edled b y
GEORGE L. GILBERT Denison University Oranvilie, Ohio 43023
A Dramatic and Relevant Demonstrationof Ring Strain Submitted by: Checked by:
T. Ross Kelly' Boston College Chestnut Hill, Massachusetts 02167 Allan R. Burkett Division of Natural Sciences Dillard University New Orleans, Louisiana 70122
The ring strain inherent in eyelopropanes and cyclohutanes and the exceotional reactivitv which results are fundamental concepts of organic ~ h e m i & ~ . Wwish e to report a demonstration of these ~rincioleswhich is at once both dramatic and relevant. It is generally observed that reaction of olefins with iodine is sluggish a t best.3 For example, addition of l-methylcyclohexene (I) to crystalline iodine does not produce any visually observable reaction. In spectacular contrast, addition of the cyclohutane a-pinene (11) to crystalline iodine results in an exceptionally exothermic reactibn attended by the evolution of a large cloud of purple smoke (12 vapor). The exothermicity is presumably due to the relief of strain associated with the rearrangement of the initial cyclobutylcarhinyl carbonium ion (Ill) to the less strained norhornyl cation (IV).'
The reaction of (11) descrihed above appears to he the principle which underlies a rather primitive hut nonetheless impressive method of veterinary antisepsis recounted hy the English veterinarian dames Herriot in his recent hestseller ''All Creatures Great and Small." T h u s external wounds of large animals were packed with crystalline iodine and then doused with turpentine (turpentine contains 60% a-pinene (11) and 30% 8-pinene (V))=.The resulting eruption not only forced the vaoorized iodine into the inaccessible extremes of the wound, h i t also left the farmer marveling a t the wonders of veterinarv science. Reactions of the oatients remain unrecorded. For purposes of demonstration the following procedure is ~
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'Recipient of NIH Research Career Development Award, 1975Rn~
ZCf.Morrison, R. T.. and Boyd, R. N., "Organic Chemistry," 3rd Ed., Allyn and Bacon, he., Boston, 1973, pp. 28a294. 3Ref.(2), p. 186. &Forrelated rearrangements of o-pinene see, inter alio, Butler, G. B., and Berlin, K. D., "Fundamentals of Organic Chemistry," The Ronald Press, New Yark, 1972, pp. 90a913. 5J.Herriot, "All Creatures Great and Small," Bantam Books, New York, 1972, pp. 27-28. "'The Merck Index," 8th Ed., Merck & Co., Rahway, N.J., 1968, p. 835. 220 1 Journal of Chemical Education
suggested. (Caution: The reaction is very exothermic. Adequate ventilation is required and prior testing is strongly recommended. The demonstrator should wear safety glasses. The reaction flask should he mounted on a ring stand or equivalent, as i t attains an internal temperature of >130°C and becomes too hot t o hold.) Place 30 g crystalline iodine in a 500-ml round-bottomed flask. Add 15 ml a-pinene or turpentine rapidly, in one portion. A stream of purple smoke immediately gushes from the flask. Prior demonstration of the nonreactivity of another olefin ((I), cyclohexene, etc.) heightens the contrast.
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Selective Solubility: "Like Dissolves Like" Submitted by: Checked by:
Wayne L. Smith Colby College Waterville, Maine Leonard C. Grotz University of Wisconsin, Waukesha Waukesha, 53816
When attempting t o demonstrate solubility principles or illustrate the "like dissolves like" concept in connection with storage of DDT in fatty tissues, the usual demonstration is to show that iodine dissolves in carbon tetrachloride or chloroform hut not in water. This demonstration works well hut suffers from several disadvantages: (1) Although iodine is indisputably covalent i t is not an organic compound, and its formula has few similarities to that of DDT. (2) The density of iodine (4.93gml-I) is sufficiently high that iodine platelets settle to the bottom when added to water and are not easily ohsewed from a distance. (3) Except a t very low concentrations, solutions of iodine in carbon tetrachloride appear black from a distance and are thus not obviously solutions. Azobenzene is clearly an oreanic comoound with a formula that has features in rotnmon'kith DD.I'.The density of this dark orange solid ( 1 . 2 0 ~ml-') issufficientlv close to that of water so that the solid issuspended in the liquid on swirling. Azohenzene readily dissolves in carhon tetrachloride or chloroform, which are more dense than water, and in ether or hexane, which are less dense than water. In all cases, a clear orange solution results. The most visually effective procedure is t o add ambenzene to a tube containing water and swirl to show the lack of solubility. Addition of the organic solvent and mixinr! oroduces a colorless water layer a i d bright orange organic-layer. Depending on the organic solvent chosen, the oranae lnyer can be on top, on the bottom, or in both places (if firstcarbon tetrachloride and then hexane are used). Materials and Equipment Large (25 X 200-mm)test tubes, azobenzene,earban tetrachloride, hexane, water.
