Organic Chemistry

536 CHAPTER 14 NMR Spectroscopy

δ (ppm)

87654 3210

frequency

Figure 14.6 (continued)

1 HNMR spectra for Problem 10.

14.7 Characteristic Values of Chemical Shifts

Approximate values of chemical shifts for different kinds of protons are shown in

Table 14.1. (A more extensive compilation of chemical shifts is given in Appen-

dix VI.) An NMR spectrum can be divided into six regions. Rather than memoriz-

ing chemical shift values, if you remember the kinds of protons that are in each region,

you will be able to tell what kinds of protons a molecule has from a quick look at its

NMR spectrum.

Table 14.1 shows that the chemical shift of methyl protons is at a lower frequency

(0.9 ppm) than is the chemical shift of methylene protons (1.3 ppm) in a similar envi-

ronment and that the chemical shift of methylene protons is at a lower frequency than

is the chemical shift of a methine proton (1.4 ppm) in a similar environment. (When

an carbon is bonded to only one hydrogen, the hydrogen is called a methine

hydrogen.) For example, the NMR spectrum of butanone shows three signals.

The signal for the aprotons of butanone is the signal at the lowest frequency because

the protons are farthest from the electron-withdrawing carbonyl group. (In correlat-

ing an NMR spectrum with a structure, the set of protons responsible for the signal at

the lowest frequency will be labeled a, the next set will be labeled b, the next set c,

etc.) The band cprotons are the same distance from the carbonyl group, but the signal

1 H

sp^3

1 H

CC

CH H

O

C

C

O

C C

H

C

H

C

12 9.0 8.0 6.5 4.5 2.5 1.5 0

H

Z

COH

O

H

vinylic Z = O, N, halogen

CHC

allylic saturated

δ (ppm)