Chemistry Solutions

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Twenty years ago, I was tasked with creating a one-semester, weighted chemistry elective course. The intent of this new course was to provide an opportunity for students to glimpse “what’s next” after having completed a Chemistry, Honors Chemistry, or AP Chemistry course. Our curriculum team wanted more than just an abbreviated version of AP Chemistry, as we wanted to expose students to chemistry in a context broader than what they’d normally see in a classroom. Our hope was to allow them to delve into some practical applications of chemistry at a deeper level.

The “magic” of forensic science

At the time, CSI: Crime Scene Investigation was a popular television show, bringing an awareness of forensic science to the general public. In light of that show’s popularity, we felt a Forensic Chemistry course with a spectroscopy component would be well-received, meeting the needs of our students and appealing to their sense of curiosity.

As mentioned earlier, we did not want a repeat of or a replacement for an AP Chemistry course, so our first step was to find out exactly what a forensic chemist does. To meet students’ expectations, we knew we would need to retain some of the techniques depicted in the television shows and movies.

Behind the drama, cool science

If you’ve ever seen those old CSI shows, you may remember some of the characters, such as Gil Grissom, whose signature line was, “Follow the evidence.” There was also Greg Sanders, the former lab technician who trained up to become an investigator, and David Hodges, the quirky trace evidence expert. There were several episodes where viewers would see Sanders or Hodges insert or inject an unknown sample into some very sophisticated piece of equipment and then learn the identity instantly. The CSI team would then use this known piece of evidence (and many others) to solve a crime — all in under 43 minutes. Magic! If only science could move at warp speed like that!

I remember students in my regular chemistry classes talking about episodes of CSI, and how cool it would be to solve mysteries. Some students asked how the “boxes hooked up to the computers” were able to identify the substances. The shows often featured microscopes and various pieces of glassware, but my kids weren’t very impressed by that. What they really wanted to know about was the graphs the show would feature, and what they meant. Based on what my students were telling me, I knew a Forensic Chemistry course was the way to go.

The data typically collected in the process of identifying these unknown substances were primarily gathered using gas chromatography (GC), mass spectrometry (MS), infrared spectroscopy (IR), and nuclear magnetic resonance (NMR). As my graduate school work involved the construction and characterization (identification) of various organic molecules, a Forensic Chemistry class would allow me to merge my passion for organic chemistry with the use of spectroscopy to present forensic puzzles to students in a meaningful way.

Exploring the unfamiliar

Do all chemistry teachers have a background in organic chemistry? Probably not. If anything, it may have been one of their most difficult courses in college. Some teachers may not have even taken the course. In my one-semester high school Forensic Chemistry class, I don’t try to compress college level organic chemistry into 18 weeks of student contact. That would be impossible! Instead, I have taken two major components of organic chemistry and incorporated them into my course: organic nomenclature and spectroscopy. With that said, the unknown compounds that students identify are simple compounds. My goal is not to replace their future college experiences. It is to expose them to the “what’s next.”

Wherever possible, I encourage teachers to reach out to nearby higher education institutions or chemical companies that have the specific chemicals and equipment capable of providing the types of spectra you want your students to work with. Partnering with a professor or chemist provides a resource most teachers could never duplicate in the classroom. Professors and chemists can benefit by exposing more students to their fields while simultaneously recruiting potential talent. Students benefit by seeing the real-world application of content learned in the classroom.

Forensic chemists and students can use chemical and physical testing to help identify an unknown substance. For example, students can use pH, thin layer chromatography (TLC), and melting point analysis to determine the identity of a specific carboxylic acid if they are provided with a list of potential candidates. Of course, forensic chemists are not afforded the luxury of choosing from multiple-choice answers when identifying an unknown substance. Instead, they must compile evidence from a variety of techniques, including different types of spectroscopies, to identify the unknown substance from an infinite list of possibilities. If we want our students to see more real-life applications of science, we should look for authentic opportunities whenever possible.

In my classes, I have seen that treating spectroscopic analysis as a puzzle takes a bit of the anxiety away for the students. Each type of spectroscopy can yield evidence of specific structural aspects, or “puzzle pieces” of a given molecule. Each puzzle piece provides students with the ability to make stronger claims regarding the identity of the unknown substance. Figure 1 summarizes the evidence students can collect using different types of spectroscopies.

Let’s take a peek at some of the information we can glean from each type of spectroscopy. For this example, consider two different unknowns (compounds A and B). Though students would only get a single unknown, these example compounds will show clearly how different, and therefore characteristic, the spectra can be, even for molecules of similar size and composition.

