Chapter 1
Application of Benchtop Nuclear Magnetic Resonance for
Structure Elucidation in a Multi
-Outcome Experiment:
Microwave
-Promoted Reduction of Unknown Aldehydes and
Ketones
Mengqi Zhang and Richard W. Morrison*
Department of Chemistry, University of Georgia, 140 Cedar Street, Athens, Georgia 30602, United States
*Email: [email protected].
Multi-outcome experiments (MOEs) develop critical thinking abilities and spectrum analysis skills by requiring students to deduce unknown starting materials based upon Fourier-transform infrared spectroscopy (FTIR) and benchtop nuclear magnetic resonance (NMR) spectroscopic analysis of their reaction products. In this way, MOEs create a probative laboratory experience for undergraduate students. An MOE experiment for the reduction of unknown aldehydes and ketones that illustrated the utility of benchtop NMR spectroscopy in multi-outcome experiments is herein described for sophomore Organic Chemistry II laboratory. Microwave is utilized as a high efficiency heating alternative.
Introduction
Multi-outcome experiments (MOEs) have been recently developed for the University of Georgia organic chemistry instructional laboratories to help facilitate students’ ability to interpret spectral results and get familiarized with modern analytical techniques(1–3). In multi-outcome experiments, there must be at least one unknown component to identify. For example, the unknown can be the starting material, the catalyst load, or the reagent. Students utilize spectroscopic analyses to characterize the final product and then deduce the identity of unknown.
Compared to traditional single-outcome experiments, aka “cookbook” experiments, MOEs have several advantages. In the traditional “cookbook” experiment, students know the identity of the final product. They perform the same experiment and record the same observations. Multi-outcome experiments require students to identify their individualized unknown reaction components through analysis of their experimental and analytical data. This process more closely resembles an actual research inquiry and thus helps students to prepare for subsequent analytical research.
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For the reduction of aldehydes and ketones described in this chapter, Fourier-transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H NMR) spectroscopy were employed as characterization techniques. Both are modern analytical tools that are of vital importance in chemical structure elucidation.
Fourier
-Transform Infrared Spectroscopy and Nuclear Magnetic Resonance
Spectroscopy in Introductory Organic Chemistry Courses and the Integrated
Laboratory Course at UGA
FTIR and NMR spectroscopy are essential analytical tools for structure elucidation and chemical determination. Both techniques are widely applied in areas of chemistry, biology and medicine. Students are introduced to FTIR and NMR spectroscopic analyses in the first semester organic chemistry lecture course at the University of Georgia. Questions involving both FTIR and NMR spectra are incorporated into homework problem sets and exams.
Multi-outcome experiments utilizing these spectroscopic techniques have been incorporated into the co-requisite instructional laboratory. Students gain hands-on experience with these modern analytical tools and learn their practical utility. MOEs help reinforce and consolidate student mastery of these techniques.
The Utility of Benchtop NMR at UGA
Every organic chemistry instructional laboratory at the University of Georgia is equipped with an FTIR spectrometer and an 82 MHz benchtop NMR spectrometer. Each student enrolled in organic chemistry instructional laboratory courses is trained to use both FTIR and NMR spectrometers for spectroscopic analyses.
Benchtop NMR spectrometers have many advantages over a high-resolution NMR spectrometer(4, 5). The 82 MHz picoSpin benchtop NMR spectrometer model used at UGA, for example, is cryogen-free and requires minimal daily maintenance. The benchtop spectrometer fits easily in the fume hood or on the benchtop without the need for new infrastructure. And, at a fraction of the cost of high-field NMR spectrometers, the benchtop NMR spectrometer is more easily acquired.
