Article pubs.acs.org/jchemeduc
Synthesis−Spectroscopy Roadmap Problems: Discovering Organic Chemistry Laurie L. Kurth and Mark J. Kurth* University of California, Department of Chemistry, Davis, California 95616, United States S Supporting Information *
ABSTRACT: Organic chemistry problems that interrelate and integrate synthesis with spectroscopy are presented. These synthesis−spectroscopy roadmap (SSR) problems uniquely engage second-year undergraduate organic chemistry students in the personal discovery of organic chemistry. SSR problems counter the memorize-or-bust strategy that many students tend to employ in the study of organic chemistry with an instructional strategy that fosters “seeing” connections, “hearing” what a problem has to say, and “feeling” the exhilaration that comes with discovery. KEYWORDS: Second-Year Undergraduate, IR Spectroscopy, Organic Chemistry, Inquiry-Based/Discovery Learning, Problem Solving/Decision Making, Reactions, Mass Spectrometry, NMR Spectroscopy
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INTRODUCTION
SYNTHESIS−SPECTROSCOPY ROADMAP PROBLEMS What information is presented in an SSR problem and how are SSR problems constructed? A typical example is presented in Figure 1. Each problem presents four pieces of spectral data: typically, one infrared (IR) spectrum, one mass spectrum (MS), one 1H nuclear magnetic resonance (NMR) spectrum, and one 13 C distortionless enhancement by polarization transfer (DEPT) spectrum (SSR problems employ calculated DEPT-135 spectra that show all nonequivalent carbons, including quaternary Cs that would not be seen in the experimental DEPT-135, where signals from CH2 and quaternary C are negative and CH and CH3 are positive), along with at least three reactions. Also, the problem-to-problem spectra order is variable (IR → 1H NMR → MS → 13C DEPT in one problem, 13C DEPT → IR → MS → 1 H NMR in another, etc.). On the basis of the presented data, an organic chemistry student unravels the problem and provides structures for each of the four unknowns. How are organic chemistry students instructed to tackle SSR problems? First, a student must realize that each problem is unique and that an SSR solution strategy must, by design, offer problem-to-problem flexibility. Second, a student must realize that these are composite problems; consequently, SSR problems cannot be solved as synthesis problems,8 and they cannot be solved as spectroscopy problems.9 SSR problems can only be solved as integrated synthesis and spectroscopy problems.10 Therefore, an appropriate solution strategy must offer the opportunity for students to see connections within the problem. For example, the number of carbon atoms in the DEPT spectrum for compound 3 (Figure 1), taken together with the reactions in
For some, the study of organic chemistry is exhilarating and enabling, but many students find the second-year undergraduate organic chemistry experience to be disarming or overwhelming.1 There are, no doubt, a multitude of explanations for these negative experiences: a first experience with major-specific coursework, an unwanted major-required hurdle, grade competition in what is perceived to be a “weeder” course, an endless onslaught of new and challenging material, etc.1a,2 Perhaps a unifying basis for these negative perspectives can be found in the memorize-to-bust approach,3 on which many students rely to navigate an undergraduate organic chemistry course. Indeed, the breadth of the subjectstructure and bonding, nomenclature, chemical reactivity, stereochemistry, mechanism, spectroscopy, synthesiscan be seen as underwriting what is often a desperate attempt to “survive” the experience. A counter4 to the memorize-to-bust strategy is a challenge that every organic chemistry instructor faces. 5 A synthesis− spectroscopy roadmap (SSR) instructional tool is presented here that aims to (i) foster “seeing” connections, (ii) “hearing” what a problem has to say, and (iii) “feeling” the exhilaration that comes with discovery.6 SSR problems are employed in the second and third quarters of a year-long introductory organic chemistry sequence. At this point in their progression, students are in a position to tackle SSR problems, which actively engage all seven of the breadth aspects listed above. Moreover, SSR problems help organic chemistry students to make connections between what they know and what they need to learn.7 © XXXX American Chemical Society and Division of Chemical Education, Inc.
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4)
5) 6)
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8) Figure 1. A typical example of an SSR problem. Each problem presents four pieces of spectral data along with at least three reactions (see SSR problem 1, Supporting Information for the full problem with spectra).
