1. Section 19.18 Spectroscopic Analysis of Carboxylic Acids

2. Infrared Spectroscopy
A carboxylic acid is characterized by peaks due to OH and C=O groups in its infrared spectrum.
C=O stretching gives an intense absorption near 1700 cm-1.
OH peak is broad and overlaps with C—H absorptions. 6

3. Figure 19.8 Infrared Spectrum of 4-Phenylbutanoic acid
C6H5CH2CH2CH2CO2H
O—H and C—H stretch
C=O
monosubstituted benzene
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. 1H NMR
proton of OH group of a carboxylic acid is normally the least shielded of all of the protons in a 1H NMR spectrum: (d ppm; broad). 6

5. Figure 19.9 O CH2CH2CH2COH 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
11.0
12.0
Chemical shift (d, ppm) 1

6. 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).

7. UV-VIS
Carboxylic acids absorb near 210 nm, but UV-VIS spectroscopy has not proven to be very useful for structure determination of carboxylic acids.

8. Aliphatic carboxylic acids undergo a variety of fragmentations.
Mass Spectrometry
Aliphatic carboxylic acids undergo a variety of fragmentations.
Aromatic carboxylic acids first form acylium ions, which then lose CO.
ArCOH •• O
• •
ArCOH •+ O
• •
ArC O
• • + Ar +