Experimental Illustration of Chemical
Paul Habefield
Brooklyn College of CUNY Brooklyn, New York 11210
Principles in Organic Chemistry Lectures
In order to distinguish the natural sciences from some of the more arbitrary productions of the human imagination, it has been traditional to subject their principles to experimental tests in the teaching laboratory or in the lecture hall whenever possible. Such demonstrations, in addition to establishing the validity of the principles taught, can also be quite entertaining and can help the student understand and remember the subject matter. Unfortunately, unlike general chemistry, where a fine tradition of lecture demonstrations exists, no such tradition is to be found in organic chemistry. I could not find a set of organic lecture demonstrations either in published form or by questioning older colleagues. This is a pity, since many of the principles of organic chemistry lend themselves to immediate experimental demonstration a t least as well as do the principles taught in general chemistry. To help anyone who wishes to add such demonstrations to his organic chemistry lectures, I list below some tested (2 terms) demonstrations which I have been using during my lectures for the second semester of our oneyear organic chemistry course.' (These demonstrations should in no way be regarded as a substitute for the experiments performed by students in the laboratory part of an organic chemistry course.) Demonstrations Activating and Deactivating Groups in Electrophilic Aromatic Substitution
To 10 drops of benzene in a 10-in. test tuhe add about 15 ml of cano. HzSOe Shake. Add 10 dropsof a40% fo~maldehydesolution and shake again. A rusty red precipitate is obtained. Repeat, using in place of benzene some ethyl benzoate (no reaction); pdichlorahenzene (no reaction); chlorobenzene (red precipitate); pchlorotoluene (bright red precipitate). The relative activatingdeactivating power of four groups (-CHa, -H, -C1, -COr CSH.) can he deduced from this e~periment.~ The Stabilify of the Trityl Radical and the Existence of a Mobile Radical-Dimer Equilibrium
A solution of triphenylmethyl chloride in dry benzene is shaken with some Zn dust. Decantation from the Zn yields a yellow solution. When a half full test tuhe of this yellow solution is On peroxide). shaken in air the color vanishes (radical On allowing this test tuhe to stand for a minute the color reappears (reestablishment of dimer % radical equilibrium). After several shakings, the color no longer reappears. (There is no more radical or dimer left.)'
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Electron Delocalizatian in Phenols ond Phenoxide Ions
To dilute solutions of phenol, 4nitropheno1, and 2,4-dinitrophenol in methanol add some 0.1 N NaOH solution and discuss the significance of the color changes in terms of resonance st* bilization.
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Phenol (colorless- colorless) 4Nitrophenol (colodess- yellow) deep yellow-orange) 2,4Dinitrophenol (pale yellow
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Nitrosation of Primary, Secondary, and Tertiary Aromatic Amines
To about 0.3 g of milininm chloride dissolved in 20 ml Ha0 add 1ml conc. HCI and cool to 0°C. Addition of one drop of 5 M NaNOn yields a colorless solution [C6Hb-N--Nt]. Repeat using N-methylmiline instead of anilinium chloride and obtain yellow emulsion [CaHs-N(CHa)-N=O]. Repeat using N,Ndimethylaniline end get green solution [ p N O CsH*N(CHs)rl. If dimethylaniline is not pure, the green color is masked by the orange color of a diazonium coupling product. Diazonium Coupling Reactions and the Colors of Azo Dyes
solved in 20 ml of ethanol (1) anilinium chloride (turns yellow) (2) dimethylaniline (turns red) (3) phenol (turns yellow) (4) Znaphthol (turns red) Discuss the colors of the azo compounds in terms of the degree of electron delocalization. Now add some 0.1 N NaOH to each of the above (1) (2) (3) (4)
turns reddish-purple remains the same turns red turns deep red to purple
Discuss color change in terms of increased electron delocalieation in anions of aso compounds 1, 3, and 4. Show that this second color change is reversible by adding some 0.1 N HCI, and discuss acid-base indicators. Reactivity in Nucleophilic Aromatic Substitution
To three test tubes containing, respectively, p-nitroiodoheneene, pnitrofluorohenzene, and 2,4dinitrochlorohenzene dissolved in 30 mole % aqueous dimethylsulfoxide (made from 43 mlDMSO and 25ml water) add 1 drop of 1.0 N NaOH p(OgN) CsHJ does not react within 45 minutes p-(OnN)CsHIF turns yellow in about 7 minutes 2,4 (O*N)ZCsHaC1turns yellow in 1 minute The effect of electron withdrawal in the ortho and para. suhstituent as well ss in the group being displaced can he deduced from this experiment. 'This report is limited to lecture demonstrations for the second half of an organic course, because that is what I have been teaching recently. I did not want to publish experiments which had not been tested for several terms before large classes. The design of similar experiments suitable for illustrating the principles usually taught in the first half of a one year organic chemistry course is no more difficult and is left up to the reader. D.,"AGuide to A d a p t e d f m m D ~ v ~ ~D., s o A~N, D PEARLMAN, Qualitative Organic Analysis," (3rd ed.), The Brooklyn College Bookstore, Brooklyn, N. Y., 1966. NOLLER,C. R., J. CHEM.EDUC.,26, 429 (1949). The lecturer may or may not deem it advisable to discuss the stmelure of Gomherg's dimer at this point [LENKAMP, H., NAUTA,W. T., A N D MACLEAN, C., Tetrahedron Lett., 249 (1968)l.
Mechanism of Hologenotion of Ketones
Fill five 10-in. test tubes half full with acetone. Add 2 drops of 5% B n i n CC4 to each of the first four test tubes. The orange bromine color disappears in 30-90 seo. The color disappears at a slightly different time in each test tube. Draw attention to the fact that the color in each particular test tube does not fade, but disappears suddenly. Now add a. few drops of bromine to each of the four test tubes again and note that this time the color disappears instmtly. To the fifth test tube add some Brs followed by a drop of 0.1 N HCI, and note that the bromine color disappears instantly. Show haw the bromination of cyelohexanone (in CC4 solution) takes place instantaneously (because of the far higher en01 content of cyclohexrtnone). (The acetone can be of technical grade; however if it is eztremely wet then the reaction is slower.) Warning
It may be worth repeating that the above set of lecture demonstrations is intended to enrich an organic
chemistry course, and to strengthen its connection with the experimental basis upon which chemistry is built. Such demonstrations cannot replace a regular course of student laboratory work, without which the teaching of chemistry, and indeed of any science, is quite meaningless. May the angry spirits of Berzelius, Pasteur, E. Fischer, et al., descend upon any miscreant who attempts to misuse these demonstrations in such a way.4
'The requirement for experiments of this type is that they illustrate some chemical principle by means of an obvious, visual effect which must take place within a fairly brief period of time. Some other experiments that fill this requirement are: "Stable Carbmians," LANSBURY, P. T.,J. CAEM.EDUC.38, 307 (1961); "Charge Transfer Complex Preceding a Diels-Alder Reaction," 36, A379 (1959). SCHIYELPFENIG, C. W., J. CHEM.EDUC.,
Volume 49, Number 10, October 1972
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