NaOH/Δ can hydrolyze a nitrile to either an amide or further to an acid.13 D2O treatment (NMR inset; compare integrals at 7.7 ppm) suggests that two protons are exchanged, which indicates that this nitrile hydrolysis produced an amide [R′′CN → R′′C(O)NH2]. Taken together, what does a student know to this point? A ketone was reduced to an alcohol; that alcohol delivered a nitrile-containing ether; partial hydrolysis of the nitrile delivered a primary amide. By deduction, compound 3 has the structure: R-O− CH2C6H4CN, and the aromatic ring is meta-substituted (from the alkylating agent structure). A first-pass inspection of the 13C DEPT of compound 3 shows 11 unique carbons; eight of these derive from the CH2C6H4CN introduced in step 2 of the reaction. By deduction, compound 2 must have at least three carbons (11 − 8 = 3). The student should also note that the final product, compound 4, has a six-proton methyl doublet and a three-proton methyl doublet. Both of these must have been present in compound 2 (i.e., reactions two and three did not add these methyl groups). Consequently, the peak at ∼18 ppm in the 13C DEPT of 3 must, in fact, be two peaks that represent three methyl carbons. As a consequence, compound 2 must have at least five carbons and an OH (MW of 2 = 88 g/mol; C5HO = 77 g/mol; 88 − 77 = 11 g/mol). Therefore, the formula of 2 is, most probably, C5H12O (MW = 88 g/mol). By inspecting the 1H NMR spectrum of 4, which is derived from that of 2, the student can deduce that compound 2 (C5H11O) has two identical methyl groups (0.95 ppm in 4), another methyl group (1.15 ppm in 4), and two methine groups (2.1 and 3.2 ppm in 4). Compound 2 also incorporates an OH. By deduction, the solution to this problem is depicted in Scheme 1.
Scheme 1. Solution for the SSR Problem Shown in Figure 1
steps one and two, informs a student about the number of carbon atoms in the starting material (compound 1). SSR problems relate a great deal of information to a student, and a student’s solution strategy must allow them to “listen” to all of it. In other words, a holistic top-to-bottom/bottom-to-top analysis is required. Often, molecular complexity builds top-to-bottom. To realize this suggests that an effective strategy will often distill down to the determination of the formula, degree of unsaturation (°U), and, ultimately, the structure of the starting material (compound 1). With this information, the ability to define the remaining three structures in the SSR problem becomes much easier. The application of this strategy to the SSR problem in Figure 1 (see SSR problem 1, Supporting Information for the full problem with spectra) might unfold something like this. 1) What is happening in step one? The IR suggests a CO in compound 1, and that is reinforced with the step one reagents (NaBH4 + CO → CHOH).11 2) What is happening in step two? An alcohol plus a base followed by an alkylating agent suggests a Williamson ether synthesis12 (ROH → ROR′; note that the alkylating agent supplies the structure for R′). 3) What is happening in step three? A student notes that the alkylating agent in step two brought along a nitrile; aq.
Two additional worked and discussed SSR problems are presented in the Supporting Information. The first of these (SSR problem 2) makes the point that each SSR is unique and that each requires a slightly different approach to find the solution. Another main point made by SSR problem 2 is that students are instructed to hear what the SSR problem has to say and solve it using the necessary information. Indeed, students quickly learn that SSR spectra often present more information than is needed to solve the problem. SSR problem 3 (Supporting Information) makes clear the close interplay between synthesis (learning reactions) and spectroscopy (learning how to interpret spectral data) in solving an SSR problem by pointing out that both an understanding of the chemistry and an understanding of the spectroscopy are essential to successfully tackle an SSR problem. This interplay is what makes SSR problems a useful learning tool for the organic chemistry student.
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DISCUSSION The real power of SSR problems is two-fold. First, and perhaps foremost, a student is called on to integrate many aspects of their B
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become more sophisticated and less transparent as the course progresses. To help address that, solutions to several SSR problems are illustrated in class over the quarter; these in-class examples are provided to students ahead of time so that they have the opportunity to spend time before the lecture working on the problem. Well over 100 self-study SSR problems are posted on the course website,18 and they are organized by book chapter (for example, chemistry of aldehydes and ketones or chemistry of carboxylic acids). Students are encouraged to work alone or in small, self-formed groups to solve these practice problems. Also, any individual SSR problem can employ any chemistry/ functional group/synthetic concept/spectroscopy that had been covered to that point in the course. Once SSR problems are introduced to the class, they are employed on the course exams. An important caveat in the group versus self-study issue is that students must understand that group effort and individual effort are two very different things in open-ended problems such as SSR problems; this is an important point to make to students since exams are individual effort only. Exam SSR problems are not graded “right” or “wrong,” but are very amenable to partial credit grading. Students are told that they must answer with structures that embrace their thinking, not bits and pieces of information gleaned from the problem. Indeed, wrong structures can and do clearly express a student’s level of understanding.
