13_BRCLoudon_pgs5-0.qxd 12/9/08 1:13 PM Page 612 612 CHAPTER 13 • NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY PROBLEMS 13.22 The d 1.2–1.5 region of the 300-MHz NMR spectrum of 1-chlorohexane, given in Fig. 13.12, is complex and not first-order. Assuming you could synthesize the needed com- pounds, explain how to use deuterium substitution to determine the chemical shifts of the protons that absorb in this region of the spectrum. Explain what you would see and how you would interpret the results. 13.23. Explain how the NMR spectra of (a) 3-methyl-2-buten-2-ol and (b) 1,2,2-trimethyl-1- propanol would change following a D2O shake. CHARACTERISTIC FUNCTIONAL-GROUP 13.7 NMR ABSORPTIONS This section surveys the important NMR absorptions of the major functional groups that we’ve already studied. The NMR spectra of other functional groups will be considered in the chapters devoted to those groups. A summary table of chemical shift information is given in Appendix III. A. NMR Spectra of Alkenes Two characteristic proton NMR absorptions for alkenes are the absorptions for the protons on the double bond, called vinylic protons (red in the following structures), and the protons on carbons adjacent to the double bond, called allylic protons (blue in the following structures). Don’t confuse these two types of protons. Typical alkene chemical shifts are illustrated in the following structures and are summarized in Fig. 13.4. vinylic proton allylic protons d 4.92 d 1.99 d 0.88 H d 5.58 terminal H CH2 CH2 CH3 vinylic Ld 1.32 L M protons $CCA ) (13.12) y d 4.88 HH) $ d 5.68 "H d 1.97 internal vinylic proton allylic proton In these structures, allylic protons have greater chemical shifts than ordinary alkyl protons, but considerably smaller chemical shifts than vinylic protons. Additionally, the chemical shifts of internal vinylic protons are greater than those of terminal vinylic protons. Recall from Sec. 13.3C that the same trend of chemical shift with branching is evident in the relative shifts of methyl, methylene, and methine protons on saturated carbon atoms. The chemical shifts of vinylic protons are much greater than would be predicted from the electronegativity of the alkene functional group and can be understood in the following way. Imagine that an alkene molecule in an NMR spectrometer is oriented with respect to the exter- nal applied field B0 as shown in Figure 13.14. The applied field induces a circulation of the p electrons in closed loops above and below the plane of the alkene. This electron circulation gives rise to an induced magnetic field Bi that opposes the applied field B0 at the center of the 13_BRCLoudon_pgs5-0.qxd 12/9/08 1:13 PM Page 613 13.7 CHARACTERISTIC FUNCTIONAL-GROUP NMR ABSORPTIONS 613 induced field Bi opposes B0 at the p bond Bi (induced field) induced p-electron circulation H H induced field Bi reinforces C C B0 at the vinylic proton H H B0 (external applied field) Figure 13.14 In an alkene,the induced field Bi (red) of the circulating p electrons augments the external applied field at the vinylic protons.As a result,vinylic protons have NMR absorptions at relatively large chemical shift (high frequency). loop. This induced field can be described as contours of closed circles. Although the induced field opposes the applied field B0 in the region of the p bond, the curvature of the induced field causes it to lie in the same direction as B0 at the vinylic protons. The induced field, therefore, augments the local field at the vinylic protons. As a result, the vinylic protons are subjected to a greater local field. This means that a greater frequency is required to bring them into reso- nance (Eq. 13.4). Consequently, their NMR absorptions occur at relatively high chemical shift. Because molecules in solution are constantly in motion and tumbling wildly, at any given time only a small fraction of the alkene molecules are oriented with respect to the external applied field as shown in Fig. 13.14. The chemical shift of a vinylic proton is an average over all orientations of the molecule. However, this particular orientation makes such a large contribution that it dominates the chemical shift. Splitting between vinylic protons in alkenes depends strongly on the geometrical relation- ship of the coupled protons. Typical coupling constants are given in Table 13.3 on p. 614. The spectra shown in Fig. 13.15 on p. 615, illustrate the very important observation that vinylic protons of cis-alkenes have smaller coupling constants than those of their trans isomers. (The same point is evident in