Piaget and Organic Chemistry The Equilibrium-Kinetic Approach for Teaching Introductory Organic Chemistry R. Daniel Libby Colby College, Watewille. ME 04901
Introductory organic chemistry has long been a course that has struck fear into the hearts of prospective students ( I ) . According to the learning theory proposed by the develo ~ m e n t aosvcholoeist l Jean Piaeet, a t least one cause of this fear may be the &scontinuitqst'udents experience when beeinnine the course (2). The current "functional moup" a p ~ r o a c ~to e sorganic chemistry do not build directl; up& the chemical backerounds that students acquire in their introductory c h e m k r y courses. Instead, these methods beein the studv of unfamiliar oreanic molecules with theoretical conside&.tions of concept;ally complex kinetically controlled reactions. Whether the first reaction considered is free radical halogenation (3), nucleophilic aliphatic suhstitution ( 4 ) ,or electrophilic addition (5),i t requires completely different methods of analysis from the equilihrium-controlled reactions that dominate most introductory chemistry courses. Thus, functional group approaches to teaching the introductory organic chemistry course produce a great discontinuitv in our students' chemical education. Piaget viewed knowledge as a mental framework that allows individuals to manipulate obiects and ideas ( 2 ) .Learning, according to Piaget, is an active process that consists of small steps in which new elements areadded torhe student's mental framework expanding herhis ability to understand new situations. In each step, the student encounters unfamiliar information, an object or concept, and attempts to understand it. When the information is found to be inconsistent with the student's mental framework, s h e h e becomes confused. This confusion is termed "disequilihriurn" or "cognitiveconflict." If the disequilibriumisnot toosevere, it engenders in the student a desire to understand the new situation. The student then beeins the process of "equilibration", where sheihe works with the new informacon and attempts to modify herhis mental structure to accommodate the new data. Finally, if the original information relates closelv enoueh to the student's urevious experience, sheihe can reach mental "equilibrium"%here herhis mental structure is appropriately modified to allow assimilation of the new data. The key factors in the process are its active nature and the need for the new information to he sufficiently limited and related to the student's previous experience to produce only moderate disequilibrium. Thus, students must be able to relate adequately to the new data so that the disequilibrium is not so great as t o render them unable to
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equilibrate with it or to discourage the students from any attempt a t equilibration. So, if Piaeet's theory is correct, large discontinuities betweenour students' preiious experi: ence and the new material presented in our courses might leave them few alternatives hut to memorize facts with little or no initial understanding. In short, new material must build upon already existing mental framework rather than creating a whole new framework of its own. An Analysls of the Problem Considering the apprehension that students seem to bring into introductorv oreanic chemistrv and the considerable volume of information covered in two short semesters, one would exnect that heeiunine the course hv buildineuoon the elementary principiks wi& which the-students gave some facilitv would he essential. However. the currentlv course designs seem to ignore the students' hackeround. Over the last 20 vears introductory oreanic c h e n k r y courses have moved from mostly desciiptGe to largely theoretical. The increasing inclusion of reaction mechanisms has changed the focus of most of the course discussion, but in most cases the organization of courses has changed only slightly. In general the material is organized in order of increasing complexity of molecular structure. This was loeical20 or 30 vears aeo when reactions were treated as facts connected with theufunctional group structure and there was little consideration of the theoretical explanations for the reactivity of individual functional groups. Now, essentially all textbooks, regardless of their organization, use reactionmechanisms to relate the structure of organic molecules to their chemical behavior. The use of mechanisms as bases for analyzing reactions has been a very valuable tool for practicing physical organic chemists. Also, the organizations and presentations of material in all current textbooks is logical and clear for anyone who is familiar with organic reactions and the relationships among them. However, these texts seem to overlook the limited experience of the typical organic student. Thus, the students, who in their earlier chemistry courses have mastered equilibrium concepts, are immediatelv oronelled into the world of activation enereies. , transition states, approximate structures of transition states. ~redictinestabilities of transition states. choosine rate-li&iting steps, and understanding the difference he-
