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218 Chapter 11 ALKENES; INFRARED SPECTROSCOPY AND MASS SPECTROMETRY processes. A classic example is encountered in attempted syntheses of terminal alkenes such as 1-butene. The only 100% reliable method is base-promoted E2 elimination of a suitable 1-butyl compound (e.g., 1-bromo-butane, 1-butyl tosylate). Any other method will give mixtures: The only elimination product (CH₃)₃COH + Major Minor, cis and trans CH₃CH₂CH=CH₂ + Minor Major, cis and trans Conc. Either or 2-butanol + Minor Major, cis and trans Additional information pertaining to this process is presented in Chapter 12. 11-8. Infrared Spectroscopy Once the most important spectroscopic technique, infrared spectroscopy is now used to complement NMR data. The IR technique helps confirm the presence or absence of common functional groups in a molecule. It is most diagnostic for the following: HO, C=N, C=C, C=0, and C=C. Different types of C-H bonds can be readily helping to confirm information obtained by NMR. Although occasionally the detailed data in Table 11-4 and in the text may be necessary to solve a problem, for the most part you will only need to look for bands in certain general regions of the IR spectrum, much the same way you have learned to divide up the NMR spectrum into rather general segments (e.g., alkane C-H and alkene C-H). The fol- lowing derived from the data in Table 11-4, shows these regions. For example, a compound exhibiting a strong band somewhere between 1680 and 1800 cm⁻¹ contains a group. However, IR data tells us about both the presence and the absence of functional groups in a molecule. Don't neglect the usefulness of the latter! For instance, a molecule lacking absorption between 3200 and 3700 cm⁻¹ cannot be an alcohol. Combining information from a molecular formula with NMR and IR data often permits com- plete determination of the structure of an unknown molecule. The text problems give you opportunities to practice. Regions of the Infrared Spectrum O-H C-H and Fingerprint region 4000 3500 3000 2500 2000 1800 1680 1500 1000 600 Wavenumber (cm⁻¹) 11-9 and 11-10. Mass Spectrometry The kind of information available from mass spectrometry falls into two categories. the m/z value for the molecular ion provides information useful in calculating the molecular formula of the molecule. Second. the lower molecular weight fragments that appear in the mass spectrum contain clues concerning structural features of the mol- ecule in question. Be sure that you understand how to extract these kinds of information from mass spectral data.