Organic Chemistry

or bonds. We may deduce, then, that the compound is an ether. Its

absorption bands show that it has only hybridized carbons and that it has a methyl

group. We also know that the compound has fewer than four adjacent methylene

groups, because there is no absorption at The compound is actually

diethyl ether.

PROBLEM 23

How does one know that the absorption band at in Figure 13.26 is due to a

bond and not to a bond?

PROBLEM 24

a. An oxygen-containing compound shows an absorption band at and no

absorption bands at or. What class of com-

pound is it?

b. A nitrogen-containing compound shows no absorption band at and no

absorption bands between and. What class of compound is it?

PROBLEM 25

How could IR spectroscopy distinguish between the following?

a. a ketone and an aldehyde d.cis-2-hexene and trans-2-hexene

b. a cyclic ketone and an open-chain ketone e. cyclohexene and cyclohexane

c. benzene and cyclohexene f. a primary amine and a tertiary amine

PROBLEM 26

For each of the following pairs of compounds, give one absorption band that could be used

to distinguish between them:

a. and

b.

c.

d.

e.

f.

13.14 Infrared Inactive Vibrations

Not all vibrations give rise to absorption bands. In order for a vibration to absorb IR

radiation, the dipole moment of the molecule must change when the vibration occurs.

For example, the bond in 1-butene has a dipole moment because the molecule

is not symmetrical about this bond. Recall from Section 1.3 that the dipole moment is

equal to the magnitude of the charge on the atoms multiplied by the distance between

C“C

and

and

CH 3

CH 3 CH 2 C CCH 3 and CH 3 CH 2 C CH

O

CH 3 CH 2 COH and CH 3 CH 2 CH 2 OH

OO

CH 3 CH 2 COCH 3 and CH 3 CH 2 COH

CH 3 CH 2 CH 2 CH 3 CH 3 CH 2 OCH 3

1700 cm-^11600 cm-^1

' 3400 cm-^1

' 3300 cm-^1 ,' 2700 cm-^1 , ' 1100 cm-^1

' 1700 cm-^1

C¬O C¬N

' 1100 cm-^1

' 720 cm-^1.

sp^3

C¬N C¬H

512 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy