In the Laboratory
Qualitative Analysis in the Beginning Organic Laboratory James H. Cooley and Richard Vaughan Williams* Department of Chemistry, University of Idaho, Moscow, ID, 83844-2343; *
[email protected] Simple questions from students are frequently provocative enough to make us rethink our teaching methods. Such was a question that came from a student during the beginning part of an organic course. It was, “How do you know all these things?” Many of the textbooks of 25 years ago presented more of the methods by which the subject is studied (1). Organic chemistry, as it is presented in textbooks today, is a collection of facts originating from conclusions reached by interpretation of data collected in some laboratory at some time. Except for the description and interpretation of spectra, description of other data obtained in the laboratory is not found. This approach adds to the students’ perception that organic chemistry is an abstract subject. As an example, most organic texts start with a description of bonding and move on to discuss molecular structure. The structures are presented as factual information to be remembered—and for all too many students, just memorized. A New Approach We have developed an approach to teaching the organic laboratory course (2) which emphasizes the collection and interpretation of data in order to solve a problem. This approach provides a more balanced introduction to organic chemistry and an answer to the question posed by our student. One topic which we include in our laboratory course, and which is very popular with our students, is qualitative organic analysis. The debate over whether to include classical methods of qualitative analysis in the laboratory course has been relatively quiescent in this Journal recently. However, two papers on this topic appeared a few years ago (3, 4 ). The first claimed that with the advent of a computer library of organic compounds attached to an IR instrument, students always got the unknown correct and learned very little from the experience (3). The second defended classical qualitative analysis as a good way to learn organic chemistry (4 ). Our own, admittedly limited, poll suggests that at least a third of professors feel that because of spectroscopy, classical “qual” is no longer needed. In “qual” the student is exposed to many of the reactions discussed in the lecture course. For most students this is a very positive experience, which reinforces their understanding of organic chemistry. All these reactions give highly visual and pleasing results such as a color change or formation of a precipitate. Reactions covered in a lecture text, such as the reaction of potassium permanganate with an alkene or 2,4dinitrophenylhydrazine with an aldehyde or ketone, are reviewed and performed in the laboratory. The standard textbook presentation tends to treat each functional group in isolation. In qual, the very different reactivities of the various functional groups are compared. This comparison of the reactivity of different functional groups strengthens the student’s understanding of the subject. The shift in emphasis
away from synthesis experiments, where the objective is to obtain a product, to using reactions to deduce information about an unknown is usually a welcome change. Data such as boiling points and melting points, as well as spectra, must be determined carefully in order for the student to justify reaching a conclusion about the structure of the unknown. When formation of a derivative is required, students must adapt a general procedure to a specific compound. They frequently learn a great deal from such an experience. For example, when a derivative is required, we find that students gain a greater understanding of recrystallization and the relationship of purity to melting point. Finally, introduction of a very systematic and logical approach to determining structure is something students like. We consider qual to be an invaluable part of our organic chemistry course. In classical qualitative analysis, as presented in Shriner, Fuson, Curtin, and Morrill (5) or Cheronis, Entrikin, and Hodnett (6 ), the student is directed to identify the unknown from tables that list possible compounds. The tables are organized to collect together compounds with a particular functional group and to index these compounds in order of increasing boiling point for liquids or increasing melting point for solids. To use these tables, the student must determine the functional group that is present in the unknown and the unknown’s boiling or melting point. Since it is crucial that the boiling point or melting point be determined correctly, we advise students to repeat the determination of these properties until they are certain that the results can be reproduced. In an earlier experiment we introduce IR spectroscopy as the best method for identifying a functional group; in these experiments we introduce solubility and classification tests as a means of identifying functional groups. We ask students to classify unknowns by using solubility and classification tests before recording any spectra. Students welcome the verification of results that comes from using both methods. Method
