Philip 1. Bayless
Wilmington College Wilrnington, Ohio
An Introductory Course in Organic Chemistry
O n American college campuses today the most harried expressions are likely to be worn not by presidents, or faculty deans, or even admissions officers, but by the many students of organic chemistry. An exaggeration? Obviously. But to the undergraduate struggling to unwind the braided strands of nomenclature, structure, physical properties, uses, syntheses, and analyses, the possession of merely human faculties seems to place him at a disadvantage. In few other courses is he charged with sorting and ordering and predicting so much, and with such attention to detail, as in Organic Chemistry-An Introductory Course. The treatment of the subject by functional groups, beginning with alkanes or alcohols and comprehensively dealing with all aspects of each compound type, is prevalent and classic. Recently, several texts have made possible an organization by reaction type, stressing similarities between functional groups rather than differences. Both of these approaches suffer (although not inherently) from weaknesses which are glimpsed hurriedly by the instructor in early fall, but become invisible among the natural products of late spring. The first mentioned treatment demands simultaneous insights into the many facets of each compound type, while the second requires an immediate plunge into the depths of reaction mechanisms. Common to both is the diierence in viewpoint between most organic courses and the preceding general chemistry course: a shift in emphasis from analysis to synthesis-a shift for which a summer vacation or a year in the analytical laboratory provides no transition. In our attempts to remedy these shortcomings we desired an understandable transition from general chemistry, and a gradual accumulation of the many viewpoints from which a contemporary organic chemist considers his science. Such a transition should build upon the familiar and the convincing. The following discussion relates our experience over the past three years with students in a four-year liberal arts college. The interests of these stndents have ranged the sciences and the pre-sciences; their abilities vary from the 16 to the 99+ percentile;' they attend the same lectures, perform the same laboratory experiments, and are graded on the same scale. Two beginnings seemed worth trying. A survey of the reactions of the common monofnnctional compounds using a limited number of familiar reagents (HOH, HC1, NaOH, Br,, Na, Hz) gives the student a brief look at principal reaction types. Bond polarity and the re-
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ducing action of metals is stressed in this treatment, and analogies are drawn to HOH, NHa, and H2C03 (the theory of types). Thus RNH,
+ HOH
-
+ OH-
RNHZt
(1)
This is a competition for H + between the unshared electron pairs on the nitrogen and oxygen atoms. RNH2
+ HaO+ + (CL-)
+ HOH + (CI-)
RNHaC
(2)
This is the same as reaction (I), with equilibrium "shifted to right" because of the higher H30+ concentration.
+
+
RNH* Na+ O H -No reaction except (1) which by the high concentration of OHis ~upp~.eased RNH*
-
+ Bn
RNHBr
+ HBr
(3)
(4)
Also, the alkyl group reacts as a hydrocarbon would with Br?. RNH*
+ Na
RNH- Na+
+ '/pH2
(5)
The metal reduces the amino hydrogen, since it is the most positive atom in the compound. RNH?
+ H2
-
No reaction with the amino group
(6)
I t should be pointed out that ammonia undergoes thc same set of reactions. Since a fairly complete qualitative analysis scheme can be developed by the students from these six reagents, the students can experimentally distinguish between classes of compounds early in the laboratory work. The second approach makes use of acid-base chemistry to initiate the students to the manifestations of resonance and induction. The nucleophilic displacement on hydrogen [Bronsted acid-base reactions as represented by equations (1) and (2), in which the amino group displaces OH- or H20 from the H+, and the OH- or H,O displaces RNH, from the H + ] is perhaps the reaction most familiar to beginners. The emphasis on the equilibria of weak acids and weak bases in the general and analytical courses allows the discussion to be based upon quantitative evidence. Both beginnings have been successful. We have chosen, however, to preface these with a few lectures dealing with structures, nomenclature, and the relation of structure to physical properties. Treating these first allows a review of bond type-physical constant relationships before the more complex comparisons of reactivities. Even the nomenclature survey is popular--especially with the pre-medics-this early in the semester. Petroleum is placed here as descriptive relief from the preceding generalizations, and to illustrate structure-property relationships.
Following the surveys of structures, physical properties, and chemical properties, displacements on hydrogen are dealt with in detail. Displacements in general are then presented using rate studies to differentiate details. Thus primary alkyl halides undergo nucleophilic displacements faster than do secondary alkyl halides by an 8 x 2 mechanism, because of the inductive and steric effects of the alkyl groups. Tertiary halides react by a first order (&I) mechanism in which the inductive and steric effects of the alkyl groups help stabilize the carbonium ion intermediate.% Substitutions introduce the free radical mechanism which complements the ionic reactions studied earlier. Addition and elimination reactions prepare the student for the study of oxidation and reduction methods. Oxidations are treated here primarily as degradatory and analytical procedures; current reagentsa are added to the procedures for reduction given to the student, but he is responsible for remembering only a few of the reaction conditions. A lecture and a mimeographed sheet of qualitative analytical procedures provide a review of the work to this point. The semester closes with the structural study of carbohydrates. The reactions dealt with during the first semester are used to illustrate structural relationships, not syntheses. No transformations are given on examinations; questions involving reactions are of the "supply the product" or "supply the reagent" variety, or "differentiate between compounds by some simple analytical reaction." The student's response is to accept a structure given h i without considering its preparation, and to speculate on its physical and chemical properties (and frequently its name). Examples of such structures are vitamin A and quinine. The laboratory work attempts to provide an experimental basis for the lectures without depending on specific prior knowledge furnished by the lectures. The initial survey of properties gives the student sufficient analogies to correlate the behavior of unfamiliar compounds encountered in the laboratory. At the beginning of the first laboratory period, each student is given a sample of a different liquid aldehyde with which he learns the unit operations of purification and identification. After a few weeks of determining physical constants and the melting points of derivatives, the aldehyde has gradually been identified. During this time he makes free use of identifiration tables and dictionaries of organic compounds. Only after identifying his aldehyde and performing several simple reactions with it does he undertake a preparation. Each student prepares phenyl magnesium bromide and condenses it with his aldehyde, having modified the t,ext procedure accordiigly. This allows a comparison of yields for substituent and structural effects. To check his technique and to learn other methods for separation each student attempts (!) to recover and identify all the products of his reaction mixture. He is aided in this by reference to the original literature. In t,he past, different classes of compounds were
For a discussion see HINE, J., "Physical Organic Chemistry," 1956, McGraw-Hill Book Co., Inc., New York. 3 For a review see BROWN, H. C., J. CHEM EDUC.,38. 173 1
(1961).
