Alkaline hexacyanoferrate(III) oxidation of substituted 4‐oxo acids: a mechanistic study

Abstract

The kinetics of the oxidation of substituted 4‐oxo‐4‐arylbutanoic acids by hexacyanoferrate(III) in aqueous alkaline medium were studied. The reactions are found to be first order in each of the reagents, the oxo acid, hydroxide ion and the hexacyanoferrate(III) ion. A mechanism involving the formation of enolate anion from the oxo compound and subsequent rate‐determining electron transfer is proposed. Application of the Olson–Simonson rule substantiated the participation of negatively charged ions in the rate‐determining step. The experimental rate constant is found to be composite, being the product of the equilibrium constant for the enolate anion formation and the rate constant of the oxidation step (Kk2). The effect of addition of solvents such as methanol and DMSO is discussed and a quantitative relationship is derived between the ionic strength and the composite rate constant. The low activation energies and even the negative activation energy in the oxidation of 4‐(3‐nitrophey1)‐4‐oxobutanoic acid are explained. The ρ values are positive and decrease with temperature. Contrary to the expected electronic effect, the methyl group in the ortho position accelerates the reaction owing to steric factors. From the intersection of the lines in the Hammett and Arrhenius plots, the isokinetic relationship is discussed. Copyright © 2005 John Wiley & Sons, Ltd.

Aquachlororuthenium(III) catalysis in the oxidation of substituted 4-oxo-4-arylbutanoic acids by bromate in acid medium: a kinetic and mechanistic study and validity of linear free-energy relationships

“…The Os(VI) is reoxidized to Os(VIII) by [Fe(CN) 6 ] 3-in a fast step [32]. On the other hand, in the alkaline hexacyanoferrate(III) oxidation of 4-oxo acids, a mechanism involving the formation of enolate anion from the oxo compound and subsequent rate-determining electron transfer by the oxidant has been proposed [33]. In the present investigation, the experimental results are entirely different from the earlier reports and a different mechanism wherein neighboring group participation in the intramolecular catalysis is proposed.…”

Kinetic Study of the Hexacyanoferrate (III) Oxidation of Dihydroxyfumaric Acid in Acid Media

“…In alkaline media, a large number of oxidations have been carried out, the mechanisms being dependent on the particular substrate and the catalyst used. [14][15][16][17][18][19][20][21][22][23][24] Although the oxidizing ability enhances in acidic solvents, only few contributions have been published in such media. 14 In acidic micelar media (sodium dioctylsulfosuccinate) the oxidation of cysteine is first-order in oxidant and reductant.…”

“…Although a relatively poor oxidant, hexacyanoferrate (III) (also known as ferricyanide) is a selective outer-sphere reactant applicable to the most easily oxidizable substrates, and it is frequently used as an interceptor of free radicals; this feature turns this species into an efficient one-electron oxidant particularly interesting in the comparative study of octahedral complexes. In alkaline media, a large number of oxidations have been carried out, the mechanisms being dependent on the particular substrate and the catalyst used. – Although the oxidizing ability enhances in acidic solvents, only few contributions have been published in such media . In acidic micelar media (sodium dioctylsulfosuccinate) the oxidation of cysteine is first-order in oxidant and reductant …”

A kinetic and mechanistic study of the osmium(VIII)-catalysed oxidation of crotyl alcohol by hexacyanoferrate(III) in aqueous Alkaline medium

“…The use of hexacyanoferrate(III) in acidic medium results in certain complications due to complexation of the oxidant by one of its reduced products, hexacyanoferrate(II). However, reactions in alkaline medium [1][2][3][4][5][6][7][8] are not fast probably due to the reduced oxidation potential of the [Fe(CN) 6 ] 3− /[Fe(CN) 6 ] 4− redox couple. 9 It is for this reason that a large number of transition metal ions such as Os(VIII), [10][11][12][13] Ru(III), [14][15][16][17] Ru(VI), 18 Ru(IV), 19 Rh(III), 20 Ir(III), 21 Ru(VIII) 22 and Pd(II) 23,24 have been employed as catalysts in alkaline medium.…”