Teaching Organic Chemistry with Student-Generated Information Ronald M. Jarret and Paul D. McMaster College of the Holy Cross, Worcester, MA01610 Five years ago, the general chemistry courses a t Holy Cross College were modified to promote greater student involvement. I n our new approach students conduct experiments and gather information that, when pooled, allows for discovery of a particular concept (I).The underlyi n g philosophy h a s now been extended to organic chemistry. We have a traditional course content, but the experiments are designed to be more than a mere vehicle for carrying out standard organic laboratory techniques and reactions. The student-generated data is also used to introduce various topics and to elaborate on them, rather than simply to verify presented material. Thus, the novelty of this approach is in the timing and interplay of the laboratory exercise and classroom lecture-discnssion. The flow between lecture and laboratory is greatly improved, and the significance of each is amplified through the cooperation. The laboratory has become the driving force for the course, where students are made partners in the learning process. Our model of discovery calls for the instructor to encourage student creativity within the confines of the structure imposed by available laboratory resources. An observation is introduced and questions are raised to guide students in proposing testable explanations. Experiments are conducted a s originally planned by the instructor, and the student results are pooled. Discussions are resumed in light of the original questions and extended to include all aspects of a particular topic normally covered in the traditional course. This indirect approach certainly takes more lecture4scussion time than the traditional style, but the discoverybased laboratory provides a greater opportunity for learning (both process and content) than the more direct method. As such, the individuals involved in overseeing the laboratory must be sufficiently trained to teach process, content, and technique within this discovery format. The %and holding" that pies on durine the teachine. (.~ r o m ng ., of process . . ~ t..i student response in expenment dcslgn and interpwtation of data! 1s at the same level as that normallv found in the teaching of content in the classroom. Time is spent on the distinction between observation and theory, but the students still need to work problems, read the text, and think about the material on their own for true learning to take place. n
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Representative Experiments Our selection of experiments represents two fundamental types of investigations. One set centers on the physical properties of organic compounds and the other (larger) group of experiments foeus on some aspect of an organic reaction.
Solubility and Extraction In the first group of experiments, students learn to handle liquids and solids. During the lab check in of the first week of class, each student mustmix the two chemicals or solutions listed on the back of their sign-in card and record whether or not they are miscible-soluble. Among the possible items tested are substances that will be used in a n upcoming extraction-recrystallization lab (e.g., benzoic acid, Caminoacetophenone, dilute sodium hydroxide, dilute hydrochloricacid, water, ethyl acetate, methanol, etc.). We then pool student data with two purposes: to point out that likes dissolve likes and to dacide (in appro+ate extraction and recrystallization solvents. The exercise is concluded by recording melting points of rrude and recrystallized materials, as well as carrying out a mixed melting point for compound identification Distillation a n d Boiling Point Dends The concepts of boiling point; distillation, and separation efficiency are also introduced in the first few weeks of the semester. Each of the three laboratory benches (with up to ten students) are assigned a functional group, with several representative members of a homologous series. Each student uses simple distillation to record the boiling point of a single compound. By pooling data, students discover boiling point trends in a homologous series and between different functional = u p s in compounds of comparable mass. A disrussion of ;nttmnolecular force; is c edispersion a s well as dipolar -generated. The i m p ~ ~ n a n of and hydrogen-bonding interactions is discovered. The students are also asked to exchanee ., a ort ti on of their distilled compound with the distillate ofsomeone else at their bench. Then one student uses simole distillation and the other fractional distillation to separate the common two-component mixture. By plotting volume of distillate vs. temperature for each set-up, the student pair can evaluate the two methods of distillation for their system. Class data can be pooled to generalize the findings and provide guidelines for selection of simple or fractional distillation in future uses. ~
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Organic Reactions In addition to the experiments described below that focus on rate order, students explore various topics using the discovery approach: the stereochemistry in Diels-Alder reactions (endo-rule)(6)and bromine addition to alkenes (68); regiochemistry in electrophilic aromatic substitution reactions (4,941); stoichiometry of Grignard reactions involving a ketone, ester, and carbonate-ester (2, 6, 7,9,1214); and the outcome of various synthetic transformations.
Physical Properties of Organic Compounds
Alkenes by Elimination (Dehydration)
In addition to the experiments described below, students discover trends in pKds for substituted benzoic acids ( 2 4 ) and discover chirality with molecular models, a polarimeter, and molecular mechanics (5).
