Learning Chemistry from Good and (Why Not?) Problematic Results: Kinetics of the pH-Independent Hydrolysis of 4‑Nitrophenyl Chloroformate Omar A. El Seoud,* Paula D. Galgano, Elizabeth P. G. Are ̂ as, and Jamille M. Moraes Institute of Chemistry, University of Sã o Paulo, P.O. Box 26077, 05513-970, Sã o Paulo, Sã o Paulo, Brazil * S Supporting Information ABSTRACT: The determination of kinetic data is central to reaction mechanism; science courses usually include experi- ments on chemical kinetics. Thanks to PC-controlled data acquisition and availability of software, the students calculate rate constants, whether the experiment has been done properly or not. This contrasts with their experience in, e.g., organic synthesis, where a broad melting point indicates an impure product. In order to teach quality kinetics and link theory to experiment, we used a multistep project, based on the (convenient) pH-independent hydrolysis of 4-nitrophenyl chloroformate in aqueous organic solvents. The steps of the project included statement of the experiment’s objective; a quiz on reaction mechanism and experimental techniques in chemical kinetics; students’ decision on the organic solvent to be employed; extraclass activity to assess their choice of the solvent; carrying out the experiment; and discussion of the results obtained. We have applied the constructivist approach to illustrate that controlling the experimental conditions (solution temperature and homogeneity) is a prerequisite for obtaining quality kinetic data. The students’ evaluation was highly positive because they participated in the different steps of the project. KEYWORDS: Upper-Division Undergraduate, Organic Chemistry, Physical Chemistry, Hands-On Learning/Manipulatives, Esters, Kinetics, Reactions, UV-Vis Spectroscopy ■ INTRODUCTION Most of our knowledge about reaction mechanism came from, and still comes from, kinetic data. Therefore, chemistry courses for science students include experiments on chemical kinetics, e.g., catalyzed acyl transfers 1-3 and sugar reactions. 4 In addition to acquiring practical skills, the students attach much importance to linking theory to experiment. 5 Representative examples of this approach include the introduction of thermodynamic versus kinetic control of reactions 6 and the interplay between chemistry and visual art. 7 Chemical kinetics experiments can be deceptively simple, especially because data acquisition is mostly done by PCs and software is available to calculate rate constants. This convenience may tempt a few students to use a “black-box” approach: mix the reagents, acquire experimental data, and then calculate the results. Therefore, they are usually able to calculate rate con- stants, independent of their quality. Some may not give proper attention to eventual problems with data fit, e.g., the agree- ment between the calculated “infinity” reading and the experi- mentally determined one; the magnitude and variation of the residuals (di fferences between experimentally and theoretically calculated data points, e.g., by iteration) with time (t). Ideally, the residuals should be small and vary randomly with t. To illustrate this situation, compare two aspects of acetyl- salicylic acid (aspirin), namely, its synthesis (organic chemistry laboratory) and hydrolysis (physical chemistry laboratory). In synthesis, the student assesses the outcome of his or her work by comparing any of the following product properties with literature data: mp 135 °C; IR spectroscopy (ν CO peak of the acetyl group at ca. 1754 cm -1 ); 1 H NMR spectroscopy (CH 3 CO- peak, at 2.352 ppm). 8 Hydrolysis of aspirin requires attention because it is subject to general acid- base catalysis; its rate constant depends on the temperature (T); solution pH; and the nature and concentration of the buffer. 9-13 If the student does not control the first two experimental variables properly, the calculated rate constants may appear in order if examined for a single run. The problem appears, however, when the data from different students are employed jointly, e.g., in plots of observed rate constant, k obs , versus catalyst concentration, or log k 2 (second-order rate constant) versus 1/T (Arrhenius plot). When this problem occurs, the student may feel frustrated, justifiably so especially because repeating the experiment may not be feasible. Obtaining quality kinetic data, therefore, requires attention because the effect of problems, if they do Laboratory Experiment pubs.acs.org/jchemeduc © XXXX American Chemical Society and Division of Chemical Education, Inc. A DOI: 10.1021/ed5007426 J. Chem. Educ. XXXX, XXX, XXX-XXX