A kinetic study of oxidation of β-cyclodextrin by permanganate in aqueous media

Colloids and Surfaces A: Physicochem. Eng. Aspects 295 (2007) 165–171

A kinetic study of oxidation of ␤-cyclodextrin by permanganate in aqueous media Manmeet Singh Manhas, Faqeer Mohammed, Zaheer khan ∗ Department of Chemistry, Jamia Millia Islamia (Central University), Jamia Nagar, New Delhi 110025, India Received 23 March 2006; received in revised form 18 August 2006; accepted 28 August 2006 Available online 1 September 2006

Abstract Kinetic data for the permanganate—␤-cyclodextrin redox system are reported for the first time. Conventional spectroscopic method was used to monitor the progress of the reaction. The reaction is first order in both [MnO4 − ] and [␤-cyclodextrin]. The [H+ ] markedly increase the oxidation rate. Formation of an intermediate (Mn(IV)) observed during the course of reaction at 420 nm. Mn(III) is also formed in a rapid step by reaction of ␤-cyclodextrin and Mn(IV). Increase in [F− ] causes the rate to decrease. One of the primary OH group of ␣-d-glucopyranose unit of ␤-cyclodextrin is responsible for the oxidation. An autocatalytic effect has been observed due to Mn(II) ions formed as a product of the reaction. The rate of oxidation of d-glucose is higher in comparison to ␤-cyclodextrin for the same concentration. Reduction of mercuric chloride indicates free radical mechanism operates during the course of reaction. The energy, enthalpy and entropy of activation have been calculated and mechanism agreement with the observation is suggested. © 2007 Elsevier B.V. All rights reserved. Keywords: Oxidation; ␤-Cyclodextrin; Permanganate; HClO4 ; d-Glucose

1. Introduction The chemistry of cyclodextrins in terms of their applications has come a long way, and its industrial uses to encapsulate foods, drugs and dyes are unlimited [1–10]. Cyclodextrins are cyclic oligoglycosides of six, seven, or eight ␣-d-glucopyranose units linked by ␣ (1 → 4) bonds [11,12]. Cyclodextrin is a well constructed miniature of an enzyme because it has a hydrophobic (interior) cavity of appropriate size [13]. It has been established that cyclodextrins are in the shape of hollow truncated cones with primary OH groups projecting from the narrow side of the cones and secondary OH group from the wide side. Cyclodextrins are capable of forming 1:1 cage complexes with many guests (surfactant tails [13–17], dyes and drugs) molecules of the appropriate size, shape and polarity [17]. Cavity of ␤-cyclodextrin normally filled with 12 water molecules held in place by hydrogen bonds.

∗

Corresponding author. E-mail address: [email protected] (Z. khan).

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.08.048

Carbohydrates are biologically important substances whose activities depend largely in their redox behaviour. Oxidation of monosaccharides by different oxidant in acid (Cr(VI), V(V), Ce(IV), Mn(III), Tl(III), Mn(IV)) as well as in alkaline (Cu(II), Fe(III), Ag(I)) medium are of special importance due to their biological relevance [18–30]. The kinetic and mechanistic features of a particular oxidation reaction are likely to be affected by the nature of the reducing sugar in solution and the active species of the oxidant. ␤-aldopyranose form has been considered in preference to ␣-aldopyranose form in the oxidation of d-glucose [19,21–23] (the most reactive centre is C-1). Although information is available on the kinetic and mechanism of oxidation of various inorganic and inorganic compounds by versatile oxidant like potassium permanganate, kinetic studies of the oxidation of aldohexoses by this powerful oxidant, until recently, been lacking. Cyclodextrin is one of the most favoured oligosaccharide which provide a unique space for carrying out synthesis and investigating reaction kinetics. The only interest, therefore, in the work was to investigate the oxidation of ␤-cyclodextrin by permanganate which was not studied by previous workers. The main objective of the present investigation is to elucidate the suitable mechanism. For the

