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Reactions of Carboxylic Acids

Carbonic Acid and its Derivatives

Carbonic acid (H2CO3 or HO(C=O)OH) is in itself not stable. It decomposes easily into water and carbon dioxide through decarboxylation. However, the salts of carbonic acid - hydrogen carbonates and carbonates - are, in fact, stable and isolable. Sodium hydrogen carbonate (NaHCO3) and sodium carbonate (soda, Na2CO3) are popular examples of carbonic acid's salts.

Tab.1
Carbonic acid and some of its salts
Carbonic acid (unstable)Sodium hydrogencarbonate (stable)Sodium carbonate (stable)
Fig.1
Fig.2
Fig.3
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Fig.4
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Fig.5
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Fig.6

Monoesters of carbonic acids are also unstable and easily decarboxylate. Decarboxylation is irreversible, as the gaseous carbon dioxide escapes. However, diesters of carbonic acid are stable.

Fig.7
Monoester
Fig.8
Diester

Stable carbonate

The stability of carbonic acid's monoamides and diamides resembles that of carbonic acid's monoesters and diesters. While monoamides of carbonic acid are unstable, diamides are stable. The simplest carbonic acid diamide is urea. Carbamates (view the illustration below) are also stable.

Tab.2
Primary amides of carbonic acid.
Carbamic acid (amidocarbonic acid, aminoformic acid) is unstableUrea (stable)Methyl carbamate (stable)
Fig.9
Fig.10
Fig.11
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Fig.12
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Fig.13
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Fig.14

Carboxylic acids that, in contrast to β-keto acids and 1,3-dicarboxylic acids, do not easily decarboxylate may be decarboxylated by an electrochemical reaction, which is known as the Kolbe electrolysis reaction. In Hermann Kolbe's electrolysis, carboxylate anions are electrochemically oxidized. The resulting carboxyl radicals spontaneously decarboxylate and the two remaining alkyl radicals subsequently combine, thus yielding a new hydrocarbon.

Tab.3
Kolbe electrolysis reaction.
Reaction scheme
Fig.15
An example
Fig.16

Exercise 1

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