2- Butanol carbon-13 chemical shift

Each alcohol has a saturated carbon atom next to oxygen, all close together. Then there are carbons next door but one to oxygen they are back in the 0-50 p.p.m. region but at its low field end— about 30-35 p.p.m.. Notice the similarity of these chemical shifts to those of carbons next to a carbonyl group (Table 3.5 on p. 63). In each case we have C-C-O and the effects are about the same. Two of the alcohols have carbon(s) one further away still at yet smaller chemical shift (further upfield, more shielded) at about 20 p.p.m., but only the n-butanol has a more remote carbon still at 15.2. The number and the chemical shift of the signals identify the molecules very clearly. 

Hybridized carbons are more shielded than sp as the chemical shifts for C-2 in pentane versus 1-pentene and C-1 in 1-butanol versus butanal demonstrate. The effect of substituent electronegativity is evident when comparing pentane with 1-butanol 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. 

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