In the Classroom
Teaching Introductory Organic Chemistry: ‘Blooming’ beyond a Simple Taxonomy Michael D. Pungente*† Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, BC, Canada V6T 1Z3; *
[email protected] Rodney A. Badger Professor Emeritus, Department of Chemistry, Southern Oregon University, Ashland, OR 97520
Students enter introductory organic chemistry with a certain amount of apprehension, usually the result of rumors from more senior students who have found ways to negotiate their way through the course. Sadly, organic chemistry is viewed by some students as little more than a rite of passage, or the academic equivalent of hazing. Students simply need to hang-on for the ride and hope that they will memorize enough to pass the course. As teachers of organic chemistry, our concern is that too many students try to just manage or memorize the material rather than understand it. Taagepera and Noori (1) tracked the development of students’ conceptual understanding of organic chemistry during a one-year sophomore course. They found that the students’ knowledge base increased as expected, but their cognitive organization of the knowledge was surprisingly weak. The authors concluded that instructors should spend more time making effective connections, helping students to construct a knowledge space based on general principles. Furthermore, if through continuous monitoring the instructor concludes that the students cannot relate new information to general principles, there is little purpose in moving forward in the content. We agree with Taagepera and Noori’s suggestion that “…we need to be more aware of our own knowledge structure and make it more transparent for the students. Otherwise the students’ cognitive structure will remain weak.” (1). We also agree with Barrow’s statement that students must be able to fit the new material into their own mental framework and then build their own understanding (2). This will not be achieved if students function only at the lower cognitive levels of knowledge and comprehension. Development of their own mental framework requires higher-level cognitive processes such as application, analysis, and synthesis. Bodner reported on the constructivist model of learning, which he summarized in a single statement: “Knowledge is constructed in the mind of the learner” (3). Besides being an obvious requirement for the chemistry major, introductory organic chemistry is a prerequisite course for many disciplines of study, particularly areas within the life sciences (such as biology, biochemistry, micro-biology), and professional schools (e.g., dentistry, medicine, and pharmacy). In fact, by far the majority of students who take introductory organic chemistry never take advanced organic chemistry courses. It has been our observation that a large number of students taking the introductory level organic chemistry consequently view the course as little more than †
Current address: Department of Chemistry, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1, Canada.
another obstacle along the way to reaching their ultimate destination. Too often, in our opinion, organic chemistry is viewed by students as a “grade-point-average breaker”, rather than as a necessary pillar in the foundation of the life sciences. In an attempt to thwart this predisposition, our first meeting with a new group of students begins by making the claim that organic chemistry is not hard. It is different from most other courses that students have taken, and the study skills used for other courses probably will not be effective for mastery of organic chemistry. Furthermore, we explain the importance of learning organic chemistry, emphasizing as Gillespie et al. have, that “...an understanding of chemistry will be useful to them years later in whatever branch of science, medicine, or technology they ultimately work, whereas the information that they learned to manage…will have been long forgotten” (ref 4, emphasis added). We recognize that these words can, and often do, mean very little to students at the beginning of the course, so these statements are reemphasized throughout the course. Having said that, we recognize that words of inspiration alone, while absolutely essential, will not make the difference for students in a course such as introductory organic chemistry. Teaching the course content through an implementation of these objectives is what will really make the difference in fostering critical thinking and higher cognitive processing necessary to do well in organic courses. Our primary goal when teaching introductory organic chemistry is to take students beyond the simple cognitive levels of knowledge and comprehension. We take a mechanistic approach to teaching organic chemistry. This is reinforced by connections to fundamental chemical principles emphasizing a unification of knowledge. Once students begin to appreciate the explanation of organic reaction mechanisms, they start to see these fundamental principles reappear regularly throughout the study of organic chemistry. True connections emerge and students begin to view organic reactions and interactions from a basis of understanding—using skills of synthesis and analysis—rather than rote memory. This ability to understand the connections between general principles and how they unlock the seemingly complex and confusing reactions in organic chemistry is an empowering experience for students. As empowerment replaces the fear, student confidence grows. Bloom’s Taxonomy of Cognitive Processes Benjamin Bloom has broken down the cognitive learning domain into six hierarchical categories, from the most simple and concrete to the most complex and abstract
