For many students, the three-dimensional nature of organic chemistry raises the difficulty of the course to a new level. This difficulty stems from the fact that the three dimensionality of a molecule is depicted on a two-dimensional surface using any of a variety of representations. Accepted representations include dash-wedge notation, sawhorse projections, Newman projections, Fischer projections, Haworth projections, and chair representations. Even some of the most basic problems in organic chemistry may require a student to convert a two-dimensional representation of a molecule into three dimensions, carry out a particular manipulation of the molecule, and then return the molecule back to a two-dimensional representation in order to provide an answer. Some students can carry out these tasks in their head with no trouble, but I have found over the years that the large majority of students cannot. So then, how do we help these students?

For me, the answer is molecular model kits. The idea is for students to allow the model kit to do the three-dimensional work for them. Students still need to know the conventions inherent to each of the various representations in order to convert between two and three dimensions accurately. And students still must understand the actual chemistry—i.e., the concepts that relate structure to stability and reactivity. But once the molecular model is constructed, students can manipulate it in their hands rather than in their heads.

However, despite the immense utility of model kits, students by and large seem to naturally shy away from using them. One reason is that most students enter the course without a working knowledge of model kits. Model kits are new to them, so they have to be coached in their usefulness. I make sure to take time in my class periods to show students not only the basics of how to construct a molecular model, but also how to actually put the model to use when solving certain types of problems. Furthermore, I have incorporated a number of these lessons into my textbook in sections titled “Strategies for Success.” One such section demonstrates how molecular models can be used to arrive at various dash-wedge representations of the same molecule. Another demonstrates how to use molecular models to predict and draw the most stable chair conformations of di- and trisubstituted cyclohexanes. And yet another section teaches students how to use molecular models to convert between Fischer projections and zig-zag representations.

A second reason that students tend to shy away from using model kits has to do with their concern about using them during exams. I allow, and even encourage, my students to use model kits on exams, but students initially worry that if they depend on model kits that they might not have enough time to complete the exam comfortably. I combat this in two ways: The first is by emphasizing to students the need to become proficient with the model kits; they must challenge themselves (long before the exam) to construct models and carry out tasks not only accurately, but also quickly. The second is by allowing students to come to the exam with structures partially built. If, for example, a student knows that cyclohexane chair conformations will be covered on an exam, they might wish to come with one or two cyclohexane rings already constructed. It then becomes a matter of simply adding substituents to the ring, which takes very little time.

Now, I realize that many professors do not allow model kits on exams, but even in such situations I believe that a student can benefit tremendously by working with model kits early in the learning process. The more a student uses molecular models to solve three-dimensional problems, the more familiar he/she will become with the necessary manipulations of molecules. In turn, the more that students use molecular models, the less dependent on the models they will become when solving organic problems.

— Joel Karty

## Leave a Reply