To collect a1H NMR spectrum, students inject approximately 0.5 mL of concentrated dissolved product (deuterated solvent not required) or neat liquid product into the benchtop NMR instrument. From the author’s teaching experience with this MOE, most of the reduction products exist in liquid form and can be injected neat into the benchtop NMR(solvent-free). Solid products were dissolved in as little methylene chloride as possible for benchtop NMR analyses. It requires ca. 30 seconds or so to collect the1H NMR spectrum of the sample. Within the traditional (2 hours and 45 minutes) laboratory period students collect all the spectroscopic data needed to elucidate the structures of the unknown components. These hands-on experiences with modern analytical techniques greatly encourage undergraduate students to pursue the experimental sciences and better prepares them for subsequent research and coursework.
Microwave Promotion and Its Application at UGA
Microwave promotion, also known as dielectric heating, was used as the alternative heating source in the reduction of unknown aldehydes and ketones. Microwave promotion uses microwave (light with frequency between 300 MHz - 300 GHz) to heat polar molecules. Polar molecules
possessing electric dipole moments align themselves in an electromagnetic field. When the field is oscillating, as in a microwave oven, these polar molecules continuously rotate to align with the oscillating field. Thus, rotating molecules push, pull and collide with other molecules, distributing energy to adjacent molecules in the process. The temperature increases due to energy transferred from the remote radiative energy through molecular motion.
The use of microwave promotion in these reductions dramatically decreased reaction times. Although the reduction reaction of aldehydes and ketones experiment using sodium borohydride can be completed at room temperature, microwave heating increased the reaction rate and abbreviated the reaction time. As a result, the reduction reaction is completed within 1 minute at 100 °C.
Experimental Section
The microwave-promoted reduction of unknown aldehydes and ketones experiment is conducted in the sophomore Organic Chemistry II laboratory. A standard instructional organic laboratory at UGA is a 2 hour and 45-minute laboratory period comprised of up to 24 students. Students complete the experiment as well as characterize their final products within the laboratory period. In the post-lab report, students are asked to identify the unknown starting material based on their experimental observation and spectral analyses.
Sodium borohydride was used to reduce aldehydes and ketones to 1° and 2° alcohols, respectively, in 95% ethanol solution (6). Microwave temperature profile parameters were shown in Table 1.
Table 1. Microwave Set-Up Parameters
Duration (min) Temperature profile (°C)
2 25-100
1 Remain at 100
Approximately 10 Cool down to < 60
The graduate teaching assistant provided a 2.00 mL ethanolic solution of one of the nine possible aldehydes or ketones shown in Figure 1 to each student. To this solution, the student combines 0.5 g of sodium borohydride and 20.0 mL 95% ethanol in a Teflon microwave pressure reaction vessel. After microwave heating was completed, the final reduction product was extracted with dichloromethane and washed with 60 mL of deionized water three times. The organic solution was dried over magnesium sulfate and the excess dichloromethane was evaporated off in a fume hood. The mass of dried final product was recorded, and the product was characterized by FTIR and1H NMR spectroscopy.
Benchtop NMR Spectrometer Parameters and Sample Preparation
One drop of tetramethylsilane was added into the liquid product before injection into the benchtop NMR spectrometer. If reduction product was solid, the solid was dissolved in a minimum amount of dichloromethane and then one drop of tetramethylsilane was added for preparation of sample.
Figure 1. Nine unknown starting aldehyde and ketone candidates. Numerical assignments for the nine unknown aldehydes and ketones are shown for reference.
The liquid mixture(approximately 0.5 mL) was injected into the benchtop NMR spectrometer. Parameters shown in Figure 2 are a screenshot of the picoSpin interface. Eight scans took approximately 30 seconds. Following acquisition, the1H NMR spectra are integrated with assistance from the graduate teaching assistant as necessary. All spectra are collected within one lab period and students receive all collected spectra by the end of lab day.