organic chemistry knowledge into the formulation of a solution to an SSR problem. Organic Chemistry is, in large part, the study of functional group interconversions. SSR problems instill that point by connecting a book-stated fact, for example, a nitrile can be partially hydrolyzed to an amide, with visual (i.e., spectral) signatures. Indeed, the SSR spectra bring the facts of each interconversion to light with tangible visual clues. Second, a student can enjoy the thrill-of-discovery in working SSR problems.14 For example, a student may know (learned the book-stated fact) that an alkene that reacts with Br2 and a primary ROH leads to bromoether formation; the connection of that knowledge to a heterocyclization (cf., 5 → 6 in SSR problem 2 in the Supporting Information) may be a discovery for that same student. The advantage is that a student can make that discovery by pondering an SSR problem. How does the instructor gather the spectra needed for SSR problems? The possible answers could be to take the spectra, use programs to calculate the spectra, or find the spectra in the chemical literature or spectral catalogs. The approach taken here was generally to use spectral catalogs to find the required IR and MS spectra and to use spectral calculation programs to generate the 1H NMR and 13C DEPT spectra. IR and MS spectra were obtained from the Spectral Database System (SDBS).15 This very useful and user-friendly database provided, for example, a perfectly functional SSR mass spectrum for phenethyl alcohol. On the other hand, some spectra were not quite ideal to present in an SSR problem. For example, the SDBS MS for (1-bromoethyl)benzene only marginally hints at a molecular ion cluster at 184/186, and to ask a student to draw that conclusion while solving an SSR problem might be unreasonable. If a vertical adjustment is made (i.e., the 184/ 186 peaks are increased in intensity), this problem can be corrected. The Mac program Paintbrush was used to make these types of modifications.16 Likewise, the SDBS provided useful and user-friendly IR spectra for the SSR problems. An appropriate retouch of the SDBS IR or MS spectra may be required to construct the desired SSR problem, since these touchups can positively affect the resulting SSR problem as a learning tool. For the 1H NMR and 13C DEPT spectra, MestReNova’s NMR prediction software (NMRPredict) was used.17 The 1H NMR spectra were presented with integrations, and the 13C NMR spectra were presented as DEPT spectra (showing all carbons). As illustrated in Figure 1, the SSR 1H NMR spectrum, the spectra can be manipulated to show integrations as well as spectral inserts (in the case depicted, a region expansion and a D2Oadded spectrum). If the 13C DEPT spectrum is shown (calculated DEPT-135 spectra are presented showing all nonequivalent carbons, including quaternary Cs that would not be seen in the experimental DEPT-135 spectra), this enables a student to sort out quickly the number and kinds of nonequivalent carbons in the unknown and come to a solution. The instructional strategy is to introduce SSR problems to an introductory organic chemistry class as soon as the spectroscopy chapters (IR, MS, and NMR) have been covered, which is generally in the third week of the second quarter (i.e., ∼13th week of a 30-week organic chemistry series). This introduction is accomplished with fairly straightforward and transparent SSR problems that draw on reactions, functional group reactivity, and functional group spectroscopy that have been covered to that point. As new reactions, synthetic strategies, and spectroscopy topics are covered, these are included in the SSR problems. By the very nature of this growing repertoire, SSR problems will
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STUDENT RESPONSE
SSR problems were employed in introductory organic chemistry for more than 20 years in both organic chemistry for majors (∼100 students per quarter) as well as organic chemistry for health and life sciences (∼400 students per quarter). The majority of students over those 20+ years found SSR problems to be a useful learning tool that uniquely integrated many aspects of studying and learning organic chemistry. For example, some students have said, “your reaction AND spec knowledge had to be up to par,” “SSR problems are a great way to use most of the main concepts,” and “SSR problems are the “real life” application of synthesis and spectroscopy.” Of course, some students found SSR problems to be a bit too challenging and commented, for example, “the only problem is that they are very difficult and result in low test scores” and “more time should be spent learning spectroscopy and synthesis before introducing SSR problems.” As summarized in Table 1, students are quite enthusiastic about the utility of SSR problems as a study device for organic chemistry (full evaluation in the Supporting Information). Perhaps the most important insight is that students report the close interplay between synthesis and spectroscopy as what makes SSR problems such a useful learning tool (relative agreement = 4.45 of 5). Students also report that the successful navigation of SSR problems requires that their knowledge of reactions and spectroscopy must be “up to par.” SSR problems were also generally described as “a great way to use most of the main concepts” taught in second-year organic chemistry and that SSR problems are “fun, logical puzzles as opposed to bland memorization” in the study of organic chemistry. Representative comments from these organic chemistry students were: C
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Table 1. Student Evaluations of SSR Problemsa
a
statements for response
relative agreementb
SSR problems helped me “see” connections in organic chemistry concepts. SSR problems helped me learn how to “hear” all of the information presented. SSR problems allowed me to “feel” the exhilaration that comes with discovery. SSR problems cannot be solved as synthesis problems alone. SSR problems cannot be solved as spectroscopy problems alone. SSR problems are composite problems that require the integration of synthesis and spectroscopy. The close interplay between synthesis and spectroscopy is what makes SSR problems useful learning tools.
