Free-radical-mediated oxidation of ascorbic acid by peroxomonosulphate

The kinetics of the oxidation of ascorbic acid (AH₂) by peroxomonosulphate (PMS) were determined in aqueous medium in acidic (pH 4.4), neutral (pH 7.0) and alkaline (pH 9.0) conditions over the temperature range 13-28°C. The reactions were found to obey total second-order kinetics, first-order each with respect to peroxomonosulphate and ascorbic acid concentration, obeying-d[AH₂]/dt = k ₂[PMS][AH₂]. Dehydroascorbic acid was detected as the product of the reaction. The stoichiometry of the reaction, [peroxomonosulphate]/[ascorbicacid] = 1 : 1, indicated the absence of self-decomposition of peroxomonosulphate. The influence of neutral salt (NaClO₄) was found to increase the reaction rate. Evidence for the formation of radical intermediates was obtained. A mechanism involving the formation of hydroxyl, sulphate and ascorbate free radicals as intermediates is proposed. The rate and activation parameters were evaluated to substantiate the mechanism proposed. A comparison is made with the corresponding reactions of the similar peroxides, S₂O₈ ^2- and P₂O₈ ^4-.     

A mechanistic study on oxidation of benzylic alcohols with PPh4HSO5 catalysed by manganese(III) porphyrins in homogeneous solution

The oxidation of variously ring-substituted 1-phenylethanols with Ph₄PHSO₅ catalysed by Mn(TMP)Cl and Mn(TDCPP)Cl in the presence of 4-tert-butylpyridine was studied in 1,2-dichloroethane homogeneous solution. The process leads only to C–H bond cleavage products, namely acetophenones. The oxidation rates are independent of the substrate concentration and, when Mn(TMP)Cl is the catalyst, even of the substrate nature. By increasing the concentration of 4-tert-butylpyridine, which acts as an axial ligand of the catalyst, a bell-shaped curve for the rate constants trend is observed. Hammett plots obtained by changing the substituents on the phenyl ring of the benzylic alcohol give different ρ values depending on the technique employed for rate constants determination, i.e., individual or competitive experiment. The observations reported above, together with a KIE of 2.5 in 1- -1-phenylethanol oxidation measured by competitive experiment, are rationalised on the basis of a mechanistic scheme in which the oxo-manganese derivative is formed in the rate determining step of the catalytic process. Furthermore, it is suggested that alcohol dehydrogenation proceeds through a hydride abstraction involving an alcohol-oxo-porphyrinato complex.     

Chemical Modification of A Vinyl Ester Resin by Acid Anhydrides and Characterization of the Products by l3C NMR

A vinyl ester resin was modified into half ester-acids by reaction with phthalic, maleic, and succinic anhydrides, using pyridine as catalyst. GPC analysis indicated that the reaction proceeds without crosslinking or polymerization. ¹³C-NMR analysis showed that the β-isomer of the vinyl ester resin containing a primary hydroxyl group reacts much more rapidly than the α-isomer with a secondary hydroxyl group.     

Kinetics and mechanism of oxidation of l-ascorbic acid by peroxomonosulphate in acid perchlorate medium. Role of copper (II) as a trace metal-ion catalyst

The kinetics of oxidation of l-ascorbic acid by peroxomonosulphate (PMS) in presence and absence of Cu(II) catalyst has been studied. The stoichiometry of the reaction is represented by the following:

H₂A + HSO₅⁻ → A + HSO₄⁻ + H₂O     

Mechanism of the oxidation of alkyl aryl and diphenyl sulphoxides by peroxomonosulphate

Detailed kinetic investigations of the oxidation of methyl phenyl sulphoxide and diphenyl sulphoxide by peroxomonosulphate in aqueous acetic acid medium reveal that the reactions are first-order, both in the sulphoxide and in the oxidant. Studies with substituted phenyl methyl sulphoxides and 4,4′-disubstituted diphenyl sulphoxides show that electron-releasing groups accelerate the rate of oxidation and electron-withdrawing groups retard it. A fair correlation between log k₂ and Hammett substituent constants has been observed in the two series. The mechanism proposed involves the rate-determining nucleophilic attack of the sulphoxide sulphur at the outer terminal peroxo oxygen atom of HSO ₅ ⁻ .