Organic Chemistry

PROBLEM 11

In each of the following compounds, which of the underlined protons has the greater

chemical shift (i.e., the farther downfield signal or the higher frequency signal)?

a. c. e.

b. d.

PROBLEM 12

In each of the following pairs of compounds, which of the underlined protons has the

greater chemical shift (i.e., the farther downfield signal or the higher frequency signal)?

a. c.

b.

PROBLEM 13

Without referring to Table 14.1, label the protons in the following compounds. The proton

that gives the signal at the lowest frequency should be labeled a, the next b, etc.

a. d. g.

b. e. h.

c. f. i.

14.8 Integration of NMR Signals

The two signals in the NMR spectrum of 1-bromo-2,2-dimethylpropane in

Figure 14.5 are not the same size because the area under each signal is proportional to

the number of protons that gives rise to the signal. (The spectrum is shown again in

Figure 14.7.) The area under the signal occurring at the lower frequency is larger be-

cause the signal is caused by ninemethyl protons, while the smaller, higher-frequency

signal results from twomethylene protons.

You probably remember from a calculus course that the area under a curve can be

determined by integration. An NMR spectrometer is equipped with a computer

that calculates the integrals electronically. Modern spectrometers print out the

integrals as numbers on the spectrum. The integrals can also be displayed by a line of

integration superimposed on the original spectrum (Figure 14.7). The height of each

integration step is proportional to the area under that signal, which, in turn, is propor-

tional to the number of protons giving rise to the signal. By measuring the heights of

1 H

1 H

CH 3 CHCHCH 3

Cl

CH 3

CH 3 CH 2 CH 2 OCHCH 3

CH 3

ClCH 2 CH 2 CH 2 Cl

CH 3 CHCH 2 OCH 3

CH 3

CH 3 CH 2 CHCH 2 CH 3

OCH 3

CH 3 CH 2 CHCH 3

OCH 3

CH 3 CH 2 CH 2 CCH 3

O

CH 3 CH 2 CH 2 COCH 3

O

CH 3 CH 2 CH

O

CH 3 CH 2 CH 2 Cl or CH 3 CH 2 CHCH 3

Cl

O O

CH 3 CH 2 CH 2 Cl or CH 3 CH 2 CH 2 Br CH 3 CH 2 CH or CH 3 CH 2 COCH 3

O

CH 3

CH 3 CHCCH 2 CH 3

CH 3

CH 3 CHOCH 3

CH 3 CH 2 CHCH 3 CH 3 CH 2 CH CH 2

Br Br Cl

CH 3 CHCHBr

538 CHAPTER 14 NMR Spectroscopy