Mass spectrometry

First, I give my students the percent composition for their unknown compound. Using this information, they can determine the empirical formula. Next, they use mass spectrometry to help determine the molecular formula of the unknown compound. The main evidence I expect students to find from the spectrum is the molar mass, although providing students with a table of various fragmentation patterns (Figure 2) can be a great help to those who wish to delve deeper into the theory. This infographic, along with many others created and maintained on the Compound Interest website, has been especially helpful for my students.

The spectra in Figure 3 show that the two compounds (A and B) have both the same empirical formula (C₂H₄O) and the same molecular formula (C₄H₈O₂), yet the “peaks” in the two spectra are different, leading to the conclusion that the compounds must be different. Does this tell us the identity of the compounds? No, but we do know the molecular formulas, i.e. the number of carbons, hydrogens, and oxygens present in a molecule of each compound. 

Infrared spectrophotometry

Drawing on the information they previously deduced, students are now able to use infrared spectrophotometry to identify any functional groups present in the unknown compound. The presence or absence of peaks at specific wavelengths in the IR region provides students with another puzzle piece of the chemical structure for the unknown molecules. Figure 4 shows a table of IR bond vibration and stretches the students use for reference. Using the information in this table, along with the previously determined molecular formula, I expect students to be able to determine the major functional group(s) present in a molecule of their unknown sample.

Recall that compounds A and B were both found to have the same molecular formula (C₄H₈O₂), yet their mass spectra were different. The IR spectra (Figure 5) are also distinctly different for these compounds. I have annotated the spectra to show the major groups I expect students to identify in a given spectrum. The differences in these spectra further confirm that the two example substances are different. Both compounds contain a carbonyl group (C=O) in their molecules. One of them also contains an alcohol functional group (O-H) in its molecules.

At this point, a student might consider classifications such as carboxylic acids, esters, alcohols, and ketones, but without additional information they could not make a definite conclusion. A forensic chemist would not stop here in identifying unknown substances, and neither should our students!

¹H NMR spectroscopy

The workhorse technology for forensic chemists trying to determine unknown organic compounds is NMR. Two types of NMR are the most common, ¹H and ¹³C. The ¹³C NMR variety is usually used as a means of verification of a potential identity, but ¹H NMR is viewed as the better tool in more directly determining the identity of an unknown compound. A reference page for ¹H NMR is shown in Figure 6.

Comparing compounds A and B, as shown in Figure 7, we can see that they have different molecular structures. Earlier, students used MS and IR to determine the molecular formula and identity of functional groups, respectively. Using ¹H NMR, students analyze the peaks for their integration, value of shift (ppm) on the x-axis, and the splitting pattern to identify the number and types of hydrogens in the unknown molecule.

With multiple forms of evidence, students can finally put all the puzzle pieces together and draw a conclusion about the identity of their unknown. Students are not given a list of possible structures, so they cannot simply use a process of elimination to come to a final determination of identity. Instead, they make a claim regarding the identity of an unknown and explain how each piece of spectroscopic evidence supports that claim.

During the spectroscopy unit, I like to have students work together in groups of 2-3. I conclude the unit with a field trip to our local college (North Central College) and our mentor, Dr. Jeff Bjorklund. During the field trip, student groups work under Dr. Bjorklund’s guidance to physically use all three instruments — Gas Chromatograph-Mass Spectrometer, Infrared Spectrophotometer, and ¹H NMR Spectrometer — to collect real data for their analysis. Using this data, students work together with their group to make a claim, including the name and structure for their unknown compound, and to support the claim with reasoning that explains the evidence they have collected. The collaborative nature of this authentic end-of-unit assessment gives students a glimpse into how science really works, while also providing them with a unique experience that can lead to future opportunities.

Over the years I have taught, many students have come back to visit after their first one or two years in college. Many inform me that their professors were shocked to learn they have even seen this equipment, let alone interacted with it and interpreted the spectra obtained. This experience has opened doors for several of my former students, allowing them to join research groups as undergraduates. Students who entered STEM or healthcare professions have even come back to let me know that organic chemistry wasn’t nearly as scary as it could have been.

The positive feedback I receive from both current and former students encourages me to continue challenging students with content and experiences that would otherwise be limited to standard classroom space and curriculum. What could you do in your own classroom (or outside of it) to show your students “what’s next”?

Acknowledgement

Dr. Jeff Bjorklund, of North Central College in Naperville, IL, has been an integral part of the success of the project described in this article. Dr. Bjorklund's research exploits the use of NMR spectroscopy to better understand food chemistry and fermentation processes.