Ten weeks ago I walked into a classroom of 180 non-science majors to teach a course in Organic Chemistry. Many of these non-science majors were anxious about the experience to come, knowing only that people taking such a class often fell to repeat it once or twice to obtain a passing grade. Most of them had just completed one quarter of introductory general chemistry, and had no idea exactly how much they were about to learn.
Proton Nuclear Magnetic Resonance (NMR) Spectroscopy is a method of analysis, particularly useful to organic chemists, wherein a compound’s structure may be elucidated utilizing an applied external magnetic field (Bo) in conjunction with an electric field (E = hv). The first generation of NMR spectrometers were called continuous wave (CW) instruments because the electric field component was held constant as the magnetic field was incrementally varied by a small degree. Today’s modern Fourier Transform (FT) instruments utilize a homogeneous magnetic field whilst “pulsing” the sample of organic substance with a broad range of radio frequencies to effect the same results.
Infrared spectroscopy (IR) is a vintage technique of using mid-infrared frequency electromagnetic radiation (400-4000 cm-1) to induce characteristic vibrations in functional groups in terms of frequency. For a molecule to be “visible” to infrared spectroscopy, it must be capable of undergoing a change in dipole moment. Hence homonuclear diatomics such as H2 or N2 will be unsuitable for this method of analysis. Although the various stretching and bending modes for a given pair of nuclei are manifest in the IR spectrum, complete interpretation of all vibrational modes is something known mostly to Organic Chemists over the age of 80 years old. Once nuclear magnetic resonance (NMR) spectroscopy became affordable in the early 1980s, most chemists utilized NMR as the primary means of characterizing organic compounds, and detailed infrared spectroscopy problem solving skills were mastered by fewer and fewer people.
Most students of Organic Chemistry are introduced to the carbocation rearrangement when learning the SN1 and E1 processes, as this is their first exposure to carbocations. It’s common to see a 1,2-alkyl shift or a 1,2-hydride shift. Sometimes, depending upon the level of challenge presented by the professor, there will be tandem 1,2-hydride and 1,2-alkyl shifts (Scheme I).