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purpose of comparison, oxidation of d-glucose has also been studied. 2. Experimental 2.1. Materials ␤-Cyclodextrin (Across, 99%), potassium permanganate (Merck India, 99%), d-glucose (Merck India, 99%), perchloric acid (Thomas Baker, 70% Reagent), sodium fluoride, manganese(II) chloride, and mercuric(II) chloride were used as supplied by Merck India. Doubly distilled, deionised and CO2 free water from an all glass apparatus was used to prepare standard solutions. Permanganate solution was prepared by the method of vogel and stored in amber coloured bottle in a refrigerator. KMnO4 was standardized by titration against oxalate. Stock solution of perchloric acid was standardized against previously standardized sodium hydroxide solution. 2.2. Kinetic measurements Reactions were carried out in two-necked glass-stopper flasks fitted with a double walled condenser (to arrest evaporation). Requisite amounts of the each reagent were mixed and the reaction flask brought to constant temperature. The ␤-cyclodextrin solution, previously thermostated, was then mixed rapidly. Zero- time was taken when half the cyclodextrin solution had been added. The rate of disappearance of MnO4 − was followed spectrophotometrically by monitoring the absorbance due to unchanged MnO4 − at known time intervals. Neither cyclodextrin nor products showed any absorbance at 525 nm. All the kinetic run carried out for a period of more than 80% completion with cyclodextrin in excess in order work under pseudo-first order conditions. Observed pseudo-first order rare constants (kobs , s−1 ) were calculated from the slopes of log (A)525 versus time plots (average correlation coefficient = 0.997). Other experimental procedure details are described elsewhere [31,32]. 2.3. Free radical detection The addition of a saturated solution of mercuric(II) chloride to the reaction mixture containing permanganate, ␤-cyclodextrin and perchloric acid indicated that the formation of precipitate of mercurous chloride suggesting the intervention of free radical as an intermediate during the course of oxidation. 3. Results and discussion 3.1. Stoichiometry and product identification A warm solution of ␤-cyclodextrin (1.0 × 10−3 mol dm−3 ) was added dropwise to a warm solution (40–45 ◦ C) of MnO4 − (10 cm3 , 1.0 × 10−3 mol dm−3 ) containing HClO4 (0.93 mol dm−3 ) until permanganate colour disappear completely. Different titrimetric experiments verified that one mole of permanganate reacts with 2.5 mol. of ␤-cyclodextrin. The oxi-

dation product of ␤-cyclodextrin was characterized as follows. In a typical experiment, ␤-cyclodextrin  MnO4 − (␤-cyclodextrin (2.0 × 10−3 mol dm−3 ), MnO4 − (2.0 × 10−4 mol dm−3 ) and HClO4 (0.93 mol dm−3 )) were allowed to react at 40 ◦ C. After completion of the reaction (complete disappearance of purple colour), a saturated solution of 2,4-dinitrophenylhydrazine in 2 M HCl was added to the reaction mixtures. The dinitrohydrazone formed was determined spectrophotometrically in a methanol–KOH solution at 420 nm [33]. On the other hand, 50 cm3 of the reaction mixture was treated with 100 cm3 of a saturated solution of 2,4-dinitrophenyl hydrazine in 2 N HCl and was left overnight in a refrigerator. The precipitated 2,4-dinitrophenyl hydrazone was filtered, washed and dried. The nature of the aldehyde was confirmed by 2,4-dinitrophenyl hydrazone IR spectrum, carbonyl stretching at 1729 cm−1 and a band at 2928 cm−1 due to the aldehyde stretching. Obviously, aldehyde was formed by the oxidation of ␤-cyclodextrin. It was also observed that the aldehyde does not undergo further oxidation under the present kinetic conditions. A spot test for acid was negative. 3.2. Hydrogen ion dependence The kinetics of this reaction remained unaffected by increasing the ionic strength of the medium by adding neutral salt, KClO4 . Therefore, ionic strength of the medium was not maintained. Variation of the [H+ ] from 0.69 to 2.15 mol dm−3 , employing HClO4 at [MnO4 − ] (2.0 × 10−4 mol dm−3 ), [␤cyclodextrin] (2.0 × 10−3 mol dm−3 ) and temperature (40 ◦ C) showed an interesting effect on the reaction rate. The observed results are shown in Fig. 1 as log (absorbance)–time profile which shows that the oxidation of ␤-cyclodextrin proceeds in two stages and that the first stage (noncatalytic) is relatively slower than the second (autoacceleration) [18,19,31,34]. The kinetic curves show inflection and the kobs values were estimated from such curves (Fig. 1A). As the reaction proceeds, a gradual curvature was observed in the plot of log (absorbance) versus time (see Fig. 1B for [H+ ] = 1.39 mol dm−3 ). At lower [H+ ] (0.69 to 0.93 mol dm−3 ), the plots of log(absorbance) versus time are linear whereas at higher [H+ ] (≥1.16 mol dm−3 ), the plots deviate from linearity. The time at which deviation commenced was found to decrease with an increase in [H+ ]. Therefore, choice of the best experimental conditions for the kinetic experiments is a crucial problem that we address first. In order to examine the effects of variables, experiments were tried at constant [H+ ] (0.93 mol dm−3 ). HClO4 was used as an acidifying agent due to the non-complexing nature of ClO4 − . Increase in acid concentration also increases the rate of oxidation of ␤-cyclodextrin (Table 1). This indicates that the reaction is of an ion-dipole type. The plot of log kobs versus log [HClO4 ] (Zucker-Hammett plot [35], Fig. 2) is linear with slope = 1.4 which cannot be considered as order with respect to [HClO4 ] (ref. [36]). A perfect fit would give a straight line with slope = 1.00 when log kobs is plotted versus log [HClO4 ]. In some cases, the slopes were less or greater than unity and the plots were not always linear. In view of his departure (slope = 1.4) from the ideal value of slope, the Bunnett’s hypothesis [37]

