Organic Chemistry

Section 13.11 C¬HAbsorption Bands 509

sp^3 C H

CH 3

C H

Wavelength (μm)

Wavenumber (cm−^1 )

2.52.6 2.7 2.8 2.9 3 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 15 16

4000380036003400320030002800260024002200 2000 1800 1600 1400 1200 1000 800 600

% Transmittance

CH 3

Figure 13.21
The IR spectrum of methylcyclohexane.

2.52.6 2.7 2.8 2.9 3 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 15 16

4000380036003400320030002800260024002200 2000 1800 1600 1400 1200 1000 800 600

Wavelength (μm)

Wavenumber (cm−^1 )

CH

sp^2

CH

bend

sp^3

CC

CH

% Transmittance

Figure 13.22
The IR spectrum of cyclohexene.

2.52.6 2.7 2.8 2.9 3 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 15 16

4000380036003400320030002800260024002200 2000 1800 1600 1400 1200 1000 800 600

Wavelength (μm)

Wavenumber (cm−^1 )

% Transmittance

CH 3

CH

CH 2 CH 3

Figure 13.23
The IR spectrum of ethylbenzene.

bands slightly to the left and slightly to the right of indicating that the com-
pounds that produced those spectra contain hydrogens bonded to and carbons.
Once we know that a compound has hydrogens bonded to carbons, we need to
determine whether those carbons are the carbons of an alkene or of a benzene ring.
A benzene ring is indicated by sharp absorption bands at and
whereas an alkene is indicated by a band at only (Table 13.4).
The compound with the spectrum shown in Figure 13.22 is, therefore, an alkene, while
that shown in Figure 13.23 has a benzene ring. Be aware that N¬Hbending vibrations

1430 cm-^1 , ' 1600 cm-^1

' 1600 cm-^11500 –

sp^2

sp^2

sp^2 sp^3

3000 cm-^1 ,