Organic Chemistry

Section 13.13 Absence of Absorption Bands 511

bending vibrations for hydrogens bonded to carbons give rise to absorp-
tion bands in the region (Table 13.5). As the table shows, the frequen-
cy of the bending vibrations of an alkene depends on the number of alkyl groups
attached to the double bond and the configuration of the alkene. It is important to real-
ize that these absorption bands can be shifted out of the characteristic regions if
strongly electron-withdrawing or electron-donating substituents are close to the double
bond (Section 13.10). Open-chain compounds with more than four adjacent methylene
groups show a characteristic absorption band at that is due to in-phase
rocking of the methylene groups (Figure 13.19).

13.12 The Shape of Absorption Bands

The shape of an absorption band can be helpful in identifying the compound responsi-
ble for an IR spectrum. For example, both and bonds stretch at
wavenumbers above but the shapes of their stretches are distinctive. No-
tice the difference in the shape of these absorption bands in the IR spectra of 1-hexanol
(Figure 13.19), pentanoic acid (Figure 13.20), and isopentylamine (Figure 13.25). An
absorption band is narrower and less intense than an
absorption band and the absorption band of a carboxylic acid
is broader than the absorption band of an alcohol (Sec-
tions 13.9 and 13.10). Notice that two absorption bands are detectable in Figure 13.25
for the stretch because there are two bonds in the compound.

PROBLEM 22

a. Why is an stretch more intense than an stretch?

b. Why is the stretch of a carboxylic acid broader than the stretch of an

alcohol?

13.13 Absence of Absorption Bands

The absence of an absorption band can be as useful as the presence of a band in iden-
tifying a compound by IR spectroscopy. For example, the spectrum in Figure 13.26
shows a strong absorption at indicating the presence of a bond.
Clearly, the compound is not an alcohol because there is no absorption above
Nor is it an ester or any other kind of carbonyl compound because there is
no absorption at ' 1700 cm-^1 .The compound has no C‚C,C“C,C‚N,C“N,

3100 cm-^1.

' 1100 cm-^1 , C¬O

O¬H O¬H

O¬H N¬H

N¬H N¬H

1 ' 3300 – 2500 cm-^12 O¬H

1 ' 3300 cm-^12 , O¬H

N¬H 1 ' 3300 cm-^12 O¬H

3100 cm-^1 ,

O¬H N¬H

1 CH 22 720 cm-^1

C¬H

1000 – 600 cm-^1

C¬H sp^2

The position, intensity, and shape

of an absorption band are helpful in

identifying functional groups.

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 13.26
The IR spectrum of diethyl ether.