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tween transition states and intermediates. So, by introducing our students t o the simplest classes of organic compounds, we are boggling their minds with complex theories of reactions. In addition, consideration of each functional group as a separate entity needlessly segments the course material introducing additional discontinuities within the course. The combination of all of these discontinuities is a tremendous shock t o most students, and I maintain that these standard approaches are placing needless hurdles in the paths of our students' progress. An Overvlew of A Proposed Solutlon To deal with these discontinuity problems, I have reorganized the course material into a continuous mechanistic development of organic reactions that builds upon the facility with chemical equilibria that most students bring into the course. I believe that this new organization, "the equilibrium-kinetic approach", produces a course that begins a t the incoming students' level and helps them work their way throueh the maior concepts of organic chemistry. Essentially the s&e m a t e h is covered &with the funitional group approach, but reactions are introduced on the basis of mechanism, and then synthetic applications of each type of reaction are considered. I have used this course organization a t three different institutions, Skidmore, Barnard, and Colby Colleee. over the last 12 years and have found that it seems to s o G m a n y of the problems indicated above. The course begins considerations of reactions with simple equilibrium controlled acid-base reactions. By studying hybridization, electronegativity, resonance, and the inductive effects on the strengths of acids and bases, students acquire a sense of how charges are stabilized in organic molecules. This sense is then used to probe the more complex reaction mechanisms involved in ihe equilibrium-conholled reactions of carbonvl-containine com~ounds. The carbonyl reaction mechanisms can for the most part be understood as processes in which reasonable steps are those that form relatively stable intermediates. Thus, the reactions can be analvzed on the basis of stabilities of completely bonded &l&les or ions. There is no need to conaider transition states with partial bonds and partial charges. The idea of equilibrium'formation of the most stable reaction intermediates isemphasized in predicting the pathsof reactions, the identity of products, &d the relative reactivities of compounds. After mastering equilibrium-controlled processes, students are prepared to deal with kinetically controlled reactions. Since they are comfortable with judging stabilities of intermediates, the students can more easily understand how to analyze the structures and stabilities of transition states and to use these analyses to predict structural effects on reaction rates. The syllabus for my course isshown in the table. Theorder is similar to that used in one textbook (6). However, of tooics ..this text does not emphasize the equilibrium vs. kinetic control that is central to my approach. Instead, it seems to present reaction mechanisms along with examples as groups of items to be memorized. I have found that, although this book follows acomplementary order of topics, it is less useful to students in my course than are others (4b,c).
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Speclllos ot the Course Unit I: Bondlng, lsomerlsm, and Organlc Functlonal Groups
This unit is designed to build upon the concepts of bonding introduced in general chemistry courses. We quickly review Lewis theory and valence bond hybridization descriptions of bonding in organic molecules. These ideas naturally introduce the possibilities for structural isomerism. Finally, the multitude of possible structural isomers for a particular molecular formula (e.g., CbH9NO) are limited to the most likely possibilities by the introduction of functional
Course Syllabus I. Bondlng. Isomerism, and Organic Functional Groups I!. S t e r e ~ l ~ o m e r land ~ mConformational AnalysisB A. Geometrical Isomerism 8. Configurational lsomerlsm C. Conformational Analysls Ill. lnmd~ctlonto Organic Reactions-Equlllbrlum Controlled Reactlons IV. Add-Base Reactions V. Reactions of Carbonyl Compounds A. lntraductlon 8. Addltian Reactlons of Aldehydes and Ketones C. Addltlan-Ellminatlon Reactlons of Aldehydes and Ketones 0 . Acyl Substitution Reactions of Carboxyllc Acids and Their Deriv* tives VI. DBterminatlonof the Structure of Organlc Moleculesd A. Mass Spectromehy B. IR Spectroscopy C. NMR Spectroscopy VII, lnnod~ctionto Kinetically Controlled Organlc Reactions ~ VIII. Nucleophilic Aliphatic Substitution-S.1 and S NReactions IX. @Ellmination Resctlons-E, and Ea Reactions X. Addition Reactions of Alkenes and Alkynes XI. Aromatic Compounds and Elecnophlllc Aromstlc Substitution Reactlons XII. Organic Free Radlcal Reactlons A. Organlc Free Radlcals 0. Addition to Alkenes C. Sub~tItUflonReactlons with Alkanes
m i s unncan bedoneatany point beforeunit VII. Altlmss I have placed ilalterunit V. and it works well. amis unit can be done at any wlnt In me course. I usually do it near the end of first semester or el the &ginning of second semester 80 that the teohnlquer can be used for qualitative organ10 analyolo In the ssoond-semester laboratory.