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Fig. 1. (A) Plots of log (absorbance) vs. time for the oxidation of ␤-cyclodextrin (2.0 × 10−3 mol dm−3 ) by MnO4 − (2.0 × 10−4 mol dm−3 ) at 40 ◦ C. (B) Plot of log (absorbance) vs. time for [H+ ] = 1.39 mol dm−3 .

was tested. According to Bunnett’s empirical observations the value of slope indicates that the water molecule should act as a proton abstracting agent in the rate-determining step. 3.3. MnO4 − dependence The order with respect to [MnO4 − ] was determined from the measurement of rate constant at several [MnO4 − ] with fixed [␤-cyclodextrin] (2.0 × 10−3 mol dm−3 ), [HClO4 ] (0.93 mol dm−3 ) and temperature 40 ◦ C. Under these conditions, the plot of log (absorbance) versus time was linear (Fig. 1) indicating the order of reaction with respect to [MnO4 − ] is one (Table 1). 3.4. β-Cyclodextrin dependence The plot of kobs versus [␤-cyclodextrin] gives a straight line passing through the origin (Fig. 3), thus showing that the order

of reaction with respect to ␤-cyclodextrin is one. There is no kinetic evidence for intermediate complex formation between ␤-cyclodextrin and MnO4 − ; if any complex is formed, its formation constant would be extremely small [38]. The values of observed pseudo-first order rate constants are summarized in Table 1. 3.5. Temperature dependence The reaction was studied at 40, 50 and 60 ◦ C at constant [MnO4 − ] (2.0 × 10−4 mol dm−3 ), [␤-cyclodextrin] (2.0 × 10−3 mol dm−3 ) and [H+ ] (0.93 mol dm−3 ). The Arrhenius plot between log kobs and 1/T was linear (Fig. 4). The value of activation energy was calculated from the slope of log kobs versus 1/T. The values of activation parameters were found to be Ea = 48 k J mol−1 , H# = 45 k J mol−1 and S# = −163 J K−1 mol−1 . The fairly high positive value of H# indicates that the transition state is highly solvated.

