1 Introduction

It has been recognized that permanganate ion still remains the most powerful oxidant for oxidation of most organic and inorganic compounds for over a century. This is owing to its high capability for application in organic synthesis in neutral [1,2,3,4,5,6,7], alkaline [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28] and acidic [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62] media.

In alkaline solutions, permanganate ion is reduced from heptavalent (MnVII) to its soluble colloidal quadrivalent state (Mn IV))as follows,

$${\text{MnO}}_{{4}}^{ - } + {\text{ 3H}}_{{2}} {\text{O }} + {\text{ 3e}}^{ - } = {\text{ MnO}}_{{2}} + {\text{ 4OH}}^{ - }$$
(1)

Again, in very strong alkaline solutions and the presence of MnO4− in more excess, the green manganate (VI) species are formed,

$${\text{MnO}}_{{4}}^{ - } + {\text{ e}}^{ - } = {\text{ MnO}}_{{4}}^{{{2} - }}$$
(2)

On the other hand, in acidic solutions the permanganate ion tends to reduce from heptavalent (Mn(VII) to its divalent (Mn(II)) oxidation states as illustrated by Eq. (3)

$${\text{MnO}}_{{4}}^{ - } + {\text{ 8 H}}^{ + } + {\text{ 5 e}}^{ - } = {\text{ Mn}}^{{{2} + }} + {\text{ 4 H}}_{{2}} {\text{O}}$$
(3)

In addition, this oxidant possesses a high tendency for protonation in acidic media according to the following equilibrium

$${\text{MnO}}_{{4}}^{ - } + {\text{ H}}^{ + } \mathop \rightleftharpoons \limits^{{K_{p} }} {\text{HMnO}}_{{4}}$$
(4)

where Kp is the protonation constant which equals to 2.99 × 10–3 mol3 dm−1 at 25 °C [63]. In addition, the permanganate oxidant in presence of excess Mn (II) is further reduced to give both Mn3+ and/or Mn4+ species in the acid media as follows

$${\text{MnO}}_{{4}}^{ - } + {\text{ 3 Mn}}^{{{2} + }} + {\text{ 8 H}}^{ + } = {\text{ 3 Mn}}^{{{3} + }} + {\text{ Mn}}^{{{4} + }} + {\text{ 4 H}}_{{2}} {\text{O}}$$
(5)

The relevant redox potentials of the couples (MnVII//MnII,); (MnVII/ MnIV); (MnVII/ MnVI) and (MnIV/ MnIII) were reported as 1.51,1.23,0.56 and 1.51 V, respectively [64].

Unfortunately, a little attention has been focused on the kinetics of oxidation of alcoholic macromolecules by permanganate ion in either natural polymers (NP) or synthetic polymers (SP), however, a lot of work was reported on the kinetics and mechanistic of oxidation of organic and inorganic substrates by this oxidant as a strong oxidizing agent. This fact may be owing to the expected kinetic complexity for oxidation of large macromolecules. Indeed, Hassan and coworkers investigated the kinetics of permanganate oxidation of alginate (AlG) [65, 66]; pectate (PEC) [67,68,69]; pectin (PECN) [70, 71]; methyl cellulose (MC) [72, 73], carboxymethyl cellulose(CMC) [73,74,75] as homo-polysaccharides and carrageenan`s (CAR) such as kappa-carrageenan (KCAR); [76,77,78,79] and chondroitin-4-sulfate(CS) as hetero-polysaccharides [80] as natural polymers in comparison with that for oxidation of poly(vinyl alcohol) (PVA) [81, 82] and poly (ethylene glycol) (PEG) [86, 87] as a synthetic polymers in strong alkaline solutions of pH’s ≥ 12. Moreover, there are still several unsolved problems regarding the kinetics and mechanisms of reduction of alkaline permanganate by macromolecules, therefore, some debate has been arisen about the nature of the electron-transfer and transition states in the rate-determining steps. In connection with the multi-equivalent oxidants such as in case of permanganate ion as strong oxidizing agent, whether electron transfers occur simultaneously two-electron changes MnVII → MnV → MnIII or through one-electron transfer MnVII → MnVI → MnV → MnIV in sequence is the fundamentally interesting topic. Moreover, is whether the inner-sphere or outer-sphere type will be the more suitable and prevailing mechanism for transfer of electrons. Such a difficulty in most redox systems involving permanganate ion as an oxidant of multi-equivalent nature may be attributed to the complexity kinetics arises from the formation of unstable intermediates such as green manganate (VI) and /or blue hypomanganate (V) formed during the reduction of MnO4− ion from (+ 7) to (+ 4) oxidation states in alkaline media Furthermore, the factors which affect the reaction pathway route such as the type of solvent, pH, nature of substrate and other variables will participate in such complexity.

