1. 18.16 Decarboxylation of Malonic Acid and Related Compounds
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2. Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylate readily.
RCOH O RH +
CO2
But malonic acid does.
150°C
CH3COH O +
CO2
HOCCH2COH 28

3. Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other. O OH HO H O HO H H OH HO + C O
This compound is the enol form of acetic acid. 29

4. Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other. O OH HO H O HO H
These hydrogens play no role. H OH HO + C O
HOCCH3 O 29

5. Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other. O OH HO R R’ O HO R R’ H
Groups other than H may be present. R R’ OH HO + C O
HOCCHR' O R 29

6. Decarboxylation is a general reaction for 1,3-dicarboxylic acids
CO2H
CO2H H
(74%)
185°C
CH(CO2H)2
160°C
(96-99%)
CH2CO2H 30

7. Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other. O OH HO R R’ O HO R R’ H
This OH group plays no role. R R’ OH HO + C O
HOCCHR' O R 29

8. Mechanism of Decarboxylation of a b-Keto AcidOH
R" R R' 
This kind of compound is called a -keto acid. 
R"CCHR' O R
Decarboxylation of a -keto acid gives a ketone. 29

9. Decarboxylation of a -Keto Acid
CH3C O
CH3
CO2H C
CH3C O
CH3 H
25°C +
CO2 32

10. Mechanism of Decarboxylation of a b-Keto Acid
One carbonyl group assists the loss of the other. O OH
R" R R' O R R' H
R"
Groups other than OH may be present.
R"CCHR' O R C O R' OH +
R" R 29

11. Section 18.17 Spectroscopic Analysis of Carboxylic Acids

12. 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

13. Figure 18.8 Infrared Spectrum of 4-Phenylbutanoic acid
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2030 The McGraw-Hill Companies, Inc. All rights reserved. 8

14. 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: ( ppm; broad). 6

15. Chemical shift (, ppm) 1

16. 13C NMR
Carbonyl carbon is at low field ( ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone ( ppm).

17. UV-VIS
Carboxylic acids absorb near 210 nm, but UV-VIS spectroscopy is not very useful for structure determination of carboxylic acids.

18. 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 +