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The Undergraduate CurricuIum in Organic Chemistry
The presentation of organic chemistry to undergraduate students has never l ~ r e n a neasy task. In onlpr t o und~rsrand the suhiect the student must a h w r t ~ numhcr a ofconcepts that seem difficult to a beginner and must also memorize a fair amount of factual material. As a student Emil Fischer (possihlv the ereatest of all organic chemists). des~airedof mas" tering the suhject ( l ) ,and its rapid and continual expansion increases the difficulties. I t is surprising that the question of what should he taught to undergraduates and the order and method in which the material ought to he presented has atl the last few tracted little attention in this ~ o i r n a (2iover years. Apparently most organic chemists agree with the view expressed in 1976 by the Organic Subcommittee of the ACS Division of Chemical Education Curriculum Committee that "undergraduate organic chemistry education is in an excellent state of health" (3).We do not agree entirely with this and think the suhiect deserves some debate. The general approach to teaching organic chemistry to undermaduates in the U S has settled into a well-established pattern. In their freshman year most students receive some instruction in organic chemistry; in many cases they go from methane to Vitamin BIZin two "easy" chapters and perhaps 6-12 lectures and, not surprisingly, they retain very little of what they covered. In the following year students who are majoring in chemistry, or those who have a need for suhstantial training in organic chemistry (e.g., chemical engineers and premedical students) take a course of about 90 lectures given over a year. Manv departments also offer shorter courses leg., one semester) which cater to the needs of students studvina hiolom and amiculture. As will become obvious later in this & t i d e & think that the difficultiei dpresenring a n adequate (:ourse in onr year are formidable and so find it hard to see how shorter courses in organic chemistrv can reallv " provide enough training to he useful as a preparation for the studvof hiochemistrv. In this article we will onlv he concerned with courses extending over one year.' The text chosen for the course usually influences the way it is taught, both in the topics covered and the way they are presented, and therefore we can use the popular texts as a Morrison guide to how courses are taught. For nearli20 and Boyd (4) has dominated the textbook market in the U.S. (at onetime being adonted hv nearlv 90% of chemistrv debartments) and is stili very widely "used. Its reputatibn is ~ r o h a h l vdue to its claritv. readihilitv. accuracv. and the The one-semester course was abandoned 12years ago in Nebraska, and we now run two types of year courses: one whichis primarily for maion in chemistrv and the other for non-maims. The main difference
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462 / Joornai of Chemical Education
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thoroughness of its coverage. Its success has influenced the format of many of its competitors. In general, texts have some introductory material recapitulating topics from general chemistry; alkanes and nomenclature are described followed by other hydrocarbons (usually including aromatic compounds); and then there is a detailed discussion of other functional groups. There are of course variations, e.g., aromatic chemistry may he treated a t a later stage, hut the general principle is to go through the various structural types in detail one by one. Stereochemistry and spectroscopy (especially proton magnetic resonance spectroscopy) are also dealt with early and in detail. Reaction theory and the concepts of reaction mechanisms are frequently introduced by an extended discussion of the halogenation of alkanes. At the end of the hook some special topics are presented: carbohydrates, amino acids, proteins, and polymers are usually included, with perhaps some rather perfunctory chapters on other natural products such as isoprenoids. In general, the one-year course is the only training in organic chemistry that a student receives as an undergraduate. Based on our experience of teaching undergraduates over many years, and on impressions of new graduate students, we do not think that the system works very well. Undergraduates often only begin to find organic chemistry interesting in the last few weeks of the year when they see how the suhject fits together. The impressions one gets from graduate students are even more significant. Many of them are good students who have received good grades as undergraduates in reputahle departments, and most of them do well later on in their graduate career. Yet a t the start of their graduate training many seem to have little real understanding of the suhject, in the sense that they propose answers that clearly are wrong (e.g., a nucleophilic reagent attacking a negative center or proposing to oxidize an alcohol to an acid using horohydride as the reagent!), and they have, at best, only poor ideas of how to set about even simple syntheses. We think that the prohlem lies tosome extent in the current method of teaching organic chemistry in which topic after topic is taken up in turn and dealt with exhaustively. From the point of view of a trained organic chemist this is an admirable approach, hut is it a good one for the beginner? The student is exposed to a succession of new ideas (some of which are difficult to absorb), hut many of these ideas are not essential to an understanding of organic chemistry, and indeed many of them will he used very little, if a t all, later in the course. The student going doggedly through the present courses must feel, like Wohler in the early 1830's, that he is hacking his way through a primeval forest in the tropics without any clear idea of where he is going. But in fact organic
