2- Butanol diastereotopic hydrogens

B The fourth possibility arises in chiral molecules, such as (R)-2-butanol. The two — CH2- hydrogens at C3 are neither homotopic nor enantiotopic. Since replacement of a hydrogen at C3 would form a second chirality center, different diastereomers (Section 9.6) would result depending on whether the pro-R or pro-S hydrogen were replaced. Such hydrogens, whose replacement by X leads to different diastereomers, are said to be diastereotopic. Diastereotopic hydrogens are neither chemically nor electronically equivalent. They are completely different and would likely show different NMR absorptions. 

The diastereotopic nature of H and at C3 in 2-butanol can also be appreciated by viewing Newman projections. In the conformations shown below (Fig. 9.17), as is the case for every possible conformation of 2-butanol, H and experience different environments because of the asymmetry from the chirality center at C2. That is, the molecailar landscape of 2-butanol appears different to each of these diastereotopic hydrogens. H and... 

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. 

Figure 9.16 and H (on C3, the front carbon in the Newman projection) experience different environments in these three conformations, as well as in every other possible conformation of 2-butanol, because of the chirality center at C2 (the back carbon in the Newman projection). In other words, the molecular landscape as viewed from one diastereotopic hydrogen will always appear different from that viewed by the other. Hence, and experience different magnetic environments and therefore should have different chemical shifts (though the difference may t>e small). They are not chemical shift equivalent. 

When identical groups or atoms on the same carbon are in in-equivalent environments, they are termed diastereotopic. For instance, the hydrogen atoms labeled Ha and Hb in (.S )-2-butanol 183 are diastereotopic. There is no symmetry operation that can interconvert these two hydrogen so that one assumes the characteristics of the other. These hydrogen atoms are different from each other in all meaningful ways, such as NMR shifts, ct( n bond lengths and, hence, bond dissociation energies. 

The two methylene hydrogens labeled H and at C3 in 2-butanol are diastereotopic. We can illustrate this by imagining replacement of H or with some imaginary group Q. The result is a pair of diastereomers. As diastereomers, they have different physical properties, including chemical shifts, especially for those protons near the chirality center. 

Consider the CH2 group of 2-butanol. There are no symmetry operations in 2-butanol, and as such the two hydrogens of the CH2 cannot be interconverted by a symmetry operation. Therefore, these two hydrogens are expected to be different from one another in all meaningful ways, such as NMR shift, acidity, C-H bond length, bond dissociation energy, reactivity, etc. They have the same connectivity, but there is no symmetry operation that interconverts them in any conformation. They are stereoheterotopic, and defined specifically as diastereotopic. 

Butanol is asymmetric, so the two hydrogens of the CH2 group are diastereotopic. Shouldn t the three hydrogens of the CH3 group at Cl (or C4) be diastereotopic also It depends. In particular, it depends on the timescaleof our observation of the molecule. 

---