Organic Chemistry

548 CHAPTER 14 NMR Spectroscopy

There are five sets of chemically equivalent protons in ethylbenzene (Figure 14.18).

We see the expected triplet for the protons and the quartet for the protons. (This is

a characteristic pattern for an ethyl group.) We expect the signal for the protons to be

a doublet and the signal for the proton to be a triplet. Because the and protons

are not equivalent, they must be considered separately in determining the splitting of the

signal for the protons. Therefore, we expect the signal for the protons to be split

into a doublet by the protons and each peak of the doublet to be split into another

doublet by the proton, forming a doublet of doublets. However, we do not see three

distinct signals for the and protons in Figure 14.18. Instead, we see overlap-

ping signals. Apparently, the electronic effect (i.e., the electron-donating/electron-

withdrawing ability) of an ethyl substituent is not sufficiently different from that of a

hydrogen to cause a difference in the environments of the and protons that is

large enough to allow them to appear as separate signals.

Hc, Hd, He

Hc, Hd, He

He

Hc

Hd Hd

He Hc He

Hc

Ha Hb

δ (ppm)

10 9 8765 4 3 2 1 0

HH

O 2 N H

HH

b

ca

frequency

Figure 14.19N
NMR spectrum of
nitrobenzene. The
signals for the
and protons do not
overlap.

Hc

Ha , Hb ,

1 H

δ (ppm)

87654 3210

HH

H CH 2 CH 3

HH

b

e

c

d

d

c

a

frequency

In contrast to the and protons of ethylbenzene, the and

protons of nitrobenzene show three distinct signals (Figure 14.19), and the multiplicity

of each signal is what we predicted for the signals for the benzene ring protons in

Hc, Hd, He Ha, Hb, Hc

Figure 14.18N
NMR spectrum of
ethylbenzene. The
signals for the
and protons
overlap.

He

Hc , Hd ,

1 H