groups as the most often encountered groupings of atoms in organic molecules. Unit 11: Stereoisomerismand Conformatlonal Analysis
In Unit I isomers are defined as compounds with the same molecular formula but having non-superimposable threedimensional structures. Studeitsnatur& see how the definition applies to structural isomers, and with a minimum of encouragkment they can extend the concept to stereoisomers. Geometrical (E-2)isomers are considered first and then configurational (R-S) isomers are introduced. Configurational isomers are the most difficult for students to recognize. However. I have found that. bv buildine two enantiomeric structurks that appear to bk iientical and then showine how thev cannot be s u ~ e r i m ~ o s e dI . can convince &dents that" these isomers a;e reai. Once the existence of enantiomers is established, the structural causes of configurational isomerism can be developed with little problem. Finallv. a distinction is made between configurations and conforkations so that conformational analysiiof acyclic and cyclohexane systems can be considered. The stereoisomerism and conformational analysis unit builds logically upon the concepts introduced in Unit I and works well at this point; however, covering this material here tends to delay considerably the consideration of reactions. Facility with stereoisomerism is not really needed for the consideration of carbonyl reactions. So a t times I have placed this unit after carbonyl reactions (Unit V). I believe that it works equally well in either position. ~
Unlt 111: Introduction to Organlc Reactions-EqulllbrlumControlled Reactions
This unit is the break between structure and reaction of oreanic molecules. I t is verv short, usually only one class p&iod, and briefly reviews the coneept of kquilibrium that students first encountered in their general chemistry courses. The differences between kinetic and equilibrium control are briefly discussed, but the emphasis is placed on Volume 68 Number 8 August 1991
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the important characteristics of equilibrium controlled reactions. Specifically: A. All reactions are considered ta be fast; thus they must have relatively low free energies of activation. B. Since reactions are fast, the overall free energychange (AGO) of s reaction step is the important characteristicin determining how favorable a reaction is. Also, the more negative the A G O , the more product will be produced at equilibrium. So, reasonable reaction mechanisms must consist only of relatively stable intermediate species and products. C. If estimates can be made of relative stabilities of the reactants and products, the relative tendency of a reaction to proceed to products can be estimated. Unit IV: Acid-Base Reactions
This is the key unit for the course and is probably where mv a ~ ~ r o a differs ch most from those of most textbook authors; I utilize Bransted-Lowry acids and bases to teach a method for utilizing charge stabilization effects to predict relative AGO values for acid-base reactions. First, we consider the large variations in pK, values among organic acids and bases. Then we explore the way these variations can be explained using electronegativity, hybridization, charge state, resonance, and inductive effects on the stabilities of charges on oreanic ions. Finallv, we use these effects to predict relative p ~ values , of additional organic acids and bases. These predictions are all qualitative. We focus on ~redictinethe order of acidities or~basicitieswithin a "eiven --& ~ u of p &mpounds rather than the magnitude ofthedifferences between comoounds. Also. we consider all tvves -. of organic acids and bases, including carbon as well as oxygen and nitrogen acids. Thus, students begin toget a sense of the stabilities and reactivities of carbanions and protonated nitrogen and oxygen species. An understanding of these stabilities is essential to being able to work with the more complex oreanic reactions studied later in the course. The charge stabilization effects develop rather naturally from the hmdina and structure disrussions in Unit I and are usually discussed in both general and introductory organic chemistry courses, but rarely are they connected with predirting positions of equilibria. L'sually they are invoked in analyses of organic reactions to explain relative stabilities of transition states where partial charges are present. Thus, in most courses, the first real application of this complex group of effects comes in dealing with a new and complex entity, the transition state. My reason for emphasizing this unit is its use of a familiar vehicle. Brhsted-Lowrv acid-base reactions, to introduce the ap&ic&ion of a vaguely understood, by students, group of charge stabilization effects. Thus, continuity is maintained in the students' development by establishine the process for analnine reaction stem with a famil. iar reactiodtype before new reactions are introduced.