Given the aforementioned justifications and our interest in redox processes involving permanganate ion particularly in alkaline solutions [64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84], the main goal of the current review aims for focusing more highlights on the kinetics and mechanistic of oxidation of alcoholic macromolecules in particularly polysaccharides by alkaline permanganate based on the type of both transfer of electron nature and states of transition in the rate determining stage through a pertinent discussion on oxidation mechanistic and model structures. An isokinetic linear correlation between the kinetic parameters (LFER) will be examined.

2 Chemical structures, properties and synthesis

2.1 Structure of alcoholic macromolecules

2.1.1 Natural polymers

2.1.1.1 Homopolysaccharides

This type of polysaccharides (PS) belongs to natural polymer macromolecules and is comprising of β-D-mannuronic and α-L-guluronic acid residues joined in (1 \(\to\) 4) positions by glycosidic linkages of linear structure manner [85,86,87,88,89,90]. It characterized by the presence of the same repeating units of monosaccharide's and may also called homoglycan such as alginate, pectate, methyl cellulose and carboxymethyl cellulose which contain two secondary alcoholic groups on C-2 and C-3 positions. They can be illustrated by the configuration (a) where R1 represents to –COOH in alganic and pectic acids, –CH2OCH3 in methyl cellulose and –CH2OCH2COOH in carboxymethyl cellulose, respectively. The position of –OH groups on C-2 and C-3 are cis in case of alginic acid and trans-position for all other polysaccharides.

figure a
2.1.1.2 Heteropolysaccharides

When the polysaccharide contains more than one type of monosaccharides they are termed by hetropolysaccharides or heteroglycan such as carrageenan and chnodrointin-4-sulfate. Such two polysaccharides are possessing both primary and secondary alcohols with presence of number of sulfur atoms within their macromolecular chains. The carrageena`s (CAR) builds up of units of (1 → 3) β -D galactose-4-sulfate and (1 → 4) 3,6-anhydro- α -D-galactose as displayed in the structure (b). Whereas, the, chondroitin-4-sulfate (CS) consists of N-acetyl-D-galactosamine 4-sulfate with D-glucuronic acid repeating units as shown in structure (C) [9].

figure b

2.2 Synthetic polymers

2.2.1 Poly (vinyl alcohol) and poly (ethylene glycol)

Poly (vinyl alcohol) and poly (ethylene glycol) are synthetic alcoholic polymers of linear structure nature [81,82,83,84] containing only secondary alcoholic groups as illustrated by the structures (d) and (e), respectively:

figure c

2.3 Behavior of alcoholic groups in alkaline solutions

The alcoholic groups of the natural polymer macromolecular chains such as polysaccharides tend to deprotonate in alkaline solutions giving its more reactive alkoxides [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80],

figure d

whereas, the alcoholic OH groups in synthetic polymers give the enolate forms (c) and (d) such as in (PVA) and (PEG)), respectively [81,82,83,84]

figure e

This behavior was found to be on contrary to that in acidic media where both the primary –CH2OH and secondary –CH–OH alcoholic groups tend to protonate [91] giving the corresponding alkoxinum ions as expressed by Eqs. (8) and (9), respectively,

figure f
figure g

where Kd and Kp are the deprotonation and protonation constants, respectively (Table 1) [68,69,70,71, 75, 77,78,79,80, 82,83,84, 86].

Table 1 Deprotonation Constant at 25 °C and Activation Parameters of k′ (Formation) and k″ (Decomposition) in the Oxidation of Some Macromolecules Containing Secondary and/or Primary Alcoholic Groups by Alkaline Permanganate. [MnO4−] = (2−4) × 10–4, [S] = (2–5) × 10–3, [OH−] = 2 × 10–2 and I = 0.1 mol dm−3
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2.4 Preparation

2.4.1 Preparation of sols of alcoholic macromolecules

The cited alcoholic macromolecules (NP and SP) are characterized by its hydrophilic nature which is easily soluble in water forming viscous colloidal solutions. The –OH, –COO, and –OSO3 moieties, which have an extreme tendency to interact with water, are responsible for this solubility. The spherical or coiled colloids will be transferred to the linear block copolymer structures in aqueous solutions by swelling (orientation). As a result, the moiety group (s) forms an interface between the macromolecule and the water. This property may facilitate the macromolecule to interact with the oxidant forming the corresponding coordination biopolymer intermediate complexes as transient a species prior to the transfer of electrons from the macromolecule substrate to the oxidant in the slow rate step.

