Organic Lecture Demonstrations Ernest F. Silversmith Morgan State University, Baltimore. MD 21239 General chemistry lends itself beautifully to lecture demonstrations. Most of the "Tested Demonstrations" that avpear in this Journal are best suited for general chemistry, and two volumes of an excellent compilation have appeared (i).Most reactions studied in organic chemistry are relatively slow and unspectacular (dramatic color changes, precipitations, etc., are rare) and therefore less suitable for demonstration purposes. Nevertheless, I have found some that work well, help to put points across and generate interest. With this article I hope to (1) provide a convenient source of demonstrations for teachers of oreanic chemistrv and (2) urge others to add to the collection.- he demoustrkions are erouoed accordine to the tooics that are treated in most modern organic tLxtbooks. The Opening Lecture Household Items T o generate interest and to show that organic compounds are part of our lives, one can bring food boxes (e.g., from margarine, gelatin dessert, cereal, or soup mix), read off the ingredients, and point out which ones are organic. Familiar Odors Compounds with familiar (pleasant) odors are placed in stoooered vials. and students are invited to take a whiff and t r i t b identify the odor. The structure of the compound is written on the side of the vial; the identity of the odor is written on the bottom. Specific examples are: 2-phenylprapanal (hyacinth(2)) 2-phenylethanol (roses (2)) 1-octen-3-01(mushrooms (3)) 5%acetic acid (vinegar) octyl acetate (oranges) (R)-cawone(caraway) LS-carvone (soearmint) geraniol (geraniums) menthol (e.g., mentholated shaving cream) vanillin (vanilla) pentyl acetate (bananas) Nylon Rope Trick Morgan and Kwolek's procedure (4) for this sure-fire interest-sparker works best in our hands. Bieber has added some useful hints (5). Molecular Structure and Its Relatlonshlp to Properties Molecular Geometry Molecular models are used to show the shapes of simple molecules. A plastic tetrahedron helps students to understand why methane is described as "tetrahedral". Two rhombi with 60" and 120' corners are cut from stiff, clear plastic (e.g., the kind used with overhead projectors). The sides of the rhombi should be 1.05 times as long as the distance between hydrogen atoms in the methane model. The rhombi are folded in half and brought together to form a tetrahedron. The two parts are taped together along two 70
Journal of Chemical Education
adjacent edges; this leaves one side of the tetrahedron untaped so that the model can be inserted. Solubility Kelter and Crouse's "Dissolving Polystyrene Cup" (6) is an entertaining demonstration of "like dissolves like". We use toluene instead of methylene chloride as the solvent, as it is easier for less sophisticated students to realize that a hydrocarbon has very low polarity, making it "like" polystyrene. Acids and Bases The gas-phase reaction between HCI and NH3 is a dramatic examole of the Bronsted-Lowrv concevt of acids and bases. A 5 0 - m ~beaker containing mL or concentrated hydrochloric acid is vlaced into a large vacuum desiccator fnrm which the porcelain plate has been removed. The cover, with the collar rotated to the "closed" position, is put on. Cunrentrated ammonia solution (5 n1LJ is placed into a 23mL Erlenmeyer flask. and a one-hole stopper containing a short elass tuhe is attached. The flask is ulaccd next to the desiccitor, and the two are connected with'as short a piece of rubber tubing as possible. For the demonstration, the collar is rotated to "open" and the flask is immersed in a warm (-50') water bath. A thick cloud of NH&l fills the desiccator. After 10 min the lid can be removed (Caution: fumes) and the solid product can be scraped from the walls. Halogenation of Hydrocarbons Doheny and L~udon'sexperiment (7)on light-initiated brominatiun of alkylbenzenes demonstrates that r 1) energy must he intruduced to start the reaction, (2) ease offormation of free radicals follows the sequence tertiary > secondary > primary, and (3) cunsiderable HBr is liberated. Nucleophlilc Substitution Mechanisms A three-legged metal music stand provides an excellent simulation of the SN2 and S N mechanisms. ~ The top of the stand is cut off with a hacksaw; the tube that remains should be the same length as the legs. Two rods (made of wood, metal, or plastic) that can slide through the tube are cut so that they are 7 cm longer than the tube. Styrofoam spheres [representing atoms orgroups) are attachedto the three legs and two rods. The spheres on the ruds should differ in color; one is the "nucleophile" and the other the "leaving group". The apparatus is clamped firmly, using a three-pronged clam^ near the mid~ointof the tuhe. with the tube oointinr to the right as viewed by the audience The legs are adjustei so that they and the tubeare in the tetrahedralarrangement. The rod with the "leaving group" is inserted into the tube from the audience's right. T o simulate the SNZmechanism, the tuhe with the "nucleophile" is inserted from the audience's left; the "leaving group" is ejected automatically. At the same time, the demonstrator uses his other hand to move the legs toward the audience's right. T o simulate S N ~the , "leaving group" rod is pulled out while the legs are moved until they are coplanar. Then, the
"nucleophile" rodis inserted while the legs are moved to give tetrahedral geometry. I t is made clear that the second step can occur from either side with equal probability.
the reaction occurs rapidly in the dark, and no (or only a trace (7) of) fuming occurs.
