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IJC-A Vol.49A(04) [April 2010] >

Title: Kinetics and mechanism of acid bromate oxidation of substituted 4-oxo acids
Authors: Reddy, Cherkupally Sanjeeva
Manjari, Padma Sunitha
Keywords: Kinetics
Reaction mechanisms
Oxo acids
Issue Date: Apr-2010
Publisher: CSIR
IPC CodeInt. Cl.9 C07B33/00
Abstract: Kinetics and mechanism of oxidation of substituted 4-oxo-4-arylbutanoic acids (4-oxo acids) by acid bromate (uncontaminated with Br2) in aqueous acetic acid medium has been studied. The reaction exhibits first order each in [bromate] and [4-oxo acid] and second order in [acid]. Variation in ionic strength has no effect on the reaction rate, while the reaction rates are enhanced on lowering the dielectric constant of the reaction medium. Changing the solvent from H2O to D2O increases the rate of oxidation (kD2O/kH2O ~1.8). Electron releasing substituents in the aromatic ring accelerate the reaction rate and electron withdrawing substituents retard it. The order of reactivity among the studied 4-oxo acids is: p-methoxy>> p-methyl>-H> p-chloro> m-nitro. The Hammett’s plot of log k versus σ is a smooth curve and concaves downward. However, the plot of log k against exalted sigma (Brown’s σ+) values is found to be linear with a slope of –1.50 at 313 K. The reaction constant (ρ+) decreases with increase in temperature. The mechanism proposed involves the attack of the protonated bromate (H2Br+O3) on the enol form of the 4-oxo acid giving the carbocationic bromate ester in the slow step, which undergoes cyclization and then on carbon-carbon bond cleavage yields the products in the fast step. From the intersection of lines in the Hammett and Arrhenius plots, the isokinetic relationship is discussed.
Page(s): 418-424
Source:IJC-A Vol.49A(04) [April 2010]

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