What's Wrong With Carbonyl Chemistry? - American Chemical Society

Chapter 7

What’s Wrong With Carbonyl Chemistry?

Downloaded by UNIV OF ARIZONA on November 5, 2012 | http://pubs.acs.org Publication Date (Web): November 5, 2012 | doi: 10.1021/bk-2012-1108.ch007

Scott T. Handy* Department of Chemistry, Middle Tennessee State University, Murfreesboro, TN 37132 *[email protected]

In general, students struggle with the sections of Organic Chemistry that pertain to the properties, reactivity, and synthesis of carbonyls. Although each and every new Organic text claims to have a new and different approach to teaching carbonyl chemistry, the results, in terms of student performance on exams and retention of material, appear to be largely the same regardless of the text. In struggling with how to improve student comprehension and retention of carbonyl chemistry, one possible source of the problem was identified as its location. In most cases, it is covered near the end of the second semester of a two semester long sequence of Organic Chemistry. As a result, it could be simple fatigue that is part of the problem. To study this hypothesis, Organic II has been taught with a revised order: carbonyl chemistry at the start of the second semester and aromatic chemistry at the end.

Introduction Organic Chemistry is a challenging course for many students. Many approaches have been tried over the years to make it more accessible to the typical student, in particular the increasing emphasis on biological applications in virtually every textbook. In addition, various pedagogical approaches have been applied to Organic Chemistry (1–3). While continued efforts to address the problem students have with the class overall are certainly important, I have noted that students seem to have a particularly serious issue with carbonyl chemistry. The most obvious means of identifying that students have an issue with carbonyl chemistry is by examining the class average on exams throughout the second semester of Organic Chemistry. Although there are some differences from © 2012 American Chemical Society In Advances in Teaching Organic Chemistry; Duffy-Matzner, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

instructor to instructor, a fairly typical sequence involves the following main topics: radicals, dienes, aromatics, carbonyl addition and substitution, carbonyl condensations (aldol chemistry), and biomolecules. In my class, I cover all of these topics except biomolecules (which is integrated in the appropriate sections of other chapters – for example, amino acid chemistry is included in with carbonyl substitutions). The division of these topics in exams for my class can be seen in Table I.


Table I. Exam and Topic Breakdown Exam Number

Topic

1

Radicals

1

Dienes

2

Aromatics

3

Carbonyl Addition/Substitution

4

Carbonyl Condensations (Aldol)

Even a quick glance at the exam average information in Figure 1 shows that students, on average, start the semester off well and then dramatically have issues when carbonyl chemistry comes around. This is an unfortunate situation for a number of reasons. Of course, it does not help students earn a good grade in the class (although many still do). More importantly, though, carbonyl chemistry is really the single most important topic for the majority of students taking Organic Chemistry. A good understanding of carbonyl chemistry is important for success and understanding in Biochemistry and Molecular Biology – courses that virtually every student in Organic will take in the future. This material also appears heavily on standardized tests, including MCAT, DAT, PCAT, etc. As a result, it will have a disproportionate influence on their future success. Indeed, the recently released AAMC/HHMI report on creating scientifically literate physicions specifically highlights carbonyl chemistry as a key part of Organic Chemistry that is essentially for quality preparation of future physicians (4). Carbonyl chemistry also receives significant emphasis on standardized tests, including the ACS full year Organic exam. In light of this endorsement, it appears very worthwhile to consider the source of this dramatic drop in performance and understanding (as measured by class exam averages).

116 In Advances in Teaching Organic Chemistry; Duffy-Matzner, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 1. Typical Class Average Trend in Organic II

The Exams Since exam averages will be used throughout this discussion, it is important to outline my exam style. My exams are a hybrid exam, with 1/3 being multiple choice questions and the remainder being short answer, fill in the missing reagents/ products, synthesis, and mechanisms. There is always one extra credit question worth 9% of the total points possible on the exam. Most students easily complete these exams within the time allotted, as I attempt to write the exams for a 45 minute time period, even though the students have 55 minutes to take it.

The Source of the Problem Why do test scores drop so precipitisely? There can be a number of possible answers to this question. One is that the instructor (myself) is just doing a poor job covering the material. In talking to other Organic professors, though, I found that they encountered similar trends in their class. So, instructor issues did not seem likely to be the answer.



The Textbook A next idea was that it was the textbook. Over my time at Middle Tennessee State University (MTSU), we have used 3 different texts: Vollhardt and Schore Organic Chemistry 5th Edition, Solomons and Fryhle Organic Chemistry 9th Edition, and Smith Organic Chemistry 2nd and 3rd Editions (5–7). As can been seen in Table II, there are differences in exam performance for the different texts, although most of these differences are slight. The one possible exception is on exam 3, where both the Volhardt and Smith texts afford significantly higher class averages than the Solomons test. Still, it is worth pointing out that this difference may simply be an artifact of that one class and not a reflection of the material being clearer in the Volhardt and Smith texts. Further, attributing the difference to the textbook assumes that the students carefully read and use the textbook, which may or may not be the case.

Table II. Class Exam Averages by Textbook Exam

Solomons

Volhardt

Smith

1

65%

69%

70%

2

82%

75%

77%

3

46%

56%

60%

4

50%

49%

43%

Number of Reactions Another thought was that maybe the number of reactions in each given section was the problem. Students very often complain about how there are so many reactions and that they cannot possibly remember (memorize) them all. Using the current (3rd) edition of Smith’s text, the number of reactions were determined (Table III). The reason that there are two columns for number of reactions is that student and professor determinations of what makes a reaction “different” are not the same. For example, a student would likely consider the Claisen and Dieckmann condensations to be two different reactions even though both are the condensation of two esters, with the only difference being inter- versus intramolecularity. Indeed, the reason for the large difference in the carbonyl substitution/elimination section is because I view the conversion of either an acid chloride or an acid anhydride into an ester or an amide as fundamnentally the same reaction, whereas from the student perspective, they are all different.