S d Reactivities Relative reactivities in SNZreactions can be demonstrated by scaling up the sodium iodide-acetone test (10.A solution uf30 g of.iodium iodide in 200 mL of acetone is prepared and stored in a ioil-covered 250-mL Erlenmeyer flask. For the into a second demonstration, half of the solution is flask. Simultaneously, 10.0 mL (0.134 mol) of bromoethane is added to one flask and 12.5mL (0.134 mol) of 2-hromopropane to the other. Both flasks are shaken. A large amount of sodium bromide precipitates in the first flask in 15-30 s, while only a trace forms in the second flask in 30 min.
The chemiluminescence of tetrakis(dimethy1amino)ethylene (10) prohahly involves the addition of oxygen to a carbon-carbon double bond. Also, it is eye-catching and has the virtue of illustrating several other concepts. The dioxetane intermediate that may he involved (10-13) has high enerev ..- content because of its 0-0 bond and anele strain. Thus, one of the tetramethylurea molecules is formed in an electronically excited state, which transfers the energy to a molecule of tetrakis(dimethylamino)ethylene. The latter emits light as it drops to the ground state (1.1).
S N Reactivities ~ Relative rates in an S Nreaction ~ can he demonstrated by the method of Danen and Blecha (9).A solution of 140mL of acetone, 60 mL of water, 0.2 mL of 1M NaOH, and 1mL of Universal Indicator is stored in a 250-mL Erlenmeyer flask. (Other indicators that change color at pH between 5 and 9 can he used.) For the demonstration, half of the solution is poured into another flask. Equimolar amounts of 2-bromo2-methylpropane (1.0 mL) and 2-hromopropane (0.8 mL) are added to t,he flasks, and both are shaken. The tertiary bromide gives a color change in -20 s, while the secondary one takes about a day. In the same way, one can show that chlorides are less reactive than bromides; 2-chloro-2-methylpropane (0.9 mL) produces a color change in -12 min. T o show the high reactivity of henzylic halides, compare henzyl chloride (1.0 mL, 5 s) with 1-chlorohutane (0.9 mL, 3 days).
Schimelpfenig has worked out an effective demonstration of the Diels-Alder reaction (15).
Rate-Determining Step
The concept of a two-step mechanism in which the first step is rate-determining can be simulated with a separatory funnel containing water. (A dye may be added for better visibility.) The stopcock is opened slightly so that the liquid drips out. Clearly, the slow step, which determines the overall rate, is the travel through the narrow passage in the stopcock. The second step (falling into the beaker) is much faster. Furthermore, one can control the overall rate by controlling the slow step (by rotating the stopcock). Alkenes and Dienes Addition of Bromine
The addition of hromine to cyclohexane is demonstrated in the same way as substitution by bromine (7). This time,
Chemiiuminescence
Dienes: the Diels-Alder Reaction
Stereochemistry Chirality
A dove makes i t ~ossihleto convince students that a hand is indeedchiral. A left glove is put on the right hand,md the demonstrator frowns to dramatize the discomfi,rt. The glove is transferred to the right hand, and the expression is changed to a smile. Enantiomerism
Two models of methane help to introduce the concept of chiral molecules. The two models are placed together to show that they are superposahle. One hydrogen atom in each is replaced by, say, a green atom; the models are shown to he still superposahle. A second hydrogen in each is replaced by aredatom; the models are again shown to he superposahle. A third hydrogen in each is replaced by a blue atom, being sure to select hydrogens that will result in enantiomers. The resulting models are shown to he nonsuperposahle mirror images. Vibration of Light Waves
The concept that a light wave vibrates in a certain plane can he shown by bending very stiff wire (e.g., from a metal coat hanger) into the shape of a wave. The end view of this model is a straight line; thus the wave vibrates in a plane. The model is then rotated to show that other waves rotate in other planes. I t is pointed out that a polarizer shunts out all hut one of these planes. Optical Activity
A,
Ph H
yellow
I
P~'~'H yellow
Figure 1. Apparatus used to demonstrate optical activity. A, flashlight: B. stand; C, polarizing film, taped to jar; D, glass jar; E. wooden stick: F, polarizing film.
