overhead projector dernonrtrotion/ Change In Optlcal Rotation with Wavelength E. Koubek and H. Oulnn US. Naval Academy Annapolis. MD 21402
Recently in this column Hambly' described a demonstration involving a solution of sucrose contained in a 50-mL graduated cylinder. This is a simplified version of that demonstration. It is easier to prepare and to carry out, and the visual image is much larger. The demonstration can be used to illustrate the effect of an ooticallv active material on plane-polarized light, as well as i o show that the light rotation is a function of the waveleneth of the liaht used. Procedure: Place a polarizing film2on an overhead projector. Set a small crystallizing dish containing about 1in. of Karo light corn syrup on top of the film, and then place another piece of polarizing film on top of the dish. By rotating the top sheet of film, it can be shown that hlue light is transmitted first, and then other colors of the rainbow apDear as the film is rotated through larger aneles. Not onlv is this a colorful demonstration, b; it also expiains why monochromatic sodium D light is used in a polarimeter.
' Hambly, G. F. J. Chem. Educ 1988, 65, 623. Available from Edmund Scientific Co..
Barrington, NJ 08007.
edited by
DORIS KOLB Bradley University Psoria. IL 61625
mixed with TCNE. On the other hand, benzene derivatives containing an increasing number of methyl groups form TCNE charge-transfer complexes increasingly of greater color in terms of depth and wavelength shift. The formation of the colored complexes can be illustrated by adding a few droos of colorless aromatic com~ound(e.e.. methvlbenzene) to few milliliters OF a colorless so~utioiof TCNE (1% in CHzCIz) in a small beaker on the stage of an overhead projector. Immediately upon mixing, the solution turns bright vellow-oranee. With mesitvlene.. 1.3.5-trimethvlbenzene. . . the color produced is a muih deeper orange-red. An eas; way to demonstrate these colored charge-transfer complexes, graphically illustrating the correlation of color and reactivity, is to prepare a set of small French square bottles containing various aromatic compounds plus TCNE and to compare the colors. To prepare the samples add 2.0 mL of TCNE solution (1% in CHzC12) to a half-ounce French square bottle; then fdl the bottle with an aromatic compound, and screw on the cap. (Caution:Samples should be prepared in a well-ventilated hood, and gloves should be worn when handling TCNE.) For solid aromatic compounds (e.g., hexamethylbenzene) or for those compounds whose absorption is too great to transmit light (e.g., methoxybenzene), CHzClz can be used to dilute the solution until it is transparent. Prepared in this manner, the capped French square bottles can be laid side by side on the stage of an overhead projector to allow the whole class to see the array of colors associated with the various aromatic comoounds. The mixtures are usually stable for several months. 'An interestine informative disnlav . " can be made bv using some of the following benzene derivatives:
a
-
Aromatic Pi Cloud Availability: Formation of Colored Charge-Transfer Complexes Kenneth E. Kolb Bradley University Peoria, IL 61625
In organic lectures the comparative availability of the pi electron cloud of the benzene ring is correlated with the reactivity of various substituted benzenes. One way to demonstrate the variance of pi electron availability in the benzene ring is to observe the color of the charge-transfer complex formed between an aromatic compound and tetracyanoethylene (TCNE). The absorptivity and color of the resulting complex correlates well, in a qualitative manner, with the availability of pi electrons in the aromatic compound and hence with its ease of electroohilic aromatic substitution. Thus benzene compounds containing meta-directing deactivating substituents, such as trifluoromethylbenzene, form no pi complex with TCNE and give no colo; when
Aromatic
&lor of Complex
nifluoronwhylbenzene benzonlniie benzene methylbenzene 1,3dimethylbenzene 1,3,5aimelhyibenrene hexamethylbenzene (diluted) methowbenzene (diluted)
none none light yellow yellow-nge orange orange-red purple-violet dark lavender
flUDTObenzBne chlorobenzene bromobenrene lodobenzene
lemon yellow yellow darker yellow red-orange
In the first erouo of comoounds. which eoes from deactivated aromatic rings to very active ones, ihe color changes progressively from colorless to deep lavender. In the second group the colors show that with bromobenzene and iodobenzene the pi cloud is more available for complexation than with fluorobenzene or chlorobenzene.
Volume 66
Number 10
October
1989
853
