Organic Chemistry

564 CHAPTER 14 NMR Spectroscopy

giving rise to the signal. Thus, the number of carbons giving rise to a NMR signal

cannot routinely be determined by integration.

The NMR spectrum of 2-butanol is shown in Figure 14.32. 2-Butanol has car-

bons in four different environments, so there are four signals in the spectrum. The rela-

tive positions of the signals depend on the same factors that determine the relative

positions of the proton signals in NMR. Carbons in electron-dense environments pro-

duce low-frequency signals, and carbons close to electron-withdrawing groups produce

high-frequency signals. This means that the signals for the carbons of 2-butanol are in

the same relative order that we would expect for the signals of the protons on those car-

bons in the NMR spectrum. Thus, the carbon of the methyl group farther away from

the electron-withdrawing OH group gives the lowest-frequency signal. As the frequency

increases, the other methyl carbon comes next, followed by the methylene carbon; and

the carbon attached to the OH group gives the highest-frequency signal.

1 H

1 H

13 C

13 C

80 60 40 20 0

bdca

CH 3 CHCH 2 CH 3

OH

δ (ppm)

frequency

Figure 14.32

Proton-decoupled 13 C NMRspectrum of 2-butanol.

bbdcd c aa

CH 3 CHCH 2 CH 3

OH

80 60 40 20 0

δ (ppm)

frequency

Figure 14.33

Proton-coupled spectrum of 2-butanol. If the spectrometer is run in a

proton-coupled mode, splitting is observed in a 13 C NMRspectrum.

13 C NMR

The signals are not normally split by neighboring carbons because there is little like-

lihood of an adjacent carbon being a The probability of two carbons being

next to each other is (about 1 in 10,000). (Because does not have a

magnetic moment, it cannot split the signal of an adjacent 13 C.)

1.11%*1.11% 12 C

13 C. 13 C