overhead projector demon1tratiow
edited by DORIS
KOLB
Bradley Unwers~ty Peona, IL 61625
A Demonstration of the Optical Activity of a Pair of Enantiomers Bruce Knauer SUNY at Oneonta Oneonta. NY 13820
A major problem with the use of sugar solutions for an introductory demonstration of optical activity is that the equal but opposite rotation of plane-polarized light exhibited by apair of enantiomers cannot be demonstratedowing to the general unavailability of one of the saccharide enantiomers. The demonstration that is described here uses the enantiomers (S)-(+)-carvone and (R)-(-)-cawone [5-isopropenyl-2-methyl-2-cyelohexenone],both of which are commercially available.' Polarlmeter Construction Two polarizing plastic sheets2 are cut so that each is approximately 10 in. square. A &in. clear plastic protractor is placed on one of these sheets so that its straight edge is exactly perpendicular to the optically transmitting orientation of the polarizing material and about 1in. from its bottom edge; tbe index hole in the protractor should he about midway between the two sides of the sheet. In this orientation the 90' mark is parallel, and the O0 mark perpendicular to the optically transmitting direction of the polarizer. A sharp knife is then used to cut out the polarizing plastic under the protractor, thus creating a hole into which the protractor can just fit. Demonstration The polarizing sheet with inserted protractor is placed on the overhead projector. The second piece of polarizing material is placed flat on the first and positioned so that one side edge is aligned with the index hole on the protractor so that the edge may pivot about the hole and act as a degree pointer (see the figure). When the second sheet is rotated so that the side edge points to 90°, there is maximum transmission of light; when it points t o 0°, there is an optical null. (In this demonstration readings are taken relative to the null rather than to the position of greatest brightness, because, inpractice, i t is much easier to get an accurate reading of the null.) A 50-mL beaker almost full of water or other achiral material is placed between the two polarizing sheets, and, by rotating the second polarizing sheet around the index hole in the protractor as a pivot, it is demonstrated that the null occurs a t OD both for the water and air. A 50-mL beaker, marked (R)-(-)-carvone, is almost filled with the levorotatory enantiomer, and a second 50-mL beaker, marked (S)-(+)-camone, is filled to exactly the same height with the dextrorotatory enantiomer. These beakers are placed next to each other, above the protractor, on the first polarizer. The second polarizer is held just above the beakers and rotated as before to obtain an optical null for each of the samples. The rotations measured from the protractor will be essentially equal in magnitude but opposite in direction.
Dlscusslon Depending on the class, discussion may be limited to the equal but opposite rotation of plane-polarized light by the two enantiomers or may be carried further. One additional thing which can be done is to calculate the specific rotation of each of the enantiomers. T o get the value closest to the value supplied on the bottle label, which was measured using the sodium D line, the demonstrator should try to have the polarizers block the yellow portion of the spectrum that is passing through the sample. In practice, if one looks directly a t the beaker holding the sample being observed and adjusts the second polarizer to an angle such that the color of the transmitted light is its darkest blue just before turning purple, a good result will be obtained (usually within 2' of the reported specific rotation value). (The projected image will appear black through a very small range of angles as a portion of the optical rotatory dispersion (ORD) effect is observed bv lookine directlv a t the beaker. The students will also be able to s& aportion of the ORD e f f e c t the extrema-on the ~roiectionscreen.) The s~ecificrotation can then he calcuiated from the formula:
where 1 is the path length of the light through the sample in decimeters and d is the density of the substance in g/mL. (Using a 50-mL beaker, the path length will turn out to be about 38 to 40 mm (= 0.38 to 0.40 dm) and the observed rotation will be about f21 or 2Z0.) Chemicals that are purchased are not 100% pure. Consequently, the specific rotations and densities of the two car-
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Aldrich Chemical Co.,940 W. St. Paul Ave., Milwaukee. WI 53233; Fluka Chemical Corp., 980 South Second St., Ronkonkoma, NY 11779; or Lancaster Synthesis, P.O. Box 1000, Windham, NH 03087. Sheets of polarizing material, 10 in. X 24 in., are available for less than $20 from Edmund Scientific Company. 101 E. Gioucester Pike, Barrington, NJ 08007. Stock flD70.887. Volume 66 Number 12 December 1989
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vone enantiomers that are listed on the bottles as supplied by Aldrich are slightly different. For example, the materials we have are labeled +5E0 2", d = 0.965 g/mL and -61°, d = 0.959 g/mL. In classes with science majors, especially, this can lead t o a short discussion of chemical purity and the use of physical constants, like specific rotation, to gauge it. In less sophisticated classes the slight difference (about 1") that shows u p in the magnitude of the observed rotation as a result of impurities can be chalked up to "experimental error." Another advantage of using the enantiomeric carvones to demonstrate optical activity is that they have different odors. (+)-Carvone is a maior constituent of caraway seed oil, and (-)-cawone is a major constituent of spearmint oil, and, for most people, this is reflected in how they smell. This aspect of the carvones is discussed a t some length by Murov and P i c k e ~ i n gwho , ~ also provide IR and NMR spectra. The aroma differGnce in these enantiomers can lead t o a discussion of the occurrence of chiral molecules in nature (olfactory receptors, presumably) and the differences in properties that result when enantiomers form diastereomeric complexes with a third chiral structure.
Colors Observed wlth Meihyl Vlolet and Various Adds
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Murov, S. L.; Pickering, M. J. Chem. Educ. 1973, 50, 74-75.
Illustrating the Inductive Effect on Acid Strength of Carboxylic Acids Kenneth E. Kolb and Doris Kolb Bradley University Peoria, IL 61625 The effect of a substituent on the strength of an organic acid was early noted and rationalized by G. N. Lewis in his classical book Valence in 1923'. Lewis explained that the chlorine atom in chloroacetic acid draws electrons toward
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Journal of Chemical Education
Add (IM)
C O I O T O ~ ~ ~ PK. ~ ~11n.1
acetic ~ h l w o a ~add ~tl~ acid di~hioroaceti~ hichloloa~eti~ m d hifiuoroacetic acid 3~hloropmpan0icacid hydroxyacetic acid oxslic acid
yellow-green "lolet blue blue*reen green yellow-green blue-violet blue~ioiet green
malonlc
blue
HCI
sbong 4.74 2.86 1.26 0.64
shong 3.9
3.8 1.5 2.8
itself and thus produces a stronger, more highly ionized acid than acetic acid. The inductive effect of the halogens and other substituents is usually tabulated in organic t&tbooks and discussed in lecture. The effect of various substituents on acid streneth can be easilv demonstrated in lecture usine aqueous solutions of various carboxylic acids containing a suitable indicator. The solutions are convenientlv laced on the stage of an overhead projector in order to compare the projected colors.
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Procedure Using 1M aqueous solutions of various carboxylic acids, place 510 mL of acid solution in a 30-50-mL beaker and add 6 drops of 0.05%methyl violet (in 95%ethanol).This proceduregives aseries of highly colored solutions. By comparing other acids with 1M hydrochloric and acetic acids, conclusions can be drawn concerning electrical effects of substituents on the carboxylie group in various aliphatic acids. Acids that have been found useful in carrying out this demonstration are listed in the table.
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Lewis, G. N. Valence and the Structure ofAtoms and Molecules; Chemical Catalog Company: 1923.