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Unit vl Reactions of Carbonyl Compounds
Armed with the methods for analyzing equilihrium reaction steps developed in unit IV, the students are ready to deal with new, more complicated organic reactions. First, we review salient features of the carbonyl bond and introduce examples of all three types of reactions of carbonyl compounds to he considered in this unit. Since the unit includes consideration of aldol and Claisen condensations, the Wittig reaction. Michael addition. and Robinson annelation. a numlrer different chapters of most organic textbooks (e;g., aldehvdes and ketones. carboxvlic acids. carhoxvlic acid derivatiGes, and reactions of enois and end~ates[darbanions]) are used as readine assienments. My approach is %st point out that compounds containine the carbonvl - moup - - undereo different t m e s of nucleophi~icreactions a t the carbong carbon but that these reactions can be loosely grouped into three categories:
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A. Addition to the carbonyl group (nucleophilicaddition). B. Substitution for the carbonyl oxygen (addition-elimination). 636
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C. Substitution for an atom that is attached to the carbonyl carbon (acyl substitution). We then identifv the structural differences between --- the groups of molecGes that seem to favor each path. From this analvsis. we find that reactions A and B involve onlv aldehydes &d ketones, while reaction C occurs primarify with carhoxvlic acids and their derivatives. Reactions A. R.and C are found t o have a common pathway of additibu'to the carbonvl carbon in the initial s t e ~ sand , we proceed to trv to understand the structural causes for differences that &cur as the reactions progress. The choice of reaction C vs. reactions A and B is related t o the ability of the atom attached to the carbonyl carbon of an a w l compound to stabilize aneeative charge after the bond between the atom and the carbonyl carbon had been broken, thus allowing the erour, containing this atom LO act as a leaving group. A carbon r' hydrogen atom attached to the carbonyl carbon of an aldehyde or ketone usually cannot act as a leaving group; thus, in most rases, aldehydes and ketones are hlocked from the acyl substitution path. The decision for aldehvdes and ketones between reactions A and B is then found to-depend upon the availabilitv of a second acidic Droton on the orieinal nucleophile. c em oval of this proton,after the nucleopbe h i s added to the carhowl carbon. ~ r o v i d ethe s lone oair of electrons needed to aid ih the eli&nation of the o;iginal carbonyl oxygen as either hydroxide ion or water, depending on the basicity or acidity of the reaction medium. In the process of our discussions, we deal with both acid and base catalysis and analyze possible pathways on the basis of the relative stabilities of the intermediate species that would be formed. Thus, we use the techniques developed with acid-base reactions in Unit IV to work step by step through the development of and relationships among the reactions of all carbonyl compounds. This approach leads the students through the process used by physical organic chemists to devise "reasonable" organic reaction mechanisms. I t gives them experience with identifvine the structural variables that seem to be responsible fbr iifferences in reactions of similar molecules. Students also develop an appreciation for the way apparently very different reactions can be connected through a general reaction mechanism. This process of identification and classification of reactions by type of mechanism seems to me to be the most difficult to erasD - * and yet the most valuable aspect of the study of organic reac~
