18.16 Decarboxylation of Malonic Acid and Related Compounds Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

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

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

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

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

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

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

Mechanism of Decarboxylation of a b-Keto Acid OH 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

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

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

Section 18.17 Spectroscopic Analysis of Carboxylic Acids

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

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

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: ( 10-12 ppm; broad). 6

Chemical shift (, ppm) 1

13C NMR Carbonyl carbon is at low field ( 160-185 ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone ( 190-215 ppm).

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

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 +