As an organic chemistry professor, I find that Fischer projections are one of the more challenging perspectives to view chiral centers. However, this projection also happens to be one of my favorites for viewing chiral carbons, chiral compounds, and meso molecules. Even though the Fischer projection can be quite challenging, it can also be the most descriptive and rewarding projection.
To be honest, Fischer projections are known to have a love-hate relationship with students. Typically, students find that Fischer projections are hard to comprehend because they have issues with the rotations. I agree that it is tough to rotate and place Fischer projections correctly in the wedge-and-dash motif. Even after having fifteen years of chemistry experience, I myself still have trouble rotating between Fischer and the wedge-and-dash motif!
One way that I’ve been able to successfully navigate around this issue is by using an analogy related to sports. I come from an area of the country where college football is huge, particularly the SEC conference. So, I like to remind myself that we, as instructors, are on the same side, or “team.” I tell my students that I want my classroom to be viewed as a team, one where I am the coach, showing my students, or the players, the best tactics to maneuver through these organic chemistry problems. More than anything, I want my students to succeed in the classroom. Thus, the phrase “if H is on the horizontal…then it is horribly wrong” is one play that I – based on my experiences as a student, tutor, and professor – have taught my students to employ when tackling any stereochemistry problems.
When I was a student learning about Fischer projections, my organic professor said the expression, “if H is on the horizontal…then it is horribly wrong.” Upon first hearing this statement, you might be confused or leery. The “it” that the professor was referring to is the chiral center, as well as the R/S assignment for the carbon within the molecule.
However, when viewing Fischer projections, students need to remember that the groups on the horizontal lines are coming out of the page when being drawn in a wedge-and-dash motif. Thus, if the lowest priority group is placed on a horizontal line, then students may have difficulty assigning the proper R and S convention. The likelihood of trouble is due to the lowest priority group being on a wedged bond and coming out of the plane. In order to properly assign the R and S convention, the lowest priority substituent needs to be in the back, on a dashed bond away from the viewer, so that the three highest priority ligands, starting at the highest priority ligand, are traced in either a clockwise or counterclockwise direction.
To keep helping my students better understand Fischer projections, I also tell them to assign the appropriate priorities to all the groups, where the highest priority (heaviest atom) is given a 1, and the lowest priority (lightest atom) is given a 4. Once these priorities are assigned, I ask my students to trace and determine the direction of the light being spun and to designate the chiral center as either the R or S convention. I remind them with the phrase … “if H is on the horizontal…then it is horribly wrong.”
So, you switch the convention, meaning that if you have found the center to be “R,” then it is truly “S.” In the same thought, if the center is determined as “S,” then the chiral center is “R.” I tell my students that this rule of thumb (ROT) can be done 100% of the time. The rationale for presenting this ROT is to eliminate stress and the mistakes that students tend to make when rotating Fischer projections.
As an instructor tasked with teaching these challenging Fischer projections to students, I find it incredibly helpful that Karty’s textbook gives a straightforward overview of chirality (chiral centers, stereoisomers, meso structures, etc.), and that its images help present Fischer projections, which are notoriously challenging, in an easy-to-understand way. For example, these images clearly show the dimensionality of the atoms as the perspective changes from 2-D to a dashed-and-wedged motif. Karty also does a nice job of showing the relationship between the enantiomers and diastereomers of sugars. And most importantly, I hear countless thanks from my students for Karty’s end-of-chapter problems, especially those on tough topics like Fischer projections being drawn in a line-dash-wedge notation.
Ultimately, persistent practice and routine application are essential when teaching organic chemistry with a mechanistic perspective. Through various examples, I try to show my students that utility rather than memorization is critical to success in the classroom, and even beyond. For instance, previous students are constantly telling me how organic chemistry content is frequently found on medical school board exams. In order to be an effective instructor and a great mentor, I’ve learned to find tremendous value in teaching organic chemistry with a mechanism-based organization, which helps me prepare my students for more than just the upcoming exam, but for the challenges that await them in the future.
-Kerri Taylor, Columbus State University
Click here to learn more about Kerri Taylor