4.25 3.85 4.20 4.65 4.50 4.85 4.45
Winter 2013, organic chemistry for majors; 89 students; average course grade, C+. bScale of agreement: 5 = strongly agree; 1 = strongly disagree.
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“Once I got the hang of them, I really enjoyed SSR problems. If you hoped to get them right, you couldn’t have any gaps in your knowledge. Your reaction AND spec knowledge had to be up to par (even understanding mechanisms helped thinking about where a problem COULD go would often lead me on the right path). Also, the satisfaction of getting the right answer was great (I am someone who likes solving puzzles though, so I could be biased). Even though I was ripping my hair trying to solve some of the SSR problems, they sure got me to learn.”“SSR problems were fun to solve, basically a huge puzzle. However, they required a lot of hours to learn how to “read” correctly.”“Although O-Chem did not come easily to me, I did enjoy the SSR problems.”“I believe that SSR problems are a great way to use most of the main concepts in CHE128B. The only problem is that they are very difficult and result in low test scores.”“SSR problems would have been a great learning tool, but they were poorly presented as the professor overestimated his students’ O-Chem understanding.”“It was a unique way to learn how to solve problems.”“Challenging, yet fun.”“SSR problems are helpful learning tools, but they take a long time to do. Having to do them on tests was overwhelming.”“More time should be spent learning spectroscopy and synthesis before introducing SSR problems.”“I quite enjoy solving SSR problems.”“During the class I did not appreciate these problems as much as I have afterwards, in further study.”“I enjoy puzzles, so these were always really fun to try to figure out during homework and exams.”“I feel that SSR problems are the “real life” application of synthesis and spectroscopy.” “SSR problems are awesome. I loved that I actually got to use the problem solving part of my head and actually got to think rather than just remembering formulas. OK I’m a physics and mathematics double major and I missed the problem-solving aspect that I thought most of the chemistry courses especially within O-Chem were lacking. This provided and motivated me to solve these problems.”“They were good learning tools but very stressful on tests.”“Every SSR problem challenged me but allowed me to enhance and improve my synthesis skill.”“SSR problems presented O-chem concepts as fun, logical puzzles as opposed to bland memorization.”“SSR problems must be practiced and practiced. They help apply all knowledge from the course.”“SSRs made the concepts of the class work with a synergistic effect rather than being removed as they are in other classes.”
chemistry and counter the memorize-to-bust strategy that many students employ in the study of organic chemistry. From an instructor’s perspective, SSR problems uniquely engage students, naturally interrelate many aspects of organic chemistry, and, importantly, put a thrill-of-discovery into the study of organic chemistry.
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ASSOCIATED CONTENT
S Supporting Information *
Survey information from introductory organic chemistry students and 22 additional SSR problems (two of these are worked and discussed problems) and their solutions. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dean Tantillo (UC Davis, Department of Chemistry) and Makhlouf Haddadin (American University of Beirut, Department of Chemistry) for their thoughtful comments and suggestions. We also thank the winter 2013 students in CHE128B (organic chemistry for majors) for their thoughtful evaluation of SSR problems.
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REFERENCES
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CONCLUDING COMMENTS Synthesis−spectroscopy roadmap problems that integrate and interrelate synthesis with spectroscopy are useful educational tools. SSR problems uniquely engage second-year undergraduate organic chemistry students in a personal discovery of organic D
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