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Mechanism of the chromic acid oxidation of alcohols. Lee, Donald Garry

Abstract

The relative rates of chromic acid oxidation of eight aryltrifluoromethylcarbinols were determined in 77.2% acetic acid medium. The primary deuterium isotope effects for these alcohols ranged from 7.40 for p-tolyltrifluoromethylcarbinol to 12.93 for 3,5-dinitrophenyltrifluoromethylcarbinol at 25°. The rates fitted a Hammett plot best when modified ϭ⁺ substituent values were used and a modified ̎rho ̎value of -1.01 was observed. The rates of oxidation of several halogenated-2-propanols as well as their corresponding 2-d analogttas were also determined in 77.2% acetic acid and in 50.1% sulfuric acid. Results indicated that while the isotope effect varied with changes in the nature of the alcohol being oxidized, it was constant for a particular alcohol in solutions of varying acidity. The thermodynamic parameters for the oxidation of two different alcohols under a variety of conditions were determined and found to be approximately the same, ΔH# = 9 kcal/mole, ΔS# = -40 cal/mole deg. An extensive investigation of the oxidation of alcohols in sulfuric acid solutions ranging from 5 to 95% was conducted and a spectroscopic study was made of the behavior of chromium VI in this entire region, A ϭ* value was obtained for the CF₃ group and this was used to determine the "true rate constant", k = kobs /K, for the chromic acid oxidation of a series of primary alcohols. A Taft ϭ*- e* plot then yielded a e* value of -0.92 for the rate determining step of this reaction. A spectroscopic investigation of the variation of the pKa of chromic acid with changes in the identity of the mineral acid solvents was made, and the results were applied to explain observed variations in the rate of oxidation of isopropyl alcohol in solutions of different mineral acids all having the same acidity. The H₀ acidity function for nitric and phosphoric acid systems with a 1:1 molar ratio of sodium perchlorate was also determined.

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