Kinetic and Mechanistic study of Oxidative Transformation of Citric acid by Pyridiniumdichromate in DMF-Water Medium

T Oxidation of citric acid by PDC was studied in the presence of perchloric acid in DMF- H2O medium at 40 C. The kinetics of the reaction was followed spectrophotometrically at  max = 355 nm. The reaction is unit dependence on each of PDC, [H+] and [substrate]. Michaelis-Menten type kinetics was observed with different alpha hydroxy acids. The reactions were studied at different temperature [30, 35, 40, 45, 50 C] and activation parameters were computed.


I. INTRODUCTION
Chromium compounds have been widely used in aqueous and non-aqueous medium for the oxidation of a variety of organic compounds [1-3]. Chromium compounds especially Cr (VI) reagents have been proved to be versatile reagents capable of oxidizing almost all the oxidizable organic functional groups. The study of both very slow and very fast reactions can pose severe experimental problems. The new reagent Pyridiniumdichromate (PDC) prepared is soluble in water, it is economic, stable, non-hygroscopic and easy to prepare in good yield (97%). The structural formula is represented as shown in Scheme 1 It is more stable and easily stored as compared to other Cr(VI) reagents. A number of n e w C r ( V I ) re a g e n t s l i k e T r i p ro p y l a m m o n i u m f luorochromate [4], Pyr idiniumf luorochromate [5], Tetrabutylammoniumchlorochromate [6] and tetraethyl ammonium chlorochromate [7] have been used to study the kinetics and mechanism of various organic compounds. The ultimate goal of our study is to build our knowledge based on the oxidation behavior of citric acids with PDC. Kinetics of oxidation of methionine [8], oximes [9], unsaturated acids [ 1 0 ] , c y s t e i n e [ 1 1 ] , a n d a l c o h o l s [ 1 2 ] b y pyridiniumchlorochromate were reported. Our literature survey reveals that the oxidation of alpha hydroxy acids by different oxidants has received limited attention; we are particularly interested to see the mechanism of the oxidation of citric acid by PDC in acidic DMF-Water media.

II. Materials And Methods
Pyridiniumdichromate was prepared by the method of Corey [13], and its purity was checked by iodometrically and melting point. Citric acids (A.R.grade) were used as supplied and purity was checked by its melting point. All other chemicals used in this investigation were of analytical reagent grade. Double distilled water was used throughout in the investigation. The rate measurements were carried outat 40ºC in HClO under the condition [citric acid ] >> [PDC], in the 4 solvent system of 50 % (v/v) DMF-H O.The progress of the 2 reaction was followed by measuring the absorbance of PDC at 355 nm in one cm cell placed in the compartment of spectrophotometer. The kinetics run were followed for more than 60-70% completion of the reaction and first order kinetics were observed.

Stoichiometry and product analysis
To determine the stoichiometry of a reaction a known slight excess of oxidant was added to a known amount of citric acid, by keeping all other condition of reaction is constant and after some hours the residual of oxidant was determined spectrophotometrically at 355nm. The product of oxidation was Acetone and was identified by its 2, 4-dinitro phenyl hydrazine derivative. The stoichiometry of the reaction was found to correspond to the equation:

i c a c i d by i n d i c a t e s t h a t
Michaelis-Menten type kinetics is followed with respect to Citric acid but the value of intercept is very small indicates that intermediate complex may be highly reactive so concentration will be very small at any time.

Effect of Dielectric constant
The influence of variation of dielectric constant of the medium was carried out by the changing DMF(% v/v) in the reaction medium, keeping other conditions remaining constants and the reaction rate was affected considerably. The rate of reaction increased with an increasing volume percentage of DMF [ Table 1]. A lot of theories have been given to quantitative explanation [14,15] for the effect of dielectric constant (D) of the reaction medium on the kinetics of liquid phase reaction. For the limiting case of a zero angle of approach between two dipoles or ion-dipole system, Amis [16] had shown that in a linear plot of logk vs. 1/D. The obs positive slope indicates a positive ion-dipole reaction, while a negative slope indicates the involvement of two dipoles or a negative ion-dipole reaction. In the present investigation a plot of logk vs. 1/D shows a straight line with a positive obs slope, clearly supporting that there is involvement of positive ion-dipole in the rate determining.
temperature [ Table 2]. From the Arrhenius plot of logk obs versus 1/T, activation energy and other thermodynamic parameters were calculated [ Table 3]. The entropy of activation is negative as expected for bimolecular reaction. The negative value also suggests the formation of a cyclic intermediate from non-cyclic reactants in the rate determining step [17]. The complex formation is proved by the plot of 1/kobs against 1/ [Citric acid]. It has been pointed out [18] that if entropy of activation is negative and small the reaction will be slow.