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GEORGE L. GILBERT Denison University Granville. Ohio 43023
A Device for Easv Demonstration Of Ootical Activity and 06ical Rotatory isp perk ion J o h n B. Kinney a n d J a m e s F. Skinner1 Williams College Williamstown. Massachusetts 01267 G. Virtes Ward Meluille High School Old Town Road Setauket, New York 11733
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A discussion of optical activity is an integral part of most introductorv oreanic chemistrv courses and optical rotatory dispersion R OR^), a spectroscopic technique of increasing importance in the elucidation of subtle structural variations, is being covered in advanced organic and biochemistry courses. Two methods for the demonstration of optical activity have been presented in this J0urnal,~.3and moderately priced student oolarimeter is available ~ommercially.~ A portable device is described here which can he used for the simultaneous demonstration of both optical activity and optical rotatory dispersion for individual students or for a class with an overhead projector. The demonstration of ORD is either overlooked in the previous device^^^^^^ or is eliminated by the use of a color filter to approximate monochromaticity. The construction can be modified depending on the machine-shop facilities available. A 20-cm piece of Plexiglas tube (9 cm 0.d.) is cut longitudinally in half. T h e two halves are cemented to a 9 cm X 20 cm X 2 mm rectangle of Plexiglas and square ends (10 cm X 10 em) are cemented to the divided tube. Holes (5 mm) are drilled a t both ends of both compartments for easy filling. A second 10 cm X 10 cm square of Plexiglas is cemented to one end, separated from the first by narrow Plexiglas strips along three edges. A square piece of rigid Polaroid5 sheet is slipped between the Plexiglas ends. I n addition, color filters could be inserted for other applications of the device. At the other end of the tube, a circle (9 cm in diameter) of Polaroid is mounted in such a manner that it can be ~ ~ material ~ rotated. The Polaroid material is fastened between two brass rines. the entire assemblv beine held to the end of the tube wi& small brackets. With the two comnartments emotv. . .. view anv white liaht source and adjust the ring to give minimum transmission. Fill one comoartment with water and the other with a 1M sucrose solution; While the water compartment continues to show
little transmission, liaht is transmitted bv the sucrose compnrtrnent. l'his obsenirion should bc explained to the stttdrnr in terms of the rotation of the plane of the polarized light hs the optically active medium. if the polarizes is now rotated slowly, increased transmission will occur t h r o u ~ hthe water compartment while the light transmitted by the sucrose solution will change from pale blue, to deep blue, purple, red, orange and yellow, an excellent demonstration of optical rotatory dispersion. The sucrose solution rotates the longer wavelengths through smaller angles. As the polarizer is rotated, it blocks a given wavelength a t one angle and the viewer sees onlv the com~lementarvwaveleneths. - . a colored beam. It is fLlt that the flexibility of this device, the simultaneous comuarison of ooticallv active and non-outicallv active solutions, and the very clear demonstration'of optical rotatory dispersion markedly increase the educational value of this device over those previously reported.
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' To whom correspondenceshould be directed.
Frank, Forrest, J., and Kidwell, Sharon M., J. CHEM. EDUC., 46,58 (1969). Hill, John W., J. CHEM. EDUC., 50,574 (1973) From Instruments for Research and Industry, 104 Franklin Ave., Cheltenham, Pa., 19012 5 Available from Edmund Scientific Co., Barrington, N.J., Catalog No. 70,418 494 1 Journal of Chemical Education
Addendum to "Simple Demonstrationof Optical Activity" Walter K.Dean Emory University Atlanta, Georzia - 30322 We have been usine the "Simole Demonstration of Ootical Activity" (~ernande;, Jack E.;J . CHEM.EDUC.. 53. 508 (1976))and find it ver~effective in dealine with a phenumenon whichis very difficuit to describe convincinglior even believablv in words. However. we have found the followine modiKcations useful. 1) Our polarizing filtersare from Edmund ScienrificCo..300 Edaeorp Blda., Barrinyon. N.J. 08007. Four 6 x &in. filters sell for 5 1 rcatalw no. F~60,6:171, a considerable saving over the prrce quotkd. ~ a r. ~ e r f i l t eare r s also available, but theseseem to be quite adequate. 2) I use a solution of about 1lb of suear in 3W ml of water. contained in an ordinsrv ~,400-ml beaker. Thy* eives a lareer imaee.'This ~" sdution needs to be prepared in advance a*it will take somp rlmp r ., rlclu. Better transmiision could probably beohmind hy wing a veirtl with a perfectly flat bottom, but I have not been able to try this; a beaker is quite adequate. 3) Another advantage of using a more concentrated solution is that the wavelength dependence of the rotation can easily be shown. Rotation of the filter a few degrees to either side of the minimum imparts a blue or red tinge to the image. 4) Adding a few ml of concentrated HC1 to the sucrose solution causes the rotation to change slowly as the sucrose is hydrolyzed. This rather slow reaction is best observed by setting the filters to minimum transmission, and watching the image brighten as hydrolysis proceeds. ~
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We hope that these suggestions will be of value to others making use of this fine demonstration.
