GUESTAUTHOR Carl H. Snyder University of Miami Coral Gables, Florida
Textbook Errors,
64
Permtlnganate Hydroxylation of Alkenes
Organic chemistry textbooks' frequently cite neutral potassium permanganate as an effective reagent for conversion of alkenes to cis-glycols. This is contrary to considerable evidence in the chemical literature that alkaline permanganate is usually a distinctly superior reagent for this oxidation and that neutral permanganate usually produces oxidation to ketols or cleavage to aldehydes. Historically, permanganate oxidation of a carboncarbon double bond appears to have been reported first by Groger (1) and Saytzeff ( 2 ) . Groger observed that oleic acid is oxidized by alkaline permanganate, while Saytzeff independently reported the product as a dihydroxystearic acid. Both workers used alkaline media presumably to effect solution of the acid. Subsequent workers used alkaline media until 1921, when Straus and Rohrbacher (3) used permanganate in the presence of magnesium sulfate to precipitate magnesium hydroxide and insure essentially neutral conditions for the oxidation of 1,2-dihydronaphthalene. Wiberg and Saegebarth (4) credit these workers with having been "the first to use a neutral solution for the hydroxylation reaction . . . presumably to avoid the . . . strongly base catalyzed [permanganate oxidation of solveut ethanol]. Subsequent workers have for the most part continued to use this procedure with either ethanol or acetone as the solvent." This use of neutral conditions may have set the pattern for permanganate hydroxylatious in textbooks, despite continued use of alkaline conditions in oxidations of unsaturated fatty acids (5), and appearance of a report (6) of "systematic experiments" which indicated that "the conditions essential for the nearly perfect conversion of oleic acid into dihydroxystearic acid [include presence of] a slight excess of alkali . . . from the beginning of the oxidation, otherwise there is a danger of hydroxyketostearic acid being formed." A more recent indictment of the term "neutral" is found in a report by Coleman, Ricciuti, and Swern (7) that, in the absence of a buffer system, the bulk of the oxidation effected by "neutral" permanganate actually occurs in an alkaline medium. In an investigation of the pH dependence of permanganate oxidation of oleic acid to the mixed ketol acids, these authors noted a rapid increase in the pH of an aqueous, "neutral" soluSuggestions of material suitable for this column and guest columns suitable for publication directly should be sent with as many details as possible, and particularly with references to modern textbooks, to W. H. Eberhardt, School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332. Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of errors discussed will not be cited. The error must occur in at least two independent standard hooks.
tion of potassiun~oleate from 10 to almost 12 on addition of only 2,5'% of the stoicbiometric quantity of potassium permanganate. Ketol acids appeared to be produced, in about 30y0 yield, primarily in the early stages of the reaction; continued reaction a t the higher pH gave 60% of 9,10-dihydroxystearic acid. With the pH controlled a t 9.0 to 9.5 throughout the reaction, 65575% of the ketol acids were obtained. Conversely, if alkali were added before addition of permanganate, 96% of the dihydroxystearic acid was produced. Other examples of this pH dependence in aqueous solution are cited by Wiherg and Saegebarth (4). These authors also investigated the effect of pH on oxidations in mixed solvents and found that the yield of glycol from cyclopentene increased from 34% in a neutral magnesium sulfate-ethanol-water system (8) to 55% at pH 13 in a tert-butyl alcohol-water system. Further, bicyclo[2.2.1]-2-heptenegave a small amount of glycol and 54-66% of cyclopentane-l,3-dicarhoxaldehydeby cleavage in a neutral magnesium sulfate-acetone-water system, hut 40% of the exo-cis-glycol in an alkaline terthutyl alcohol-water system. Although conversions of cyclohexene and cycloheptene in mixed solvents (4) and conversions of elaidic acid in an aqueous system (7) seem (curiously) rather insensitive to pH, Wiberg and Saegebarth conclude that "the oxidation in basic solution is effected more easily and gives a purer product than does the other procedure." Not only should "neutral permanganate" be revised to "alkaline permanganate" in textbook discussions of the preparation of glycols from alkenes, hut truly neutral permanganate might well he recognized as an effective reagent for oxidation of alkenes to ketols or aldehydes. Finally, although a mechanistic discussion of these permanganate oxidations does not seem pertinent to the restricted exposure of a textbook error, several cogent mechanistic comments appearing in the literature are noted (4,9, 10). Literature Cited
GR~GER,M., Ber., 18, 1268 (1885). SAYTZEFF, A,, J. Pmkt. Chem., 31, 541 (1885). STRAWS, F., AND ROHRBACAER, A,, B ~ T .54,40, , 69 (1921). WIBERG, K. B., AND SAEGEBARTH, K. A,, J . Am. Chem. Sac., 79, 2822 (1957).
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Ha~woon,H. J., Chem. Revs., 62.99, 117 (1962). LAP WORT^. A,. AND MOTTRAM. E. N.. J. Chem. SOC.,127, 1628 (19'25).'
(7) COLEMAN, J. E., RICCIUTI, C., AND SWERN, D., J. Am. Chem. Soe., 78, 5342 (1956).
(8) OWEN,L. N., AND SMITH, P. N., J. Chem. Soc., 4026 (1952). (9) LADBURY, J. W., AND CULLIS,C. F., Chern. Revs., 58, 403 (1958).
(10) GOULD,E. S., "Mechanism and Structure in Organic
Chemistry," Halt, Rinehart and Winston, New York, 1959, p. 538. Volume 43, Number
3, March 1966
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