In aqueous H 2 SO 4 media, the chromic acid oxidation of D-galactose in the presence and absence of 2,2´bipyridine (bpy) has been carried out under the conditions, [D-galactose] T >> [Cr(VI)] T at different temperatures. The monomeric species of Cr (VI) has been found to be kinetically active in the absence of bpy whereas in the bpy-catalysed path, the Cr(VI)-bpy complex has been suggested as the active oxidant. In the bpy-catalysed path, Cr(VI)-bpy complex receives a nucleophilic attack by the substrate to form a ternary complex, which subsequently experiences a redox decomposition (through 2e transfer) at the rate-determining step leading to the product lactone and Cr(IV)-bpy complex. Then the Cr(IV)-bpy complex participates in faster steps in further oxidation of D-galactose and ultimately it is converted into Cr(III)-bpy complex. In the uncatalysed path, Cr(VI)-substrate ester experiences acid catalysed redox decomposition (2e-transfer) at the rate determining step. The uncatalysed path shows second order dependence on [H + ] while the bpycatalysed path shows a first order dependence on [H + ]. Both the uncatalysed path and bpy-catalysed path show the first order dependence on both [D-galactose] T and [Cr(VI)] T. The bpy-catalysed path is first order in [bpy] T. These observations remain unaltered in the presence of externally added surfactants. Effect of the surfactants like N-cetylpyridinium chloride (CPC, a cationic surfactant) and sodium dodecyl sulfate (SDS, an anionic surfactant), on both the uncatalysed and bpycatalysed paths has been studied. CPC inhibits both the uncatalysed and bpy-catalysed path, while SDS accelerates the reactions. In the catalysed path, cationic Cr(VI)-bpy complex is the reactive species which is attracted by the anionic micellar head groups of SDS but repelled by the cationic micellar head groups of CPC. The neutral substrate is accumulated in the Stern layer of both types of micelles. Thus the observed micellar effects have been explained by considering the hydrophobic and electrostatic interactions between the reactants and surfactants in terms of the proposed mechanism.
] dependence, it has been noted that the both Ce(SO 4 ) 2+ and Ce(SO 4 ) 2 have been found kinetically active. The different rate constants in the presence and absence of surfactants have been estimated with the activation parameters. N-cetylpyridinium chloride has been found to retard the oxidation process of hexitols, whereas sodium dodecyl sulfate has been found to accelerate the rate process. All these findings including the micellar effects have been interpreted in terms of the proposed reaction mechanism and partitioning behavior of the kinetically active different species of Ce(IV) between the aqueous and pseudomicellar phase.
Chromic acid oxidation of two polyols (D-sorbitol and D-mannitol) to their corresponding aldohexoses in the presence of phenanthroline (phen) involves as the active oxidant, a Cr(VI)-phen complex formed in a rapid pre-equilibrium step. The observed micellar effect (rate acceleration by an anionic surfactant and rate retardation by a cationic surfactant) conforms with the proposed reaction mechanism.
Kinetics of the oxidation of α-hydroxy acids [HA], namely malic acid, lactic acid and mandelic acid, by Ce(IV) in aqueous sulfuric acid media have been investigated both in the absence and presence of the cationic surfactant, N-cetylpyridinium chloride (CPC) and the anionic surfactant, sodium dodecyl sulfate (SDS). Under the conditions [HA] [Ce(IV)], the reaction exhibits first-order kinetics both in [Ce(IV)] and [HA]. The rate retarding effect of HSO4- has been analysed and it has been noted that both Ce(SO4)2+ and Ce(SO4)2 are kinetically active but their relative contribution depends on the nature of the α-hydroxy acid and surfactant. The different rate constants in the presence and absence of surfactants have been determined with the corresponding activation parameters. SDS has been found to accelerate the rate process for all three α-hydroxy acids. CPC has been found to retard the oxidation of malic acid more than the other two α-hydroxy acids. In fact, for lactic acid, the rate is more or less insensitive to CPC. All these findings, including the micellar effects, have been interpreted in terms of the proposed reaction mechanism and partitioning behaviour of the different kinetically active species of Ce(IV) between the aqueous and pseudomicellar phases.
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