Content area
Full Text
ABSTRACT:
The Kinetics and mechanism of oxidation of some α hydroxy acids by pyridinium dichromate (PDC) has been studied by spectrophotometric method in 50% acetic acid - 50% water (v/v) medium at 303K. Under the conditions of pseudo first order, the reaction follows first order kinetics with respect to hydroxy acid, [H+] and [PDC]. The reaction is catalysed by perchloric acid. There is no salt effect. The rate increases with increase in the percentage of acetic acid and the plot of log Kobs Versus 1/D is linear with a positive slope indicating the ion-dipole nature of the reaction. Activation parameters have been evaluated. Based on the experimental results, a suitable mechanism has been proposed.
KEYWORDS: Pyridinium dichromate, hydroxy acids, kinetics, oxidation, perchloric acid.
(ProQuest: ... denotes formulae omitted.)
INTRODUCTION:
Chromium (VI) reagents are widely used in organic chemistry for the oxidation of primary and secondary alcohols to carbonyl compounds. A variety of compounds containing chromium (VI) has proved to be versatile reagents capable of oxidising almost every oxidisable functional group[1]. A number of new chromium (VI) containing compounds, with heterocyclic bases like pyridinium chlorochromate (PCC)[2], pyridinium bromochromate (PBC)[3-5], quinolinium chlorochromate (QCC)[6-7], quinolinium fluorochromate (QFC)[8], quinolinium bromochromate (QBC)[9], imidazolium fluorochromate (IFC) [10], pyridinium fluorochromate (PFC)[11-14], imidazolium dichromate (IDC)[15], quinolinium dichromate (QDC) [16] and pyridinium dichromate (PDC) [17], have been developed to improve the selectivity of oxidation of organic compounds. The kinetics and mechanism of oxidation of α hydroxy acids by various oxidants have been reported[18-24]. However, the kinetics of oxidation of α hydroxy acids by PDC, a Cr(VI) reagents has not yet been studied. This prompted us to undertake the present investigation. The present work reports the kinetics of oxidation of α hydroxy acids by PDC in aqueous acetic acid medium and evaluates the rate constants. Mechanistic aspects are also discussed.
EXPERIMENTAL:
Materials:
Pyridine, pyridinium dichromate and chromium (VI) oxide (CrO3) were obtained from Aldrich. The hydroxy acids namely glycollic acid (GA), lactic acid (LA) and mandelic acid (MA) were commercial products of highest purity available and were used as such. Acetic acid was purified by standard method and fraction distilling at 118°C was collected. The solution of perchloric acid is prepared by diluting known volume of acid in water and standardised by sodium...