The First Experiment We elected to introduce this systematic approach to qualitative analysis, but to limit the number of classes of compounds and to use only three experiments. In the first experiment the unknowns are limited to alkanes, alkenes, alkyl halides, primary and secondary alcohols, and ethers. The procedure followed by our students is summarized in Flow Chart I. For tables, we refer students to the CRC Handbook of Tables for Organic Compound Identification (7 ). Solubility tests introduced at this point are in water, ether, and concentrated sulfuric acid. Water and ether serve to introduce the technique used in determining solubilities and help to
JChemEd.chem.wisc.edu • Vol. 76 No. 8 August 1999 • Journal of Chemical Education
1117
In the Laboratory
distinguish a low-molecular-weight alcohol or ether from the rest. Concentrated sulfuric acid distinguishes an alkane or alkyl halide (which are insoluble) from an alkene, alcohol, or ether (which are soluble). Classification tests include, for the alkyl halide, the Beilstein test, formation of a precipitate with ethanolic silver nitrate or with sodium iodide in acetone; for the alkene, the Baeyer test with aqueous potassium permanganate and decolorization of bromine in methylene chloride; and for the alcohol (primary or secondary) oxidation with chromic acid. Because good classification tests specific for the alkane or ether are not known, the presence of these functional groups must be deduced from the solubility test in sulfuric acid, negative results on other classification tests, and eventually from IR and NMR spectra. The data students collect allow them to use the tables to select a few possibilities. While we recommend that proton and carbon NMR and IR spectra be used to confirm the identity of the unknown after its identification from “wet tests”, students are required to decide for themselves exactly what data to collect. Grading is based not only on the right answer but also on the written discussion the student makes by careful interpretation of the data that have been collected. A major aim of these experiments, emphasized in the grading scale, is that the student gain and display understanding.
tests introduced in this experiment include a 2,4-dinitrophenylhydrazine test for an aldehyde or ketone, a Tollens test for an aldehyde, formation of CO2 bubbles in the solubility test with sodium bicarbonate for a carboxylic acid, Hinsberg and nitrous acid tests for an amine, ferric chloride and bromine water tests for a phenol, and hydroxylamine, followed by ferric chloride, for an ester. As in the previous experiment, students use both the solubility and classification tests and boiling or melting point to identify the unknown.1 As before, following identification with “wet tests”, the student is encouraged to confirm the identification with IR and NMR spectra.
The Third Experiment In a third experiment, the student is given an unknown belonging to one of the 11 classes of compounds introduced in the first two experiments. We ask the student not only to identify this unknown, but also to select, prepare, and purify by crystallization a solid derivative of it. The melting point of the derivative is determined and compared with that listed in the tables. Discussion and Conclusions The qual experiments are a big departure from earlier laboratory experiences. The student is to develop a plan for “solving” the unknown by deciding what tests to run and how much data to collect to present a convincing argument to the instructor in support of their structural assignment. We find that students need time to reach an understanding of the experiment in addition to enough time to complete the assigned tasks. We inform the students in the first period how many unknowns of each type they must identify in the scheduled time (up to seven three-hour laboratory periods). We allow them to work at their own pace and encourage them to identify additional bonus compounds. At every step there is sufficient time to carry out confirmatory positive tests on
The Second Experiment In a second experiment the classes of compounds introduced are aldehyde, amine, carboxylic acid, ester, ketone, and phenol. The procedure followed is summarized in Flow Charts II and III. In this experiment solubility tests in water, 5% hydrochloric acid, 5% sodium hydroxide, and 5% sodium bicarbonate are introduced. To decide between weak and strong acids and bases, the student is directed to test water solutions with litmus or pH paper. These tests introduce the solubility behavior of acidic and basic compounds, and most students are able to use them to decide among the various functional groups without too much difficulty. Classification
Unknown A Water soluble No
Yes
Alkane, Alkene, Alkyl Halide, Alcohol (>C-5), Ether, (>C-5)
Alcohol (