A polarimeter for demonstration purposes can easily he made from a rectaneular or sauare container such as a hattery jar. A aster's-choice" coffee jar is a satisfactory suhstitute. Two circles of polarizing film are cut; their diameters should be equal to the width of the jar. One circle is taped to the outside of the iar. near the base. The other circle is held outside the opposiie face of the jar and rotated until a minimum of light gets through the polarizers. A flat wooden stick such as a tongue depressor is glued to the edge of the untaped circle so that the stick points vertically upward. A flashlight mounted on a stand fashioned from a cardboard box is a convenient light source. The apparatus is shown in Fieure 1. For the demonstration, the jar is placed on the table with the t a ~ e circle d toward the demonstrator. The lieht source is set behind the jar. The other circle is held in front of the jar with both hands and rotated. The audience is asked to say "now" when a minimum of light gets through; the stick should be vertical at this point. Water is added to the jar till it reaches the top of the circle. The untaped circle is again rotated. The stick will again be vertical when little light gets Volume 65
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throueh. showine that water is ontically inactive. The water is repiaied by a ;strong sucrose solution (500 g i n 700 mL of water works well). and the nrocedure is repeated. The stick will end up to thd right of vertical, showing that sucrose is dextrorotatory. (With sucrose, the light minimum is accompanied by a color change.) Spectroscopy Relationship between Wavelength and Color
We use a chart containing pictures of waves in the colors of the visible spectrum, drawn to scale. For example, the distance between crests of the red wave is twice as great as that in the violet wave. How Our Eyes Function as a Crude Spectrophotometer
Two objects with similar shapes but different colors (ex., a carrot and a parsnip) convince students that they are already experts in determining what wavelengths of light are absorbed, and using this information for identification. When they see the carrot, their eyes and brain tell them i t is orange and therefore ahsorhs blue light. The parsnip is yellow and therefore ahsorhs violet light. Thus, they can determine which is which. Energy Levels in NMR
The principles of energy absorption in NMR spectroscopy can be clarified by using a horseshoe magnet (to simulate the magnet in the spectrometer) and a magnetic stirring bar (to simulate the magnetic nucleus). The bar should he slightly shorter than the distance between poles of the horseshoe magnet. The north poles of both magnets are marked with colored tape or crayon. A string, tied around the center of the har magnet, is used to suspend the bar from a clamp attached to a ring stand. For the demonstration, the horseshoe magnet is brought up to the suspended bar so that the bar is between the poles. The bar will line itself up "with the field" of the horseshoe; that is, the north pole of the bar will he near the south pole of the horseshoe. Clearly, this is the more stahle (lower energy) alignment. The bar is then rotated by hand so that its north pole faces the north pole of the horseshoe magnet. I t is held in this position while the demonstrator explains that the addition of a small amount of energy has made it possihle to flip the small magnet around so that it is lined up against the external field. This is the less stable alignment; to prove it, the demonstrator lets go of the bar, which spontaneously rotates back to the original alignment ("relaxes", in NMR terminology). Precession of Nuclei in NMR
A gyroscope is caused to spin and is placed on the lecture table with the axis tilted about 20° from the vertical. The gyroscope axis, which simulates the axis of a spinning nucleus, precesses a t a steady frequency. Alcohols and Phenols
75-mL portions are poured into 125-mL Erlenmeyer flasks. An alcohol (0.067 mol, a 25-fold excess) is added to each, and the mixtures are shaken. Ethanol (3.9 mL), I-propanol (5.0 mL), and 2-propanol (5.1 mL) cause immediate color changes; within 1-3 min the mixtures become greenish blue. 2-Methyl-2-butanol(6.3 mL) produces no color change in 24 h. Aldehydes and Ketones Bisulflte Addition ( 18)
A solution of 55 g of sodium bisulfite in 200 mL of water is prepared in a 500-mL Erlenmeyer flask. For the demonstration, 20 mL of cyclohexanone is added, and the mixture is shaken vigorously. Solid appears in about 30 s; after several minutes there is so much that the flask can he inverted and no liquid runs out. Aldol Condensation
A demonstration developed by King and Ostrum (19) is verv effective. The solution of acetone. benzaldehvde. and KOH in aqueous ethanol is clear for about one minite; then, suddenlv " (as . in a clock reaction). . . i t becomes cloudv as the dibenzalacetone (1,5-diphenyl-1,4-peutadien-3-one) comes out of solution as an oil. Scratching with a glass rod causes the oil to crystallize. (Caution: The product is irritating to the skin.) Silver Mirror
A demonstration of Tollen's "silver mirror" test has been described (20). E-Z lsomerization
E-Z isomerism is oossible for hvdrazones and other deviationsof aldehydesand unsymmeirical ketones that contain carbon-nitroren double bonds. The ohotochromism of triphenylformaian (Fig. 2) involves E-z isomerization at carbon-nitrogen and a t nitrogen-nitrogen double bonds 1711) ~-.,.