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Finally, with a reasonable ability to recognize reactions by mechanism, the students are prepared to consider synthetic applications of carhonyl reactions. As with reaction mechanisms, I introduce organic synthesis as a process. We consider synthesis using the retrosynthetic or disconnection approach (7). This method of analysis relies quite heavily upon an understanding of reaction mechanisms: and thus it reinforces the students' understanding of the reactions while ~rovidinethem with new tools for devisine" svnthetic oaths. " AS eachiew reaction is mastered throughout the cour&, new disconnections are discussed. and the students' svnthetic framework builds one element at a time. Unlt VI: Determination of the Structure of Organic Molecules
This unit is essentially independent from the rest of the course. I t can be covered a t any point after Unit I. I usually do i t in the middle of the year so that the techniques can be used in the qualitative organic portion of the second semester laboratory. Unit Vll: Infroduction to Kinetically Controlled Organic Reactions
The emphasis of this unit is on specifvine the differences between kinetically controlled and eq"ili6rium-controlled reactions. Basic transition state theory is introdured, and we make clear distinctions between transition states and reaction intermediates. I use specific steps from carbonyl reac-
tions to show how t o simulate the structure of the transition state of a known reaction step. Then we discuss the way the relative free energies of activation of reactions can he estimated from differences in structures of reactants and transition states. Again we use known reactions to develop the applications of steric and electronic effects on the rates of reactions. The approach is very similar to that used with equilibrium controlled reactions in Units IV and V except that we are comparing the stahilities of reactants with transition states rather than reactants with intermediates or products. The fleeting nature of the transition state structure makes it more difficult for students to analyze, hut they rapidly begin to accept that an effect that stahilizes a positive charge also stabilizes a partial positive charge but to a lesser extent. Work in this unit prepares the students to analyze thenew kinetically controlled reactions to he considered in the next several units. Unit VIII: Nucleophilic Aliphatic S~bStit~tion-S~l and St2 Reactions
The obvious kineticallv controlled reaction to consider first is the SN2 reaction. There is a smooth transition from the introduction in Unit VII to this one-step reaction. Structural effects on these reactions are nicely accommondated by the simplest possible mechanism and everything falls nicely into place. Another strength of this unit is the convenient source of disequilibrium that can he introduced by attempting to extend the S Nmechanism ~ t o S Nreaction ~ stiuations. When the data does not fit the expected mechanism, we diacuss how to use the data to aid in modifvine the mecha- nism to fit the new results. Then we identify tLe key structural factors and reaction conditions that favor each tvpe of reaction. This approach gives students tools and experience that allow them to recoenize the characteristics of each reaction type easily and cokidently predict the correct outcome eiven a related but unfamiliar reaction situation. and 4
Unit iX: @-EliminationReactions-El
P-Elimination reactions are introduced as reactions that usuallv compete with nucleophilic substitution reactions. l.hus.-I presknt data showing how changes in structure and reaction conditions change the proportion of elimination vs. anhstitution. and we identifv the imnortant factors thattend to favor each type of reaction. hes similarities between the S Nand ~ E, mechanisms are discussed. and the Dossibilitv of a product-determining step that is separate f r k n the ratedetermining step is introduced. Again this unit builds upon and is tightly connected to the previous unit. Students generallv find that devisine - the. 3-elimination mechanism flows smo&hly and logically from their work with nucleophilic substitution reactions.
