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The oxidation of secondary and tertiary aromatic alcohols by chromium (VI) and manganese (VII) Banoo, Fariza

Abstract

The mechanism of permanganate oxidation of benzhydrol has been investigated between pH 7.00 and H₀ -2.46. The deuterium isotope effect, kH/kD, obtained by studying the oxidation rate of benzhydrol α-d, was found to fall from 7.3 at pH 7.00 to 1.08 at H₀ -1.22. Between pH 7.00 and H₀ 0.20, the reaction is of second order, first order in each of the reactants. Beyond H₀-0.50 the reaction becomes first order in carbinol and zero order in permanganate, and in this region the reaction is strongly acid-catalyzed. A study of eight substituted benzhydrols shows an excellent Hammett correlation with α⁺,ρ⁺ being -1.02. The rate-determining step at higher acidities is believed to be the scission of protonated carbinol to carbonium ion, which then reacts with permanganate in a fast step. This idea is supported by the results of a study of the rate of ionization of p-methoxybenzhydrol under conditions similar to those of the oxidation reaction. The mechanism of chromic acid oxidation of benzhydrol has been studied between H₀ 0.50 and -4.20. The reaction is acid-catalyzed in this region and was found to be of second order, first order in each of the reactants. The deuterium isotope effect with benzhydrol α-d at H₀ -1.00 is 6.81. A Hammett plot, obtained from the study of five substituted benzhydrols, shows σ to be the operative substituent constant, ρ being -0.54. The rate-determining step is believed to be the unimolecular decomposition of a chromate ester. The permanganate oxidation of triphenylcarbinol has been investigated in the region of acidity of H₀ -0.60 to -1.93. The reaction was found to be acid-catalyzed in this region and to produce benzophenone and phenol. The order of reaction was found to be the same as in the case of benzhydrol, i.e., first order in carbinol and zero order in permanganate. A study of nine substituted triphenylcarbinols gave a good Hammett correlation with σ⁺,ρ⁺+ being -1.39. As in the case of benzhydrol, the rate-determining step is believed to be the ionization of the carbinols. The chromic acid oxidation of triphenylcarbinol has been investigated between HR -2.80 and -7.48. Acid-catalysis was observed for the reaction in this region. The reaction was found to be of second order, first order in each of the reactants, as in the case of benzhydrol. The reaction gave a quantitative yield of benzophenone, phenol also being formed. A Hammett correlation with σ⁺+ was obtained from a study of eleven substituted triphenylcarbinols, ρ⁺+ was found to be -0.879. The rates of oxidation of the triarylcarbinols were also studied in the presence of added manganous ions, which showed an almost uniform reduction in rate for all the carbinols, the value of ρ⁺ in this case being -0.906. The migration aptitudes of the substituted aryl groups were determined and ρ⁺ for migration was found to be -1.44. A chromate ester mechanism, similar to that proposed for benzhydrol and other secondary alcohols, is believed to be operative in this case, except that the decomposition of the ester takes place by migration of the electron-rich ring. It is suggested that the chromic acid oxidation of primary and secondary alcohols may take place by an analogous rearrangement reaction of a chromate ester. In these cases the migrating group would be hydrogen. Permanganate oxidation of benzhydrol was also studied in frozen system between pH 1.50 and 12.40. A large increase in rate, compared to that in the liquid system, was observed. The deuterium isotope effect, kH/kD, of 7.2 to 7.5 was found in this system. The reactions are of second order, as in the liquid system. The acceleration in rate appears to be due to an increase in concentration of the reactants in the liquid phase between ice crystals.

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