ediled by GEORGE L. GILBERT Denison University Granville. Ohio 43023
Purple Benzene Revisited Submitted by:
Checked by:
Anthony J. Doheny, Jr.' Bruce Ganem Cornell University I t h a c a , NY 14853 Wayne L. Felty Pennsylvania S t a t e University Wilkes-Barre Campus Wilkes-Barre, PA
In a recent Tested Demonstration, A. W. Herriot2 described the preparation of purple ben2ene.b~phase transfer catalysis" with quaternary ammonium salts and potassium permanganate. A variant of this reaction using crown ethers4 has been used successfully a t Cornell for some years.
It is somewhat simpler in execution, and the explanation of the effect can he made less sophisticated with n o loss of scientific rigor since only one counterion and one liquid phase are involved. Demonstration T o a dry 500-ml Erlenmeyer flask containing about 350 ml dry benzene, on a lighted magnetic stirrer and about 120 mg 1 Present address: The Kendall Company, Lexington, MA 07172 Herriot, A. W., J. CHEM. EDUC., 54,229 (1977). (a) McIntosh, J. M., J. CHEM. EDUC., 55,235 (1978).(b) Gakel, G. W. and Weber, W. P., J. CHEM. EDUC., 55,350 (1978). (a) Knipe, A. C., J. CHEM. EDUC., 53,619 (1976). (b) Gokel, G. W. and DU&. (1976). Durst, H. D.. D., Aldrichimiea Acto, 9,3 (1976) 5 Prolonged stirring (>1 min) will give a pinkish tinge to the henzene. This can be avoided by drying the benzene over PzOs and filtering through fresh desiccant inta aflame-dried flask, but it is simpler to avoid overstirring. 6 Aldrich Chemical Company, Inc., Milwaukee, Wisconsin. Catalog number 18,665-1. The dicyclohexyl- and dibenzo-analogsalso work (catalog numbers 15,840-2and 15,839-9),butthe latter does not work as well. The first two are quite hygroscopic and are easier to handle if kept scrupulously dry. 7 The saturated crown ethers are readilyabsorbed through the skin and should be handled with poly-gloves. Their toxicity presumably owes much to their disruotion of these orocesses.For safetv. the flask
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ether complex can be destroyed by reducing the MnOd- to MnOn, filtering and washing the brown precipitate with benzene. After concentrating the benzene filtrate and washings to one-quarter volume,the crown ether can be extracted into three portions of 1 M aqueous HI. Decomnasition of the macrocvcle is eamolete after 72 hiand the aqueous waste can be discarded: Sam, D. J., and Simmons, H. E., J. Amer. Chem. Soc., 94,4024 (1972). Christensen, J. J., Eatough, D. J., and Izatt, R. M., Chem. Rev, 74,351 (1974). 308 / Journal of Chemical Education
KMn04. Stir for -30 see and note no color ~ h a n g eAdd . ~ about 200 m e 18-crown-6.6 ICAUTION: Use plastic e l o v e ~.. Rapid ~I . dissol;tion of K M ~ turns O ~ the benzene bright purple and the color is stable for about a day if air is excluded from solution. About 5 rnl of cyclohexenk can be added. a t this point producing brown Mn02 over 5-10 min. According to Sam and Simmonss this is an excellent oxidizing system, since reactions are quantitative and stoichiometric and the MnOl by-product, in the absence of water, is easily removed and washed. Discu~sion The K+ ion is solubilized as a 1:l complex with the hexadentate cyclic polvether. The MnOd- must also dissolve to maintain klectrin&trality. Besides the phase transfer effect, this reaction illustrates principles appropriate a t many different levels of chemicafinstr&tion~ltemphasizes the complex forming ability of unshared electron pairs on oxygen and the solvating powers of ether groups. Furthermore, it points out, by analogy, the importance of cationic hydration spheres in aqueous solution and the metabolic transport of ions across biological membranes. Students with some background in thermodvnamics mav. . ~ a i insight n into the microscopic origins < d r n t r o p yhy considering why the equilibrium umatnnt (or the reacriun is sirnif~cnntlyrrmter than ime,.'even thuugh the water-cation bond seems :omparable in strength with the ether-cation bond.
Corridor Demonstration Name the Compound Contest E d w a r d Kouhek U. S . Naval Academy Annapolis, MD 21402 Shown is a photo of a "hallway demonstration" which may be of interest to others. Periodically I construct a model of a compound t h a t is in the news, or one with interesting structural properties (Kepone, shown in the photo is an example of both) and display it outside my office in a contest. The idea of a contest usuallv eenerates the students' interest and some will stop and take; recond look. Some will then take the time t o eo t o the librarv and usuallv, .. via a formula index. manaae l o correcrl?. identit). the I armlnnlnd. Prizt:s of rourie can vary. bur I ha\,e found thut the ronte-t provides mt. with en ~.xcellent mechanism for the disposal ;f sample textbooks left by book salesmen.
