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Mechanism of permanganate oxidation of fluoral and related compounds

University of British Columbia

The mechanism of permanganate oxidation of fluoral hydrate and its analogue, substituted with deuterium at position-1, has been extensively investigated throughout the pH region 0.7 to 14 and a successful study was made of the behaviour of permanganate in the sulfuric acid region from pH 0.7 to 46.3% H₂SO₄. Compared with 2,2,2-trifluoroethanol and other substituted ethanols which ionise in the pH region, fluoral hydrate is unique since it has two acidic hydrogens, both of which can ionise. The pK[subscript a] for the first ionisation process has been determined to be 10.1. From the kinetic data as well as other supporting evidence, at least four different reaction paths can be distinguished for the permanganate oxidation of fluoral hydrate: (i) Reaction of the di-anion of fluoral hydrate, (ii) Reaction of the mono-anion, (iii) Reaction of the unionised fluoral hydrate with permanganate anion, and (iv) Tentatively proposed reaction of the unionised fluoral hydrate with permanganic acid. The deuterium isotope effects range from k[subscript H]/k[subscript D] = 5 for the oxidation of the di-anion, k[subscript H]/k[subscript D] = 10 for the mono-anion reaction, k[subscript H]/k[subscript D] = 4 for the neutral molecule reaction and k[subscript H]/k[subscript D] = 6 for the permanganic acid oxidation of unionised fluoral hydrate in strong acid. Activation parameters and a positive salt effect show that the transition state formation involves a reaction of similarly charged species; proton tunneling is considered unlikely on the basis of a temperature study. The rate determining step is most consistent with a hydride ion transfer to the permanganate ion. In connexion with the above study, 2,2,2-trifluoroethanol, 1,1,1;3,3,3-hexafluoropropanol-2 and the corresponding 2-d analogue have been examined in somewhat lesser detail. An isotope effect k[subscript H]/k[subscript D] = 20 has been observed for the oxidation of 1,1,1;3,3,3-hexafluoropropanol-2. The pK[subscript a] of permanganic acid has been determined in sulfuric acid by a spectrophotometry method, using two different methods of calculation; pK[subscript a] = -5.1. Formic acid has been oxidised by permanganate in the region 25 to 72 % H₂SO₄ and the reaction in the region from 25 to about 40 % H₂SO₄ is interpreted as the oxidation of the unionised formic acid by permanganic acid. An isotope effect of about 7 is observed in the very strongly acidic region (72 % H₂SO₄), which decreases upon going to 25 % H₂SO₄. Certain 4,4'-disubstituted benzhydrols were oxidised by permanganate in 0.1 M sodium hydroxide, however solubility difficulties prevented any extensive studies. [ ... ]

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2011-11-16

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http://hdl.handle.net/2429/39066

Chemistry

University of British Columbia

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