Despite the emphasis that a number of modem organic chemistry textbooks place on the effect of substituents on reaction rates there is a general scarcity of simple laboratory experiments that can be used to illustrate this phenomenon. In an attempt to

The mechanism of the chromic acid oxidation of 2-propanol in trifluoroacetic acid solutions has been studied. The general features of this reaction strongly resemble those observed in aqueous solutions of other strong acids. A "normal" primary kinetic isotope effect is observed, similar thermodynamic parameters are obtained, and the rate behaves in similar way towards changes in acidity.An extensive study of the effect of added salts on this reaction has substantiated that the identity of added anions has a profound effect on the rate of reaction and that any complete reaction mechanism must take this into consideration.I t has been established that chromic acid is a very effective reagent for the oxidation of alcohols and that under similar conditions of acidity and temperature the rate of oxidation is inuch faster in acetic acid solutions than in aqueous solutions (I). T h e kinetics and mechanism of this reaction have been investigated quite thoroughly and it appears that the enhanced rate in acetic acid solutions is due in part to incorporation of an acetate ligand into the chromic acid ester, which acts as an intermediate in this reaction (1,2). I t seemed probable to us that the details of the mechanism of this reaction could be further exposed by undertaking a kinetic study of the chromic acid oxidation of 2-propanol in trifluoroacetic acid solutions. The following paper reports the results of this investigation. EXPERIMENTAL Kinetic nlethodsAll kinetic measurements were made spectrophotometrically using a Unicam model S P 500 spectrophotometer with a thermostated cell compartment. Solutions were made up by adding a weighed amount of anhydrous trifluoroacetic acid (Halocarbon Products) to a dilute aqueous solution of IC2Cr2O7 (Fisher Reagent). This solution was then brought to 25.0 f 0.2" in a thermostated bath and a n excesspf 2-propano1 was added volumetrically either from a pipet or a microsyringe fitted with a Chaney adaptor. The optical density of these solutions a t 3 500 A was then determined a t intervals and a plot of log [CrVI] against time was prepared. A typical rate plot is presented in Fig. 1.