Organic Chemistry

510 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

C O

% Transmittance

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

CH 3 CH 2 CH 2 CH 2 CH

aldehyde group O

hydrogen

Bending Vibrations

If a compound has carbons, a look at will tell you whether the

compound has a methyl group. All hydrogens bonded to hybridized carbons show

a bending vibration slightly to the leftof Only methyl groups

show a bending vibration slightly to the rightof So if a compound

has a methyl group, absorption bands will appear bothto the left and to the right of

otherwise, only the band to the left of will be present. You can

see evidence of a methyl group in Figure 13.21 (methylcyclohexane) and in

Figure 13.23 (ethylbenzene), but not in Figure 13.22 (cyclohexene). Two methyl

groups attached to the same carbon can sometimes be detected by a split in the methyl

peak at ' 1380 cm-^1 (Figure 13.25).

1400 cm-^1 ; 1400 cm-^1

C¬H 1400 cm-^1.

C¬H 1400 cm-^1.

sp^3

sp^3 ' 1400 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

CH 3

NH

Wavelength (μm)

Wavenumber (cm−^1 )

CH 3

CH 3 CHCH 2 CH 2 NH 2

% Transmittance

bend

NH

stretch

Figure 13.25

The IR spectrum of isopentylamine. The double peak at indicates the presence

of an isopropyl group. Two N¬Hbonds are indicated as well.

' 1380 cm-^1

also occur at so absorption at that wavelength does not always indicate a

bond. However, absorption bands resulting from bends tend to be broad-

er (due to hydrogen bonding) and more intense (due to being more polar) than those re-

sulting from stretches (see Figure 13.25), and they will be accompanied by

stretching at (Table 13.4).

The stretch of the bond in an aldehyde group shows two absorption

bands—one at and the other at (Figure 13.24). This makes

aldehydes relatively easy to identify because essentially no other absorption occurs at

these wavenumbers.

' 2820 cm-^1 ' 2720 cm-^1

C¬H

N¬H 3500 – 3300 cm-^1

C“C

C“C N¬H

' 1600 cm-^1 ,

Figure 13.24

The IR spectrum of pentanal. The absorptions at and readily identify

an aldehyde group. Note also the intense absorption band at indicating a

C“Obond.

' 1730 cm-^1

' 2820 ' 2720 cm-^1