As a new fall semester dawns, my mind turns inevitably to the fresh crop of students that will soon be struggling with Lewis structures containing many more atoms than they are accustomed to. Teaching at a community college brings some advantages, like having organic students that you have taught through both semesters of the general chemistry sequence, and thus they know what they should be prepared for in terms of new material. There are also challenges, like the students drawn back to the study of organic chemistry from other career paths with the goal of transitioning into a health profession. These students are often a few years removed from the study of chemistry and are starting from a different place than the first group. And finally, I will see a group of students from general chemistry courses taught by different instructors with their own individual points of view on the material and/or different textbooks. My experience is that switching to Joel’s text has gone a long way towards solving the student preparedness problems I have encountered.
This preparedness issue creates a daunting challenge for the beginning of the class – to ensure that everyone is up to speed and grounded in the information they will need to succeed as the course progresses. In my experience, many organic chemistry textbooks are ill-equipped to aid in tackling this problem. Often the first chapter is a re-iteration of every general chemistry idea that has any relevance to organic chemistry with each topic being a single section of the chapter. This approach fails for students further removed from their general chemistry studies as well as those who didn’t put in as much effort as they should have.
In this early stage of learning organic chemistry from Karty’s text, students have specifically commented on the effectiveness of section 1.6 (Strategies for Success: Drawing Lewis Structures Quickly) as helping them bridge the gap between material covered in general chemistry and what will be expected in organic chemistry. The Lewis structure discussion in a general chemistry book, for example, often focuses on molecules made up of 3-5 atoms with a clear ‘central’ atom. This concept does not transfer well to even simple organic molecules with multiple carbon atoms (let alone oxygen and nitrogen). The hints that Joel provides in section 1.6 related to how many bonds each commonly encountered atom is likely to have jump-starts students’ ability to begin visualizing structures of larger molecules. This gives students a boost of confidence and clarity as they proceed through the remainder of the chapter and other issues they need to be familiar with like formal charges, likely bond/non-bonding pair architectures and recognizing resonance contributors.
After completing the first chapter from this book I believe many more of my students are well positioned to tackle the issue of nomenclature, which will be the subject of my next post.
Karty’s mechanistic approach to organic chemistry provides the content organization to facilitate student success. In a functional group approach students are more likely to apply an incorrect mechanism to solve a synthetic problem. This is because classification by functional group does not provide an organizational level that allows students to classify reactivity. Organization by functional group relies more on remembering molecular structure than on analyzing the structure for key reactive areas. Thus, a mechanistic organization is given a higher mark on Bloom’s taxonomy of learning and leads to a deeper level of student understanding. A mechanistic organization also conditions students to immediately apply previously mastered concepts in order to solve more complex problems. Homing in on functional group rather than rationalizing reactivity greatly diminishes the variety of problems a student can solve.
I have been teaching organic chemistry for a long time (several years ago I had a wonderful student who pointed out that I taught her dad!). The beginning of first semester of organic chemistry is always clunky and sometimes even painful. How does one make it through the first class without going through every detail that the students faithfully learned in general chemistry, while taking into account that their memory of general chemistry is more wobbly than a fresh batch of Jell-O? What new material can be introduced? Adding to the confusion is the lack of a common language needed to go forward in the course.
My three year old son recently has shown interest in solving puzzles. He dumps the pieces on the floor and randomly clicks them together until he finds a match. This is often the same approach that students take to problem solving in organic chemistry. To help my students work more systematically, I introduce IR early in the semester, as part of a laboratory experiment on the structural determination of an unknown solid, to model a strategic approach.
The strategy I introduce is both straightforward (there either are or are not absorbances that correspond to key functional groups) as well as creative and open-ended. Continue reading →
When I was in ninth grade, my family built a house. I remember my dad, who is an engineer, regularly checking on the progress and quality of the foundation. He knew that the foundation was the most important part of the house. Building a proper foundation took a lot of time, but it was important if the house was going to be sturdy and durable. So it is with organic chemistry. A solid foundation in certain general chemistry concepts is required in order for students to truly succeed in organic chemistry. The importance of this grounded foundation was really made apparent to me when I switched from a functional-group organization to a mechanistic one.
Most organic textbooks have one “review” chapter. Because I too considered the material in these chapters to be review, I would spend only one or two lectures on such topics as drawing Lewis structures, VSEPR, resonance, bond polarity/electronegativity, bond strength, hybridization, and Brønsted acid/base properties. However, this one- or even two-chapter “review” would not be sufficient, I discovered, and I would often see my students struggling throughout the course because, since these topics reappear in almost every subsequent chapter, without fully understanding these foundational topics, the superstructure of organic material that students constructed was often flimsy and faulty. Strong foundational knowledge, like a housing foundation, is essential to the success of whatever you are building on, and the functional-group texts I was using were not adequately laying this foundational knowledge.
One of the things my students find most challenging about aromaticity is whether to include lone pairs as part of a cyclic π system. If a lone pair is included, then the number of π electrons increases by two, and a student’s prediction about whether a species is aromatic will also change. What I think makes this challenging is that the rules appear to change depending on the nature of the ring and the nature of the atom that has the lone pair. This issue is apparent when we compare pyridine, pyrrole, and furan:
Pyridine and pyrrole both contain a nitrogen atom with a lone pair of electrons, but, whereas the lone pair in pyridine is not included in the π system, the one in pyrrole is. In furan, one of the lone pairs on oxygen is included but not the other.
Students can certainly memorize these specific results for pyridine, pyrrole, and furan, but the true problem manifests itself when students are asked to make predictions about the aromaticity of unfamiliar molecules that contain atoms with lone pairs. For many years my students struggled with this, but with an additional tool I now teach, they have become quite good at making these kinds of predictions.
There are two fundamentally different applications of molecular orbital (MO) theory in an undergraduate organic chemistry course. One application is toward various aspects of structure and stability of molecular species, including such things as the stabilization that occurs from the formation of a covalent bond, hybridization, rotational characteristics of σ and π bonds, conjugation, and aromaticity. The second application of MO theory is toward the dynamics of reactions, invoking some form of frontier MO theory. What orbitals are involved and how do electrons flow during the course of the reaction? And why should an elementary step take place at all, exhibiting the particular stereochemistry it does?
Over the years, I have learned that most instructors focus on the first of these applications in their courses, but relatively few instructors teach (or spend much time on) frontier MO theory. I think there’s good reason for this: Nearly everything that a student needs to know with regard to electron flow and stereochemistry can be explained using Lewis structures, VSEPR theory, resonance theory, and charge attraction/repulsion. These are things that students already know by the time they begin to study mechanisms.