The colloidal sols of such polysaccharides can be prepared by adding small portions of the reagent powder to doubly distilled water with simultaneous stirring the mixtures vigorously and continuously in order to prevent the formation of aggregates of difficulty swelling. The prepared sols should be left for about 5–10 h at room temperature in order to become free from air-bubbles. Then, it kept into a refrigerator to avoid the bacterial attack. Again, when the sol is taking out from the refrigerator, it should be left the ambient room temperature for about 2 h before using. The macromolecule sols are valid for using for about a couple of weeks.

2.4.2 Stoichiometric measurements

In terms of the observed kinetics complexity in such redox systems owing to formation of short-lived detectable green manganate (VI) and/or blue hypomanganate (V) transient species, the stoichiometric determination becomes of great importance transient species. All kinetic measurements in the present review were carried out at pH’s > 12. This fact was attributed to the difficulty for determining the reactions stoichiometry at pH’s < 11 due to disproportionation of the formed manganate (VI) ion which its presence depends on the pH and time [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84]. This ion species are known to be stable and accumulated as the reaction proceeded above pH > 12 [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84]. The stoichiometry was performed by mixing the reactants of different molar ratios which the permanganate ion was found in a slightly excess over that of the alcoholic macromolecule at pH’s > 12 with continuous stirring at room temperature until reaction completion (10–24 h). Then the excess of unreacted permanganate ion was determined spectophotometrically at its absorption maxima of wavelength of 525 nm. In addition, the concentration data was further checked by well-known quantitative analytical techniques. The results obtained from different techniques were discovered to be in excellent agreement with each other with negligible errors (± 3%). Such stoichiometric measurements are necessary and important for formulation the conditions applied into the experimental measurements in accordance to the selected kinetic conditions and the order model as well as such stoichiometry will support the identification of the reaction products [92].

2.4.3 Synthesis of carbonyl and acid derivatives of alcoholic macromolecules

Synthesis of such carbonyl and acid derivatives of macromolecules of either natural or synthetic polymers as alcoholic macromolecules results from combining the stoichiometric molar ratios (defined by Eqs. 10–12) of the respective macromolecule with permanganate ion oxidant in alkaline solutions of pH`s > 12 (which was previously adjusted by using NaOH) in the presence of a stoichiometric molar ratio of NaF equivalent to that required for removal of the formed manganese (IV) it equals four times the molar ratio of initial permanganate applied which are identified by stoichiometric Eqs. (10–12) in order to precipitate the reduced form obtained from the reduced form of manganese as (MnF4). The reaction mixture was vigorously stirred for about 24 h at room temperature. The precipitated MnF4 can be separated by filtration and the resulting solution is concentrated to one-fifth of the original volume using a rotary evaporator. The pH of solution is adjusted to ca 5–6 by addition drops of acetic acid. The solid product is filtered off, washed several times with ethanol, dried under vacuum to be ready for using as chelating agent for most of polyvalent metal cations.

3 Analyses of kinetic data

3.1 Stoichiometry and oxidation products

The fundamental experimental facts that form the evidence of the speculated mechanisms are essentially based on the rate laws [93]. In addition, there are many other diverse facts needed to flesh out more suitable and plausible mechanisms [94, 95]. Therefore, the correct identification of the reaction products may still considered as the most important evidence for supporting the elucidated reaction mechanism of any reaction, followed by identification of the intermediates, which are the key to produce of electron-transfer pathway route of the studied oxidation reactions.

The experimental results showed that the stoichiometric molar ratio of ([MnO4−]consumed / [Substrate]0) were 0.67 ± 0.1 for Poly (Vinyl Alcohol), 2.67 ± 0.1 for kappa- carrageenan (KCAR) & 2.7 ± 0.1 for chondroitin-4-sulfate and 1.3 ± 0.1 mol for the other polysaccharides, respectively, which are corresponding to the following stoichiometric Eqs. (10–12),

