My favorite chapter of second-semester organic chemistry is perhaps more surprising to me than anyone else, considering all the great mechanisms and reactions that fill the second-half of Karty’s textbook. My enjoyment of Chapter 16 (“Structure Determination 2: Nuclear Magnetic Resonance Spectroscopy and Mass Spectrometry”) is most likely due to my own experiences analyzing complex NMR data of carbohydrates, lipids, and peptides. Affectionately known as the “NMR Chapter,” Chapter 16 is briefly covered in the final week of our first-semester organic chemistry course, but is further expanded on and put to use in the first week of the second-semester.
At Northern Arizona University, our organic chemistry students are tested on NMR throughout the entire semester, while the mass spectroscopy is reserved for the analytical chemistry course the following semester. This gives the organic chemistry faculty nearly two weeks to dedicate to 1H, 13C, and DEPT NMR in lecture. As with many programs, these NMR techniques become very important to those chemistry students looking to carve-out a career in organic chemistry and for the other students required to take the second-semester laboratory course. Because the lab course is designed around the use of NMR analysis software, processing raw NMR data, analyzing unknown NMR spectra, and identifying the compounds the students synthesized in class, it is crucial that the chapter be covered in great detail to ensure the students’ success.
Though we cover some introductory theory about NMR in the final week of the first-semester course, we return to Karty’s chapter the first week of second-semester organic chemistry in order to further dissect and expand on the background of NMR spectroscopy. We navigate through the chapter, using Karty’s descriptions and illustrations to introduce 1H NMR chemical shifts, splittings, and couplings. From here, we head off on a tangent to discuss processing the raw data using MestReNova (an NMR processing software), making sure that the students receive the proper background, tutorials, and necessary tools to process and identify their own spectra in lecture and the laboratory. After 1H NMR, we transition into 13C spectra—discussing the absence of splitting and the identification of important carbons (like aromatic and quaternary carbons) via chemical shift. We then introduce DEPT spectra, using it in conjunction with a typical 13C spectrum to better identify their compounds.
Along with a typical homework assignment for the chapter, we also provide the students with a half-dozen unknown NMR spectra to process, analyze, and identify. Using the tools they learned in lecture, the students must analyze the 1H, 13C, and DEPT spectra of an unknown compound in order to determine the structure that corresponds to the compound’s molecular formula. They are required to identify the peak splitting, calculate the integrations, measure the J-values, and assign all peaks on the 1H spectra. Additionally, students are required to use the DEPT spectra to identify all the peaks pertaining to carbons bound to one, two, or three hydrogens. After obtaining that information, they are then able to locate their quaternary carbons. Identification of each unknown is a complex, detailed, and tedious process for the student, but they are given three weeks to complete the spectra—with the reward of finding one of these spectra verbatim on the upcoming exam.
Delving into this kind of detail the first week of second-semester organic chemistry provides the students (and myself) with a number of positive outcomes. They get an idea of the pace and difficulty of the class ahead and they get a chance to see the types of assignments they will be doing to prepare them for future exams (we also provide them with complex syntheses to practice for future exams). And for those willing to put in the time, it gives the students an idea of my dedication to their success—many students use the end-of-chapter homework, chapter examples, and my office hours to guide them along on their assignments. Perhaps I am biased because of the opportunities I am given when teaching Chapter 16 at the beginning of second-semester, but it is by far my favorite chapter of the semester.
-Cliff Coss, Northern Arizona University