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JCE Featured Molecules
William F. Coleman
Molecular Models of Leaf Extracts
Wellesley College Wellesley, MA 02481
January Featured Molecules
Relative Energy / (kJ/mol)
Our Featured Molecules this month come from the paper by Pelter et. al. on the analysis of leaf extracts by thin-layer chromatography (1). As the authors discuss, their experiment may be used in courses at various levels of the curriculum. The molecules discussed in the paper are also of wide interest both for their structural properties and their wide-ranging appearance in both natural and synthetic substances. Included in the molecule collection are all of the isomers for the molecules pictured in the text with the exception of menthyl acetate, for which only one structure is given (see below). All of these molecules have been optimized at the HF/631-G(d) level. The menthol family—enantiomeric pairs of menthol, isomenthol, neomenthol and neoisomenthol—provide a rich yet coherent group of molecules on which to base discussion of chirality, enantiomers and diastereomers. Treadwell and Black have described some of the differences in physical properties of four members of this family, and several other experiments using one or more menthols have been published in this Journal (2, 3). I have created a Web page in which the eight molecules are embedded in no particular order, and with no rational file names. This is being used in at least one of our organic sections to give students experience at identifying enantiomers, and diastereomers, and in applying R/S notation (4). As access to computational software becomes more common, and as efforts are being made to incorporate more relevant modeling experiments into all levels of the curriculum, the menthols again present some interesting possibilities. While students at the organic level know about enantiomers differing in their optical rotation, and about chiral molecules interacting with chiral and achiral environments, it is instructive for them to think of other ways in which enantiomers and diastereomers are the same or different. Three useful ways of checking to see whether two structures are truly enantiomers is to compute their total energies, vibrational spectra, and dipole moments. These calculations are available in most common computational packages. Figure 1 shows the results of energy calculations on
neoisomenthol (upper)
16.260 kJ/mol
neomenthol (lower)
16.189 kJ/mol
menthol
8.482 kJ/mol
optimized structures of the eight isomers. The enantiomeric pairs have, as expected, exactly the same total energy, while the various diastereomers differ in energy. The computation of the vibrational spectra is a very sensitive probe to determine whether two structures are optimized and enantiomeric or not. Structures that are almost enantiomeric, but not quite optimized, may exhibit similar energies, but the low frequency vibrations will be sensitive to any deviation from optimization. If two supposedly enantiomeric structures do not have the same computed vibrations, or if either shows a negative frequency, the structures need to be optimized more carefully. As with the vibrational frequencies, enantiomers should show identical dipole moments. Only one structure of the eight isomers in the menthyl acetate family is included in the collection, giving students the chance to build the other seven and verify their computed properties. Because of the central role that chirality plays in chemistry, and particularly in biochemistry, it seems appropriate to introduce some of these visualization and modeling exercises early in the curriculum, and in courses designed for students majoring in other areas. Students in various courses could pursue other aspects of these same molecules including odor and cooling properties, and green chemistry approaches to synthesizing menthols. Literature Cited 1. Pelter, Libbie S. W.; Amico, Andrea; Gordon, Natalie; Martin, Chylah; Sandifer, Dessalyn; Pelter, Michael W. J. Chem. Educ. 2008, 85, 133. 2. Treadwell, Edward M.; Black, T. Howard J. Chem. Educ. 2005, 82, 1046 3. Poiré, C.; Rabiller, C.; Chon, C.; Hudhomme, P. J. Chem. Educ. 1996, 73, 93; Furton, Kenneth G.; Perez, Maria I. J. Chem. Educ. 1991, 68, 946; Barry, J. J. Chem. Educ. 1973, 50, 292; Dunathan, H. C. J. Chem. Educ. 1963, 40, 205. 4. Interactive structures for the eight isomers of the menthol family are available at http://www.wellesley.edu/Chemistry/Flick/menthols.html (accessed Nov 2007).
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Jan/abs160.html Full text (HTML and PDF) with links to cited URLs and JCE articles isomenthol
0.000 kJ/mol
Figure 1. Relative energies of diastereomers of menthol computed at the HF/631-G(d) level.
160
(R)-(−)-carvone: the principal constituent of spearmint oil
Supplement Find Molecular Models of Leaf Extracts in the JCE Digital Library at http://www.JCE.DivCHED.org/JCEWWW/Features/ MonthlyMolecules/2008/Jan/
Journal of Chemical Education • Vol. 85 No. 1 January 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