Students have studied the structure. stabilitv. .. and properties of alkenes. The general elimination reaction is introduced in lecture bv disdavinp the production of isobutylene from t-butanh a i d theterminal and internal Volume 71 Number 12 December 1994
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draw the expected ~ r o d u c t s(based on their discussion) for thk dehy&ation reaction of 4-methyl-2~ e n t a n o land decahvdro-2-naphthol before conh c t i n g the experiment. Each student generates a gas chromatogram of their product mixture (e.g., Fig. 1). Examination of the accompanying mass spectra identify all products a s having the same molecular weight as the expected alkenes. (Library data-base searching often gives leads to the identity of methylpentenes.) The relative ratio of alkenes formed is dependent on the specific reaction conditions. For example, addition of about 10 mL of water facilitates distillation ( ~ r o d u c isolation) t but establishes a different (eaAy) equilibrium mixture of octahydronaphthalenes in the same reaction time (Fies. l a and ib). 3 At first d a n c e a t their chromatoerams. stuTime (mind dents realize that more peaks are than expected for a n E2 reaction. The similar retention time of peaks and idmttcnl mass of molecular Ions lead to the reall~atlonthat a collect~onof Isomers has been generated. Students familiar with carbocations identify the mechanism a s being E l . If students had no prior knowledge of the behavior of carbocations, they were asked to make a list of likelv s t r u c t u r e s (CcH,n for dehvdration of H ~ of ~ decahymethylpentanol, c ~ f i i~dehydratibn dro-2-naphthol) to account for the number of observed pkaks in the chromatogram. They are also asked to look a t this list in light of the original question of the experiment: How are products formed? In this context, it seems verv unlikelv to students that t h e collection of resuited from a n E2 type of mechanism. They have discovered that dehidration proceeds throigh a two-step process and that the carbocation intermediate is 2 3 4 eapahle of rearrangement. Rearrangement to the Time (min.) more highly substituted carbocation will thus yield the most highly substituted alkenes a s the major Figure 1. GC trace of isomeric octah dronaphthalenes (octalins);$1 and 2) trans A' products. In discussing this reaction, students are told and octalin; (3) mlxture of cis AY and A'.' octaiins; (4) CIS A octalin; (5) A'.= octalin; (6) hexahydronaphthalene derived from octahydronaphthol impurity; (a, that the system is actually more complicated than above) concentrated sulfuric acid or ~hos~horic acid used: fb, below) about 10 mL presented. For examole. thev are told that carbocaof water added to the reaction mixture, kons can also he formed from protonation of alkenes and that carbocations can react with nucleophiles (like water). Given this, students are then alkenes resulting from the reaction of 2-butanol with acid. asked to consider which features of the experiment drive Questions are then raised about the reaction. the equilibrium toward the alkene? To this end, GC traces Which. if an". ofthe structures would be the ~redictedmaim (for the dehydration of decahydro-2-naphthol) are preproduct k~netlcv?. thermodynnmlc mntroi,? sented that represent different stages in the reaction (Figs. W h a t is rhr rnrrhnnwn rimmgnfarrps ofthr dchydrativn l a and lb). The discussion of alkene formation via eliminareaction? tion is also extended to include the reaction of alkylhalides with base and the E2 mechanism. Both the hydrogen and the leaving group depart at the same time (E2). The leavine before the hvdraeen Nucleophilic Substitution ....e r o u, ~deoarts , . .. (Ell and generates a carbwatlon mtermedrate. Adiscovew laboratory on the SNl-SN2 reactions of alcoThe leavmg ~ m u pdeparts after the hydrogen El,,.,,, hols (with halides under acidic conditions) can be introThe E l c process ~ (with a carbanion intermediate) is immeduced with a format similar to the one presented for the diately ruled out because of the acidic conditions, leaving dehvdration reaction. Students have studied the structure the E l and E2 mechanisms. andproperties of'alcohols and discussed the nature of nuThe direction of the discussion a t this point depends on cleophilcs and leaving groups The general sul)stitution rewhether students understand carbocations and their rearaction IS introduwd in lecture hy displaying the pn,duction rangement. If so, judicious substrate choice (4-methyl-2of nlkylhalides from HX and a variety of alcohols I 1 , . 2'. or pentanol(7b, 12, 15, 16) or decahydro-2-naphthol) can be 3"). besti ions are then raised about'the reaction. used to differentiate between the two possible mechaWhat is the mechanism (timing of steps) of the substitution nisms. If not, students can discover the involvement of carreaction? bocations in the dehydration reaction of alcohols prone to rearrangement by observing more than the expected numThe hnnd ro the nurlrophde IS h n g formed while the bond to the leaving group is breaking Su2 ber of products. Either way, students are instructed to