A stock solution is prepared by dissolving LO mg of rriphenylformazan (Eastman 9033) in 40 mL of toluene. One volume is diluted with 24 volumes of toluene for the demonstration. A square glass bottle (-4 X 4 cm) is filled lo a height of about 4 cm with the diluted solution. (A duplicate can he used to comoare colors hefore and after irradiation.) The bottle is irraiiated with strong light; direct sunlight is best, but a slide projector works well also. The color changes from red to yellow in about 15 s; in the ahsence of strong light, the color reverts to red in about 4 min. The yellow-to-red change occurs instantly if adrop of acetic acid is added (presumably because the thermal isomerization is acid-catalvzed (2111, but then the cycle can not be repeated. In the ahsence of acid, the cycle can be repeated many times. Carboxylic Acids Conversion of an acid to its salt and reconversion to the acid can be demonstrated by magnetically stirring 1.0 g of
Acidity of Phenols
The acidity of phenols can be demonstrated hy interconverting indophenol (red, ether-soluble) and its sodium salt (blue, water-soluble) in a separatory funnel (16).This colorful demonstration also reinforces the "like dissolves like" concept. Oxidation of Alcohols
Kolb has developed a demonstration of the dichromate oxidation of alcohols on an overhead projector (17). The demonstration can also he done without a projector. A stock solution of 60 mL of concentrated sulfuric acid and 2.0 g of potassium dichromate in 500 mL of water is prepared, and 72
Journal of Chemical Education
Figure 2. The photochromism of triphenylformazan.
finely divided p-chlorobenzoic acid with 150 mL of water in a 250-mL Erlenmeyer flask. An acid-base indicator may be added to dramatize pH changes. Addition of 1 mL of 6 M NaOH causes the solid to dissolve, and addition of 1mL of 6 M HC1 causes i t to reappear.
boiling-water bath. Within one minute, the blue color is replaced by a red solid in the tube to which glucose was added; the other solution remains unchanged for at least 20 min.
Amines
Astock solution is prepared from 10 g of phenylbydrazine hvdrochloride. 15 e ot'sodium acetate and 100 m L of water. 1; separate 2dO-mi test tubes, 30 mL of stock solution is mixed with 1.2 g of glucose, fructose, or sucrose. The tubes are immersed in a boiling-water bath. Yellow solid forms in the glucose solution within 4 min and in the fructose solution within 2 min. Sucrose gives no solid in a t least 30 min.
Salt Formation
The demonstration described under "Carboxylic Acids" is carried out on p-chloroaniline. In this case, the HCI is added first. lsomerization of Azo Compounds
The photochromism of triphenylformazan described above can be used here also, since i t involves E-Z isomerization around an azo group. Carbohydrates
Methylation
The hydroxyl groups of di- and polysaccharides are often "tagged" by converting them to methoxyl groups. The methylated compound is hydrolyzed, and the positions of untagged hydroxyls in the products reveals just which carbons were originally involved in the linkages (between monosaccharide units) that were "cut". This concept can be simulated with string, scissors, and brightly colored tape. A piece of string about 60 cm long is cut into two equalpieces, and these are balled upand shaken in cupped hands. The strings are stretched out on the lecture table; no one can tell which ends resulted from the cut. A second 60-cm piece is "tagged" by attaching tape to both ends. The cutting and shaking are repeated. This time it is obvious that the "tagged" ends (i.e., methoxyl groups) were not involved in the "cut" (i.e., hydrolysis).
Osazone Formation 123 . .
Protelns
Chirpich has described a way to "rubberize" a bone by soakine it in hvdrochloric acid (24). The soaking removes the substaLces, such as calcium phosphate, that makes the bone rigid. The rubberized bone consists largely of proteins, showing that there is considerable protein in bone. We find that turkey drumstick bones work well, and we bring an untreated one for comparison purposes. Literature CHed ~
~~
1. Shakhashiri, B. Chomieol Domomtmtione. Univenify of Wismnsin: Madison. WI, 1983 and 1985:Vola. Iand 2. 2. ~ i k h ~S.u The , pHiltsr (Ameri- Chemical Soeicty Student Alfiiiata Newaletterl L985,17(3), Id. 3. Wmd, W.F.;Fealcr, M..I Chem. Edue. 1986,63.92. 4. Morgao,P. W.;Kwolek.S.L.J. Chem.Edue. 1959.36.182184. 5. B1eher.T. I.J. Chem.Edu. 1919.56. W 1 0 .
.
.
10. Gill. S. K. J. Chem.Edue. 1984.61.713. 11. Wiben. N.: Buehler. J. W. Z Ndurfomeh. 1361.19b.M.
Benedict's Test
The published procedure (22) can be scaled up for use as a lecture demonstration. A mixture of 15 mL of Benedict's solution and 15 mL of water is placed into each of two 200mm test tubes. Glucose (0.6 g) is added to one tube and sucrose (0.6 g) to the other. The tubes are immersed in a
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