In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
A Demonstration of Structure–Reactivity Relationships in Organic Chemistry submitted by:
Thomas A. Newton Chemistry Department, University of Southern Maine, Portland, ME 04104;
[email protected] checked by:
Grant Forman and Robert Stick Department of Chemistry, The University of Western Australia, Nedlands, WA, 6907 Australia
In recent years there has been an increased emphasis in organic chemistry texts on the use of pKa values of organic compounds to predict and rationalize the outcome of chemical reactions. Virtually all introductory organic chemistry textbooks now include tables of pKa values of the different classes of organic compounds. It is possible to demonstrate a correlation between the pKa values of various compounds and the conductivity of aqueous solutions of these compounds. This correlation, in turn, enables students to develop a better understanding of structure–reactivity relationships that are central to our understanding of organic reactions. Several articles describing the use of conductivity devices to demonstrate the concept of electrolytes have appeared in this Journal (1–3). These simple devices may be purchased,1 or they may be constructed from readily available materials (4). While conductivity devices are normally used to demonstrate the concept of electrolytes in general chemistry courses, repetition of this demonstration in organic chemistry is not only an effective way to refresh students’ memories about ions and conductivity, it also helps them to understand and predict the electrolytic properties of organic solutes from their Lewis structures. The development of such structure– reactivity relationships is central to understanding many of the subtleties of organic chemistry. Many students find it difficult to distinguish between strong and weak electrolytes on the one hand and between weak electrolytes and nonelectrolytes on the other. This is understandable since the sharp dividing line that they would like to use to separate one class from another does not exist. It is possible, by careful choice of solutes, to demonstrate the continuum between strong and weak electrolytes and between weak electrolytes and nonelectrolytes, and to give students semi-quantitative ideas of what we mean when we describe a compound as a weak electrolyte or a nonelectrolyte. Furthermore, the demonstrations help students make the connection between the conductivity of a solution and the pKa of the solute. Most organic chemistry textbooks include a discussion of acid–base chemistry and the use of pKa values to predict the outcome of a variety of organic transformations. Two demonstrations involving organic electrolytes are described. To be most effective, three conductivity devices, each equipped with a 7.5-watt bulb, are required. Small differences in conductivity are not as apparent with higher wattage bulbs. Also, the electrodes should be made from the same material and the distance between the electrodes should be the same in each apparatus. Side-by-side comparisons of each 294
set of solutions with the room lights dimmed make these demonstrations most effective. Demonstration 1: Strong Electrolytes versus Weak Electrolytes Prepare 0.01 M aqueous solutions of the following solutes: acetic acid (pKa = 4.76), chloroacetic acid (pKa = 2.86), and trichloroacetic acid2 (pKa = 0.64). Immerse a conductivity apparatus into each sample. The difference in brightness of the three bulbs is obvious. This demonstration also provides an opportunity for students to correlate the inductive effect of substituents attached to the α-carbon of a carboxylic acid with the acidity of that acid. Demonstration 2: Weak Electrolytes versus Nonelectrolytes Prepare 0.01 M and 0.1 M solutions of phenol2 (pKa = 9.9) and 4-nitrophenol (pKa = 7.1). The only solution in which the bulb glows is the 0.1 M 4-nitrophenol. The concentrations of H3O+ in 0.01 M and 0.1 M 4-nitrophenol are 2.8 × 10᎑5 M and 8.9 × 10᎑5 M, respectively. These values allow students to bracket the range of solute concentrations that spans the gulf between weak electrolytes and nonelectrolytes.3 The conductivity of the 0.1 M 4-nitrophenol solution also allows for a discussion of the resonance effect of the nitro group on the acidity of the phenolic proton, and more to the point, of the stabilization of the phenolate ion. Notes 1. Fisher Scientific Educational Catalog, number CQS43478 or www.fishersci.com (accessed Oct 2002), product number S43478, which uses the 7.5-watt bulb, CQS43478BLB. 2. Trichloroacetic acid and phenol are readily purified by distillation at atmospheric pressure. The former boils at 196–198 ⬚C, while the latter boils at 182–183 ⬚C. 3. The definition of the term nonelectrolyte is necessarily an operational one. In the present demonstration it means that the concentration of ions is so low that there is no detectable current using a 7.5-watt bulb. A lower wattage bulb would provide greater sensitivity and move the “boundary” between weak electrolytes and nonelectrolytes to lower concentrations.
Literature Cited 1. 2. 3. 4.
Gilbert, E. C.; Pease, C. S. J. Chem. Educ. 1927, 4, 1297. Suter, Hans A.; Kaelber, Lorraine. J. Chem. Educ. 1955, 32, 640. Thomas, William B. J. Chem. Educ. 1962, 39, 531. Mercer, Gary D. J. Chem. Educ. 1991, 68, 619.
Journal of Chemical Education • Vol. 80 No. 3 March 2003 • JChemEd.chem.wisc.edu