chemistrv is an entirelv loeical subiect in which the Darts are interconnected; to grasp &is a person needs to know how to interconvert functional groups and modify molecular skeletons, so a beginner must be introduced as quickly as possible to the simple reactions that relate the main classes of organic (.c,mpoundsto each other. Yet the early chaptelr, of mans texts discuss such topics as ~~mlwnzenoid an,maticcompv~~ndior various forms of dissvmmetrv before discussine" Shll .. and S..N ~ and other basic reactions. Most chemists (even academic ones) can " eo through a.oroductive career without ever havine to deal u with a nonbenzenoid aromatic compound or encountering optical isomerism arising from biphenyls. Again in the chapters on alkyl halides various organometallic reactions are discussed in some detail. Here the problem is more subtle. Most practicing organic chemists use many types of nrgannmetallic reactions besides the Grignard reaction, but the question for the instructor is "Does knowledge of several common nrganometallic reagents during the first eight or ten weeks of a course reallv h e .l one's ~ overall understandine of organic chem~stry?"We think not-Grignard reagentsire enoueh . " in the beeinnine" to illustrate the ootential of this tvoe .. of reaction. Because so much time is spent on discussion of these types of subjects in the early part of the course, the reactions of carhonyl compounds, which are so important in organic synthesis, are done in the second semester leaving little time for the student to get a feel for reactions using carbonyl comnounds. As a result we find students. even a t the end of the ~-~~~~ year, whose approach to any synthetic problem is to produce an alkane, mysteriously halogenate it a t a specific position, form the required skeleton by a Wurtz reaction, and again use free radical halogenation to put in a required functional group. The students' reliance on free radical halogenation stems from the fact that it is the first reaction they study and is used as their introduction to reaction mechan&ms. But the vast majority of reactions go by a heterolytic mechanism in solution, and very little of what is learned in an extended discussion of free radical halogenations (including discussions of bond energies) is used later in the course. We think that a different approach could help the student master oreanic chemistrv more easilv. It would involve a change in t h e order of the topics rather than a change in the material taueht over the vear. The criterion for introduction of material early in the conrse should be whether it contributes to the students' understanding of the general picture of organic chemistry; nothing should he introduced in the early stages unless it will be used to build on later. Nonbenzenoid hvdmcarbons. radical reactions. metals other than maenesium should come late in the course rather than early, and even the treatment of benzenoid hydrocarbons should be deferred. The best time to introduce spectroscopy is difficult to decide. While spectroscopy IS an essential tool for anyone practicing organic chemistry, it is not necessary for an understanding of the subject. As it is not easy to make much meaningful use of nmr and ir spectroscopy early in an elementary course, they are best out off in a lecture course until earlv in the second semester. Essentially the first eight weeks of the course should he a selective survey of the major types of compounds, their nomenclature, their principal reactions, and the basic reaction mechanisms involved. The object would be to make the student familiar with several important functional groups and some of their kev interconversions. and to show how these intrrcmew&,ns tit into 3 limited number of merhanisric puttcrns.'l'h~.itudeut s h d d then ser that nraanic ~ h e r n i ~ t r v is a logical subject in which the various partsfit together in H definite pattern. It is difficult to be dogmatic about what reaction is best discussed first with students. Substitution a t saturated carbon has a lot to recommend it; apart from the importance of the reaction, it can he used to introduce the idea of the energy curve and the transition state. In any case, whatever reaction
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is chosen should be one that eoes hv a heterolvtic mechanism. While the specific topics to h'k covered in the 6eginning would be to some extent a matter of individual taste,. thev. should include the nommclntureand structure ofhydrocarbons, a]cohuls, rther.;, alkyl ha~idts,acids land their drrivati\.ez),aldehydes and ketones. It would he necessary to do enough stereochemistry to understand additions to double bonds, eliminations to form alkenes, and nucleophilic substitutions. Fundamental reactions and their mechanisms would include electronhilic addition (haloeens. hvdroeen . . . halides. and water) to, ;and hydroyenariun of, dmble and triplt: h n d s , nuclec,philic substitutim at saturafecl (SKI m d Sh2) and unsatu. rated car11011t i t l t e r c ~ n \ . e r i ~ ~~i ncarboxylic s acids and their dt:rn.ativesj, (:rignard reagents and their reactions with illdehydes and ketones ( n ~ ~ c l e o p h iaddition l~c at ~ ~ n w t u r : l w d carlxm). Reduction of curh(~nylcompoundi t g r alcohols and oxidation of alcc,hoIs ;hnuld lw mvntwned. The stres.: would he on modification of functional groups; the only chain lengthening reactions would be the Grignard addition and the displacement of halides with cyanide and alkynyl anions. The reactions mentioned here are those covered in the earlv chapters of the original t!dirion c g f ('ram and Hammc~ndL ~ I . (Jnev this marerial 1s cwnplered the instructor could set synthetic problem.
