Organic Chemistry

bonds show stretching vibrations in the region from 1200 to but these

vibrations are weak and of little value in identifying compounds. Similarly, a

bond stretches at a higher frequency than does a bond

and a bond stretches at a higher frequency than

does a bond which in turn stretches at a higher frequency than

does a bond (Table 13.4).

PROBLEM 17

a. Which will occur at a larger wavenumber?

1. a stretch or a stretch


2. a stretch or a bend


3. a stretch or a stretch
   b. Assuming that the force constants are the same, which will occur at a larger wavenumber?


4. a stretch or a stretch


5. a stretch or a stretch

Resonance and Inductive Electronic Effects

Table 13.4 shows a range of wavenumbers for each stretch because the exact position

of the absorption band depends on other structural features of the molecule, such as

electron delocalization, the electronic effect of neighboring substituents, and hydrogen

bonding. Important details about the structure of a compound can be revealed by the

exact position of the absorption band.

For example, the IR spectrum in Figure 13.15 shows that the carbonyl group

of 2-pentanone absorbs at whereas the IR spectrum in Figure 13.16

shows that the carbonyl group of 2-cyclohexenone absorbs at a lower frequency

2-Cyclohexenone absorbs at a lower frequency because the carbonyl group

has less double-bond character due to electron delocalization. Because a single bond is

weaker than a double bond, a carbonyl group with significant single-bond character will

stretch at a lower frequency than will one with little or no single-bond character.

O−

+

O

2-pentanone

CH 3 CCH 2 CH 2 CH 3

O

C Oat 1720cm−^1

2-cyclohexenone

C Oat 1680cm−^1

11680 cm-^12.

1720 cm-^1 ,

1 C“O 2

C¬O C¬C

C¬O C¬Cl

C¬N C“N

C¬H C¬H

C‚C C“C

C¬N 1 ' 1100 cm-^12

C“N 1 ' 1600 cm-^12 ,

1 ' 1100 cm-^12 , C‚N 1 ' 2200 cm-^12

1 ' 1700 cm-^12 C¬O

C“O

800 cm-^1 ,

504 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

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 3 CCH 2 CH 2 CH 3

O

% Transmittance

C O

Figure 13.15

The IR spectrum of 2-pentanone. The intense absorption band at 1720 indicates a

C“Obond.

'