Table 1 Variation of rate constant with [MnO4 − ], [␤-cyclodextrin], and [HClO4 ] at 40 ◦ C 104 [MnO4 − ] (mol dm−3 )

103 [cyclodextrin] (mol dm−3 )

[HClO4 ] (mol dm−3 )

Temperature (◦ C)

104 kobs (s−1 )

0.3 0.5 1.0 1.5 2.0 2.0

2.0

0.93

40

0.93

40

2.0

1.0 2.0 3.0 4.0 5.0 2.0

40

2.0

2.0

0.69 0.93 1.16 1.39 1.62 1.82 0.93

2.2 2.3 2.0 2.3 2.2 1.2 2.3 3.6 5.3 6.5 1.5 2.3 3.0 3.4 5.3 6.1 2.3 3.8 6.8

40 50 60

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−.

Fig. 2. Zucker-Hammett plot for the oxidation of ␤-cyclodextrin by MnO4 Reaction conditions: [MnO4 − ] (2.0 × 10−4 mol dm−3 ), [␤-cyclodextrin] (2.0 × 10−3 mol dm−3 ), temperature (40 ◦ C).

3.6. Identification of Intermediate (Mn(IV)) It is well known that in the case of permanganate as oxidizing agent the possible oxidizing species are Mn(VII), Mn(VI), Mn(V), Mn(IV) and Mn(III). The presence of Mn(VI) and Mn(V) were ruled out, as they are highly unstable in acid medium [39]. As for as the formation of Mn(IV) is concerned, the oxidation of ␤-cyclodextrin was also studied by monitoring the absorbance at 420 nm, where the contribution from MnO4 −

Fig. 4. Arrhenius plot for the oxidation of ␤-cyclodextrin by MnO4 − . Reaction conditions: [MnO4 − ] (2.0 × 10−4 mol dm−3 ), [␤-cyclodextrin] (2.0 × 10−3 mol dm−3 ), [H+ ] (0.93 mol dm−3 ).

is negligible [40]. Fig. 5 shows the formation of Mn(IV) as an intermediate during the reaction of MnO4 − by ␤-cyclodextrin. It was also observed that formation of Mn(IV) was very sensitive to [HClO4 ]. At higher [HClO4 ], the formation of Mn(IV) was not observed. These results indicate that amount of Mn(IV) species is decreasing with increasing acidity of the medium. The spectra of Mn(IV) depend on the experimental conditions as well as the nature of the reducing agent. These results are in good agreement with the observations of other investigators [41,42]. Thus, the Mn(IV) species can exist only in neutral aqueous medium. 3.7. Identification of Intermediate (Mn(III)) It is well known that fluoride ions form complexes with Mn(III) [34]. To substantiate the formation of Mn(III) during the redox process, the effect of adding sodium fluoride has been studied. The kobs values decreased with increasing [F− ] (Fig. 6). These results indicate that Mn(III) species are readily removed by F− by complex formation, it is highly probable that Mn(III) is

Fig. 3. Dependence of pseudo-first order rate constant (kobs ) on [␤cyclodextrin]. Inset: log–log plot between kobs and [␤-cyclodextrin]. Reaction conditions: [MnO4 − ] (2.0 × 10−4 mol dm−3 ), [HClO4 ] (0.93 mol dm−3 ), temperature (40 ◦ C).

Fig. 5. Plots showing the formation of Mn(IV) as an intermediate at 420 nm during the oxidation of ␤-cyclodextrin by MnO4 − . Reaction conditions: [MnO4 − ] (2.0 × 10−4 mol dm−3 ), [␤-cyclodextrin](2.0 × 10−3 mol dm−3 ), temperature (40 ◦ C), [H+ ] (0.69 (A) and 0.11 mol dm−3 (B)).

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Fig. 6. Plots of kobs vs. [F− ] and [Mn2+ ]. Reaction conditions were the same as Fig. 3.

also formed as an intermediate. Our results seem to indicate that Mn(IV) and Mn(III) are the probable oxidizing intermediate(s) species. 4. Mechanism Scheme 1.