$${3 }\left( {{\text{C}}_{{2}} {\text{H}}_{{4}} {\text{O}}} \right)_{{\text{n}}} + {\text{ 2 MnO}}_{{4}}^{ - } = {3 }\left( {{\text{C}}_{{2}} {\text{H}}_{{2}} {\text{O}}} \right)_{{\text{n}}} + {\text{ 2 MnO}}_{{2}} + {\text{ 2 OH}}^{ - } + {\text{ 2 H}}_{{2}} {\text{O}}$$
(10.1)
$${\text{3H}} - \left( {{\text{OC}}_{{2}} {\text{H}}_{{4}} } \right)_{{\text{n}}} - {\text{O}} - {\text{CH}}_{{2}} {\text{CH}}_{{2}} {\text{OH }} + {\text{4MnO}}_{{4}}^{ - } = {\text{3H}} - \left( { - {\text{OC}}_{{2}} {\text{H}}_{{4}} } \right)_{{\text{n}}} {\text{OCH}}_{{2}} {\text{COOH }} + {\text{ 4MnO}}_{{2}} + {\text{ 4OH}}^{ - } + {\text{H}}_{{2}} {\text{O}}$$
(10.2)
$${3 }\left( {{\text{C}}_{{{12}}} {\text{H}}_{{{17}}} {\text{O}}_{{{12}}} {\text{S}}} \right)_{{\text{n}}} + {\text{ 8 MnO}}_{{4}}^{ - } = { 3 }\left( {{\text{C}}_{{{12}}} {\text{H}}_{{{11}}} {\text{O}}_{{{13}}} {\text{S}}} \right)_{{\text{n}}} + {\text{ 8 MnO}}_{{2}} + {\text{ 8 OH}}^{ - } + {\text{ 5 H}}_{{2}} {\text{O}}$$
(11.1)
$${3}\left( {{\text{C}}_{{{14}}} {\text{H}}_{{{21}}} {\text{NO}}_{{{14}}} {\text{S}}^{ - } } \right)_{{\text{n}}} + {\text{ 8MnO}}_{{4}}^{ - } = { 3}\left( {{\text{C}}_{{{14}}} {\text{H}}_{{{15}}} {\text{NO}}_{{{15}}} {\text{S}}^{ - } } \right)_{{\text{n}}} + {\text{8MnO}}_{{2}} + {\text{8OH}}^{ - } + {\text{5H}}_{{2}} {\text{O}}$$
(11.2)
$${3 }\left( {{\text{C}}_{{5}} {\text{H}}_{{7}} {\text{O}}_{{6}} {\text{R}}} \right)_{{\text{n}}} + {\text{ 4 MnO}}_{{4}}^{ - } = { 3 }\left( {{\text{C}}_{{5}} {\text{H}}_{{3}} {\text{O}}_{{6}} {\text{R}}} \right)_{{\text{n}}} + {\text{ 4 MnO}}_{{2}} + {\text{ 4 OH}}^{ - } + {\text{ 4 H}}_{{2}} {\text{O}}$$
(12)

where (C2 H4 O)n; H–(OC2H4) n–O–C2H4 OH; (C12 H17 O12 S)n & (C14H21NO14S−)n; and (C5 H7 O6 R)n denote to the Poly (Vinyl Alcohol), PEG, Kapa CAR and CS and PS, respectively, and (C2H2O)n, (H–(–OC2H4)nOCH2COOH; (C12H11O13S)n & (C14H15NO15S−)n; and (C5H3O6 R)n which corresponding to monocarbonyl, dicarbonyl and carbonyl acid groups depending on the oxidized groups of polysaccharides and (S) represents to the polymer substrate [92].

3.2 FTIR spectra

The broad bands appeared in the IR spectra at 1730–1760 were indicative to the transformation of the hydroxyl groups into the carbonyl groups which is further confirmed by the disappearance of the absorption bands of the –OH groups in the IR spectra. Again, both C6H3(NO2)2NHNH2 and NH2OH were applied for identification of the carbonyl groups. The formation of 2,4-dinitrophenyl hydrazone and oxime derivatives, respectively, were indicative for formation of such carbonyl forms [92].

3.3 Curves of reaction –times

All kinetic measurements for oxidation reaction using cited macromolecules were performed in pseudo-first-order settings, in which the concentrations of the substrates [S] (PS or SP) were kept in a large excess over that of permanganate ion concentration. The pseudo first-order plots of oxidation reactions were found to be of sigmoidal nature consisting of two distinct stages. The magnitudes of the rate constants in such two stages were allowed to follow each stage separately using a conventional spectrophotometer technique. It is possible to watch the reactions' courses by keeping an eye on either the decline in permanganate ion's absorbance at its maximum absorption of 525 nm wavelength or the increase in absorbance of green manganate (IV) at its maximum absorbance wavelength of 606–610 nm. The two rates were found to be directly proportional to the concentration of permanganate (MnO4−) or manganate (MnO42−) species, and the results showed that the resultant pseudo-first-order kinetic measurements were reproducible since they agreed with one another within negligible experimental errors (4%).

The reaction mixtures were kept constant in terms of ionic strength using sodium perchlorate as an inert electrolyte.

