ABSTRACT
Cr (VI) + S Cr (IV) + P6slow (12.1)The reactive chromium (IV) formed may undergo further oxidation to give chromium (V) Cr (IV) + Cr (VI) Cr (V) (12.2)followed by either the following fast reaction Cr (V) + S Cr (IV) + S• (12.3)or the oxidation of the formed substrate radical (S•) in Eq. 12.3 to give rise to the oxidation products Cr (IV) + S Cr (III) + P4 (12.4)According to the above mechanism, the oxidizable substrate can be oxidized principally into two different products. This
mechanism is characterized by the absence of any interaction between Cr (IV) and the substrate, implying that such reaction must be slow compared with the rapid reaction between Cr (IV) and Cr (VI) (Eq. 12.2). The previous mechanism requires that the formed Cr(V) defined by Eq.(12.2) should be more effective oxidant for the oxidizable organic substrate. The interaction between Cr (IV) and Cr (VI) had for long time been widely accepted as the most probable next step that followed the rate-determining step in reactions of chromic acid oxidation. However, this suggestion is most unlikely on the basis of thermodynamic grounds for the estimated equilibrium constant (K = 4 × 10-14), which makes this process highly improbable [12, 14].The second mechanism corresponds to a rate-limiting of two-electron changes of the substrate (S) by Cr (VI), followed by oxidation of a further substrate molecule via either Cr (IV) formed in the first slow stage of reaction: Cr (VI) + S slow Cr (IV) + P6 (12.5) Cr (IV) + S Cr (III) + S• (12.6)and/or the oxidation of the substrate radical formed by Cr (VI) oxidant as follows: Cr (VI) + S• Cr (V) + P6 (12.7) Cr (V) + S Cr (III) + P5 (12.8)A third mechanism has been suggested; it involves the formation of only chromium (IV) as an intermediate species as follows: Cr (VI) + S slow Cr (IV) + P6 (12.9) Cr (IV) + S Cr (III) + S• (12.10) Cr (IV) + S• Cr (III) + P4 (12.11) Cr (IV) + Cr (VI) 2Cr (V) (12.12)A further possible mechanism was suggested for the oxidation of oxalic acid by chromic acid, which is based on the disproportionation of Cr (IV) formed after the initial rate-determining step as follows:
Cr (VI) + S slow Cr (IV) + P6 (12.13) 2Cr (IV) Cr (V) + Cr (III) (12.14) Cr (V) + S Cr (III) + P5 (12.15)For the above mechanism to occur, the accumulation of Cr (IV) intermediate concentration should be high enough to make the following second-order disproportionation possible (Eq. 12.14). This could happen only if reaction (12.15) was quite slow. The objection of the above mechanism to occur based on the high equilibrium constant of disproportionation (7.6 × 1012) estimated from the redox potentials of various Cr (V)/Cr (IV) (+1.34 V) and Cr (IV)/Cr(III) (+2.1 V) couples. Again, Hasan and Rocek [36] reported that the formation of Cr (IV) as an intermediate is completely avoided in redox reactions that involve the formation of stable complexes such as in the following redox reaction: Cr (VI) + S Complex (12.16) Complex slow Cr (V) + P6 + S• (12.17) Cr (VI) + S• Cr (V) + P6 (12.18) Cr (V) + S Cr (III) + P5, (12.19)where (S) is the oxidizable substrate, (S•) is the substrate radical formed, and P6, P5,and P4 are the final products originating from the reactions of the substrate molecules or substrate radicals with hexa-, penta-, and tetravalent chromium species, respectively.Although the synthesis of aldehydes, ketones, and acids as carbonyl groups by oxidation of organic substrates with chromic acid as a multi-equivalent oxidant has attracted numerous investigators over a period of many years [1-37] owing to their wide applications in industrial engineering, a little attention has been focused to the oxidation of macromolecules containing oxidizable alcoholic groups such as polysaccharides, which are known to have great importance and wide applications. For example, polysaccharides can be used in industrial technology such as stabilizers, detergents, emulsifiers, paper textile, paint, latex, and ceramic glazen; in cosmetic pharmaceutics as binders,
thickeners, hand lotion, shampoo, and hair treatment components; in medicine as blood anti-agulant, stomach ulcer controllers, and toothpaste binders; and in the food industry, such as drinks, jellies, relishes, pizza, fish gels, pet foods, and milk products [38-41]. Today, those polysaccharides and their derivatives have become more applicable in biomedical sensors, drug delivery, tissues engineering, skin grafting, medical adhesive, and wound dressing applications [42, 43], pervaporization dehydration processes [44], ethanol manufacturing [45], chelating agent for removal of toxic heavy metals and radionuclides [46, 47], and corrosion inhibition [48].Although the kinetics of the oxidation of some macromolecules by permanganate ion as a multi-equivalent oxidant has been investigated in both alkaline [49-60] and acidic solutions [61-67] as well as the kinetics of oxidation of either poly (vinyl alcohol) as a synthetic polymer [68] or kappa-carrageenan [69], carboxymethyl cellulose [70], and chondroitin-4-sulfate [71] polysaccharides as natural polymers by chromic acid, it is obvious that a lack of information about the nature of electron transfer and the transition states in the rate-determining step still remains incomplete and poorly understood. On the other hand, the kinetics of oxidation of studied alcoholic macromolecules [68-71] by chromic acid with respect to the nature of electron transfer and the transition states in the rate-determining step were found to be contrary to that reported by Hasan and Rocek elsewhere [36, 37].In view of the above discrepancies and our interest in the oxidation of alcoholic macromolecules in acidic solutions by either multi-equivalent oxidants such as chromium (VI) [69-71] and permanganate [61-67] or one-equivalent oxidants such as hexacyanoferrate (III) [72] and cerium (IV) [73], the present work explores the nature of electron transfer and transition state in the rate-determining step as well as the influence of homogeneous catalysis and model structure of the oxidant on the kinetics of oxidation by chromic acid in acidic solutions. Furthermore, this work includes pertinent discussion with a great deal of emphasis on the oxidation mechanisms and the chemistry of interaction of such macromolecules in aqueous solutions in order to compensate such lack of information of the nature of transfer of electrons and transition states in the rate-determining step in the oxidation of macromolecules containing alcoholic groups such as polysaccharides by chromic acid. Moreover, it is of interest to
present novel methods for the synthesis of coordination biopolymer keto-derivative precursors by the methods of oxidation processes. These keto-derivative oxidation products have wide applications in the technological industry as already mentioned. In addition, the synthesized products can be used as biodegradable and safety chelating agents for the removal of toxic heavy metals from contaminated wastes in the environment. 12.2 Nature and Physical Properties of
12.2.1 Nature of Chromium Ion SpeciesThere are various oxidation states of chromium ion ranging from –2 to +6, but only the trivalent and hexavalent states are the most stable under most natural environments and are more prevalent in aqueous phases. These two stable states, Cr (III) and Cr (VI), exhibit very different toxicities and mobilities. It is well known that chromium (III) is relatively insoluble in aqueous systems and exhibits a little toxicity and mobility [74-76]. On the contrary, chromium (VI) occurs as a highly soluble and more toxic species [31, 32]. Hence, the more conventional methods for removing the Cr (VI) from wastewater were based on the chemical reduction of Cr (VI) present using suitable chemical reducing agents. Here, the cited safe, nontoxic, and biodegradable polysaccharides may be used efficiently as reductants for this purpose.