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1030
Journal of Chemical Education
The leaving group departs before the nucleaphile binds (SN1)and generates a carbocation intermediate (in the rate-determining step). The nucleophile binds before the leaving group departs. The third mechanism (with a n intermediate that violates the octet rule) is immediately ruled out, leaving the SN1and SN2 mechanisms. In looking for a way to distinguish between the SN2and SN1pathways, students note the involvement (or lack of involvement) of the nucleophile in the rate-determining step. Primary (1-hutanol(l4) or l-hexanol), secondary (2-hexanol), and tertiary (t-bntanol (14) or 3-methyl-3-pentanol) alcohols are individually halogenated through a competition for chloride and bromide ion under acidic conditions. If the reaction goes by a n S Nprocess, ~ the major product should be the one generated by the stronger nucleophile (Br-1. However, a n S N path ~ should generate a nearly equal mixture of alkylbromide and alkylchloride. Each student generates a gas chromatogram of their product mixture (Fig. 2). Inspection of the GC trace allows the student to quickly distinguish be-
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dent to discover the need for different mechanisms based on the substrate. Closer inspection of the GC trace for the secondary system (in comparison with that of the primary system) indicates that some product may be generated by both routes. Discussions can also be extended to include other leaving groups and nucleophiles, as well as related E 1 - E 2 mechanisms and carhonium ion rearrangements. Conclusion We are promoting a sequence of introductory courses that call students to be active, rather than passive, participants in chemical education. The course content outlined is traditional, but a n emphasis is placed on teaching and learning chemist r y by doing chemistry through t h e scientific method. With each undergraduate carrying out a slight modification of a general experiment, the student is able to claim ownership, feel personal involvement, and make significant contributions. By pooling class results we also support cooperative, rather than competitive, learning.
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Acknowledgment We wish to thank Jamie New for assistance in developing and testing the experiments. We recognize Michael McGrath for his assistance in planning and participating in this program. The authors are grateIS 2 23 3 ful to the Pew Charitable 'Rust (through NECUSE) Time (mln.) for of laboratory and the Keck Figure 2. GC trace of alkylhalides (C6ti13X. X = Br. CI);(a)l-bromohexane(major), renovations' R' M. Foundation for I-chlorohexane (minor);(b) 2-bromohexane (major),2-chlorohexane (minor);(c) acknowledges the National Science Foundation- 3.ch~~ro-3-methylpentane. 3-bromo-3-methylpentane. Instrument and Laboratory Improvement Pro7. (a, hhman, J. w operotiond organicclrpmistry: ~ ~ andy ~ anc o n :on ton, 1988; gram (USE-8852774, USE-9052318), Hewlett-Packard, i b ~ e h m a n J. , w o p m t i a n a l Omagonic Chemistry, 2nd ed.; Allyn and Bacon: the Kresge Foundation, and the College of the Holy Cross Bostan, 1981. for instrument acquisition and maintenance. 8. Reimer. M . J A m r Chem. Sm. 1948.64,2510. 9. D m & H. D.: Dokel, 0.W EzpeCmentd 01ganic Chemistry; MeCraw-Hill: New York. 1987. H ~ ~ W O L. O ~M.: , M O O ~ XC . J . E X ~ P orgonic ~ ~ ~ chemistry : ~ I Pnncipksandhcti-; Blackwell Scientific: Oxford, 1989. 11. Fulkmd. J. E. J. Chem. Educ. 1974,51,115. Wkwi C. F..Jr.&m"monlalOm~icChemisfrv: McMiltim:NewYork, 1984. 12. Robe*. .R.M.:Gilbert. J.C.:Rodewald. L. B.: Winmve.A. S.M o & m E z m r i m n t ~ l . Ovanic Chemistry;~mndem:hil lad el phi., 19%. Most. C. F. Jr ErpP~rp~imrnlal Organic Chamkfry; Wiley: New York, 1988. 13. Fieser, L. F:Williamson, K L. O m a n i e E x ~ e ~ i m = l s ; H e a tLe*ngtan. h: MA.1987. S,E~~rimelsinPhplcolorgonicChamistry~M~Millian:hndon, 1969, 14. Pavia, D.L.: Lampman, G. M.: M z , G. S. Inlmduclion Lo OlgonicLobomlory &hJarret, R.. M.: Syr,N. J. Chem. Educ. 1930.67.153-155. niquea; Saunders: Philadelphia, 1988. 15. Nienhouse. E. J. J C k m . Educ 1868.46.765-766. Nimitz, J. S. Erperimrnts i n Organic Chemistry; Prenfice Hall: Englewood Cliffs, 16. Henne, A. L.; Mahlszak,A. H. J. Anwr Chrm. Soe. 1944.66.164P-1652. NJ. 1991.
Literature Cited 1. We,R.W: lhtzler, M. A J. Chem Edue. 1991,66,228-231. 2. 3. 4.
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