3.4 Scanning of spectral changes

The disappearance of the distinct permanganate absorbance band located at wavelengths ranging from 490 to 590 nm is considered as the sign and indication of the reduction of permanganate ion. Typical examples for the spectral changes during oxidation of some cited natural polymer(polysaccharides) and synthetic polymers by alkaline permanganate ion are shown in Figs. 1, 2, 3. In such figures, it is clear that the spectral features of permanganate ion are the most pronounced species. On the other hand, it noticed the display of other species as the redox reaction starts to proceed. The overall spectral changes are the gradual disappearance of the band of MnO4− ion at its absorption maximum wavelength of 525 nm through a continuous appearance of different bands at absorptions λmax = 606, 435, 350 and 315 nm which are characterized to the interference of the formed intermediate species. This behavior was surprising and of interesting which gave us more flexibility by allowing us to monitor the rate of reaction development when the permanganate ion either disappear or when the intermediate complexes appear at appropriate wavelengths with no detectable interference from other reagents. The experimental results at both two cases of 525 and 606 nm, respectively, gave identical rate constants of relatively fast formation of intermediate complexes at the first stage within negligible experimental errors. This finding suggests that the reactions were first-order in terms of ion concentration permanganate kinetics.

Fig. 1
figure 1

a–d Spectral changes during the formation of the intermediate complexes in the oxidation of a sodium alginate, b KCAR, c sodium pectate and d poly(vinyl alcohol) by alkaline permanganate

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Fig. 2
figure 2

UV–Vis spectra (200–800 nm) of the kappa-carrageenan, permanganate ion and intermediate complex in the oxidation of KCAR by permanganate ion. [MnO4−] = 4 × 10−4, [KCAR] = 4 × 10−3, [OH−] = 0.01, I = 0.1 mol dm−3 at 20 °C

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Fig. 3
figure 3

a–c Spectral changes during the formation (a, b) and decomposition (a’, b’) of the intermediate complexes in the oxidation of a sodium alginate and b KCAR by alkaline permanganate. Reference cell (MnO4− and OH− of the same mixture concentration)

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It is well known that the 606 nm bands correspond to the appearance of transient species of green manganese (VI) which is observed within the visible region [99,100,101,102]. At this wavelength, the absorption of MnO4− is much weaker, thus the detection of MnO42− is not difficult. The absence of the absorption band of hypomanganate (V) around 700 nm during the progress of reactions in particularly at elevated temperatures and high substrate concentrations could be accounted for by the fact that it has a very brief lifetime and disproportionate quickly [97,98,99]. This rapid disproportionation is autocatalyzed by the formed manganese (IV) [96] as reduced product of permanganate ion and may be considered as the reason for the diffuse signal at 700 nm.

$${\text{Mn}}^{{\text{V}}} {\text{O}}_{{4}}^{{{3} - }} + {\text{ Mn}}^{{\text{V}}} {\text{O}}_{{4}}^{{{3} - }} = {\text{ Mn}}^{{{\text{VI}}}} {\text{O}}_{{4}}^{{{2} - }} + {\text{ Mn}}^{{{\text{IV}}}}$$
(13)

The visual change in color of the solution mixtures from purple-pink (heptavalent manganese) to blue (pentavalent manganese) to green (hexavalent manganese) to yellow (tetravalent manganese) as the reaction progressed may suggest the formation of hypomanganate (V) at firstly in the oxidation process (Fig. 4). This expectation was met with success of keen trials for detection of Mn(V) at low concentration of reactants and at lower temperatures in cases of oxidation of carrageenan [79,80,81,82], methyl cellulose [72, 73], carboxymethyl cellulose [74, 75], PVA [81, 82] and PEG [83, 84] by permanganate ion. A typical illustration is provided in Fig. 5. The band of MnVO43− was appeared at a wavelength of around 738 nm in good agreements with that reported elsewhere [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84, 96,97,98,99].

Fig. 4
figure 4

The color degradation of KMnO4 during the oxidation process: Oxidation states: a Violet Mn (VII) ion; b Blue Mn(V) ion; c Green Mn (VI) ion in the oxidation by alkaline permanganate

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Fig. 5
figure 5

Spectral changes (550–800 nm) during the oxidation of kappa-carrageenan by permanganate ion at [MnO4−] = 1 × 10−4, [KCAR] = 4 × 10−3, [OH−] = 0.05, I = 0.1 mol dm −3 at 0 °C

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Instead of a suspension of MnO2, the yellow color that remains even after all of the MnO4− ions disappeared could indicate the production of stable water-soluble colloidal manganese (IV) as a final reduced form [103, 104]. Water-soluble manganese (IV) has been postulated in many redox reactions involving permanganate ion as an oxidant in alkaline media [102, 103]. This manganese (IV) does not precipitate immediately as MnO2 but forms a stable yellow solution at first and may be coagulated on aging.