assigned but uneven difficulty and versatility resulted. Aldehydes suit the scheme admirably, and most students benefit from the additional exposure to carbonyl chemistry. The first semester of Organic Chemistry thus emphasizes analysis: simple reactions and physical properties. The principal emphasis during the second semester is synthesis. The student is asked to review the first semester's reactions as methods for preparing the various classes of compounds (rather than viewing them as reaction types). If he has maintained a card file of reactions, as suggested, he has only to rearrange them according to functional groups. With the overall reaction on one side of the card and the mechanism on the reverse, yields and key references may easily be added. During this review the lectures deal with the behavior of reactive intermediates (carbanions, carbonium ions, free radicals), followed by a concerted attack on carbon-carbon condensations, aromatic syntheses, rearrangements, and a series of descriptive examples of these. We have found that the common factors influencing syntheses and the similarities between the various types are more readily presented,
Outline of the Course Tezt: Fieser, L. F. and Fieser, M., "Basic Organic Chemistry," 1959, D. C. Heath & Co., Boston, Mass. Supplementary Reading: Cram, D. J. and Hammond, G. S., "Oreanic Chemistrv." 1959. McGraw-Hill Book Co.. New Y&K, N. Y . Laboratory tezt: Fieser, L. F., "Experiments in Organic Chemistry," 1955, D. C. Heath & Co., Boston, Mass.
..
Fiwt Semester Lecture tollics (number) Laboratory topics Structures and nomenclature Purification o f adehyde by f.7, fractional distillation. Determination of boiling point and refractive index. Petroleum (i) Reactions and solubilities ( 3 ) Selection of possible aldehydes from identification tables. Acids and bases ( 3 ) Preparation of derivatives. Dis~laeements( 3 ) Crystallization and melting Suhititutions (i) points of derivatives. Aromatic compounds ( 3 ) Completion of identification. Additions (3) Oxidation of aldehyde and pKa Eliminations ( 3 ) of resulting acid.. Cycloalkanes (2) Comparisons of pK. values. Oxidation and reduction ( 3 ) Griznard reaction of aldehvde. Quditative analysis ( 1 ) 1soI%on of uroducts by kluStructure proof ( 2 ) tion chromitagraphy. Optical isomerism ( 4 ) Identification of products. Carbohydrates (4) Second Semester
Synthesis and preparative procedures
,-,
Grignard syntheses ( I ) Friedel-Crafts syntheses ( 1 ) Diazo reactions f 1)
~e&rmgements( 3 ) Mechanism of the Sommelet rearrangement ( 1 ) Heteroevclies and alkaloids ( 3 ) Industrial pioeeases ( 3 ) 0rg.mometsllica ( 2 ) Nitrogen and sulfur compounds (9, ,a1
Silicon compounds ( 1 ) Volume 39, Number 6, June 1962
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and acquired, as a unit. Presentation of syntheses withm a three week period has resulted in better comprehension, greater facility, and increased interest in this phase of the course. A single lecture describing some of the most useful literature of organic chemistry is given near the middle of the second semester. Each student then chooses an outstandmg contemporary chemist whose work he is to review during the remaining weeks; he will thus become familiar with some chemistry of current interest. Such work can be easily traced in the Author Indexes of Chemical Abstracts, and might include H. C. Brown's papers on strained carbonium ions or J. D. Roberts' studies of small ring compounds or the alkaloid syntheses of R. B. Woodward. From this point in the course the lecture material is largely descriptive. The assigned reading is not extensive. The second semester laboratory consists of the usual sequence of syntheses which illustrate many preparative procedures and reagents. A more integrated laboratory with literature searches could be instituted, but our students need the disciplme of "yield and purity."
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We have been pleased with the comprehension shown by our students, and by their ability to read the current literature with understanding. Instructors soon become inured to direct compliments from students, so that the unintentional ones become that much more rewarding. Following one of the examinations during the second semester, a very average (39 percentile) pre-medical student apologized for being unable to remember the product of the pinacol rearrangement; he had had to derive it from the mechanism. Because of the small size (25-30) of our classes, the ever-changing (up) abilities of our students, and a constitutional skepticism toward comparison data, we cannot claim that this treatment is superior to our past efforts or to those of other colleges. We still have some excellent students, some average students, and even some indierent students. But the instructor's accommodation to and enthusiasm for a given presentation is certainly not the least important factor in its success. So, if the reader should lind the logic of this treatment appealing, I urge him to cut it to his measure, and wear it in good health.