Table III. Number of Reactions per Exam Exam

# of Reactions (Professor)

# of Reactions (Student)

Average

1

4

7

70%

2

9

11

77%

3

10/19

61

60%

4

6

18

43%

Even with this difference in opinion as to the exact number of reactions, it is clear that the number of reactions does not consistently track with class exam average, as by this figuring, exam averages on aldol chemistry should be comparable to that of aromatic chemistry.

Timing Another possible reason that has been considered is that of timing. Carbonyl chemistry is near the end of the second semester of a challenging two semester course sequence. By that point in time, it may be that most students are simply worn out and incapable of putting forth their best effort on the material. In that case, moving carbonyl chemistry to earlier in the second semester by flipping it and the radical, dienes, and aromatic chapters should result in an improvement in performance on the carbonyl exams. It would also be expected that performance on the radical, diene and aromatics exams would suffer. To test this idea, the carbonyl chapters were moved to the start of the semester in Organic II in spring of 2011. Operationally, this is not difficult as there is very little use of radical, diene, and aromatic chemistry in the carbonyl chapters and what little that does appear, such as Friedel-Crafts acylation for the synthesis of aromatic ketones, was covered as it occurred. The results of this experiment can be seen in Table IV. The comparison is of the class taught in spring of 2011 (revised order) with spring of 2010 (standard order). The class sizes were similar (roughly 100 students in both cases), had the same instructor, and used similar (though not identical) exams. In examining the results, there is a definite difference. Class averages on the two carbonyl exams (exams 3 and 4 in the normal order and 1 and 2 in the carbonyls first order) increase modestly, while the average on the aromatics exam does decrease slightly (exam 2 in the normal order and exam 4 in the carbonyls first order). Interestingly, the average on the radicals/dienes exam (exam 1 in the normal order and exam 3 in the carbonyls first order) actually increases significantly in the carbonyl first approach. As a result, order (and thus fatigue) is clearly not the only issue driving student performance.



Table IV. Effect of Topic Order Exam

Normal Order

Carbonyls First

Exam

1 (dienes, radicals)

70%

66%

1 (carbonyl additions/substitutions)

2 (aromatics)

77%

54%

2 (aldols)

3 (carbonyl additions/substitutions)

60%

85%

3 (dienes, radicals)

4 (aldols)

43%

68%

4 (aromatics)

Number of Mechanisms Upon further reflection, one other possible source of the problem that students have with carbonyl chemistry might be the number of reaction mechanisms. Even though students say that there are too many reactions, in fact, students often do not differentiate between reactions and reaction mechanisms. At the same time, it is well established that students struggle with arrow formalism (8, 9). As seen in Table V, there are far more mechanisms in the carbonyl sections than in the radicals, dienes and aromatic ones.

Table V. Mechanism Analysis Section

Number of Mehanisms

Radicals

2

Dienes

2

Aromatics

1

Carbonyl Addition/Substitution

9

Carbonyl Condensation (Aldol)

9

Although this is a plausible hypothesis, it is one that cannot be readily tested. Arrow formalism is an intrinsic part of Organic Chemistry and to teach a class without “mechanisms” would be doing the students a serious disservice. Still, this possibility might serve to focus more attention on how to more effectively teach the idea of mechanism in Organic I and II.


Conclusions In summary, carbonyl chemistry is a very important topic in the standard Organic Chemistry sequence. It is considerably more challenging to many students and yet is of vital importance to their future success in more advanced courses as well as standardized tests. Several potential reasons for student difficulty were explored, with none adequately rationalizing the problem.


Acknowledgments I would like to thank Rachael Hall for assisting in the student determination of number of reactions.

References 1. 2.

3. 4. 5. 6. 7. 8. 9.

Katz, M. Teaching Organic Chemistry via Student-Directed Learning. J. Chem. Educ. 1996, 73, 440–445. Karty, J. M.; Gooch, G.; Bowman, B. G. Teaching a Modified Hendrickson, Cram, and Hammond Curriculum in Organic Chemistry. J. Chem. Educ. 2007, 84, 1209–1216. Wheeler, D. M. S.; Wheeler, M. M. The Undergraduate Curriculum in Organic Chemistry. J. Chem. Educ. 1979, 56, 462–464. HHMI. Scientific Foundations for Future Physicians; Report of the AAMCHHMI Committee, 2009. Vollhardt, P.; Schore, N. Organic Chemistry: Structure and Function, 5th ed.; W.H. Freeman and Company: New York, 2005. Solomons, T. W. G.; Fryhle, C. B. Organic Chemisry, 9th ed.; John Wily & Sons: Hoboken, 2007. Smith, J. G. Organic Chemistry, 3rd ed.; McGraw-Hill: New York, 2011. Bhattacharyya, G.; Bodner, G. M. ‘It Gets Me to the Product’: How Students Propose Organic Mechanisms. J. Chem. Educ. 2005, 82, 1402–4107. Penn, J. H.; Al-Shammari, A. G. Teaching Reaction Mechanisms Using the Curved Arrow Neglect (CAN) Method. J. Chem. Educ. 2008, 85, 1291–1295.


What's Wrong With Carbonyl Chemistry? - American Chemical Society

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