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(Figure 1, Table 1, and ref 5 ). Bloom’s taxonomy of cognitive processes shows that learning follows certain steps. Bloom suggests that we need to follow those steps up the hierarchy to build true critical thinking skills with which we not only simply repeat information but also truly understand it and use it creatively. The six steps are shown in Figure 1. Within Bloom’s taxonomy, the acts of recalling and reporting knowledge are seen as less sophisticated than the alternatives of translating information into new forms, applying information to new contexts, analyzing, synthesizing, and evaluating information. However, more often than not, classroom time is spent at the lowest cognitive levels. Instructors feel overwhelmed, as do the students, by the volume of content that makes up the introductory organic chemistry syllabus. Traditionally, one of the most efficient ways to cover massive amounts of course material is the didactic lecture approach. Those teachers who truly want to develop student potential must help learners move up the cognitive ladder beyond simply functioning as information gatherers. In classroom sessions, whether during lectures or problem-solving exercises, students must be challenged at the application and synthesis level, and distinctions between levels must be made clear. Once these critical thinking skills are understood and practiced, students will better understand the objectives behind the examinations set by the instructor.
Evaluation Synthesis Analysis Application Comprehension Knowledge
Figure 1. Bloom classifies learning behaviors into six hierarchical categories from Knowledge (the most simple and concrete) to Evaluation (the most complex and abstract).
Table 1. A Description of the Skills Required at Each of the Cognitive Levels within Bloom’s Taxonomy Cognitive Level Evaluation
Compare and discriminate between ideas, assess value of theories, make choices based on argument, verify value of evidence
Synthesis
Put together elements or parts to form a whole
Analysis
See patterns, organize the parts, recognize hidden meanings, identify components
Application
Use information, methods, concepts, and theories in new situations to solve problems; use required skills or knowledge
Comprehension
Understand information, grasp meanings, interpret facts
Knowledge
Recall information
Applying Bloom’s Taxonomy to Introductory Organic Chemistry Classes Like learning a new language, introductory organic chemistry typically begins with the grammar or taxonomy of organic chemistry. This introduction allows the instructor to speak the language of organic chemistry, re-examine principles, and lay the groundwork for advancement into reactions and mechanisms (applications and analysis). However, too often when the instructor kicks into “higher-level cognitive gear”, and begins delving into applications, the students are still functioning at the lower knowledge and comprehension cognitive levels, memorizing seemingly unrelated facts. This discrepancy between the instructor’s expectations and student performance becomes painfully obvious at exam time. Often, unintentionally or unknowingly, the instructor teaches at the lower knowledge and comprehension cognitive levels but examines at the higher analysis and synthesis levels while the students’ exam expectations remain at the lower knowledge and comprehension cognitive levels. The results: students complain that the exams are too hard; the instructor concludes while marking the papers that the students don’t “understand” basic concepts. Pungente recalls the first time he introduced Bloom’s taxonomy to his introductory organic chemistry class. Approximately four weeks into the course the various cognitive levels of Bloom’s Taxonomy (Figure 1 and Table 1) were discussed, and students were told at what level they would need to work or function to succeed in this course. The importance of moving beyond the lower knowledge and comprehension levels (where many students tend to remain throughout the entire course) was emphasized. Students were informed that they would be tested beyond simple knowledge and comprehension and that it was in their best interest to look for patterns between reaction mechanisms and fundamental chemical prin780
Skills Demonstrated (The learner must ...)
