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JCE Featured Molecules
William F. Coleman
Molecular Models of Chiral Molecules
Wellesley College Wellesley, MA 02481
Metolachlor, Chiralane, Nanotubes, Helicenes, Hexol, and BINAP September 2009 Featured Molecules September’s featured molecules are inspired by the paper by Albrecht Mannschreck and Erwin von Angerer on the herbicide metolachlor (1). In that paper the authors emphasize the chiral nature of the molecule, chiral approaches to its synthesis, and, ultimately, the herbicidal efficiency of the various isomers. Of particular interest from a structural point of view is the chirality of the molecule that arises not from the stereogenic center on one of the nitrogen substituents, but rather from the hindered rotation about the nitrogen-ring bond, often referred to as axial chirality. In addition to the molecules from the featured paper, we have added a number of molecules to the collection that demonstrate various ways of achieving chirality. Beginning with the basic definition that a molecule is chiral if it cannot be superimposed on its mirror image, we can define what might be called static chirality for those cases where the molecule and its mirror image are not easily converted from the one to the other at a given temperature. As an example, the two rotamers of 1-chloro-2-fluoroethane included in the collection are mirror images of one another and are non-superimposable. Therefore both are chiral, but that chirality is rendered moot by rapid rotation about the C–C bond. However, introduction of a third carbon atom to give the 1-chloro-3-fluoroallenes, dramatically increases the rotational barrier and results in enantiomers that should be separable by chiral chromatography. In the language of group theory a molecule is chiral if it does not possess an improper axis of rotation Sn (sometimes referred to as a rotation–reflection axis). An alternative way of saying this is that a molecule is chiral if the only symmetry elements other than the identity are proper rotation axes. Given that S1 = σ (plane of symmetry) and S2 = i (inversion center), we see the source of the often stated, but incomplete, idea that a molecule is chiral if it has no plane of symmetry or center of inversion. For the most
[6.6]chiralane: an enantiomeric pair with T symmetry
common chemical point groups those which result in chirality are C1, with only the identity, and the pure rotational groups Cn, Dn, T, O, and I, the latter three being quite rare. An example of an enantiomeric pair with T symmetry ([6.6]chiralane; Figure 1) is included in the molecule collection (2). Additional enantiomeric pairs in this month’s collection are chiral nanotubes (Figure 1), several helicenes of C2 symmetry, hexol (the first non-carbon-containing molecule to be resolved), and the BINAP ligand of Ryoji Noyori and coworkers that was recognized as part of the 2001 Nobel Prize in Chemistry (3). Previous columns have emphasized the stereochemistry of nickel– ethylenediamine complexes (4) and many examples of chirality resulting from stereogenic centers (5–7). This range of examples should help students appreciate that chirality is a property of many molecules (perhaps the vast majority of molecules) and that the chirality is an important consideration when no mechanism exists for converting one enantiomer to another. If there is no relatively low-energy conversion pathway that carries one enantiomeric form through a point group containing an Sn axis to the other enantiomeric form, then static chirality will be found. Literature Cited (all Web sites accessed Jul 2009) 1. Mannschreck, Albrecht; von Angerer, Erwin. J. Chem. Educ. 2009, 86, 1054–1059. 2. Parity Divergent Molecules; original coordinates from http:// www.mazepath.com/uncleal/chiralan.htm. 3. The Nobel Prize in Chemistry 2001. http://nobelprize.org/nobel_prizes/chemistry/laureates/2001/. 4. JCE Featured Molecules June 2006: Copper and Nickel Complex Ions. http://www.jce.divched.org/JCEWWW/Features/MonthlyMolecules/2006/Aug/index.html 5. JCE Featured Molecules July 2006: Amino Acids. http://www. jce.divched.org/JCEWWW/Features/MonthlyMolecules/2006/Jul/ index.html. 6. JCE Featured Molecules July 2005: Menthol Stereoisomers. http://www.jce.divched.org/JCEWWW/Features/MonthlyMolecules/2005/Jul/index.html. 7. JCE Featured Molecules July 2004: Enantiomer Specificity in Pharmaceuticals. http://www.jce.divched.org/JCEWWW/Features/MonthlyMolecules/2004/Jul/index.html.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Sep/abs1104.html Full text (HTML and PDF) with images in color Links to cited URLs and JCE article
chiral nanotubes
Supplement Find Molecular Models of Chiral Molecules in the JCE Digital Library at http://www.JCE.DivCHED.org/JCEWWW/Features/ MonthlyMolecules/2009/Sep/
Figure 1. Several of the molecules added to this month’s collection.
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The molecules added to the collection this month include: BINAP ligand; chiral nanotubes; [6.6]chiralane; 1-chloro-3fluoroallene; 1-chloro-2-fluoroethane; helicenes of C2 symmetry; hexol; metolachlor; model anilide from ref 1
Journal of Chemical Education • Vol. 86 No. 9 September 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