Procedure Fill a test tube approximately one-third full with water. Add a few crystals (0.1-0.2 g) of azobenzene and swirl to demonstrate the lack of solubility. Add approximately one-half as much carbon tetrachloride as water and swirl to obtain a bright orange carbon tetrachloride layer. Additional azobenzene may be floated on top of the water and dissolved in heaane to produce an orange-clear-orange arrangement. This demonstration may be used to show that organic compounds dissolve in nonpolar solvents ("like dissolves like"). I t is especially useful when discussing the concentration of DDT in the fatty tissues of man and other animals. Azobenzene is obviously an organic compound, and its formula has some similarities to that of DDT. The demonstration is also suitable for leading into a discussion of soaps or detergents. Chloroform may be substituted for carbon tetrachloride; diethyl ether may he substituted for hexane.
Purple Benzene: Solubilization of Anlons in Organic Solvents
Prepare a solution of a large hydrophobic quaternary onium ion using about 0.5 g of salt in 3 ml of water. Aliquat 336 (a solution of tricaprylmetbylammonium chloride manufactured by General Mills, Kankakee, Illinois), is an ideal extracting agent. Tetrabutylammonium bromide, tetrahutylphosphonium chloride, and tetraphenylarsonium chloride also work well; less organic onium ions give very incomplete extraction. Demonstration Shake the se~sratowfunnel and note that the o m l e color remains entirely in thr i o w r layer. Thrn add a few drops oi the quaternary ammonium ion solution and shnke the aeparatory funnel again. After the Inyersseparate,therolor wrll healrnusr exclusivel?.in the upper layer. The demonstration can be extended by adding a few drops of cyclohexene. Brief shaking of the separatory funnel converts the purple into the brown of manganese dioxide. Remarks The initial preference of a typical salt like potassium permanganate for solution in water rather than the nonpolar solvent benzene is apparent. The quaternary ammonium salt is soluhle in water, but because of the "greasy" hydrocarbon oerinherv. it is more soluble in benzene. For electroneutralitv. the quaternm ammonium cation must be accompanied in t& organic phaseby an anion. The permanganate ion is larger and less dependent on aqueous solvation than chloride or bromide ion and so is ~referentiallvextracted into the organic phase. The sodium chloride serves to decrease the solLbilitybf the quaternary salt in the aqueous phase and also to enhance the separation of the layers. This tecbniaue is now widely used in organic synthesis to solubilize anionic nucleophiles and hases i& orianic phase. This techniaue, called phase transfer catalysis, can readily he incorporated in this demonstration by showing the rapid reaction of vermannanate with an alkene. The stability of benzene to p&nang&ate compared to cyclohexene may also be noted. ~~~
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Submitted by: Checked by:
A r t h u r W. Herriott Florida International University Miami, Florida 33199 Lisa Gilbert Granuille High School Granville, Ohio 43023
Preparation
Dissolve a few crystals (30 mg) of potassium permanganate in 75 ml of 10%sodium chloride solution to obtain a brightly colored solution. Transfer the solution to a 250-ml separatory funnel mounted on a portable ring stand and add 75 ml of benzene.
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Volume 54, Number 4, April 1977 1 228
