Organic Chemistry

Section 13.14 Infrared Inactive Vibrations 513

them. When the bond stretches, the increasing distance between the atoms in-
creases the dipole moment. Because the dipole moment changes when the bond
stretches, an absorption band is observed for the stretching vibration.

2,3-Dimethyl-2-butene, in contrast, is a symmetrical molecule, so its bond has no
dipole moment. When the bond stretches, it still has no dipole moment. Since stretching
is not accompanied by a change in dipole moment, no absorption band is observed. The
vibration is infrared inactive.2,3-Dimethyl-2-heptene experiences a very small change
in dipole moment when its bond stretches, so only an extremely weak absorption
band (if any) will be detected for the stretching vibration of the bond.

PROBLEM 27

Which of the following compounds has a vibration that is infrared inactive: acetone,

1-butyne, 2-butyne, ethene?

PROBLEM 28

The mass spectrum and infrared spectrum of an unknown compound are shown in

Figures 13.27 and 13.28, respectively. Identify the compound.

H 2 ,H 2 O,Cl 2 ,

C“C

C“C

CC

1-butene 2,3-dimethyl-2-butene 2,3-dimethyl-2-heptene

CH 3 H 3 C CH 2 CH 2 CH 2 CH 3

H 3 C

H 3 C

H 3 C

C

H

H CH 3 CH 3

C

H

CH 2 CH 3

CC

C“C

C“C

m/z

100

80

60

40

20

0

20 40 60 80

84

100

Relative abundance

55

>Figure 13.27

The mass spectrum for Problem 28.

2.5 2.6 2.72.82.9 3 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 15 16

4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600

Wavelength (μm)

Wavenumber (cm−^1 )

% Transmittance

Figure 13.28

The IR spectrum for Problem 28.