1. Section 20.21 Spectroscopic Analysis of Carboxylic Acid Derivatives

2. Infrared Spectroscopy
C=O stretching frequency depends on whether the compound is an acyl chloride, anhydride, ester, or amide.
C=O stretching frequency n
CH3CCl O
CH3COCCH3 O
CH3COCH3 O
CH3CNH2 O
1822 cm-1
1748
and
1815 cm-1
1736 cm-1
1694 cm-1 6

3. Infrared Spectroscopy
Anhydrides have two peaks due to C=O stretching. One results from symmetrical stretching of the C=O unit, the other from an antisymmetrical stretch.
C=O stretching frequency n
CH3COCCH3 O
1748
and
1815 cm-1 6

4. Infrared Spectroscopy
Nitriles are readily identified by absorption due to carbon-nitrogen triple bond stretching in the cm-1 region. 6

5. 1H NMR readily distinguishes between isomeric esters of the type:
RCOR' O
R'COR O
and O C H O C H
is less shielded than 6

6. 1H NMR For example: CH3COCH2CH3 O O and CH3CH2COCH3
Both have a triplet-quartet pattern for an ethyl group and a methyl singlet. They can be identified, however, on the basis of chemical shifts. 6

7. O O Figure 20.9 CH3CH2COCH3 CH3COCH2CH3 1 Chemical shift (d, ppm) 1.0
1.0
2.0
3.0
4.0
5.0
1.0
2.0
3.0
4.0
5.0
Chemical shift (d, ppm) 1

8. 13C NMR
Carbonyl carbon is at low field (d ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone (d ppm).
The carbon of a CN group appears near d 120 ppm.

9. UV-VIS nÆp* absorption: lmax CH3CCl O CH3COCCH3 O CH3COCH3 O CH3CNH2 O
235 nm
225 nm
207 nm
214 nm 6

10. Mass Spectrometry
Most carboxylic acid derivatives give a prominent peak for an acylium ion derived by the fragmentation shown.
RCX •• O
• •
RCX •+ O
• • RC O +
• • + •
• • X

11. Mass Spectrometry
Amides, however, cleave in the direction that gives a nitrogen-stabilized cation.
RCNR'2 •• O
• • •+
RCNR'2 O
• • •• + O C
• •
NR'2 •• • R +