Miklós Jáky

Oxidations with soluble manganese(IV) phosphate

Manganese(IV) phosphate was prepared in 3 M phosphoric acid. Ultra-centrifugal and electron microscopic studies did not indicate colloid properties. Manganese(IV) phosphate oxidizes mainly oxygen-containing bifunctional substrates in an autocatalytic way. Its reactivity was found to be higher in every case than that of manganese(III) phosphate.

Formation of manganate(V) in oxidations by permanganate ion in strongly alkaline solutions

Abstract The oxidation of tartronate and mesoxalate ion by permanganate has been studied in 0.1–2.0 M aqueous NaOH. Manganate(VI) ion is formed in the first rapid phase, which is kinetically first order with respect to both MnO 4 − and the substrate. The second-order rate constant depends on the OH − ion concentration, which implies higher reactivity of the species with deprotonated 2-hydroxy groups. The proposed mechanism consists of outer sphere electron transfer to the MnO 4 − ion, generating an anion-radical and manganate(VI). The latter reacts with both substrates in the second, slower phase, affording manganate(V) as a relatively stable product in strong alkali. The anion-radical is oxidized by MnO 4 − in a rapid step.

Mechanism of the permanganate oxidation of unsaturated compounds. Part III. Intermediates in the oxidation of maleic and fumaric acids

The intermediates in the permanganate oxidation of maleic and fumaric acids have been studied in acidic solutions. The accumulation and decay of manganese(III) has been demonstrated by the stopped-flow technique. The concomitant four-electron oxidation of the substrates leads to the formation of formyl(hydroxy)acetic acid. The subsequent reactions reveal a complex pattern in which hydroxymalonic, glyoxylic, and oxalic acid are further intermediates. The product distribution has been determined as a function of the pH and the mole ratio of the reactants. A reaction scheme is suggested which rationalises the observed behaviour.

Mechanism of the permanganate oxidation of unsaturated compounds. Part VI. Kinetic investigation of the oxidation of methylmaleic acid, methylfumaric acid, and dimethylmaleic acid

The kinetics of the permanganate oxidation of methylmaleic (MMA), methylfumaric (MFA), and dimethylmaleic (DMA) acids have been studied by the stopped-flow technique in aqueous solution between pH 0.0 and 5.0. Manganese(III) has been detected as a short-lived intermediate in all cases. The reactions are first order with respect to both MnO4– and the substrate. The second-order rate constant depends strongly on the pH. The rate of interconversions of the acid, the mono-, and the di-anion is much higher than the rate of oxidation. The rate constants are, in the above order (at 25°C, in dm3 mol–1 s–1) : 150, 660, and 310 (MMA); 1 240, 370, and 98 (MFA); 240, 90, and 54 (DMA). Low activation enthalpies and high negative activation entropies are observed in all cases. The rate-determining step is the cis-attack of permanganate on the double bond, resulting in the formation of a short-lived, undetectable cyclic hypomanganate (MnO43–) ester. The relative reactivities reflect the influence of steric factors.

Mechanism of the permanganate oxidation of unsaturated compounds. Part IV. Kinetic investigation of the oxidation of maleic and fumaric acid

The kinetics of the permanganate oxidation of maleic and fumaric acids to formyl(hydroxy)acetic acid have been studied by the stopped-flow technique in aqueous solution between pH 0·5 and 5·5. The reactions are first order with respect to both MnO4– and the substrates. The second-order rate constant depends strongly on the pH. The rate of interconversion of the acid, the mono-, and the di-anion is much higher than the rate of oxidation. The resolved rate constants are, in the above order (at 25°C): 1080, 1880, and 1000 dm3 mol–1s–1(maleic acid and 15,000, 3000, and 1180 dm3 mol–1 s–1(fumaric acid). Low activation enthalpies and high negative activation entropies are observed for both substrates. The rate-determining step is the cis-attack of permanganate on the double bond, resulting in the formation of a short-lived cyclic intermediate which contains manganese(V). The 14-fold difference between the reactivities of fumaric and maleic acids points to steric hindrance in the latter against forcing the cis-carboxy-groups towards each other.

