DEMONSTRATION pubs.acs.org/jchemeduc
Demonstrating Optical Activity Using an iPad Pauline M. Schwartz,*,† Dante M. Lepore,† Brandy N. Morneau,† and Carl Barratt‡ †
Department of Chemistry and Chemical Engineering and ‡Department of Mechanical, Civil and Environmental Engineering, Tagliatela College of Engineering, University of New Haven, West Haven, Connecticut 06516, United States ABSTRACT: Optical activity using an iPad as a source of polarized light is demonstrated. A sample crystal or solution can be placed on the iPad running a white screen app. The sample is viewed through a polarized filter that can be rotated. This setup can be used in the laboratory or with a document camera to easily project in a large lecture hall. KEYWORDS: First-Year Undergraduate/General, General Public, High School/Introductory Chemistry, Second-Year Undergraduate, Upper-Division Undergraduate, Demonstrations, Organic Chemistry, Inquiry-Based/Discovery Learning, Chirality/Optical Activity, Laboratory Equipment/Apparatus
O
ptical activity is an intriguing characteristic of asymmetric substances. It has an interesting history commencing in the early 1800s; in 1831, Louis Pasteur observed “handedness” in tartaric acid crystals.1 Although a fundamental understanding of optical activity is complex,2,3 demonstrating optical activity is straightforward and certainly fascinates students in all areas of chemistry.4 6 Light incident on a filter that passes only one plane of light emerges polarized, and if this polarized light is incident on a second plane polarized filter, it will emerge with full intensity only if the two filters are aligned. Otherwise, as the filters are rotated relative to one another, the light is dimmed until it is finally blocked when the filters are perpendicularly oriented. Because optically active substances rotate the plane of polarized light, it follows that placing a substance or its solution between two such filters can be used to determine if the substance is optically active7 and also enables characterization of an optically active substance as rotating light clockwise (+ or d for dextrorotary) or counterclockwise ( or l for levorotary). Many common substances display optical activity because their structure is asymmetric. Optically active molecules such as sucrose possess one or more chiral centers and lack an internal plane of symmetry. For organic compounds, a chiral center has four different groups bonded to a carbon atom.7 Many inorganic substances are also chiral and optically active.8 It is important for a student to recognize that there is no relationship between optical activity, a laboratory measurement, and structural asymmetry designated as an absolute configuration, (R) or (S). It is also useful to note that not all optically active substances necessarily require a chiral center.8 Sodium chlorate (NaClO3), for example, is an interesting case because it is not chiral or optically active in solution, but NaClO3 forms two crystal configurations, which are optically active.9 In the course of our research, we made several collections of NaClO3 crystals under different conditions and wanted a quick means to identify and separate the two optical forms. When we used a white light source from an iPad running an app called Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Figure 1. (A) Demonstration of polarized light emitted from an iPad. (B) Note the marker on the polarizing filter shows that it has been rotated approximately 90° from the initial position.
Flashlight (from John Haney Software, version 3.2, 2008), we found, to our initial surprise, that the light from the iPad was polarized (Figure 1). An iPad or any similar device, such as an iPhone or a tablet PC, is useful for demonstrating optical activity. The iPad screen has a special backlit IPS LCD (in-plane switching liquid crystal display) of 1024 768 pixels (32 pixels per inch);10 the screen is composed of two cross polarized panels, and color is achieved by using three liquid crystal molecules in each pixel that are oriented by an electric field. A person wearing polarized sunglasses can clearly see that light emanating from an iPad is polarized! To observe optical activity using an iPad, crystals were placed on the iPad screen and viewed through a second polarized filter. The two different crystal forms of NaClO3 could easily be identify and distinguished by their behavior from a crystal of NaCl that is not optically active (Figure 2). The use of an iPad for viewing optical activity of a sucrose solution is shown in Figure 3. This setup can be used in the laboratory as a “hands on” activity or with a document camera and projector to easily demonstrate Published: October 04, 2011 1692
dx.doi.org/10.1021/ed200014m | J. Chem. Educ. 2011, 88, 1692–1693
Journal of Chemical Education
DEMONSTRATION
Figure 2. (A) Setup for observing optical activity of crystals with an iPad. (B) Top crystal is NaCl (control) and bottom crystals are two forms of NaClO3.
Figure 3. (A) Setup for observing optical activity of a solution. (B) shows optical activity of a solution of D-sucrose (25 mL of a solution of approximately 50 g of D-sucrose and 100 mL of water in a flat-bottom 25 mL graduated cylinder).
optical activity in a large lecture hall. Figures 1 3 were made under room lighting, but optical activity is better displayed if stray light is removed by working in a dark room. Polarized sunglasses provide a fun variation in demonstrating the optical activity of materials using an iPad.
’ AUTHOR INFORMATION
(7) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; John Wiley and Sons: New York, 1994. (8) Claborn, K.; Isborn, C. Kaminsky, W. Kahr, B. Angew. Chem., Int. Ed. 2008, 47, 5706 5717 (10.1002/anie.200704559). (9) Kondepudi, D. K.; Kaufman, R. J.; Singh, N. Science 1990, 250, 975–976. (10) iPad Technical Specifications, 2010 at http://www.apple.com/ ipad/specs/ (accessed Sep 2011).
Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge funding through the Connecticut Space Grant Consortium and the University of New Haven Faculty Research Support to C.B. and P.M.S. We thank the University for supporting undergraduate summer fellowships and for a NASA CT Space Grant Fellowship for D.M.L. ’ REFERENCES (1) Flack, H. D. Acta Crystallogr. 2009, A65, 371–389. (2) Charney, E. The Molecular Basis of Optical Activity; John Wiley and Sons: New York, 1979. (3) Frolov, A. M.; Wardlaw, D. M. 2010; available from http://arxiv. org/abs/1009.0889v3 (accessed Sep 2011). (4) Mehta, A.; Greenbowe, T. J. J. Chem. Educ. 2011, 88, 1194–1197. (5) Knauer, B. J. Chem. Educ. 1989, 66, 1033–1034. (6) Hambly, G. F. J. Chem. Educ. 1988, 65, 623. 1693
dx.doi.org/10.1021/ed200014m |J. Chem. Educ. 2011, 88, 1692–1693