The conversion of MnO4− to MnVIO42− may be supported by an isobestic point showing at 575 nm during the reaction progression. However, isobestic point was observed at 475 nm shows that the creation of a detectable manganate (VI) intermediate during the reduction of permanganate by alcoholic or synthetic polymers is not necessarily excluded by the presence of MnO4− or the newly created MnIV. In accordance with these facts, the absorbance noticed at 606 nm wavelength was attributed to the initial reduction of MnO4− to MnO42− together with the production of transient coordination intermediate complexes with the formula [S, MnO42−]. Additionally, the rise of the produced soluble manganese (IV), which does not absorb above 540 nm, caused to the occurrence shift of the bands which was detected at wavelengths of 435 and 350 nm. The molar extinction coefficient (εmix) of manganite (VI) was calculated by the well-known methods and found to be in the range of 1250 ± 100 dm3 mol−1 cm−1 [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84, 99, 104,105,106,107].

3.5 Identification of the oxidation products

The products of oxidations were identified by using the spectral data, elemental analysis as well as the reactions with both 2,4-dinitrophenyl hydrazine and hydroxyl amine to afford the corresponding hydzone and oxime derivatives, respectively, as shown in Tables (2, 3, 4) [68,69,70,71, 75, 77,78,79,80, 82,83,84, 86].

Table 2 Elemental Analysis of Product of Oxidation of Some Polysaccharides
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Table 3 Elemental Analysis of 2,4-Dinitrophenyl Hydrazone Derivatives
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Table 4 Elemental Analysis of Dioxime Derivatives
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4 Reaction mechanism

According to the results of experimental kinetics [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84], the oxidation of alcoholic groups in either natural polymers (polysaccharides PS) or synthetic polymers (PVA and PEG) by alkaline permanganate can be expressed by two separately distinct stages. The first stage is relatively fast (autoacceleration period) which proceeds through formation of transient coordination intermediate complexes involving green MnVI and/or blue MnV short-lived transient species. The second stage is slow (induction period) and corresponding to the decomposition of such intermediates to give soluble colloidal MnIV and dicarbonyl- or dicarbonyl-acid derivatives in case of (PS); carbonyl derivatives in case of (PVA) and acid in case of (PEG), respectively, as final oxidation products.

Generally, two parts with differ in the degree of sophistication are usually involved in determination of reaction mechanisms. The first part is how to recognize all elementary steps which support the experimental stoichiometry obtained. The second and more difficult one is how to develop a detailed stereochemical picture of each elementary step as it occurs [108,109,110]. Furthermore, a bulk of information concerning the mechanistic behavior of permanganate oxidation has been provided by organic systems particularly in aqueous acidic solutions [100].

In view of the above aspects and the kinetic observations and interpretations [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84], the more suitable mechanism which may be suggested and agreed with the experimental kinetic results, involves a fast deprotonation of the alcoholic groups to form alkoxides in case of polysaccharides or enolates in case of poly (vinyl alcohol) and poly (ethylene glycol) substrates before the rate-determining stages. The deprotonation process is followed by the attack of permanganate ion on the reactive alkoxides or enolate forms through three reaction pathways. The first and second pathway routes correspond to the attack of permanganate ion oxidant on the center of alkoxide or enolate forms to give the intermediate complexes (1) and (2). The presence of Na+ cations tends to lower the net charge of the complexes generated (a) or to polarize the Mn–O bond (b), which facilitates the production of such intermediates [111].

figure h

The third pathway is corresponding to the attack of the oxidant to –C–H hydrogen bonding forming the intermediate complex (3).

A slow decomposition of complexes (1), (2) or (3) will take place as the Mn(VI) concentration is built up with formation of the corresponding the reaction products. Two probable processes for the decomposition of such complexes may be suggested on the basis of the [OH−] dependency of the rate constants (1–3). A quick deprotonation of the intermediates by alkali, then an electron move from the substrate (S−) to MnVI (intermediates 1 and 2), is what the initial mechanism looks like. The second process, however, is the rate-determining step (intermediate 3) transfer of the hydride ion from the substrate (S−) to MnO4−. It was found that the former reaction mechanism (which involving the reaction pathways I and II) is the most plausible mechanism which consistent by the experimental results of the rate constants for the [OH−]-dependences for decomposition. Consequently, the second mechanism represented by pathway route (III) may be excluded. But this suggestion is not conclusive. The formation and decomposition mechanisms of the intermediate complexes are illustrated in Schemes (I, I`), (II), (III) and (IV), respectively.