Figure 2. A screenshot of benchtop NMR interface (picoSpin model) and parameter as reference. Water was injected as the test run sample. Eight scans took ~ 30 seconds and provided quality spectra. “Tx frequency”
Spectral Analysis
The starting materials and the final products of the reduction were very similar in state and color, as shown in Figure 3. Both photos were taken by the author using cellphone prior and after the reaction completion with white paper as background. Numerical assignments for the nine unknown aldehydes and ketones were listed in Figure 1. All nine starting materials were colorless liquid which cannot be distinguished from mere observation. All nine reduction products were colorless liquid except p-tolualdehyde (6) product would appear as a white solid. Methylene chloride may still be present in the product if evaporation did not go to completion but will not interfere with the spectra collection. As evidenced in Figure 3, it is not possible to determine the identity of the unknown starting materials or products by visual inspection.
Figure 3. a) Visual comparison of the nine starting aldehydes and ketones candidates. b) Visual comparison of the nine alcohol products. Numerical assignments for the nine unknowns were listed in Figure 1. Both
photos were taken by the author prior to and after the reaction with white paper as background.
Figure 4. FTIR spectrum of 4-ethoxybenzaldehyde (4) reduction product collected from student data in
2018 fall. The presence of O-H bond stretch around 3400 cm-1and disappearance of carbonyl bond stretch
Figure 5. FTIR spectrum of propiophenone (8) reduction product collected from student data in 2018 fall.
The presence of O-H bond stretch around 3400 cm-1and the disappearance of the carbonyl bond stretch
around 1700 cm-1indicated reaction completion.
Figure 6.1H NMR spectrum of 4-ethoxybenzaldehyde (4) reduction product with peak assignments. The
peak at 5.07 ppm corresponds to residual dichloromethane solvent. The tetramethylsilane (TMS) reference signal was set to 0.0 ppm.
FTIR spectra for all products showed a characteristic O-H bond stretch at 3300 cm-1and the disappearance of carbonyl bond stretch(around 1725 cm-1), which indicated reduction completion. Example FTIR spectra of 4-ethoxybenzaldehyde (4) and propiophenone (8) reduction product are shown in Figure 4 and Figure 5. Csp2-H and Csp3-H stretches were also present in the FTIR spectra
around 3100 cm-1. Students were required to label distinctive absorptions, or lack of distinctive absorptions, mentioned on the spectrum. As anticipated, this did not help with the identification of the starting unknown which made1H NMR essential for product structure elucidation.
1H NMR spectroscopic analysis of chemical shifts, signal integrations and splitting patterns were necessary to elucidate the structure of the unknown product. Students were required to correlate protons for their product structure to peaks on the1H NMR spectrum. Figure 6 and Figure 7 provide two examples of1H NMR spectra acquired using the benchtop NMR spectrometer. Spectra were taken from student data using the parameters given in Figure 2.
Figure 7.1H NMR spectrum of propiophenone (8) reduction product with peak assignments. The peak at
5.07 ppm corresponds to residual dichloromethane solvent. The tetramethylsilane (TMS) reference signal was set to 0.0 ppm.
Because the nine starting material candidates share very similar chemical structures, some being constitutional isomers, the structure elucidation of the corresponding reduction products is a challenge. Three different aldehyde or ketone candidates were assigned to each lab section. At the end of lab period, the three sets of product spectra collected for the entire class were sent to every student. Thus, students in all sections had access to spectra for all nine products and knew that their unknown was one of the nine. Detailed analysis and comparison assisted students in identifying their assigned unknown starting material.
An example flow chart of the identification process is shown in Figure 8. This flow chart was not provided to students but shown here for illustration purposes. From the flow chart, it is clear that students must fully extract all data(chemical shifts, splitting patterns, integrations etc.) from the1H
NMR spectra to identify the unknown. As a result, students are well rehearsed in1H NMR spectrum interpretation through this experiment and more fully appreciate the importance of spectroscopic analysis in organic chemistry.
Figure 8. Flow chart illustrating the identification process for the unknown starting material candidate based
on1H NMR spectrum. This chart was not provided to students.
