Reaction mechanisms, permanganate

The neutralisation of acids with bases provides many valuable volumetric methods of chemical analysis and redox titrations are useful as well. But here we encounter an important difference between acid/base and redox reactions in solution. Acid/base reactions which involve the transfer of protons are very fast indeed they are usually instantaneous for all practical purposes. In protonic solvents, polar H-X bonds are very labile and undergo rapid proton exchange. For example, if B(OH)3 - a very weak acid - is recrystallised from D20, we obtain a fully-deuterated product. Redox reactions, on the other hand, are often very slow under ordinary conditions. To return to the analogy between acid/base and redox titrations, many readers will be familiar with the reaction between permanganate and oxalic acid the reaction is very slow at room temperature and, for titrimetric purposes, should be carried out at about 60 °C. The mechanism whereby a redox reaction takes place tends to be... 

Additional examples of the oxidative cleavage of double bonds by permanganate to produce aldehydes and ketones are summarized in Table 4. A detailed study of the reaction mechanism has also been reported. ... 

In this section, brief fundamental reaction mechanisms for each AOP are addressed. Included as AOPs are individual and combinational processes in the use of ultraviolet (UV) irradiation, catalyzed titanium dioxide oxidation, Fenton s reagent oxidation, ozonation, peroxone oxidation, and permanganate oxidation. 

Unsaturated halogenated compounds with a central C = C or C = C bond undergo cleavage on oxidation in the middle of the molecule and form two carboxylic acids without loss of a carbon atom. The reaction mechanism of the oxidation of internal fluoroalkenes by potassium permanganate includes hydroxylation of the C = C bond in 2 by permanganate, accompanied by dehydrofluorination of the geminal fluoroalkane-1,2-diol 3 and formation of an intermediate fluorocarbonyl compound 4 oxidative cleavage of the C-C bond in 4 leads to the final products 5.1... 

As we previously did for permanganate, we investigated the reaction mechanism for the different possible PES, wondering whether osmium tetroxide shows a similar behavior. We also calculated a possible addition of water as weU as the intermediate reoxidation [65]. 

It has been difficult to formulate detailed reaction mechanisms for these oxidations, since several metal oxidation states are undoubtedly involved during the course of the reaction. In the case of permanganate, it is considered likely that... 

Even though the mechanism of the reaction between oxalate and permanganate is extremely compHcated, titration under acidic conditions is extremely accurate. This is the recommended method for standardi2ation of permanganate solutions. 

Phase-transfer catalysed oxidation of sulphoxides to sulphones using copper(II) permanganate or a mixture of potassium permanganate and copper(II) sulphate is also possible156. In this case hexane is used as the solvent for the organic phase and the reaction is carried out under reflux for 24 hours. Sulphones are prepared by this method in quantitative yields and the mechanism proposed is given in equation (51). 

Stewart and Van der Linden also examined the incorporation of into the cyanate from labelled permanganate. The percentage of transfer varied with alkalinity and the authors believe that significant oxygen-transfer occurs in the second-order reaction, but not in the complex reaction. Accordingly the mechanism for the second-order reaction is proposed to be... 

A subsequent detailed analysis of the permanganate oxidation of the tertiary hydrogen atom of 4-phenylvaleric acid in 2.5 M potassium hydroxide solution supports the caged radical mechanism. The reaction order is two overall, A h/ d is ca. 11.5, ring substitution has little elfect on the rate (p 0) and the oxidation proceeds with a net 30-40 % retention of optical configuration. 

A few results have been reported on the oxidation of cyclohexanol by acidic permanganate In the absence of added fluoride ions the reaction is first-order in both alcohol and oxidant , the apparent first-order rate coefficient (for excess alcohol) at 25 °C following an acidity dependence k = 3.5-1-16.0 [H30 ]sec fcg/A , depends on acidity (3.2 in dilute acid, 2.4 in 1 M acid) and D2o/ H20 is f-74. Addition of fluoride permitted observation of the reaction for longer periods (before precipitation) and under these conditions methanol is attacked at about the same rates as di-isopropyl ether, although dioxan is oxidised over twenty times more slowly. The lack of specificity and the isotope effect indicates that a hydride-ion abstraction mechanism operates under these conditions. (The reactivity of di-isopropyl ether towards two-equivalent oxidants is illustrated by its reaction with Hg(II).) Similar results were obtained with buffered permanganate. 

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

TL5981>. The proposed mechanism involves the oxidation of the amine to an imine, tautomerization to an enamine, and a sequence of nucleophilic attacks on the pyridazine rings followed by oxidation steps. The oxidant of choice is (bispyridine)silver permanganate <1982TL1847>, which is easily prepared, mild in action, and is soluble in organic media. If R1 = H in the product 77, electrophilic substitution (e.g., bromination, nitration, Mannich, and Vilsmeier-Haack-Arnold reactions) occurs at this position. 

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