During the oxidation, it is evident that permanganate is reduced to various oxidation states, i.e., Mn(VI), Mn(V), Mn(IV) and Mn(III) in acidic, alkaline and neutral media. The reactivity of these intermediates states depend on the nature of the reductant and on the pH of the medium. The standard redox potentials of Mn(VII)/Mn(II), Mn(VII)/Mn(IV), Mn(IV)/Mn(III) and Mn(III)/Mn(II) are +1.51, +1.69, +0.95, and +1.51 V at 25 ◦ C in acidic media, respectively. As expected from their E◦ values, their reactions with ␤-cyclodextrin will be fast as compared to the hydrolysis of glycosidic linkage. An aqueous solution of a sugar is an equilibrium mixture of ␣- and ␤-anomers. ␤-Cyclodextrin is water soluble oligosaccharide which has seven ␣-glucose units and forms a ring structure. Despite uncertainty of the exact oxidation site for the complicated structure of ␤-cyclodextrin, the glucose unit can at least be considered. It has been established in the oxidation of aldoses by metal ions that the anomer having OH-1 equatorial undergoes faster oxidation than the corresponding anomer having OH-1 axial [31,43] d-glucose exists in equilibrium between ␣- and ␤-pyranose forms with free aldehyde form as intermediates.

In ␤-cyclodextrin, seven ␣-d-glucopyranose units linked by ␣-(1 → 4) bonds. Therefore, OH-1 of ␣-d-glucopyranose unit is not responsible for the reduction of MnO4 − ion. ␤-Cyclodextrin has a relatively inflexible doughnut shaped structure where the top of the molecule has 14 secondary OH groups and the bottom has the 7 primary OH groups from position 6. As a result, outside of the ␤-cyclodextrin has hydrophilic OH groups. In the

present reaction it has been assumed that C-1 position is blocked by glycosidic linkage, the reactive site is to be C-6. Thus, it is the CH2 OH group of ␤-cyclodextrin which is oxidized [44,45]. The nature of the product formed by the oxidation of aldoses depends on the oxidizing agent used. Mild oxidizing reagents oxidize only the aldehyde group to give aldonic acids. Aldoses are completely broken down by periodic acid. On the other hand, the ring in glycosides is stable in the presence of periodic acid. Thus, the hydrolysis of glycosidic bonds did not occur under the reaction conditions used in this study [44,46]. In the light of above observations and discussion, the proposed oxidation–reduction mechanisms are given in Schemes 1 and 2: (i) for the noncatalytic pathway, (ii) for the autoacceleration pathway. In Scheme 1, Eq. (1) represent, protonation of MnO4 − . The next reaction shows formation of a ␤-cyclodextrin–Mn(VII) complex (C1). In analogy with previous studies [32,47] we assume that it decomposes in a one-step, two electron oxidation–reduction mechanism to Mn(V) and other oxidation products (Eq. (3)). Mn(V) is highly unstable in an acidic medium [41] with respect to disproportionation and immediately gets converted into Mn(IV) (Mn(IV) is commonly involved in the MnO4 − oxidation of organic reductants (Eqs. (4) and (5)); other oxidation states of Mn are obviously involved in the reaction; they are extremely unstable in acidic solutions). In Scheme 2, reaction (6) represents formation of another complex (C2) with Mn(IV) and ␤-cyclodextrin. Eq. (7) is the rate determining step of autoacceleration pathway to yield Mn(III) as an intermediate. Therefore, Mn(III) would also participate

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M.S. Manhas et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 295 (2007) 165–171 Table 2 Variation of rate constant with [d-glucose], [MnO4 − ] and [HClO4 ] at 40 ◦ C 103 [d-glucose] (mol dm−3 )

104 [MnO4 − ] (mol dm−3 )

[HClO4 ] (mol dm−3 )

104 kobs (s−1 )

2.0 2.5 3.0 3.5 4.0 2.0

2.0

0.11

1.2 1.4 1.6 1.8 2.0 2.0

0.11

1.5 1.9 2.6 3.3 4.6 1.5 1.5 1.5 1.5 1.5 0.7 1.5 2.3 8.4 Very fast

2.0

Scheme 2.