Student Feedback in Post-Lab Online Survey
Students were given one week to complete a voluntary online post-lab survey via Google form after the completion of the lab. Surveyed students answered a series of questions pertinent to experiment, including identification result, yield, and feedback on characterization techniques. Of the 121 students that participated in the survey in 2018 fall, the correct identification rate of the starting candidate was 70.2%. Correct identifications varied from 44.4% in p-tolualdehyde (6) to 90% in 4-fluoroacetophenone (5). The reason(s) for the varied identification rate is unknown. These results demonstrate that the difficulty level for this multi-outcome experiment was moderate to challenging and was appropriate for students in the Organic Chemistry II laboratory.
When asked to select the most important characterization technique for the identification of the unknown, 96.7% of the students chose NMR spectroscopy, compared to IR spectroscopy and observation. Survey comments reflected that students recognized the importance and fully used the NMR data during the identification process. One student wrote in the post-lab survey, “Since the
final product was an alcohol, all of the products would have had the same functional groups and therefore, the same IR. The 1H NMR allowed you to piece together the carbon skeleton of the product, which would have been distinct. From there, you could work backwards to determine the identity of the unknown ketone/ aldehyde reagent. I could not draw any conclusion from the smell or color. Everyone’s product looked/smelled the same to me.” This coincided with the author’s prediction and suggested the experiment design
was successful.
Virtual Practice for Multi
-Outcome Experiments
The unique circumstances related to Covid-19 created a daunting challenge for the instructional laboratory. During the 2020 summer semester, no in-person instructional laboratory work was allowed. The Chemistry Department at UGA disseminated recorded benchtop experiments associated with every multi-outcome experiment. Pre-lab lecture videos and detailed experiment videos were shared with students via Zoom meetings. Students received FTIR and1H NMR spectra for their unknown products electronically to analyze and identify. Thus, students enrolled in the
Organic Chemistry II laboratory course completed the reduction of an unknown aldehyde or ketone experiment remotely.
Compared to the single-outcome analog of this reduction experiments, this MOE and all other multi-outcome experiments that emphasize analysis of spectroscopic results were less significantly impacted by the transition to virtual laboratory experiments. The most substantial negative impact was the absence of hands-on benchtop work and direct exposure to modern analytical instrumentation. Nevertheless, based upon students results and feedback, multi-outcome experiments provided students with exposure to characterization techniques and an array of structure elucidation examples for group discussions and collaborative problems solving.
Conclusions
Survey feedback from students and graduate teaching assistants uniformly evaluated this experiment as successful and constituting a valuable contribution to the array of MOEs performed in the organic instructional laboratories.
In this chapter, we presented a new multi-outcome experiment for the microwave-promoted reduction of aldehydes and ketones. This MOE was incorporated into the second-semester organic chemistry laboratory curriculum and emphasizes benchtop NMR spectroscopic analysis. With the advent of benchtop NMR spectroscopy and its introduction into organic chemistry instructional laboratory courses students are able to bridge the gap between NMR concepts taught in lecture and their hands-on experiences in the co-requisite laboratory using modern analytical techniques. In addition, this new MOE reinforces the transformation of aldehydes and ketones into their corresponding alcohols using sodium borohydride as a reducing agent. MOEs not only cultivate students’ spectral analysis skills but also promote independent thinking, which better prepares them for future research inquiry. Student comments proclaim that they develop more enthusiasm toward organic laboratory experiments via exploration and identification in MOEs than through simple structure confirmation in traditional single-outcome “cookbook” experiments. MOEs provide students with individualized laboratory experiences and increased ownership of their acquired results.
Acknowledgments
The authors would like to thank Dr. Richard Hubbard and Fabian Tejedor Rojas for their help in the preparation of digital videos and in the coordination of instrument maintenance. In addition, acknowledge the help of Thaddeus Paulsel and all organic teaching assistants in the UGA chemistry department for their collective efforts in the implementation of new MOEs.
References
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