NOTE: This table is adapted from reference 1.
ciples, to compare and contrast, to begin developing their personal “mental framework” when studying organic chemistry. The students were very receptive. They appreciated this insight and guidance toward approaching the course material. Furthermore, the explanation of learning psychology gave credibility to his teaching and learning approach. Students saw that there was literature support for the instructor’s approach and appreciated that the instructor actually showed concern for their learning at more meaningful levels. Simply informing students of the cognitive levels at which they must function if they are to succeed in organic chemistry is not sufficient. Students benefit further if the instructor makes a habit of pointing out during lectures from which cognitive level students should view the material. This enables students to better gauge the expectation level of assignments and examinations.
Journal of Chemical Education • Vol. 80 No. 7 July 2003 • JChemEd.chem.wisc.edu
In the Classroom
Fundamental Principles of General Chemistry Used in Introductory Organic Chemistry
I. Structure A. Atomic theory B. Molecular structure 1. Ionic covalent bonding a. Electronegativity and bond polarities b. Resonance rules 2. Hybridization and geometry
The partial list of fundamental chemistry principles that we believe are necessary to begin the study of organic chemistry are included in Figure 2. We want to emphasize the strong link between molecular structure and reactivity. Knowing the fundamental principles that govern structure and polarity of molecules will help the students deduce reactivity and understand mechanisms later in the course. By the end of this introductory organic chemistry course, we want our students to be able to look at a molecule’s structure and predict how it will react towards various reagents.
II. Reactivity A. Thermodynamics 1. Thermochemistry 2. Equilibria 3. Acid–base theory B. Kinetics Figure 2. Fundamental chemical principles of structure and reactivity necessary to study and understand organic chemistry. Most Reactive Acid Derivative
Leaving Group
Conjugate Acid
pK a
Clⴚ
HCl
⬃ −7
O R
C
Cl
R
C
O
O R
C
C
C
R'
ⴚ
C
O
ⴚ
OR'
O R
O
O
O
OR'
ⴚ
N
N
O R'
HO
C
HOR'
HN
R'
⬃5.0
⬃15–18
⬃35
Least Reactive
Figure 3. The relative reactivity of carboxylic acid derivatives toward nucleophilic substitution, and the relative pK a values for the conjugate acids of the various leaving groups.
Students’ confidence in their ability to master the content, and ultimately increase their chance of success in the course, is highest if students approach the subject from a perspective of understanding rather than memorization. Making explicit connections between general chemistry principles and organic reaction mechanisms during lectures can facilitate understanding. Ideally, these connections are being made in both the classroom and laboratory and should be re-enforced through problem sets and examinations. In the following section we outline those general chemistry fundamentals we feel are key to making connections to reactions and mechanisms in organic chemistry. We then present a detailed example of how the principles of acid–base chemistry can be connected to relative reactivity of carboxylic acid derivatives and leaving-group ability in nucleophilic substitution reactions.
Linking the Fundamental Principles to Organic Reactions: An Example The relative reactivity of carboxylic acid derivatives (Figure 3) toward nucleophilic acyl substitution can be taught from principles of acid–base chemistry and resonance stabilization. When a Lewis base with its non-bonded pair of electrons is stable, it will have little tendency to donate its pair of electrons to a Lewis acid or to remove a proton from a Brønsted–Lowry acid. The more stable the Lewis base is, the less capably it can function as nucleophile and the better it can function as a leaving group in a nucleophilic substitution reaction. The products of the reaction are lower in energy (more stable) than the reactants, so the substitution reaction is thermodynamically (and often kinetically) favored. Thus weak bases make good leaving groups. To a first approximation, one can simply look at the relative pKa values of the resultant conjugate acids of the leaving groups to determine the rankings of the leaving groups and the relative reactivities of carboxylic acid derivatives in nucleophilic substitution reactions. Reminding students of the fact that strong acids have weak conjugate bases, the fact that HCl is the strongest acid in Figure 3 makes Cl− the weakest base and hence the best leaving group. Since students more easily identify with relative pKa values than with pKb values, this approach seems to work for them. Furthermore, inductive and resonance stabilization, as well as polarizabilities, are helpful in explaining the relative reactivity toward nucleophilic substitution between acid anhydrides and esters. Upon substitution, the resulting leaving group for an acid anhydride and an ester are the carboxylate anion and an alkoxide, respectively (Figure 3). Students identify with the fact that, in general, ionic species are less stable than their neutral counterparts. Students then can appreciate that those species that are better able to accommodate ionic charges are more stable than those species that are less able to disperse the charge. Since the negative charge associated with the carboxylate anion is delocalized over two oxygen atoms and the negative charge associated with an alkoxide is localized (Figure 4), carboxylate anions are better able to bear a charge and therefore are more stable and less basic than alkoxides. An analogy that seems evocative with students is the game of “hot potato” (in this case, a molecular hot potato). If one relates a charged atom, in this case an oxygen atom, with a hot potato, the alkoxide oxygen (with a localized negative charge) is burning hot. The two
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In the Classroom
O R
ⴚ
There is room for a mix of all cognitive levels, and a welldesigned evaluation process may not only provide a grade but also pinpoint the cognitive level at which a class or individual student is performing. We believe that students are capable of critically thinking about topics in organic chemistry at higher cognitive levels when shown how to achieve this level. Furthermore, it is our experience that students gain a greater sense of ownership of the material and become more excited about the material when they can forge their own mental framework upon which they can hang new concepts.