the coupling constants of cis and trans protons shown in Figs. 13.9 and 13.10.) These coupling constants, along with the characteristic AC H bending bands from IR spectroscopy (Sec. 12.4C), provide important ways to determineL alkene stereochemistry. The very weak geminal splitting between vinylic protons on the same carbon stands in con- trast to the much larger cis and trans splittings. Geminal splitting is also illustrated in Figs. 13.9 and 13.10. The last two entries in Table 13.3 show that small splitting in alkenes is sometimes ob- served between protons separated by more than three bonds. Recall that splitting over these distances is usually not observed in saturated compounds. These long-distance interactions between protons are transmitted by the p electrons. 13_BRCLoudon_pgs5-0.qxd 12/9/08 1:13 PM Page 614 614 CHAPTER 13 • NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY TABLE 13.3 Coupling Constants for Proton Splitting in Alkenes Relationship of protons Name of relationship Coupling constant J, Hz HH $CCA ) cis 6–14 ) $ H $CCA ) trans 11–18 ) $H H $CCA ) geminal 0–3.5 ) $H H $CCA ) vicinal 4–10 ) H ) "$C % $ $ $C A C $ four-bond (allylic) 0–3.0 H C H % % (double bond$ can be cis or trans) $ % H C L $C A C $ five-bond 0–1.5 H $ $C % % (double bond can$ be cis or trans) In many spectra, geminal, four-bond, and five-bond splittings are not readily discernible as clearly separated lines, but instead are manifested as perceptibly broadened peaks. Such is the case, for example, in the NMR spectrum in Fig. 13.16 (Problem 13.24). PROBLEM 13.24 Propose a structure for a compound with the formula C7H14 with the NMR spectrum shown in Fig. 13.16. Explain in detail how you arrived at your structure. B. NMR Spectra of Alkanes and Cycloalkanes Because all of the protons in a typical alkane are in very similar chemical environments, the NMR spectra of alkanes and cycloalkanes cover a very narrow range of chemical shifts, typi- cally d 0.7–1.7. Because of this narrow range, the splitting in many of these spectra shows extensive non-first-order behavior. One interesting exception to these generalizations is the chemical shifts of protons on a cy- clopropane ring, which are unusual for alkanes; they absorb at unusually low chemical shifts, 13_BRCLoudon_pgs5-0.qxd 12/9/08 1:13 PM Page 615 13.7 CHARACTERISTIC FUNCTIONAL-GROUP NMR ABSORPTIONS 615 Ha Hb Ha Hb J 8.3 Hz J 13.5 Hz = = Ha Hb Cl Hb $CCA ) $CCA ) a Cl) $CO2H H) $CO2H cis trans d 6.86 d 6.25 d 7.51 d 6.26 (a) (b) Figure 13.15 The NMR spectra of the vinylic protons (color) of cis-trans isomers. The coupling constants are larger for the trans protons. chemical shift, Hz 2400 2100 1800 1500 1200 900 600 300 0 21459 0.07 ppm C7H14 7202 2219 2451 8 7 6543210 chemical shift, ppm (d) Figure 13.16 The NMR spectrum for Problem 13.24. The red number above each resonance is its relative inte- gral in arbitrary units. typically d 0–0.5. Some even have resonances at smaller chemical shifts than TMS (that is, negative d values). For example, the chemical shifts of the ring protons of cis-1,2-dimethylcy- clopropane shown in red are d ( 0.11). - d ( 0.11) - H H H3C CH3 13_BRCLoudon_pgs5-0.qxd 12/9/08 1:13 PM Page 616 616 CHAPTER 13 • NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY The cause of this unusual chemical shift is an induced electron current in the cyclopropane ring that is oriented so as to shield the cyclopropane protons from the applied field. As a result, these protons are subjected to a smaller local field, and their chemical shifts are decreased. C. NMR Spectra of Alkyl Halides and Ethers Several NMR spectra of alkyl halides and ethers were presented in developing the principles of NMR earlier in this chapter. The chemical shifts caused by the halogens are usually in pro- portion to their electronegativities. For the most part, chloro groups and ether oxygens have about the same chemical-shift effect on neighboring protons (Fig. 13.4). However, epoxides, like cyclopropanes, have considerably smaller chemical shifts than their open-chain analogs. d 3.65 d 2.95 d 2.4, 2.7 $ $ H3C CH O CH CH3 H3C CH CH2 L L LL LLO "CH3 "CH3 An interesting type of splitting is observed in the NMR spectra of compounds containing fluorine. The common isotope of fluorine (19F) has a nuclear spin. Proton resonances are split by neighboring fluorine in the same general way that they are split by neighboring protons; the same n 1 splitting rule applies. For example, the proton in HCCl2F appears as a doublet cen- tered at+ d 7.43 with a large coupling constant JHF of 54 Hz.