Micellar effect on the reaction of chromium(VI) oxidation of D‐fructose in the presence and absence of picolinic acid in aqueous media: a kinetic study

Abstract

The kinetics and mechanism of the Cr(VI) oxidation of D-fructose in the presence and absence of picolinic acid (PA) in aqueous acid media were studied under the conditions [D-fructose] T )[Cr(VI)] T at different temperatures. Under the kinetic conditions, the monomeric species of Cr(VI) was found to be kinetically active in the absence of PA whereas in the PA-catalysed path, the Cr(VI)-PA complex was considered to be the active oxidant. In this path, the Cr(VI)-PA complex undergoes a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a redox decomposition through glycol splitting leading to the lactone of C 5 -aldonic acid along with formaldehyde and the Cr(IV)-PA complex. The primary product formaldehyde undergoes further oxidation (in part) to form formic acid. Then the Cr(IV)-PA complex participates further in the oxidation of Dfructose and ultimately is converted into the inert Cr(III)-PA complex. In the uncatalysed path, the Cr(VI)-substrate ester experiences an acid-catalysed redox decomposition (2e transfer) in the rate-determining step giving rise to the products. The uncatalysed path shows a second-order dependence on [H ] whereas the PA catalysed path shows a fractional order in [H ]. Both paths show a first-order dependence on [D-fructose] T and [Cr(VI)] T . The PA-catalysed path is first order in [PA] T . All these patterns remain unaltered in the presence of externally added surfactants. The effects of a cationic surfactant, N-cetylpyridinium chloride (CPC), and an anionic surfactant, sodium dodecyl sulfate (SDS), on both the uncatalysed and PA-catalysed paths were studied. CPC inhibits both the uncatalysed and PAcatalysed paths whereas SDS catalyses the reactions. The observed micellar effects are explained by considering a distribution pattern of the reactants between the micellar and aqueous phases. The applicability of different kinetic models, e.g. the pseudo-phase ion-exchange model, the Menger-Portnoy model and the Piszkiewicz cooperative model, was tested to explain the observed micellar effects. The effect of [surfactant] T on the activation parameters was explored to rationalize the micellar effect.

Picolinic acid promoted oxidative decarboxylation of phenylsulfinylacetic acid by Cr(VI)

“…The UV-visible spectra of Cr(III) ion shows two peaks at 421 nm and 592 nm which are attributed to the octahedral transitions, 4 A 2g (F) → 4 T 1g (F) and 4 A 2g (F) → 4 T 2g (F) [36][37][38] whereas the spectrum of the product mixture exhibits two peaks at 408 nm and 542 nm ( Figure 4). This clearly demonstrates that Cr(VI) is reduced to Cr(III) ion but the blue shift observed in λ max values for the product mixture is due to existence of Cr(III) in the form of complex, probably the Cr(III)-PA complex as reported in the PA catalysed Cr(VI) oxidation reactions [12][13][14]. …”

“…The added Mn 2+ ion, scavenger for Cr(IV), has failed to produce any noticeable effect on the reaction rate ruling out the participation of Cr(IV) as the active species. Thus, the active species in this reaction is Cr(VI) itself, which exists in aqueous acidic solution in a variety of forms such as CrO 4 [12,14,49] it has been observed that the redox potential of Cr(VI) increases in the presence of chelating agents and believed that Cr(VI) forms complexes with the chelating agents. Thus in the present case, the increase in reaction rate with increase in [PA] may be attributed to the formation of a bimolecular cyclic complex (C 1 , Scheme 1) between Cr(VI) and PA which is assumed to be the kinetically active oxidizing species [12,[17][18][19].…”

“…Thus PA is not a true catalyst and it is better described as a promoter. Picolinic acid promoted Cr(VI) oxidation of several organic substrates [12][13][14][15][16] including organic sulfur compounds [12,[17][18][19] have been extensively studied. The catalytic effect was attributed to the activation of oxidizing species of chromium through a precursor complex formation.…”

Chromic acid oxidation of hexitols in the presence of 2,2′‐bipyridyl catalyst in aqueous micellar media: A kinetic study

“…The observation is identical with that observed by Bunton and Cerichelli [18] in the oxidation of ferrocene by ferric salts in the presence of cationic surfactant, cetyltrimethyl ammonium bromide (CTAB). The similar observations have also been noted by Panigrahi and Sahu [19] in the oxidation of acetophenone by Ce(IV) in the presence of Ndodecylpyridinium chloride (NDPC), by Reddi and coworkers [2b] in the oxalic acid-catalyzed oxidation of aromatic azo-compounds by Cr(VI) in the presence (where L = bipy) (11) (12) (13) (14) Scheme 2 Chromic acid oxidation of hexitol (i.e., D-sorbitol and D-mannitol) [20], L-sorbose [21], ethanol, and propan-1-ol [22] in the presence of CPC.…”

Micellar Catalysis on 1,10-Phenanthroline Promoted Chromic Acid Oxidation of Ethane-1,2-diol in Aqueous Media at Room Temperature

“…The concentration of Cr(VI) at different time interval was measured by a titrimetric quenching technique using excess of standard Mohr's solution and unreacted Fe(II) was estimated by a standard Ce(IV) solution using ferroin indicator. 10 The pseudo-first order rate constant were calculated from the slopes of the plots of log[Cr(VI)] T versus time (t), which were linear at least for three halflives. The scan spectra and spectrum after completion of the reaction were recorded with a UV-VIS spectrophotometer [UV-2450 (SHIMADZU)].…”