The Open Catalysis Journal, 2009, 2, 71-78 71 1876-214X/09 2009 Bentham Open Open Access Kinetics and Mechanism of Oxidation of D-Galactose by Chromium(VI) in Presence of 2,2 ´ -Bipyridine Catalyst in Aqueous Micellar Media Ruhidas Bayen and Asim K. Das * Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India Abstract: 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 bpy- catalysed 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 bpy- catalysed 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. Keywords: Kinetics, 2,2 ´ -bipyridine, D-galactose, chromium(VI), surfactants. 1. INTRODUCTION To understand the mechanistic aspects of reduction of Cr(VI) to Cr(III), several kinetic studies of chromic acid oxidation of different types of organic substrates have been carried out by different workers [1-3]. Kinetic studies of oxidation of different types of organic substrate by halochromate have also been investigated by different workers [4, 5] to explore the effect of the substituent on the redox activity of Cr(VI). In this regard, micellar effect as a powerful probe [6] has been utilised by different workers to explore the redox activity of chromium(VI) [6-9]. Among the different types of chelating agents [10-15] to catalyse the Cr(VI) oxidation of different types of organic substrates, 2,2´-bpyridyl (bpy) is quite important [16-18]. Some of the chelating agents as catalysts like oxalic acid, - hydroxy acids experience cooxidation [19]. But bpy is never co-oxidised along with the substrate. In this regard, it acts always as an oxidation catalyst. During the reaction, bpy is gradually lost due to the formation of inert Cr(III)-bpy complex. Because of this fate bpy, it cannot be defined as a true catalyst from its definition, but loosely very often it is described as an oxidation catalyst. The present paper deals with the micellar effects on Cr(VI) oxidation D-galactose in the presence of *Address correspondence to this author at the Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India; Tel: +91 9832185863; E-mail: ak_das3@yahoo.com bpy. The micellar effects have been studied to substantiate the proposed reaction mechanisms. 2. EXPERIMENTAL 2.1. Materials and Reagents 2,2´-Bipyridine (A.R. Qualigens, India), D-galactose (A.R. SRL, India), K 2 Cr 2 O 7 (A.R. BDH), sodium dodecyl sulphate (SDS) (A.R. SRL, India), N-cetyl pyridinium chloride (CPC) (A.R.SRL, India), H 2 SO 4 (E. Merck), HClO 4 (E. Merck) and all other chemicals used are of highest purity available commercially. Solutions were prepared in doubly distilled water. 2.2. Procedure and Kinetic Measurements Solutions of the oxidant and reaction mixtures containing the known quantities of the substrate (S) (i.e. D-galactose), catalyst (bpy), under the kinetic conditions, [D-galactose] T >> [Cr(VI)] T and [bpy] T >> [Cr(VI)] T , acid and other necessary chemicals were separately thermostated (± 0.1 ° C). The reaction was initiated by mixing the requisite amounts of the oxidant with the reaction mixture. Progress of the reaction was monitored by following the rate of disappearance of Cr(VI) by titrimetric quenching technique [20] as discussed earlier. The pseudo first order constants (k obs ) were calculated as usual. Under the experimental conditions, possibility of decomposition of the surfactants by Cr(VI) has been