An Overhead Projection Demonstration of Optical Activity
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With the increasing imnortance of stereochemistrv in organic chemistrv. it is desirable to be able to demonstrate the nhenomenon of ontical activitv to laree classes. e rank-and ~TdwelPhave discribed a nolsrimeter for use on the overhead pruiecror. The dwicr described in thls paper ts easier to construct and cheaper, yet makes no sacrifice ot accuracy. The materrals required, other than an owrhend projerror, are rwo polariring lenses.~a yellow filter.2 an oatmeal box, a piece ofcardboard, s 1 W - m l bcakrr, andoptically arriw compoundr surhascarbohgdrates. A "cell" is cot~atructrdfrom a cylindrral cardboard box .a 42-oz.sire Quaker Oar* box rut down t o a height of 15 cm works quite nicely,. A hole 4.5 cm in dinmeter is rut around the center of the hatcom of the box and a 5 x 5 rm square of polarizing film isplaced over the hole and glued or taped in place. A hole 1cm in diameter is cut around the cent& of the top af the box and the second polarizing film is fastened over it. The lid or "analyzer" is calibrated by placing a strip of removable tape around it, marking the angles of minimum transmission (i.e., the angles at which the polarizing axes of the two films are perpendicular). The tape is then removed, marked off into a scale with 10 intervals, and then replaced on the lid. To use the overhead projector as a "light source" it is best to block out most of the light hy placing a piece of cardboard with a 5-cm hole in the center over the light hoard. A yellow filter is placed over the hole and the cardboard box "cell" is placed over the hole. A 1000-ml beaker is placed in the box to hold the solution to he examined. At this point, it may be demonstrated that maximum transmission occurs at angles of 0" and 180" (when the polarizing axes are parallel) and minimum transmission occurs at 90" and 210' (when the polarizing axes are perpendicular). Maximum transmission shows as a large yellow circle on the projection screen. The exact maximum is difficult to determine accurately. At minimum transmission, the screen is totally dark. The exact minimum is easily determined in a darkened room to within 0.5". The projected light shows meen just on one side of the minimum and red iust on the other side. To measure the bptical aciivity-of a carbohydrate solution, it is convenient to use about 100 g of the sugar and dissolve it in enough water to make the solution 1dm deep in a 1000-ml beaker (about 820 ml of water). The beaker is then nlaeed in the box and the analvzer set to 90". If the material is onticallv active. lieht is now seen on the screen even thoueh the polarizing axes are perpendicular. The "analyzer" is rotated to the left or right to obtain minimum transmission, and the angle through which it is rotated is read directly. The specific rotation of a number of carbohydrates has been calculated from observed rotations measured in this manner. Values within 1-2" of those in the literature are easily obtained. The mutarotation of u-D(+)-glucase and the inversion of sucrose are easily demonstrated. The former, ordinarily quite slow, is brought about a t a moderately rapid rate by the addition of a few drops of concentrated ammonium hydroxide s ~ l u t i a nThe . ~ inversion of sucrose, catalyzed by addition of enough concentrated hydrochloric acid to make the ~ e r i a dFinal . readinas should be taken solution approximatelv 2 N in HC1, is best observed over a 3- or Chour laboratorv .. after a t least 48 hr. Since virtually all educational institutions have overhead projectors available, and the cost of materials (excluding chemicals and common glassware) is about $2, this system should provide an excellent end nearly universal means of demonstrating optical activity. ~~~~~~
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Frank, Forrest J., and Kidwell, Sharon M., J. CHEM. EDUC., 46.58 (1969). from Edmund Scientific Co., Barrington, N. J. Brewster, Ray Q., Vander Werf, Calvin A,, and McEwen, William E., "Unitized Experiments in Organic Chemistry," 2nd ed., D. Van Nostrand Company, Inc., Princeton, New Jersey, 1964, p. 141. 4 Daniels. Famineton. Williams. J. W.. Bender. Paul. Albertv. Robert A,. and Comweil. C. D.. "Exnerimental Phvsieal chemistry," 6th ed.; ~ c ~ r a w : ~~i al lo company, k inc., ~ e w ~ o r1962; k , p. 142. 2 Available
University of Wisconsin River Falls. 54022
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Journal of Chemical Education
John W.Hill
