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. 150C 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%) 185C CH(CO2H)2 160C (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 25C + 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 CHabsorptions. 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:( ppm;broad). 6 Chemical shift (, ppm) 1 13C NMR Carbonyl carbon is at low field ( ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone ( 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 +