Organic Chemistry

PROBLEM 28

Why is there no coupling between and or between and in cis- or trans-3-

chloropropenoic acid?

Let’s now summarize the kind of information that can be obtained from an

spectrum:

1. The number of signals indicates the number of different kinds of protons that are
   in the compound.

1 H NMR

Ha Hc Hb Hc

552 CHAPTER 14 NMR Spectroscopy

Coupling constants can be used to distinguish between the spectra of cis

and trans alkenes. The coupling constant of trans-vinylic protons is significantly

greater than the coupling constant of cis-vinylic protons (Figure 14.21), because the cou-

pling constant depends on the dihedral angle between the two bonds in the

unit. The coupling constant is greatest when the angle between

the two bonds is 180°(trans) and smaller when it is 0°(cis). Notice the differ-

ence between Jbdand Jcdin the spectrum of 3-bromo-1-propene (Figure 14.17).

C¬H

H¬C“C¬H

C¬H

1 H NMR

Table 14.3 Approximate Values of Coupling Constants

Approximate value of Jab (Hz) Approximate value of Jab (Hz)

C7

H

0

C

H

C

H

CC

H

H

H

C

C

C2

(geminal coupling)

H

15 (trans)

10 (cis)

1

(long-range

coupling)

ab

ab

a

b

a

Hb

C

C

H

C

C

b

Hb

C

Ha

C

Ha

ab

ab

a

b

b

b

a

a

a

b

b c

a

Jba = 14 Hz Jab = 14 Hz

c

a

Jba = 9 Hz Jab = 9 Hz

Hb Ha

C

H

Cl

COOH

H

C

b

C

H

Cl

C

H

COOH

Hb Ha

trans-3-chloropropenoic acid cis-3-chloropropenoic acid

Figure 14.21

The doublets observed for the and protons in the NMR spectra of trans-3-

chloropropenoic acid and cis-3-chloropropenoic acid. The coupling constant for trans

protons (14 Hz) is greater than the coupling constant for cis protons (9 Hz).

Ha Hb 1 H

The coupling contant for trans protons is
greater than the coupling constant for
cis protons.

The dependence of the coupling
constant on the angle between the two
bonds is called the Karplus
relationship after Martin Karplus,
who first observed the relationship.
Karplus was born in 1930. He
received a B.A. from Harvard
University and a Ph.D. from the
California Institute of Technology. He
is currently a professor of chemistry at
Harvard University.

C¬H