in the reaction as an autocatalyst. In presence of large amount of ␤-cyclodextrin, the Mn(III) immediately gets converted into stable product (Mn(II)). In order to explain the autoacceleration path, the oxidation kinetics of ␤-cyclodextrin was also studied in presence of externally added Mn(II) (a reaction product). The results are shown in the kobs -[Mn2+ ] profile in Fig. 5. It can be noticed that the reaction rates decrease gradually with increasing [Mn(II)] from 0.13 × 10−3 to 0.2 × 10−3 mol dm−3 while at higher [Mn(II)], the results are opposite (rate increased with increasing [Mn(II)]). The decrease in the oxidation rate with [Mn(II)] indicates that Mn(IV) is involved in the autoacceleration reaction path. The kinetics of Mn(II) oxidation to Mn(III) by MnO4 − have been reported on several occasions [48,49]. Therefore, in presence of Mn(II), there is a competition between Mn(II) and ␤-cyclodextrin to react with MnO4 − . Thus, we conclude that in presence of externally added Mn(II), the path of oxidation of ␤-cyclodextrin by MnO4 − may become more complicated (Scheme 3). Due to the complicated features of the reaction, exact dependence of kobs on [Mn(II)] cannot be estimated in presence of ␤-cyclodextrin. The mechanism of oxidation of organic reductant by permanganate is known to be quite complex. It is to be mentioned that the oxidation product, i.e., aldehyde moiety will

Scheme 3.

0.04 0.11 0.16 0.23 0.93

be more reactive towards intermediate valence states of manganese (Mn(IV) and Mn(III)). The [product] is much smaller as compared to that of the ␤-cyclodextrin. As a result, the Mn(IV) and Mn(III) will preferably react with ␤-cyclodextrin rather than with product (Eqs. (2) and (6)). 4.1. Oxidation of d-glucose In order to compare the reactivity of ␤-cyclodextrin, a series of kinetic runs were also performed for the oxidation of d-glucose (structural unit of ␤-cyclodextrin) under the same experimental conditions. It has been found that for [H+ ] (0.93 mol dm−3 ) oxidation of d-glucose was too fast. Therefore, low [H+ ] was used for rate determination. The influences of [d-glucose], [MnO4 − ] and [H+ ] on kobs are shown in Table 2.

Fig. 7. Plot of log (absorbance) vs. log (wavelength) for the intermediate (MnO2 ) obtained from the oxidation of d-glucose by permanganate. Reaction conditions: [MnO4 − ] (2.0 × 10−4 mol dm−3 ), [d-glucose](2.0 × 10−3 mol dm−3 ), temperature (40 ◦ C), [H+ ] (0.11 mol dm−3 ).

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References

Scheme 4.

In order to identify the nature of intermediate (Mn(IV)), the spectrum of the reaction mixture was recorded before completion of the reaction. A linear relationship between log (absorbance) and log (wavelength) is expected if the water soluble colloidal MnO2 species are present in solution [40]. Such plot has been realized in the present study (Fig. 7). Thus, we may safely conclude that water soluble colloidal MnO2 was formed as an intermediate during the course of the reaction. On the basis of the observed results, the following mechanism is proposed for the oxidation of d-glucose (Scheme 4). 5. Conclusion The most interesting feature of this study is the oxidation of ␤cyclodextrin by MnO4 − in aqueous medium. We are unaware of precedence in the reducing nature of ␤-cyclodextrin. A perusal of literature revealed that ␤-cyclodextrin forms a variety of inclusion compounds. To examine the nature of cage complexes formed in the cavity of cyclodextrins, it is necessary to study the reducing as well as oxidizing properties in detail. To the best of our knowledge, this is the first evidence showing the oxidative decomposition of ␤-cyclodextrin. This study opens up a new area of cyclodextrin science in which cyclodextrins can be used as reactants. Oxidation d-glucose by permanganate in perchloric acid medium proceeds at a rate where the ␤-cyclodextrin practically does not undergo substantial oxidation under the comparable experimental conditions. The faster oxidation rate of d-glucose as compared to the ␤-cyclodextrin may be due to the fact that C-1 has potential CHO group whereas C-6 possesses a CH2 OH group.

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