Scheme I
scheme 1

Speculated mechanism for the formation of coordination biopolymer intermediate complexes in the oxidation of secondary alcoholic macromolecules by alkaline permanganate

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Scheme II
scheme 2

Speculated mechanism for the decomposition of coordination biopolymer intermediate complexes in the oxidation of secondary alcoholic macromolecules by alkaline permanganate

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Scheme III
scheme 3

Speculated mechanisms for formation and decomposition of coordination biopolymer intermediate complexes in the oxidation of PVA by alkaline permanganate

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Scheme IV
scheme 4

Speculated mechanisms for formation and decomposition of coordination biopolymer intermediate complexes in the oxidation of PEG by alkaline permanganate

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Generally, the oxidation of polysaccharides by permanganate ion leads to the formation of mono- and/or dikcarbonyl derivatives depending on the initial molar concentration used of reactants as well as on the pH’s of the medium [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84]. Typical examples for oxidation of alginates, pectates, methyl cellulose and carboxymethyl cellulose can be illustrated by the following equations:

$${\text{C}}_{{6}} {\text{H}}_{{7}} {\text{O}}_{{6}} {\text{R }} + {\text{MnO}}_{{4}}^{ - } + {\text{ OH}}^{ - } = {\text{C}}_{{6}} {\text{H}}_{{5}} {\text{O}}_{{6}} {\text{R }} + {\text{ HMn}}^{{\text{V}}} {\text{O}}_{{4}}^{{{2} - }} + {\text{ H}}_{{2}} {\text{O}}$$
(15)
$${\text{C}}_{{6}} {\text{H}}_{{7}} {\text{O}}_{{6}} {\text{R }} + {\text{ 2MnO}}_{{4}}^{ - } + {\text{ 2OH}}^{ - } = {\text{C}}_{{6}} {\text{H}}_{{3}} {\text{O}}_{{6}} {\text{R }} + {\text{ 2HMn}}^{{\text{V}}} {\text{O}}_{{4}}^{{{2} - }} + {\text{ 2H}}_{{2}} {\text{O}}$$
(16)
$${\text{C}}_{{6}} {\text{H}}_{{7}} {\text{O}}_{{6}} {\text{R }} + {\text{MnO}}_{{4}}^{ - } + {\text{ 2OH}}^{ - } = {\text{C}}_{{6}} {\text{H}}_{{5}} {6}_{{4}} {\text{R }} + {\text{Mn}}^{{\text{V}}} {\text{O}}_{{4}}^{{{3} - }} + {\text{ 2H}}_{{2}} {\text{O}}$$
(17)
$${\text{C}}_{{6}} {\text{H}}_{{7}} {\text{O}}_{{6}} {\text{R }} + {\text{ 2MnO}}_{{4}}^{ - } + {\text{ 4OH}}^{ - } = {\text{C}}_{{6}} {\text{H}}_{{3}} {\text{O}}_{{6}} {\text{R }} + {\text{ 2Mn}}^{{\text{V}}} {\text{O}}_{{4}}^{{{3} - }} + {\text{ 4H}}_{{2}} {\text{O}}$$
(18)

where C6H7O6R, C6H5O6R and C6H3O6R denote the corresponding oxidation products of, monocarbonyl (Eqs.15 and 17) and dikcarbonyl derivatives (Eqs.16 and 18), respectively.

On the other hand, the oxidation of PVA by this oxidant gives poly (vinyl ketone) [81, 82], and oxidation of PEG give poly(ethylene glcolic acid); whereas the oxidation of carrageenans give monocarbonyl- or dicarbonyl-acid derivatives [76,77,78,79], respectively.

Oxidation products of the cited polysaccharides particularly the chondroitin, methyl cellulose and poly (vinyl alcohol) was found to have a high tendency for complexation with polyvalent metal ions to form coordination biopolymer complexes which can be act as a monobasic bidentate [112, 113].

The mechanism by which a multi-equivalent oxidant such as permanganate ion oxidizes the substrate depends not only on the structure of the substrate, but also on the nature of the medium [114]. In strong alkaline solutions, no mechanistic information was available to distinguish between a direct one-electron transfer and two-electron changes owing to the fast disproporation of the formed hypomanganate (V) giving MnIV and MnVI species [96]. But, some information may be deduced from the nature of the transition states. If the transition states of both oxidant and reductant are unstable, simultaneous two-electron changes are the more favorable mechanism. On the contrary, if the transition states are stable, a successive one-electron transfer is the more predominant ones [115,116,117].

