Hydrogens diastereotopic. nonequivalence

This outcome derives from the fact that the carbon that bears the fluorine is chiral, which makes the two vicinal hydrogens diastereotopic and thus magnetically nonequivalent. In such a case, the two diastereotopic protons will not only appear as separate signals (an AB system), but they usually will also couple to vicinal fluorines (and hydrogens) with different coupling constants. Examining Figure 2.8b, which represents... 

As mentioned in Sect. IV-B, enantiotopic groups even in achiral substances may show nonequivalence. For example, this property sometimes can be used to distinguish meso compounds from their chiral isomers. Thus in the presence of TFPE, the meso isomer of dimethyl 2,3-diaminosuccinate (46) shows two equally intense methoxy singlets and an AB quartet for the now diastereotopic methine hydrogens (88). This coupling clearly shows 46 to be the... 

Methylene protons adjacent to a chiral centre will be non-equivalent, despite the fact that there is free rotation about the carbon-carbon bond. Such protons are described as diastereotopic, since replacement of either of the two hydrogens in turn by a group X produces a pair of diastereoisomers. Such is the case of the two methylene protons (Ha and Hb) in l-phenylpropan-2-ol, which are nonequivalent and therefore have different chemical shifts in the p.m.r. spectrum. 

Diastereotopic protons in 1,2-dichloropropane. The proton NMR spectrum of 1,2-dichloropropane shows distinct absorptions for the methylene protons on Cl. These hydrogen atoms are diastereotopic and are chemically nonequivalent. 

Recall that two nuclei attached to the same atom can be nonequivalent [as required to produce observable coupling between them (Section 8.4)] if they are diastereotopic (Section 4.3). Even if the two nuclei are equivalent, there is a way to measure the coupling constant between them. For example, if two hydrogens are equivalent, one can (in principle, at least)... 

FIGURE 5.64 The 300-MHz spectrum of the diastereotopic methylene hydrogens in citric acid. While there is no adjacent stereocenter, restricted rotation renders these hydrogens nonequivalent. The resonances of the protons of the carboxyhc acid and hydroxyl groups are not shown. 

Molecules such as 3-methyl-2-butanol are even more complicated. If a hydrogen on one of the methyl groups on carbon-3 of 3-methyl-2-butanol is substituted with a deuterium, a new chiral center is created. Because there is already one chiral center, diastereomers are now possible. Thus, the methyl groups on carbon-3 of 3-methyl-2-butanol are diastereotopic. Diastereotopic hydrogens have different chemical shifts under all conditions, which can lead to unexpected complexity in spectra of simple compounds. The H-NMR spectrum of 3-methyl-2-butanol is shown in Figure 13.28. The methyl groups on carbon-3 are nonequivalent and give two doublets rather than one doublet of twice the intensity, which would be expected if they were equivalent. 

Typically, the chemical shifts of diastereotopic hydrogens are similar and, like other nonequivalent hydrogens, can even be the same by chance. The further the diastereotopic hydrogens are from the asymmetric center, the more similar their chemical shifts are... 

We expect the signal for the methyl protons adjacent to the diastereotopic hydrogens of 2-bromobutane to be a doublet of doublets as a result of splitting by the nonequivalent diastereotopic hydrogens. The signal, however, is a triplet. Use a splitting diagram to explain why it is a triplet rather than a doublet of doublets. 

Frequently, diastereotopic hydrogens have such close chemical shifts that their nonequivalence is not visible in an NMR spectrum. For example, in the chiral 2-bromohexane, all three methylene groups contain diastereotopic hydrogens. In this compound, only the two methylenes closest to the stereocenter reveal their presence. The third one is too far removed for the asymmetry of the stereocenter to have a measurable effect. 

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