Organic Chemistry

514 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

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

CH 2 CCH 2 CH 2 CH 3

CH 3

Figure 13.29

The IR spectrum of Compound 1.

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

O

CH

Figure 13.30

The IR spectrum of Compound 2.

13.15 Identifying Infrared Spectra

We will now look at some IR spectra and see what we can determine about the struc-

ture of the compounds that give rise to the spectra. We might not be able to identify

the compound precisely, but when we find out what it is, its structure should fit with

our observations.

Compound 1. The absorptions in the region in Figure 13.29 indi-

cate that hydrogens are attached to both carbons and carbons

Now let’s see if the carbons belong to an alkene or to a benzene

ring. The absorption at and the absorption at (Table 13.5)

suggest that the compound is a terminal alkene with two alkyl substituents at the

2-position. The lack of absorption at indicates that the compound has

fewer than four adjacent methylene groups. We are not surprised to find that the

compound is 2-methyl-1-pentene.

' 720 cm-^1

1650 cm-^1 ' 890 cm-^1

12950 cm-^12. sp^2

sp^213075 cm-^12 sp^3

3000 cm-^1

Compound 2. The absorption in the region in Figure 13.30 indicates

that hydrogens are attached to carbons but not to carbons. The ab-

sorptions at and indicate that the compound has a benzene ring.

The absorptions at and show that the compound is an aldehyde.

The absorption band for the carbonyl group is lower than normal

so the carbonyl group has partial single-bond character. Thus, it must be

attached directly to the benzene ring. The compound is benzaldehyde.

11720 cm-^12 ,

(C“O) 11700 cm-^12

2810 cm-^12730 cm-^1

1600 cm-^11460 cm-^1

sp^213050 cm-^12 sp^3

3000 cm-^1