O
C
R
C O
O
ⴚ
a carboxylate anion (delocalized)
R
O
ⴚ
an alkoxide anion (localized) Figure 4. The charge distribution associated with a carboxylate anion and an alkoxide anion.
oxygen atoms of the carboxylate anion (a delocalized negative charge) are, by contrast, not as hot (i.e., not as unstable) and therefore the carboxylate anion is a more stable anion. Finally, making the connection that a more stable anion is a weaker base ultimately explains why acid anhydrides are more reactive toward nucleophilic substitution than esters, for example. Once these connections between fundamental chemical principles and organic reaction mechanisms are made explicit and reinforced, students are better equipped to succeed in organic chemistry. Understanding will reduce short-term rote memorization and improve long-term retention. Exploiting Cognitive Levels in Exam Questions Finally, instructors can close the gap between what they expect from their students and what their students think is sufficient to master the course content by illustrating clear examples of exam-type questions at different cognitive levels. This would be a useful exercise for both the students and the instructor to gain awareness of the distinction between cognitive levels for a given topic. For example, instructors can make clear the difference between the levels of cognition required to demonstrate understanding of the relative reactivity of carboxylic acid derivatives (see the Appendix). Note that the authors are not suggesting that there is anything wrong with exam questions that test students at the lower cognitive levels, nor are we suggesting instructors should be posing exam questions only at the higher cognitive levels.
Conclusions Students build a mental framework upon which they can further develop their own understanding of organic chemistry as a result of educating them on the taxonomy of cognitive processes early in an introductory organic chemistry course. Students’ learning is profoundly improved once they begin to make connections between fundamental chemical principles and organic reaction mechanisms. Students then begin to view organic reactions and interactions from a basis of understanding rather than as a collection of unrelated facts that they must memorize for the exam. The disparity between students’ and instructor’s expectations for examination questions is thereby reduced and student performance as well as confidence increase. Acknowledgments We would like to thank Robert Perkins and Dana Zendrowski for many fruitful discussions, and Iain Taylor for his helpful suggestions during the preparation of this manuscript. Literature Cited 1. 2. 3. 4.
Taagepera, M.; Noori, S. J. Chem. Educ. 2000, 77, 1224. Barrow, G. M. J. Chem. Educ. 1998, 75, 541. Bodner, G. M. J. Chem. Educ. 1986, 63, 873. Gillespie, R. J.; Spencer, J. N.; Moog, R. S. J. Chem. Educ. 1998, 75, 541. 5. Bloom, B. S. Taxonomy of Educational Objectives: The Classification of Educational Goals: Handbook I, Cognitive Domain, 1st ed.; Longmans, Green: New York, 1956.
Appendix Below are example examination questions at the various cognitive levels towards the understanding of the relative reactivity of carboxylic acid derivatives.
