Kinetics and mechanism of the oxidative cleavage of ketones by quinolinium dichromate

The kinetics of the oxidation of ketones (cyclic and arylalkyl) by quinolinium dichromate (QDC), in an acid medium, has been investigated. The rate of the reaction was proportional to the concentrations of the ketone, QDC, and acid. The order of reactivity for the cyclic ketones was C₆ > C₈ > C₅ > C₇ (ring size) and acetophenone > propiophenone > butyrophenone > valerophenone (arylalkyl ketones). A kinetic isotope effect, k _H/k _D = 5.80, was observed (cyclohexanone and cyclohexanone-d₄). The rate data have been rationalized using conformational considerations. The mechanism involved the attack of the protonated QDC on the enol form (of the ketone), in the rate-determining step, forming a cyclic chromate ester. Subsequent cleavage of the carbon-carbon bond of the ester yielded carboxylic acids as the products. Published in Russian in Kinetika i Kataliz, 2007, Vol. 48, No. 3, pp. 403–411 This article was submitted by the authors in English.