hence, the free-radical test fails to be observed. It found that the plots of ln (absorbance) against time in the oxidation of methyl cellulose by hexacyanoferrtae(III) in alkaline solution [16] was straight lines up to 70%, then deviated from linearity in a similar manner to that observed here. Hence, this deviation can be attributed to the interference one of the products, [Fe(CN)6]4−, species as suggested elsewhere [16].

3− d FeðCNÞ6 kK1 K2 ½OH− ½ST ½Ox ¼ dt 1 þ K1 ½OH− þ K1 K2 ½OH− ½Ox

ð6Þ

where [S]T denotes the analytical concentration of the substrate. In the presence of a large excess of the substrate over that of the [Ox], the general rate law- expression is 3− Rate ¼ kobs Fe ðCNÞ6

ð7Þ

Table 3 The apparent rate constants (k′ and k″) and the deprotonation constants K1 in the oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate (III) at various temperatures. [Fe(CN)6]3− = 7 × 10−4, [CS] = 1.5 × 10−2 and I = 1.0 mol dm−3. Constants

Temp, °C

k′, dm9 mol−3 s−1 102 k˝, dm6 mol−2 s−1 K1, dm3 mol−1

30 °C

40 °C

0.14 9.89 1.41

0.41 40.7 1.01

Experimental errors ± 4%.

Table 4 The kinetic parameters of the apparent rate-constants (k΄ and k˝), second-order rate constant (kn) and the formation constant (K) in the oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate (III) at various temperatures. Constants

Parameters ΔH≠ kJ mol

k΄ k˝ kn K Fig. 4. Spectral changes (280–380 nm) during the formation of intermediate complexes in the oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate (III). Reference cell (hexacyanoferrate (III) and OH−) [Fe(CN)6] 3− = 7 × 10−4, [CS] = 2 × 10−2, [OH−] = 0.4 and I = 1.0 mol dm−3 at 25 °C (scanning time intervals = 2 min).

ΔS≠ −1

74.71 85.87 79.31 ΔH° kJ mol−1 −24.38

J mol

ΔG≠ −1

K

−1

−14.23 +21.39 −14.47 ΔS° J mol−1 K−1 −77.74

kJ mol

−1

78.95 79.495 83.62 ΔG° kJ mol−1 −1.214

Experimental errors ± 4%. kn = Second-order rate constant at 0.2 M NaOH.

E≠a

A

kJ mol−1

mol−1 s−1

77.175 88.14 82.31

3.03 × 1012 2.04 × 1014 3.63 × 1012


Comparing Eqs. (6) and (7) and rearrangement, the following relationship is obtained 1 ¼ kobs

1 1 1 ∗ 0 þ þ K k K1 K2 ½OH− k K2 ½S

ð8Þ

0

K ¼ ½Ox=k ½S

According to Eq. (8), at constant [OH−] plots of 1/kobs against 1/[S] should be linear with positive intercept or 1/kobs axis as was

357

experimentally observed (Fig. 2). Again, plots of 1/kobs against 1/[OH−] at constant [S] should be linear with positive intercepts on 1/kobs axis. The experimental results were satisfied this requirement. The small intercept observed in (Fig. 3) may lead us to simplify Eq. (8) to Eq. (9), ½OH− 1 ½ST 1 ¼ ¼ þ ″ 0 kobs kn k k

ð9Þ

According to Eq. (9), plots of [S]/kobs against 1/[OH−] (shown in Fig. 5) were found to be linear with positive intercepts on [S]/kobs

Scheme 1. Mechanism of oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate (III)

358


from whose slopes and intercepts, the apparent rate constants (k′ = kK1K2 and k″ = kK2) and the deprotonation constant K1 can be evaluated. These values were calculated by the method of least-squares and are summarized in Table 3. Unfortunately, the values of the rate constant of the elementary reaction (k) could not be calculated because of the non-availability of the formation constants (K2). Therefore, the apparent rate constants are considered to be composite quantities of the rate constants, deprotonation constants and the formation constants, respectively. The activation parameters of the apparent rate constants (k′ and k) were calculated from the dependence of those constants on temperature using the Arrhenius and Eyring equations by the least-squares method. The values of the calculated parameters are summarized in Table 4. Again, the thermodynamic parameters of the deprotonation constants (K1) were evaluated from the known thermodynamically equations and listed in Table 4. It is well known that redox reactions which involving [Fe(CN)6]3− as an inert oxidant are proceeding by a variety of reaction mechanisms. Some of these reactions proceed by either inner- or outer-sphere mechanisms for the electron transfer processes. Other reactions occurred by both outer- and inner-sphere mechanisms via intervention of freeradicals. However, the present rate-laws provide no information on whether electron transfer of inner- or outer sphere nature, some information may be expected by examining the magnitude of the rate constants of the elementary reactions or the activation parameters. It reported elsewhere [33–34] that the entropy of activation tends to be more negative for reactions of inner-sphere nature, whereas the reactions of positive ΔS≠ values tend to proceed via outer-sphere mechanism. In view of the negative entropy of activation obtained (Table 4), the electron transfer process may occur by either inner- or outer- sphere mechanism. However the values of the entropies of activation pointed to the outer-sphere mechanism, the intervention of free-radical may suggest the existing of inner-sphere type for electron- transfer process. Moreover, the formation of intermediate complex in which one cyanide ion from the inert [Fe(CN)6]3 − may act as a bridging ligand between the two reactants in the complex formed suggests that one-electron transfer of inner-sphere nature is the more plausible mechanism for oxidation of chondroitin-4-sulfate by alkaline hexacyanoferrate (III). But this suggestion is not exclusive. A suggested mechanism is illustrated in Scheme 1. The decrease of the magnitude of deprotonation constants (K1) with increasing the temperature indicates that the deprotonation process of the substrate is of exothermic nature. The large positive value of ΔG≠ is indicative to the enhanced formation of the intermediate with increasing temperature as well as to the non-spontaneity of the complex formation in the rate determining step as represented by the proposed mechanism. Although, the behavior of the ionic strength dependence of the rate constant (Fig. 4) is qualitatively as expected for the charges involved, an alternative mechanism may be suggested. It involves an ionic pairing between K+ and [Fe(CN)6]3− [5,35] which may affect the magnitude of the slope obtained from the plot of the ionic strength dependence of the rate constants. Since the addition of [K+] ions was found to have no influence on the rate constants, the possibility of existence an ion-pairing mechanism was excluded.

5. Conclusion The kinetics and mechanism of reduction of hexacyanoferrate (III) by biodegradable chondroitin-4-sulfate (CS) in alkaline solutions have been investigated spectrophotometrically. Evidence for formation of 1:1 intermediate complexes prior to the rate-determining step was

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