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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
Series/Report no.: Int. Cl.<sup>9</sup> C07B33/00
Abstract: Kinetics and mechanism of oxidation of substituted 4-oxo-4-arylbutanoic acids (4-oxo acids) by acid bromate (uncontaminated with Br<sub>2</sub>) 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 H<sub>2</sub>O to D<sub>2</sub>O increases the rate of oxidation (<i style="">k</i><sub>D2O</sub>/<i style="">k</i><sub>H2O</sub> ~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: <i style="">p</i>-methoxy>> <i style="">p</i>-methyl>-H> <i style="">p</i>-chloro> <i style="">m</i>-nitro. The Hammett’s plot of log <i style="">k</i> versus <i style="">σ</i> is a smooth curve and concaves downward. However, the plot of log <i style="">k</i> against exalted sigma (Brown’s <i style="">σ</i><i style=""><sup>+</sup></i>) values is found to be linear with a slope of –1.50 at 313 K. The reaction constant (ρ<sup>+</sup>) decreases with increase in temperature. The mechanism proposed involves the attack of the protonated bromate (H<sub>2</sub>Br<sup>+</sup>O<sub>3</sub>) 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.
Description: 418-424
Appears in Collections:IJC-A Vol.49A(04) [April 2010]

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