Stereochemistry and Adaptability

The three dimensional nature of organic molecules is a topic that all organic chemistry courses must cover. Chairs, Newman projections, Fischer projections, rotations, conformational changes, and stereochemistry are very challenging for sophomore students to grasp—especially if this is their first exposure to the concept that structure is a key element to reactivity. That said, watching students take the stereochemistry exam is very entertaining! The students problem-solve by rotating the paper to “see” a perspective better, hold up their hands and rotate them to visualize stereocenters, and even turn pages over to better view the mirror image. It is a rare opportunity for a faculty member to see student problem-solving in real time.

I imagine that stereochemistry is one of the biggest challenges for any textbook author to tackle. Any approach is going to, by the very nature of a text, commit to a singular process of solving problems of “what is the relationship between these two molecules?” Karty has chosen the path of asking students to visualize these relationships. This requires students to see, really see, that two molecules are mirror images of one another if they are enantiomers. For many of my students, with practice, they are able to picture these compounds and solve the problem. The examples in Karty are excellent, and I find that most of these students respond positively to the visuals in the text. However, I have found that there is always a subset of students, about 40% for me, that are unable to mentally manipulate molecules with confidence. In my twenty years of teaching, I have also found that this mental manipulation is not an accurate measure of overall success in organic chemistry, chemistry in general, or further pursuits that lie in other disciplinary areas.

Because of this, other modes of problem solving are requisite and necessary to validate the different learning styles of my students. I moved Cahn-Ingold-Prelog R and S nomenclature into Chapter 5 (“Isomorism 2”) when stereochemistry was taught. I also incorporated stereochemical assignment into all of the problem solving that we did in class so students who could not visualize the enantiomeric relationships could be confident that two compounds were mirror images by the assignment of R to one stereocenter and S to the stereocenter in the partner molecule. I have found that students who are not adept at mental manipulations are often very proficient at assignment of R and S. This gives these students confidence that they too can obtain the correct answer. Early in the organic chemistry sequence, a little bit of confidence can be a huge boost.

In this way, I have found the Karty text to be adaptable to my teaching style. I am able to easily move content items around to fit the needs of my students. And any moves have been well-received. Additionally, I really like the discussion of 1,2-difluorocyclohexane in Figure 5-8 (below). It clearly shows the superimposability of the meso compound. Including the discussion of R and S early allows students to understand the internal mirror plane in the molecule even if they don’t see it.

Fig. 5-8

Overcoming organization challenges with other texts has not been this easy. Often, the movement makes it difficult to assign problems or readings from the text. In this case, I simply added stereochemical assignment to the Chapter 5 problems and posted a key for the students to check. The organization of Karty does not hinder my ability to move things around in ways that I find more suitable for my student population. So, even though I don’t find Karty’s text to be absolutely “perfect” for my class as is, the stereochemical content that I need is there for me to move into the appropriate spots and the movement is far more seamless than with previous texts I have used.

-Anne Wilson, Butler University

Click here to learn more about Prof. Wilson.

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