1. Section 17.17 Spectroscopic Analysis of Aldehydes and Ketones

2. Infrared Spectroscopy
Presence of a C=O group is readily apparent in infrared spectrum
C=O stretching gives an intense absorption at cm-1
In addition to peak for C=O, aldehydes give two weak peaks near 2720 and 2820 nm for H—C=O 6

3. Figure 17.13 Infrared Spectrum of Butanal
H—C=O
CH3CH2CH2CH=O
C=O
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 8

4. Aldehydes: H—C=O proton is at very low field (d 9-10 ppm).
1H NMR
Aldehydes: H—C=O proton is at very low field (d 9-10 ppm).
Methyl ketones: CH3 singlet near d 2 ppm. 6

5. Figure 17.14 H C O CH(CH3)2 1 Chemical shift (d, ppm) 1.0 2.0 3.0 4.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (d, ppm) 1

6. Figure 17.15 CH3 C O CH3CH2 1 Chemical shift (d, ppm) 1.0 2.0 3.0 4.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (d, ppm) 1

7. 13C NMR
Carbonyl carbon is at extremely low field-near d 200 ppm
Intensity of carbonyl carbon is usually weak

8. Figure 17.16 O CH3CH2CCH2CH2CH2CH3 Chemical shift (d, ppm) 1 20 40 6020 40 60 80
100
120
140
160
180
200
Chemical shift (d, ppm) 1

9. UV-VIS
Aldehydes and ketones have two bands in the UV region: p Æ p* and n Æ p*
p Æ p*: excitation of a bonding p electron to an antibonding p * orbital
p Æ p*: excitation of a nonbonding electron on oxygen to an antibonding p * orbital

10. UV-VIS
H3C C O ••
• •
p Æ p* lmax 187 nm
n Æ p* lmax 270 nm

11. Molecular ion fragments to give an acyl cation
Mass Spectrometry
Molecular ion fragments to give an acyl cation
CH3CH2CCH2CH3 •+ O
• •
m/z 86
CH3CH2C O
• • + +
CH2CH3 •
m/z 57