2. Circle the carboxylic acid derivative most reactive toward nucleophilic substitution. O
O
C
C
Knowledge
H3C
O
H3 C
N
O
O
1. Circle the generalized structure below that represents an ester. O
R
782
O
O
O
O
C
C
C
C
NHR
R
O
R
R
R O
C R
Cl
C
O Cl ⴚ
O
C O
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C H3C
C O
CH3
In the Classroom
Comprehension
Analysis
3 a. Draw the structures of the initial leaving groups formed in the base-promoted hydrolysis of the following compounds at pH 10.
7.
O
O
Write the mechanism for the saponification of methyl acetate (below) in 0.1 M NaOH in water. Show the movement of electron pairs using curved arrows and the structures of all intermediates and final products. Identify the rate-limiting step. O
C
C H3C
H3 C
O
N
A
OCH3
H2O, ∆
B
8. O
O
O
C
C
C
H3 C
0.1 M NaOH
C H3C
Cl
H3C
O
CH3
D
C
Would the rate of the rate-limiting step in Question 7 above be faster, the same, or slower by changing the reactant to methyl trifluoroacetate? Explain your answer in terms of transition-state theory.
Synthesis Consider this transformation.
b. Rank the leaving groups from 3a in order of their basicity.
O C OH
c. Which compound in the series above would undergo nucleophilic substitution reactions the fastest? Circle the correct answer. ABCD
HOH2CH2C I
d. Which compound would react the slowest? Circle the correct answer. ABCD 4.
Several Steps
What is the relationship between the pKa of an acid, the stability of its conjugate base, and the conjugate base’s ability to be a leaving group in nucleophilic substitution reactions under basic conditions?
O C OH
O
Application 5.
Circle the compound in the pair below that would react faster with alkaline water under identical conditions of concentration, pH, and temperature. Explain your choice in terms of relative differences in rate-limiting steps or in relative stability of intermediates or products. O
O
C
C
H3C
6.
C
H3C
OCH3
H3C
N
CH2 II
H
9.
Working backwards, which one reaction below would reasonably produce II? O a.
C OCH3
O
OCF3
Identify the three functional groups in the molecule below and rank them in terms of their relative reactivity towards a solution of dilute sodium methoxide in methanol.
H3C
C
H2O
CH2
N H
O b.
C O
O
OH
HO
H
O
O
C
C O
CH2 O
CH3
c.
N B
H H
O
C OH
C O
+ 2NH2CH3
C
A C H3C O
H3Oⴙ
NaOH
HOH2CH2C
+
H3C
C N
Cl
H
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In the Classroom 10.
a. Under acid-catalyzed hydrolysis conditions, some of the t-butyl alcohol product does contain 18O. Both oxygen atoms of the carboxylic acid are labeled with 18O to the same extent.
What would be the major product of the following reaction?
O C
O
OCH3
O
+
C Cl
CH3
C
2 NH2CH3
H3C
CH2
O
C
CH3
+ H2O*
*ⴙ H3O
CH3 III
11.
Apply the clues given in the preceding reactions (from questions 9 and 10) and write a reaction sequence that would convert compound I to II.
O* C H3C
CH3 * OH
+
HO* C CH3
Evaluation 12.
Some esters may hydrolyze by competing nucleophilic substitution mechanisms when their structures allow it. When compound III below is saponified in 18 Oenriched water, none of the t-butyl alcohol product contains the 18O isotope (shown as O* below) and all of the sodium carboxylate is labeled.
b. Use these observations to explain how an SN1 or SN2 mechanism with either alkyl-oxygen or acyl-oxygen cleavage might compete with the usual mechanism for ester hydrolysis. What structural requirements of the ester must be met for the alternate mechanism to be competitive?
O CH3
C H3C
O
C
CH3
+
* NaOH
H2O*
CH3 III O* C H3C
784
CH3 O*ⴚ Naⴙ
+
HO
C
CH3
CH3
Journal of Chemical Education • Vol. 80 No. 7 July 2003 • JChemEd.chem.wisc.edu
CH3
