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A study of the mechanisms of permanganate oxidation of 2,2,2-Trifluoro-l-phenylethanol and cyanide ion Van der Linden, Ronald

Abstract

The mechanisms of permanganate oxidation of two very different substrates have been examined. Firstly, in an attempt to elucidate the mechanism of permanganate oxidation of alcohols, a number of meta and para substituted 2,2,2-trifluoro-1-phenylethanols have been prepared and the kinetics of permanganate oxidation studied over a pH range of 1 to 13.3. Secondly, the reaction between permanganate and cyanide ion has been examined over the pH range 3 to 14.6. The reaction between permanganate and 2,2,2-trifluoro-1-phenylethanol in aqueous alkaline solution was shown to give manganate and trifluoroacetophenone (hydrated form) in almost quantitative yield under kinetic conditions and is therefore represented by the equation, 2MnO⁻₄ + C₆H₅CHOHCF₃ + 20H⁻→ 2Mn0⁼₄ + C₆H₅COCF₃ + 2H₂0 The trifluorophenylethanols are more acidic than the normal hydrogen containing secondary alcohols and their pKas as determined spectrophotometrically showed the following order of decreasing acidity for substitution into the phenyl ring m-NO₂> m-Br > H > p-CH₃ >p-MeO. The rate of reduction of permanganate was followed iodometrically and the rate expression which fits the kinetic data for the alkaline catalyzed oxidation is the following: [formula omitted]. The rate of oxidation of the p-CH₃, m-Br and unsubstituted trifluorophenylethanols-l-d by permanganate was found in each case to be 16 times slower than the rate of oxidation of the corresponding protio compounds in alkaline solution. This latter result, the kinetics, i.e., the close similarity between the ionization type rate curves and the calculated ionization curves for the alcohols, the thermodynamics and positive salt effect indicate a mechanism which involves a primary ionization step to give the anion of the alcohol and then a rate controlling bimolecular step where permanganate abstracts a hydride ion from the anion. This mechanism is somewhat invalidated in view of the observation that the rate constants obtained for oxidation of the various substituted alcohols in alkaline solution are relatively insensitive to nuclear substitution. A plot of Hammett σ values versus the log of the rate constants appears to give a smooth curve. A number of mechanisms involving termolecular steps are considered to account for this latter observation and the large deuterium isotope effect is discussed in the light of present theories. Kinetic and oxygen¹⁸ tracer experiments have been performed in an attempt to elucidate the mechanism(s) of permanganate oxidation of cyanide. A mechanistic interpretation is attempted using the data obtained in basic solution above pH 12 to 14.6 where the oxidation is represented by the equation, 2MnO₄⁻ + CN⁻ + 20H⁻→ 2MnO₄⁼ + NCO⁻ + H₂0 From pH 12 to 6 the reaction was found to be complex and unstoichiometric yielding cyanate, carbon dioxide, cyanide ion and finally cyanogen at pH 9 to 6. The rate of reduction of permanganate as followed iodometrically and spectrophotometrically, is found to be markedly dependent on the pH of the medium and reactant concentration. The observation that the rate is negligible in acid solution but rapid in basic media suggests cyanide ion and not hydrocyanic acid molecule to be the primary reactive species. At pH greater than 12 two parallel processes are indicated which have been designated as Reaction A and Reaction B. Reaction A appears at low reactant concentrations < 0.0004 M cyanide and higher hydroxyl ion concentrations > pH 13. The rate of Reaction A is represented by the kinetic expression [formula omitted] where k₂ is independent of hydroxyl ion concentration and is insensitive to the effect of manganate and barium ions. A positive salt effect is observed and labeling experiments using permanganate enriched in oxygen¹⁸ showed that the oxygen introduced into the product cyanate comes mainly from the oxidant (70%-80% oxygen transferred). These observations suggest a mechanism which involves a rate determining bimolecular reaction between permanganate and cyanide ions to yield a Mn V species and cyanate ion. The possibility that a second parallel process Reaction B was occurring was indicated by the changing kinetics at higher reactant concentrations and lower basicities, by the non-linear Arrhenius plots and the observation that only 15%-25% oxygen¹⁸ transfer from permanganate to substrate had occurred at pH 13. The rate of this latter process can be tentatively represented by the kinetic expression [formula omitted]. A mode of oxidation is suggested which appears to fit these results. Permanganate, cyanide ion and a hydrocyanic acid molecule are reacted to produce a reactive species which undergoes further oxidation by permanganate to yield cyanogen. Cyanogen hydrolysis results in cyanate where oxygen is derived from the solvent.

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