Kinetic study of the ruthenium(VI)‐catalyzed oxidation of benzyl alcohol by alkaline hexacyanoferrate(III)

Abstract

The kinetics of the Ru(VI)-catalyzed oxidation of benzyl alcohol by hexacyanoferrate(III), in an alkaline medium, has been studied using a spectrophotometric technique. The initial rates method was used for the kinetic analysis. The reaction is first order in [Ru(VI)], while the order changes from one to zero for both hexacyanoferrate(III) and benzyl alcohol upon increasing their concentrations. The rate data suggest a reaction mechanism based on a catalytic cycle in which ruthenate oxidizes the substrate through formation of an intermediate complex. This complex decomposes in a reversible step to produce ruthenium(IV), which is reoxidized by hexacyanoferrate(III) in a slow step. The theoretical rate law obtained is in complete agreement with all the experimental observations.

Synthesis and Characterization of 3‐(Aryl azo)‐4‐hydroxy‐1,2‐naphthoquinone Ruthenium(III) Complexes as Catalysts for Benzyl Alcohol Oxidation

“…This catalytic cycle continues until the substrate was completely consumed. This was in agreement with the reported mechanism for the oxidation of benzyl alcohol to benzoic acid by trans ‐[Ru (VI) O 3 (OH) 2 ] 2− with alkaline K 3 [Fe(CN) 6 ] as co‐oxidant [49] …”

Alcohol oxidation catalysed by Ru(VI) in the presence of alkaline hexacyanoferrate(III)

“…As previously discussed 7,20,21,26 for Ru(VI), the dependence of the initial rate on RuO and the organic substrate: Step 7 involves a hydride transfer from the α-C-H bond of the alcohol to the oxoligand of ruthenium, a process that is favoured by the prior coordination of the organic substrate to the metal through the oxygen of the hydroxy group. 26 The occurrence of this hydride transfer is supported by the following experimental results: (a) a moderate kinetic isotopic effect, which indicates cleavage of a C-H bond and the absence of free radicals in the reaction mixture, (b) oxidation of cyclobutanol produces cyclobutanone as the sole product, and (c) the negative value of the Hammett reaction constant found for the oxidation of benzyl alcohol. 27 The following fast reaction would yield the corresponding ketone:…”

Oxidation of 2,3‐butanediol by alkaline hexacyanoferrate(III) using Ru(III) or Ru(VI) as catalyst

“…This process is favoured by the prior coordination of the organic substrate to the metal through the oxygen of the hydroxyl group and supported by the following experimental results: (a) a moderate kinetic isotope effect, which indicates cleavage of a C-H bond in the absence of free radicals in the reaction mixture, (b) the presence of cyclobutanone as the unique product in the oxidation of cyclobutanol and (c) the negative value of the Hammett reaction constant found for the oxidation of benzyl alcohol. 19 From the species involved in these decompositions, two new processes take place: the regeneration of the catalyst and the formation of final products.…”