Mechanism of the permanganate oxidation of unsaturated compounds. Part 8. Kinetics of the oxidation of halogenomaleic acids

A study of the short-lived intermediates of the permanganate oxidation of halogenomaleic acids has revealed that an initial 4-electron process occurs, followed by reactions between manganese(III) and the organic intermediates. According to stopped-flow kinetic measurements, the reaction is of the first order with respect to both reactants. The pH dependence of the second-order rate constant is constant with the reactivity order: monoanion > acid > dianion. The reactivity trendt reflected by the individual rate constants indicate the predominance of steric over electronic effects.

Mechanism of the permanganate oxidation of unsaturated compounds. Part V. Intermediates and kinetics of the oxidation of substituted propynes

The permanganate oxidation of propargyl alcohol (PA), but-2-yne-1,4-diol (BD), and propargyl chloride (PC) and bromide (PB) has been studied in aqueous perchloric acid. The stoicheiometry for PA and BD depends on the substrate: MnO4– mole ratio, three limiting cases being observed. Manganese(III) has been detected as a shortlived intermediate by the stopped-flow technique. The organic substrates yield αβ-dioxo-intermediates, which are oxidised by manganese(III) to the corresponding acids. In the presence of pyrophosphate, the dioxo-compounds are stable products. Stopped-flow kinetic measurements under conditions eliminating interference by the Guyard reaction show that the reactions are first order with respect to both MnO4– and the substrates, and independent of pH between 0·98 and 4·9. The second-order rate constants at 25°C are: 10·0 (PA), 11·5 (PC and PB), and 12·5 (BD) dm3 mol–1 s–1, with the activation parameters ΔH‡= 26·4 (PA) and 22·0 (BD) kJ mol–1; ΔS‡=–138 (PA) and –150 (BD) J mol–1 K–1. The rate-determining step is assumed to be concerted attack of MnO4– on the triple bond, resulting in the formation of a short-lived cyclic intermediate containing manganese(V).

Kinetics and mechanism of the oxidation of ketones with permanganate ions

The kinetics and mechanism of the oxidation of ketones with permanganate ions were studied in aqueous acidic and alkaline media for acetone, hydroxyacetone, butan-2-one and butane-2,3-dione. Acid catalysed nucleophilic addition of the permanganate to the carbonyl C-atom is suggested. In alkaline media, parallel with electron abstraction from the enolate, a concerted mechanism is proposed. Intermediates and end products were determined. Comparisons between halogenation and oxidation have been made.

Oxidations of α,β-unsaturated alcohols with soluble manganese(IV) phosphate

Abstract Manganese(IV) phosphate was dissolved in 3 M phosphoric acid and a detailed study was made of the oxidation of α,β-unsaturated alcohols (allyl alcohol, 2-butene-1,4-diol, propargyl alcohol and 2-butyne-1,4-diol) where the double bond remained intact and, via aldehydes, the corresponding α,β-unsaturated carboxylic acids were formed. Contrary to numerous bifunctional substrates, simple, non-autocatalytic kinetics were found. The first order related to the oxidant points to the fact that manganese(IV) does not exist in 3 M phosphoric acid as a colloid. A rate constant of 970 M−1 s−1 was found for the reaction between MnIV and Mn2+.

Mechanism of the permanganate oxidation of unsaturated compounds. Part 7. Kinetics of the oxidation of propiolic and phenylpropiolic acids

The permanganate oxidation of propiolic acid (PA) and phenylpropiolic acid (PPA) has been studied in aqueous perchloric acid. Short-lived intermediates of the reactions are manganese(III) and the corresponding dioxo-compounds. According to stopped-flow kinetic measurements, the reactions are first order with respect to both MnO4– and the acids, and the second-order rate constant is pH dependent, owing to the different reactivities of the acids and anions. The resolved rate constants at 25 °C are (dm3 mol–1 s–1) 183 and 43.2 for PA and its anion, and 38.8 and 11.6 for PPA and its anion, respectively. The relative reactivities of several acetylenic species are discussed in terms of a near-concerted nucleophilic attack of MnO4– on the triple bond.