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Unit X: Addition Reactions of Alkenes and Alkynes
Electropbilic addition is simply the reverse of the ELelimination mechanism. Thus addition reactions build nicely upon the ideas developed and discussed in Unit IX. This unit introduces the possihility that a a-bond can act as a base to yield a carhocation conjugate acid. The stereospecificity and regiospecificity of electrophilic addition reactions are used together with steric and electronic effects to devise the mechanism. Then differences in specificities are used to recognize the differences in mechanism between the electrophilic reactions and concerted addition reactions such as hydroboration, epoxidation, and Diels-Alder reactions. Unit XI: Aromatic Compounds and Electrophilic Aromatic Substitution Reactions
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Aromaticity and the Hiickel4n 2 mle are introduced as applications of molecular orhital theory to delocalized ring
systems. With an understanding of electrophilic addition and the particular stahilities of aromatic systems, students can reasonably understand why aromatic a-systems tend to undergo suhstitution rather than addition. Again this mechanistic variation results from changes in stahilities of the transition states for potential reactions of a carhocation intermediate formed by the attachment of an electrophile to a a-system. When the transition state for loss of a proton is stabilized by incipient aromaticity, suhstitution occurs, hut, when there is no possihility of producing aromaticity, the suhstitution transition state is less stahle, and addition of a nucleophile to the carbocation is more likely. Thus, the new concept of aromaticity is used to account for a new reaction. Unit XI\: Organic Free Radical Reactions
I start this unit by comparing the structures of organic free radicals to those of carhocations and carbanions. Students can easily accept that radicals are electron deficient and thus are stabilized by the same types of effects as are carhocations. We then extend the discussion t o relate hond energies to the stabilities of radicals formed by scission of the hond. Finally, we consider potential reactions of radicals and see how chain processes work. Having extensive experience with analyzing transition state stahilities, the students can relatively easily deal with radical addition and suhstitution reactions as well as the explanation of peroxide effects on addition of HBr to alkenes. Summary
Introductory organic chemistry courses may be unique in the way that most of the material considered in the course can he explained through reasonably well understood reaction m e c h i s m s . The major changes in organic courses over the last 15 years have introduced much more of the theory of reaction mechanisms. These changes a t once present t h e possihilitv of greatly simplifying the study . ~ . of organic chemistry or of making & hopelessly confusing. Sincemost practicing organic chemists use mechanisms to aid them in thinkingahout the potential paths and synthetic uses of o r ~ a n i reactions, c these mechanisms may provide the Piagetian mental framework that students can build and use to deal with organic chemistry. Mechanisms are discussed in all new oreanic textbooks. but thev seem to be add-ons to the classicorganization rather thanan integral part of the material. Thus. current functional erouD present or- . an~roaches .. ganic chemistry as a series of loosely related shbrt stories. Although each part is self-consistent and logical, the collection as a whole lacks cohesiveness and creates hreaks in the continuitv of the students chemical education. I t seems to he time to take advantage of the conceptual framework ~ r o v i d e dby reaction mechanisms to present organic c h e m k y as one continuously unfolding story. The proposed equilihrium-kinetic approach is both internally continuous and blends smoothly together with the students' previous experience. Thus, i t brings introductory organic chemistry into line with Piaget's view of optimal learning opportunities. I . '"Symposium on Reducing D s u m a in the Undergraduate Organic Chemistry Course". Abstracts of the Annual Meeting of the American Chemical Society, 1978, Miami
Beach. Oiu. of Chem. Educ.,numbers 24-27. 2. Pieget, Jean J.Rrs.Sei. Teach. I964,2. 176-186. 3. ( a ) Morri~on.R. T.:Bayd, R. N. Organic Chemistry. 5th ed.; Allyn and B s o n : Boston. c Freeman: New Y m k , 1987. 1987. (b) Vollhsrdt. K . P. O ~ g a n i Chemistry: 4. la1 . . Solomons. T . W . C . Omonic Chcmiafrv: . Wilev: . New York. 1988. ib1 Eee. S. N. Ormnic Chamistry: H&: Lexington, MA, 1989. (4 Wade. L. G . J Po ~ Chem~ istry; Prentiee-Hall: Endewwd. N J . 1987. 5. (s)Csrey,F. A. Organic Chrmirfry:MeCrsw-Hill: New York, 1987.l b l Louden,G. M. Organic C h ~ m i s l r y ,2nd ed.; BenjaminICumminps: Menlo Park, CA, 1988. (el McMurray, J. Organic Chemistry, 2nd ed.: Brook~ICole:Pacific Grove C A , ,988. 6. Pine S. H. Organic Chemistry, 5th ed.: McCraw-Hill: New York, 1987. 7. warren S. Organic Chrmistrv. The Disconnection ~ p p r o o c h :wiloy: ow York, 1982.
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