CHAPTER THIRTEEN Spectroscopy When enantiomers are generated by replacing first one proton and then another by a test group, the pair of protons are enantiotopic with respect to one another. The meth ylene protons at C-2 of 1-propanol, for example, are enantiotopic. CH CH CH,OH CHOH H CHOH Enantiotopic 1-Propanol (R)-2-Chloro-l-propanol (S)-2-Chloro-1-propanol have different chemica chemical shift, regardless field strength of the NMR spectro shifts in a chiral solvent. Be. cause the customary solvent At the beginning of this section we noted that an NMR spectrum provides struc CDCl3)used in NMR mea- tural information based on chemical shift, the number of peaks, their relative areas, and the multiplicity, or splitting, of the peaks. We have discussed the first three of these fea- phenomenon is not observed tures of H NMR spectroscopy. Lets now turn our attention to peak splitting to see what kind of information it offers 13.7 SPIN-SPIN SPLITTING IN NMR SPECTROSCOPY The H NMR spectrum of CH3OCH2CN(see Figure 13.9)discussed in the preceding section is relatively simple because both signals are singlets; that is, each one consists of a single peak. It is quite common though to see a signal for a particular proton appear not as a singlet, but as a collection of peaks. The signal may be split into two peaks(a doublet), three peaks(a triplet), four peaks(a quartet), or even more. Figure 13.10 nows the H NMR spectrum of 1, 1-dichloroethane(CH; CHCI), which is characterized by a doublet centered at 8 2 1 ppm for the methyl protons and a quartet at 8 5.9 ppm for the methine proton The number of peaks into which the signal for a particular proton is split is called its multiplicity. For simple cases the rule that allows us to predict splitting in H NMR More complicated splitting Multiplicity of signal for Ha = n+1 patterns conform to an ex- tension of the"n+1"rule where n is equal to the number of equivalent protons that are vicinal to Ha. Two pro- and will be discussed in se tons are vicinal to each other when they are bonded to adjacent atoms. Protons vicinal tion 13.11 to Ha are separated from Ha by three bonds. The three methyl protons of 1, 1 dichloroethane are vicinal to the methine proton and split its signal into a quartet. The ingle methine proton, in turn, splits the methyl protons' signal into a doublet This proton splits the signal for the methyl protons into H c-CI These three protons split the signal for the me proton into a quartet. The physical basis for peak splitting in 1, I-dichloroethane can be explained with the aid of which examines how the chemical shift of the methyl protons is affected by the spin of the methine proton. There are two magnetic environments for the methyl protons: one in which the magnetic moment of the methine proton is paral lel to the applied field, and the other in which it is antiparallel to it. When the magnetic Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteWhen enantiomers are generated by replacing first one proton and then another by a test group, the pair of protons are enantiotopic with respect to one another. The meth￾ylene protons at C-2 of 1-propanol, for example, are enantiotopic. Replacing one of these protons by chlorine as a test group gives (R)-2-chloro-1-propanol; replacing the other gives (S)-2-chloro-1-propanol. Enantiotopic protons have the same chemical shift, regardless of the field strength of the NMR spectrometer. At the beginning of this section we noted that an NMR spectrum provides struc￾tural information based on chemical shift, the number of peaks, their relative areas, and the multiplicity, or splitting, of the peaks. We have discussed the first three of these fea￾tures of 1 H NMR spectroscopy. Let’s now turn our attention to peak splitting to see what kind of information it offers. 13.7 SPIN–SPIN SPLITTING IN NMR SPECTROSCOPY The 1 H NMR spectrum of CH3OCH2CN (see Figure 13.9) discussed in the preceding section is relatively simple because both signals are singlets; that is, each one consists of a single peak. It is quite common though to see a signal for a particular proton appear not as a singlet, but as a collection of peaks. The signal may be split into two peaks (a doublet), three peaks (a triplet), four peaks (a quartet), or even more. Figure 13.10 shows the 1 H NMR spectrum of 1,1-dichloroethane (CH3CHCl2), which is characterized by a doublet centered at  2.1 ppm for the methyl protons and a quartet at  5.9 ppm for the methine proton. The number of peaks into which the signal for a particular proton is split is called its multiplicity. For simple cases the rule that allows us to predict splitting in 1 H NMR spectroscopy is Multiplicity of signal for Ha n  1 where n is equal to the number of equivalent protons that are vicinal to Ha. Two pro￾tons are vicinal to each other when they are bonded to adjacent atoms. Protons vicinal to Ha are separated from Ha by three bonds. The three methyl protons of 1,1- dichloroethane are vicinal to the methine proton and split its signal into a quartet. The single methine proton, in turn, splits the methyl protons’ signal into a doublet. The physical basis for peak splitting in 1,1-dichloroethane can be explained with the aid of Figure 13.11, which examines how the chemical shift of the methyl protons is affected by the spin of the methine proton. There are two magnetic environments for the methyl protons: one in which the magnetic moment of the methine proton is paral￾lel to the applied field, and the other in which it is antiparallel to it. When the magnetic This proton splits the signal for the methyl protons into a doublet. These three protons split the signal for the methine proton into a quartet. Cl H CH3 C Cl Enantiotopic protons C CH3 CH2OH H H H C CH2OH C CH2OH 1-Propanol CH3 Cl H (R)-2-Chloro-1-propanol CH3 Cl (S)-2-Chloro-1-propanol 500 CHAPTER THIRTEEN Spectroscopy Enantiotopic protons can have different chemical shifts in a chiral solvent. Be￾cause the customary solvent (CDCl3) used in NMR mea￾surements is achiral, this phenomenon is not observed in routine work. More complicated splitting patterns conform to an ex￾tension of the “n  1” rule and will be discussed in Sec￾tion 13.11. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website