5462
be that the oxidation takes place in a coordination complex in which the lower reactivity of the ether is more likely due to steric reasons than to the requirement of a readily replaceable hydroxyl hydrogen atom. A mechanism which is consistent with all the observations discussed above could then contain a rapid reversible formation of a complex and its rate-limiting decomposition (eq 32). The radical then can undergo
further oxidation or react with a second radical. It is possible that the existence of the radical in the form of a chromium(II1) complex adds to its earlier discussed stability. The value of the reaction constant p* is also in agreement with the proposed mechanism; hydrogen abstraction reactions generally e ~ h i b i t p~ ~values ! ~ ~ in the range from -0.5 to - 1.5. (42) (a) C. Walling and B. B. Jacknow, J . Amer. Chem. Soc., 82,6113
It should be emphasized that this mechanism is proposed only for alcohols not undergoing cleavage. For example, in cyclobutanol carbon-carbon rather than carbon-hydrogen bond cleavage is observed, and the rate-limiting step of the chromium(1V) oxidation probably involves step 33. The high reactivity of cyclo-
butanol (it reacts 21 times faster than isopropyl alcohol with chromium(lV), but -3 times faster4 with chromium(V1)) is in accord with the assumption of a different mechanism.6,’ Acknowledgment. We wish to thank Mr. Chiu Ng for measuring the effect of acidity and Mr. David Aylward for his help in the preparation of cyclobutanol. The use of computer time through a University of Illinois Research Board grant is also acknowledged. (1960); G. A. Russel and R. C. Williamson, Jr., ibid., 86, 2357 (1964); (c) E . S. Huyser, ibid., 82, 394 (1960); (d) R. D. Gilliom and B. F. Ward, Jr., ibid. ,87, 3944 (1965). (43) H. G. Kuivila and W. J. Becker, 111, ibid., 74, 5329 (1952).
Mechanism of Chromic Acid Oxidation of Isopropyl Alcohol. Evidence for Oxidation by Chromium (IV) Matiur Rahman and Jan RoCek* Contribution from the Department of Chemistry, University of Illinois at Chicago Circle, Chicago, Illinois 60680. Received December 9, 1970 Abstract: The presence of acrylonitrile or acrylamide in the oxidation of isopropyl alcohol by chromic acid leads to the formation of polymers and a considerable reduction in the yield of acetone. The observations are consistent with a mechanism involving a rapid one-electron oxidation of the alcohol by chromium(1V). The mechanism often preferred involving the reaction CrvI CrIV + 2CrV is shown to be inconsistent both with the findings of this work and with electrochemical data which lead for this reaction to an estimated equilibrium constant of 4 X 10-14. The results thus support conclusions drawn earlier from the investigation of chromium(1V) generated from
+
chromium(V1) and vanadium(1V). lthough the mechanism of chromic acid oxidation of alcohols has been the subject of numerous studies during the past 25 years, only the rate-limiting step (which is responsible for only 33% of the total products) is well understood. It is generally agreed that it involves a direct two-electron oxidation to a carbonyl compound with the concomitant formation of a chromium(1V) species2 (reactions 1 and 2). There R2CHOH + H 2 C r 0 4+ H + e R2CHOCr03H2+ H20 (1)
A
R2CHOCr03Ha++R E O
+ CrIv
+
(2)
is much less agreement on the next step of the reaction. (1) This investigation was supported by the Petroleum Research Fund (Grant No. 1529-4A), administered by the American Chemical Society, and by the National Science Foundation (Grant No. 9441). (2) (a) F. H . Westheimer, Chem. Reu., 45, 419 (1949); (b) K. B. Wiberg in “Oxidation in Organic Chemistry,” Part A, IC. B. Wiberg, Ed., Academic Press, New York, N. Y.,1965, pp 159-170; (c) W. Watanabe and F. H. Westheimer,.J. Chem. Phys., 17, 61 (1949); (d) I
