Activity pubs.acs.org/jchemeduc
Chirality and Pinwheels Emilio Rodríguez-Fernández* Department of Inorganic Chemistry, University of Salamanca, 37008 Salamanca, Spain S Supporting Information *
ABSTRACT: By using cardboard models that resemble propellers, the students of inorganic courses can easily visualizing the distinct rotation of optical isomers. These propellers rotate clockwise or counterclockwise when they are dropped from a certain height or in the presence of wind.
KEYWORDS: Second-Year Undergraduate, Inorganic Chemistry, Analogies/Transfer, Hands-On Learning Manipulatives, Chirality/Optical Activity, Coordination Compounds, Enantiomer, Molecular Properties/Structure
A
chiral molecule can occur as two stereoisomers that are mirror images of each other and cannot be superimposed upon each other. Optical isomers rotate the plane of polarized light at equal angles but opposite signs. Spatial abilities are required for visualizing optical isomers.1 To illustrate optical isomers, various objects have been used: rubber gloves, Eskimo yo-yo, chiral crackers, dishes, and king’s cut for octahedral complexes such as [Co(en)3]3+,2−7 where en is ethylenediamine.
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PINWHEEL MODEL Isomers of [Co(en)3]3+ may be visualized as an imaginary propeller or pinwheel whose blades are the three ethylenediamine ligands. A propeller and its mirror image are nonsuperimposable. The propeller will turn clockwise or counterclockwise depending on its chirality. In addition, complexes of the type cis-[Co(en)2X2]+ and Zr(acac)4, where acac is acetylacetonate, can be represented by a two-bladed propeller and a four-blade propeller, respectively (see the Supporting Information, Figures S1 and S2). The construction with paper or cardboard of a two-bladed propeller is described in the Supporting Information (Figure S3). By using these models, the students, when studying chirality, can easily visualize the distinct rotation of both optical isomers. These propellers can rotate clockwise or counterclockwise when they are dropped from a certain height or in the presence of a steam of air (Figure 1A,B). This effect is best observed by inserting a pin in the center of the propeller, which acts as a direction vector. When a pin is inserted in two two-bladed propellers of the same handedness, both rotate in the same direction as the individual helices (Figure 1C). If each helix is of different handedness (Figure 1D), the assembly does not rotate when dropped from a height, modeling the behavior of a racemic mixture in terms of its effect on polarized light. These types of models can be used to illustrate the physical properties of enantiomerically pure and racemic mixtures (see © 2013 American Chemical Society and Division of Chemical Education, Inc.
Figure 1. Double-bladed left-handed propeller rotates counterclockwise (A) and double-bladed right-handed propeller rotates clockwise (B). Two left-handed propellers (C) and two opposite hand propellers (D).
Figure S4 in the Supporting Information). About 15−20 pinwheels of the same handedness were placed in a beaker and the resulting “solution” was stirred for 1 min. After this time, an association between the pinwheels is observed, as occurs in the crystallization of the enantiomer. In another beaker, 15 righthanded pinwheels and 15 left-handed pinwheels were placed. After stirring, there was no significant association between the pinwheels. The experiment could be related to the behavior of a racemic mixture. All students participate in the preparation of the pinwheels “solutions”, and related activities. Each student is responsible for making two sets of left- and right-handed pinwheels. The preparation of the material requires approximately 20 min and 10 min is needed for the activity.
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SUMMARY The activity is directed to students of advanced inorganic chemistry and coordination chemistry courses and is aimed at Published: April 10, 2013 623
dx.doi.org/10.1021/ed3004963 | J. Chem. Educ. 2013, 90, 623−624
Journal of Chemical Education
Activity
introducing the concept of chiral molecules. The students (20 in the class) enjoyed the activity and intuitively related the rotation of the pinwheels with the structures of coordination complexes such as cis-[CoCl2(en)2] and [Co(en)3]Cl3. Students also easily visualized behavior between isomers, for example, the nonrotation of a pair of pinwheels of different orientation is directly analogous to a meso compound; it is achiral despite having chiral components. The “dissolution of pinwheels” represents the behavior of an optically active substance or a racemic mixture. This activity has been welcomed by students and helped them visualize the concepts of chirality and nonsuperimposable objects on the mirror image.
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ASSOCIATED CONTENT
S Supporting Information *
Relationship between the complex polyhedron and the appropriate propellers; construction of the propellers; photo illustration of the properties. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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