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William F. Coleman Wellesley College Wellesley, MA 02481
Structures of r-Oxocarboxylic Acids William F. Coleman Department of Chemistry, Wellesley College, Wellesley, Massachusetts 02481
[email protected] w This paper contains enhanced objects available on the Internet at http://pubs.acs.org/jchemeduc. n
JCE Featured Molecules for October 2010
In their paper on R-oxocarboxylic acids, Robert C. Kerber and Marian S. Fernando describe the role of the hydrated forms of those species in interpreting the corresponding acid dissociation constants (1). This paper is, in many ways, a companion piece to an earlier publication by Kerber on the correct structure of dehydroascorbic acid, the oxidized form of ascorbic acid (2). The Featured Molecules for this month (3) are drawn from these two papers. Table 1 lists the 3D, rotatable images in MOL format of the molecules available in the HTML version of this paper. They include one member of each enantiomeric pair of the oxidized ascorbic acid (molecule 1 in ref 1) and gas-phase and solution-phase structures of the species in Scheme 1 of ref 1 (with R = CH3), pyruvic acid and its conjugate base and hydrated pyruvic acid and its conjugate base. A number of student exercises can be designed around these interesting molecules. The stereoisomers of dehydroascorbic acid provide useful practice with using R and S notation. Students might also follow the stereochemical course of the oxidation of ascorbic acid and see whether they can explain the origin of the stereochemistry that is observed for the product. Ascorbic acid is also included in the molecule collection for this month. A computational chemistry experiment could easily be devised around the equilibria in Scheme 1 of ref 1 for pyruvic acid (Figure 1). Using the provided structures as starting points, calculations at reasonably high levels of theory would allow students to explore the thermodynamics of this scheme in both the gas-phase and, more relevantly, in solution. The structures that are provided have been
optimized at the MP2 level using the Dunning basis set cc-pVDZ. A collaborative project would enable individual students to carry out calculations, including frequencies, so as to provide thermodynamic data at several levels of theory and using several solvation models; students could then combine their data to explore the equilibrium system. The calculations done to produce the structures are consistent with the hydrated pyruvic acid being the dominant species in solution at low pH. These calculations could be extended further to include
Figure 1. This scheme portrays the equilibria in aqueous pyruvic acid solutions. Pyruvic acid (upper-left corner) is a key intermediate in the metabolism of carbohydrates, fatty acids, and proteins. In dilute water solution at 25 °C it is 70% hydrated (lower-left corner).(1).
Table 1. JCE Featured Molecules for October 2010 Featured Molecule
Description
dehydroascorbic acid (in water; 1SRR)
optimized at the DFT-B3LYP-6311þþG(d,p) level
dehydroascorbic acid (in water; 1SRS)
optimized at the DFT-B3LYP-6311þþG(d,p) level
dehydroascorbic acid (in water; 1SSR)
optimized at the DFT-B3LYP-6311þþG(d,p) level
dehydroascorbic acid (in water; 1SSS)
optimized at the DFT-B3LYP-6311þþG(d,p) level
ascorbic acid
optimized at the DFT-B3LYP-6311þþG(d,p) level
hydrated pyruvic acid anion (in gas phase)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
hydrated pyruvic acid anion (in water)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
hydrated pyruvic acid (gas phase)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
hydrated pyruvic acid (in water)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
pyruvic acid anion (in gas phase)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
pyruvic acid anion (in water)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
pyruvic acid (in gas phase)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
pyruvic acid (in water)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
glycine zwitterion (in gas phase)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
glycine zwitterion (in water)
optimized at the MP2 level using the Dunning basis set cc-pVDZ
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Vol. 87 No. 10 October 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100802d Published on Web 08/30/2010
On the Web
discussions of gas-phase ionization energies and proton and electron affinities. Another example of differences between gas-phase and solution-phase structures is found in the case of amino acid zwitterions. The video available in the HTML version of this paper, the stepwise optimization of an initial glycine zwitterion in the gas-phase at the MP2/cc-pVDZ level, clearly shows that the zwitterion structure is not stable. Rather there is a proton transfer that leads to the optimized geometry.
r 2010 American Chemical Society and Division of Chemical Education, Inc.
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Literature Cited 1. Kerber, R. C.; Fernando, M. S. J. Chem. Educ. 2010, 87, DOI: 10.1021/ed1003096. 2. Kerber, R. C. J. Chem. Educ. 2008, 85, 1237–1242. 3. JCE Featured Molecules from Jun 2002 through Dec 2009 are available at the JCE Digital Library, http://www.jce.divched.org/ JCEWWW/Features/MonthlyMolecules/ (accessed Aug 2010). JCE Featured Molecules from Jan 2010 to the present are available in the HTML version of each column.
pubs.acs.org/jchemeduc
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Vol. 87 No. 10 October 2010
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Journal of Chemical Education
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