Solving the IR Puzzle

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. An absorbance around 3,300 cm-1 requires experience in pattern-recognition to confirm what structure it signals.  Ultimately, IR only provides some logical structural possibilities not a final answer, unlike the discrete puzzles my son wants to solve.

Starting the year with an unknown powder to identify can be intimidating to students.  The first step in any systematic approach is to break the process down into steps, which happens as this puzzle is solved over the course of several laboratory weeks. First, the students take a melting point of the unknown and follow a simple flow-chart to assess solubility of their solid. During the second week, the students do an extraction to separate a mixture of unknown compounds. This emphasizes solubility related to acid and amine functional groups. In a third week, they recrystallize their unknowns and obtain IR spectra.

I cover IR in lecture concurrent to the lab experiment.  I emphasize three key areas (see the figure below) of the IR spectrum: bond to H, triple bond, and double bond stretches. Furthermore, I expect students to tell the difference between alkene/carbonyl/ carboxyl stretches and the alkene/arene stretches, as well as C-H versus O-H (alcohol/ acid) and N-H (amines). I have the students compare and contrast functional groups and resulting absorbances. For example, I might point out the aldehyde C-H or the difference in resonance between esters and amides. If you think in terms of a concept map, IR can be connected to functional groups, intermolecular forces, resonance, bond strength, bond polarity, and the mass of the atoms, all of which are early semester topics.

Overall, I have found that teaching IR facilitates systematic problem solving. Here’s why:

  • Analyzing an IR is a multi-step process, like writing good mechanisms, but it has limited outcomes, which helps students be successful more easily and earlier in the semester.
  • IR emphasizes how structure influences physical properties. Students can combine similar functional groups together, to see what they have in common, and to connect to spectral properties.
  • IR emphasizes the idea that there are many structural options for one molecular weight. In general chemistry, students rely on chemical formulas to balance chemical equations. As such, they frequently fail to grasp the importance of Lewis Structures, let alone become comfortable enough with them to move to the assumption-laden skeletal forms. So, as students are working on this skill in organic chemistry they struggle to find a) all of the possible constitutional isomers for a given chemical formula and b) to tell the difference between conformations, resonance forms, and constitutional isomers. IR and structure determination is a way for them to practice solving problems that appear to have multiple solutions, only some of which are good solutions. Like an actual puzzle, students sometimes try to force things to fit even if they don’t. There’s a certain amount of creative decision-making and big picture thinking that is required in considering the options and choosing the best possible structure to fit the data.
  • Analyzing IR to determine unknown structure is what synthetic organic chemists do on a regular basis. I believe it’s important for students to mimic, as closely as possible, what scientists do in the laboratory. This is an underlying philosophy of teaching with a mechanistic approach and moving more towards discovery/inquiry-based experiments.

Students consider this lab a rewarding experience, through which they get to “act like real organic chemists” while learning key foundational content. They are largely successful on both hour-long exams, as well as on an end of term lab practical exam, in a) identifying relevant peaks correctly, b) matching that information to the correct possible unknown, and/or c) suggesting a reasonable structure for a given chemical formula and IR. Solving puzzles can be frustrating or confidence building; the key to keeping it fun is to provide a strategic scaffold and to encourage creativity.

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-Tara Kishbaugh, Eastern Mennonite University

Click here to learn more about Prof. Kishbaugh.

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