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William F. Coleman Wellesley College Wellesley, MA 02481
Molecular Models of Natural Acid-Base Indicators William F. Coleman Department of Chemistry, Wellesley College, Wellesley, MA 02481
[email protected] JCE Featured Molecules for January 2010 “Who it is who doesn't love a good indicator.” That terrible mangling of Robert Frost brings us to our featured molecules for this month (1), taken from the paper by Brahmadeo Dewprashad and Latifa Hadir on using various natural products to examine the relative acidity of some alcohols (2). Added to the molecule library (1) are the keto and enol forms of curcumin (Figure 1), and the acid and base forms of cyanadin, of the 1, 5-diglucoside of cyanadin, and of 2-hydroxy-1,4-naphthoquinone. All structures were calculated at the DFT/B3LYP-631-G(d) level. Cyanadin, other members of the family of anthocyanins, and the structures without the attached sugars (aglycones) have been previously discussed on several occasions in this Journal (3-5). Calculated structures for the various acid-base forms of the new molecules should be compared to the resonance stabilized base structures in ref 1. For example, students could be asked to use Jmol to determine the CC and CO bond lengths in the base structure, and make and defend a decision about the relative contributions of the various resonance forms. In the case of the cyanadins, how important is the zwitterionic structure? The computed partial charges on all of the oxygens in the base form of the aglycolic cyanadin show that (a) all of the oxygens are negative and (b) the doubly bonded oxygen is slightly more negative than the others. Does that information help students answer the original question? Students might be asked to comment on the most striking difference between the structures of the glycolic and aglycolic forms of cyanadin. Of course, instructors will hope that students notice that the three rings, which are planar in the aglycolic form, are not planar in the glycolic form. They could then measure the absorption spectra of the various acid and base forms to determine whether this loss of planarity, and concomitant reduction of delocalization, affects the spectrum, or the colors of the various forms. More advanced students could be asked to perform additional calculations to determine whether any observed spectral shifts were reflected in their computations. In the case of the acid form of 2-hydroxy-1,4-naphthoquinone, the alcoholic hydrogen atom is directed toward the ketonic oxygen atom, in the minimum energy configuration. The conformer that has the H directed fully away from the oxygen atom is less stable, but represents a relative minimum in the potential energy surface (there are no negative frequencies for this form). Students should be able to offer an explanation for this observation, and might also wish to calculate the potential barrier for rotation about the C to O (alcoholic) bond. These exercises could easily be extended to the other anthocyanins and their corresponding aglycones. Using this as
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Figure 1. Several of the natural acid-base indicators added to the JCE Featured Molecules collection. Curcumin, the compound that gives turmeric its characteristic color, exists predominantly in the enol form. It is yellow under acidic conditions but changes to a red color under basic conditions. 2-Hydroxy-1,4-naphthoquinone gives henna, a natural dye used in hair colorants and in body decorations, its characteristic color. A solution of 2-hydroxy-1,4-napthoquinone is yellow under acidic conditions but changes to reddish orange under basic conditions (2).
a computational exercise in a physical chemistry laboratory would enable students to explore the relationship between the observed spectra and the predictions of the relative absorption energies from several types of calculations. Literature Cited 1. Molecular Models of Natural Acid-Base Indicators; available at the JCE Digital Library: http://www.jce.divched.org/JCEDLib/ Molecules/2010/Jan/. 2. Dewprashad, B.; Hadir, L. J. Chem. Educ. 2010, 87, DOI: 10.1021/ ed800014k. 3. Foster, M. J. Chem. Educ. 1978, 55, 107. 4. Suzuki, C. J. Chem. Educ. 1991, 68, 588. 5. Curtright, R.; Rynearson, J. A.; Markwell, J. J. Chem. Educ. 1996, 73, 307.
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r 2009 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 1 January 2010 10.1021/ed800038w Published on Web 12/18/2009
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