The change in color of the solution mixture can be seen with the naked eye to the spectrophotometric detection of hypomanganate (V) transient species [71, 99] and the absence of induced polymerization may all combined to support the two-electron transfer mechanism. But this pathway seems to be kinetically inaccessible [59, 118]. In view of these aspects, the both two reaction mechanisms may be favorable. But, comparison of the observed kinetics for the present oxidation reactions with those of other redox reactions involving alkaline permanganate as an oxidant, the successive one-electron transfer mechanism is the most probable pathway route. Indeed, this suggestion is not conclusive.

4.1 Linear free- energy relationship (EXENER`s CORRELATION)

It is well-known that the rate law provides no information regarding inner- or outer-sphere nature; however, some information can be obtained by examining the magnitude of the activation parameters in particularly that of the entropies of activation. The kinetic parameters evaluated from the Eyring equation of the absolute rate theory and from the Arrhenius equation [119] are listed in Table 1. It has been suggested that the redox reactions fall into two classes, one of positive- entropies of activation and the other posses negative values of activation entropy, respectively. The ΔS≠ values for a group of outer-sphere reactions tend to be more positive, whereas the values of ΔS≠ for a set of inner-sphere reactions are more negative [120, 121]. Again, it has been suggested that the entropies of activation are negative for a group of MnO4− reactions where direct evidence or postulate complex formation between MnVII and the substrate are formed. This suggestion agrees very well with proposition for inner-sphere mechanisms [97, 122,123,124,125,126,127].

Many reactions display an isokinetic relationship, according to Leffler and Grunwald [128] ( ΔH≠  = C + BΔS≠). Which observed in Figs. 6 and 7, plots of ΔH≠ vs. ΔS≠ values for formation (k’) and decomposition (k2) of the intermediate complexes through the oxidation of the studied polysaccharides by alkaline permanganate are linear, with C = 290 and 250 K and B = 71 and 78 kJ, for the formation and decomposition processes, respectively. This linearity suggests that identical reaction mechanisms govern the kinetics of these macromolecules by permanganate oxidation in alkaline solutions. These macromolecules' reactivity may also be shown by the B values, which are considerable and significant. a draw of the values of activation enthalpie against that of the activation entropies for the apparent constants, k1` and k2’ of formation or decomposition of the intermediate species are presented in Figs. 6 and 7, respectively.

Fig. 6
figure 6

Isokinetic relationship for the formation of intermediate complexes in the oxidation of some macromolecules by alkaline permanganate

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Fig. 7
figure 7

Isokinetic relationship for the decomposition of intermediate complexes in the oxidation of some macromolecules by alkaline permanganate

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In view of the above tentative interpretations, the redox reaction involving alcoholic macromolecule and alkaline MnO4− is predicted to proceed via successive inner-sphere one-electron transfer pathways.

5 Conclusion

Permanganate ion as a strong oxidizing agent, easily handled, readily available and versatile oxidizing agent has been widely used for more a century. The oxidizing capability of permanganate has been exploited in more modern areas such as biochemical footprint assays, nanotechnology, environmental pollution control and bioluminescence.

Application of biodegradable natural macromolecules such as the cited polysaccharides as natural polymers and both poly (vinyl alcohol) and poly(ethylene glycol) as synthetic polymers in the environmental requirements becomes of great importance in today technology. For example, cross-linked polysaccharide-based-materials have demonstrated outstanding removal capabilities for certain pollutants such as dyes, radionuclides and metal ions compared to that available in the international markets such as sorbents and commercial activated carbons high costs.

The oxidation products resulting from the cited polysaccharides are considered a promising novel synthesized coordination biopolymer precursors in particularly that synthesized from methyl cellulose and chondroitin substrates as well as that produced from the oxidation of (vinyl alcohol). These compounds were discovered to have a significant capacity for chelation, resulting in the formation of the appropriate ionotropic metal substrate gel complexes by coordination with polyvalent metal cations, i.e. it behaves as monobasic bidentates similar to that of poly (β-diketone). Hence, it may be possible to consider these products as promising chelating agents for removal of undesired poisonous heavy metal cations from environment, soil, wastewater and other contaminated matters. This means that oxidation of the cited polysaccharides aims to simultaneous synthesis of coordination biopolymer precursor derivatives besides a pertinent explanation of the nature of both electron-transfer and substrates interaction in terms of kinetics and mechanistic approach based on suggested structure models.

Biopolymers could be usually utilized for drug encapsulation to protect and deliver bioactive or functional components such as minerals, peptides, proteins, enzymes, drugs, lipids dietary fibers, conductors, selective cation sieves, semi-permeable membranes, biocatalysts and cation exchange resins. Hence, the mono- and diketo-oxidation derivatives produced from the of polysaccharides can be used for such purpose. Application of these oxidation products as coordination biopolymer precursors for elimination of pollutants from contaminated matters is in progress in our laboratory.