Organic Chemistry

− 49 °C

− 57 °C

− 60 °C

− 63 °C

− 68 °C

− 89 °C

558 CHAPTER 14 NMR Spectroscopy

room temperature to be detected by the NMR spectrometer. Because axial protons in

one chair conformer are equatorial protons in the other chair conformer, all the protons

in cyclohexane have the same average environment on the NMR time scale, so the

NMR spectrum shows one signal.

The rate of chair–chair interconversion is temperature-dependent—the lower the

temperature, the slower is the rate of interconversion. has 11 deuteri-

ums, which means that it has only one hydrogen. NMR spectra of

taken at various temperatures are shown in Figure 14.27. Cyclohexane with only one

hydrogen was used for this experiment in order to prevent splitting of the signal, which

would have complicated the spectrum. Deuterium signals are not detectable in

NMR, and splitting by a deuterium on the same or on an adjacent carbon is not

normally detectable at the operating frequency of an NMR spectrometer.

At room temperature, the NMR spectrum of shows one sharp

signal, which is an average for the axial proton of one chair and the equatorial proton

of the other chair. As the temperature decreases, the signal becomes broader and even-

tually separates into two signals, which are equidistant from the original signal. At

two sharp singlets are observed because at that temperature, the rate of

chair–chair interconversion has decreased sufficiently to allow the two kinds of pro-

tons (axial and equatorial) to be individually detected on the NMR time scale.

14.15 Protons Bonded to Oxygen and Nitrogen

The chemical shift of a proton bonded to an oxygen or a nitrogen depends on the degree

to which the proton is hydrogen bonded—the greater the extent of hydrogen bonding,

the greater is the chemical shift—because the extent of hydrogen bonding affects the

electron density around the proton. For example, the chemical shift of the OH proton of

an alcohol ranges from 2 to 5 ppm; the chemical shift of the OH proton of a carboxylic

acid, from 10 to 12 ppm; the chemical shift of the NH proton of an amine, from 1.5 to

4 ppm; and the chemical shift of the NH proton of an amide, from 5 to 8 ppm.

The NMR spectrum of pure dry ethanol is shown in Figure 14.28(a), and the

NMR spectrum of ethanol with a trace amount of acid is shown in Figure 14.28(b).

The spectrum shown in Figure 14.28(a) is what we would predict from what we have

learned so far. The signal for the proton bonded to oxygen is farthest downfield and is

split into a triplet by the neighboring methylene protons; the signal for the methylene

protons is split into a multiplet by the combined effects of the methyl protons and the

OH proton.

The spectrum shown in Figure 14.28(b) is the type of spectrum most often obtained

for alcohols. The signal for the proton bonded to oxygen is not split, and this proton

does not split the signal of the adjacent protons. So the signal for the OH proton is a

singlet, and the signal for the methylene protons is a quartet because it is split only by

the methyl protons.

The two spectra differ because protons bonded to oxygen undergo proton exchange,

which means that they are transferred from one molecule to another. Whether the OH

proton and the methylene protons split each other’s signals depends on how long a par-

ticular proton stays on the OH group.

1 H 1 H

• 89 °C,

1 H cyclohexane-d 11

1 H

1 H

1 H cyclohexane-d 11

Cyclohexane-d 11

chair–chair

interconversion

H

H

H

H

equatorial

axial

axial

equatorial

Figure 14.27
NMR spectra of cyclohexane
at various temperatures.

1 H -d 11