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JCE Featured Molecules
William F. Coleman Wellesley College Wellesley, MA 02481
Molecular Models of EDTA and Other Chelating Agents September Featured Molecules Deirdre Belle-Oudry presents a variation on an old theme in her paper on using an indirect EDTA titration for sulfate analysis (1). EDTA and (often loosely) related species are this month’s Featured Molecules.
1.0
0.8
Fraction
H5Y
á
2ź
H2Y
0.6
HY3ź
H6Y2á H4Y
0.4
Y4ź
H3Yź 0.2
0.0 0
2
4
6
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pH Figure 1. A distribution diagram for the EDTA system. At the pH of normal waters, the predominant species have one or both of the nitrogen atoms protonated. (Figure shown in color on p 1159.)
EDTA is a hexaprotic acid (H6Y2+) having the pKa values given in the featured paper (1). Figure 1 shows a distribution diagram for the EDTA system (2). At the pH of normal waters, the predominant species have one or both of the nitrogen atoms protonated. Complexation, however, requires that both nitrogens be deprotonated and it is generally assumed that the form that complexes with metal ions is Y4−. Structures of several forms of EDTA are included in the molecule collection (Figure 2). These structures are quite flexible having many conformations that are readily accessible at room temperature. An introduction to EDTA chemistry leads to broader questions of metal ion chelation or sequestration. Related chelating agents included in the molecule collection are EGTA, DCTA, NTA, BAPTA, and DTPA. Molecular dynamics and Hartree–Fock calculations on BAPTA (Figure 2) confirm that many conformations, ranging from those with the phenyl rings parallel to one another, to more elongated forms, are essentially isoenergetic in room temperature aqueous solution (3). Also included in the molecule collection are several crown ethers, an isophore (nonactin), and a cryptand. These not only provide students with a glimpse of the types of molecules being employed for metal ion sequestration but open a wide range of topics of current research in a variety of areas of inorganic, industrial, environmental, and biological chemistry. Literature Cited
EDTA (H6Y2+)
1. Belle-Oudry, Deirdre. J. Chem. Educ. 2008, 85, 1269–1270. 2. This figure is from the spreadsheet available at http://www. wellesley.edu/Chemistry/Flick/chem205/alpha6.xls (accessed July 2008). 3. A short movie of a low-level (MM2) molecular mechanics calculation on BAPTA—one picosecond in one femtosecond intervals at 298 K—can be found at http://www.wellesley.edu/Chemistry/ Flick/bapta.html (accessed July 2008).
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Sep/abs1296.html Full text (HTML and PDF) with Figures 1 and 2 in color Links to cited URLs and JCE article Supplement
Find “Molecular Models of EDTA and Other Chelating Agents” in the JCE Digital Library at http://www.JCE.DivCHED.org/ JCEWWW/Features/MonthlyMolecules/2008/Sep/.
The molecules added to the collection this month are EDTA,
BAPTA
Figure 2. Two of the molecules added to the JCE Featured Molecules collection this month.
1296
EGTA, DCTA, NTA, BAPTA, DTPA, several crown ethers, an isophore (nonactin), a cryptand, and Eriochrome Black T (a metal ion indicator).
Journal of Chemical Education • Vol. 85 No. 9 September 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
