Organic Chemistry

506 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

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 CH 2 CH 2 COCH 2 CH 3

O

C O

C O

Figure 13.18

The IR spectrum of ethyl butanoate. The electron-withdrawing oxygen atom makes the

carbonyl group of an ester harder to stretch than the carbonyl group of a

ketone (' 1720 cm-^1 ).

(' 1740 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

Wavelength (μm)

Wavenumber (cm−^1 )

% Transmittance CH

3 (CH 2 ) 4 CH 2 OH

(CH 2 ) 5

O H C O

Figure 13.19

The IR spectrum of 1-hexanol.

A bond shows a stretch between 1250 and If the bond is

in an alcohol (Figure 13.19) or an ether, the stretch will occur toward the lower end of

the range. If, however, the bond is in a carboxylic acid (Figure 13.20), the

stretch will occur at the higher end of the range. The position of the absorption

varies because the bond in an alcohol is a pure single bond, whereas the

bond in a carboxylic acid has partial double-bond character that is due to resonance

electron donation. Esters show stretches at both ends of the range (Figure 13.18)

because esters have two single bonds—one that is a pure single bond and one

that has partial double-bond character.

C¬O

C¬O

C¬O C¬O

C¬O

C¬O

C¬O 1050 cm-^1. C¬O

CH 3 CH 3

CH 3 CH 2 OH

O

~1050 cm−^1

CH 3 CH 2 O CH 2 CH 3

~1050 cm−^1

C

OH

~1250 cm−^1 ~1250 cm−^1 and ~1050 cm−^1

CH 3

O−

C

CH 3 OH+

O

C

CH 3 O

O−

C

CH 3 O+

PROBLEM 18

Which will occur at a larger wavenumber?

a. the stretch of an amine or the stretch of an amide

b. the stretch of phenol or the stretch of cyclohexanol

c. the stretch of a ketone or the stretch of an amide

d. the stretch or the bend of the C¬Obond in ethanol

C“O C“O

C¬O C¬O

C¬N C¬N