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Structure, Properties, and Nomenclature of Carboxylic Acids

Carboxylic Acids and Their Derivatives

Compounds that contain a carbonyl group are found all throughout nature. They play an important role in many biological processes. Hormones, vitamins, amino acids, medicinal drugs, and flavours are examples of compounds that may contain carbonyl groups. A carbonyl group consists of a carbon atom and an oxygen atom connected by a double bond. A molecular residue that is assembled from a carbonyl group and one alkyl or aryl substituent is known as an acyl group. If a carbonyl group carries a hydroxy group, the entire group is then known as a carboxyl group COOH (O=C-O-H). The hydrogen of the carboxyl group is acidic. Thus, compounds that contain a carboxyl group are carboxylic acids.

Fig.1
Carbonyl group and carboxyl group.

Carboxylic acids are vital starting products in many syntheses, such as the syntheses of esters, carboxamides and acyl chlorides.

Acetic acid (CH3COOH, ethanoic acid) is, aside from water, the main component of vinegar. Butyric acid (CH3CH2CH2COOH, butanoic acid) is the reason for the rancid odor of rotten butter, while caproic acid (CH3(CH2)4COOH, hexanoic acid) is responsible for the odor of goats and worn socks. Fatty acids, such as palmitic acid (CH3(CH2)1_4COOH, hexadecanoic acid), are naturally occuring carboxylic acids that possess a long hydrocarbon chain. They are, for instance, components of fats and other lipids and serve as a source of energy in biological systems. Cholic acid (3,7,12-trihydroxy-5-cholan-24-oic acid) is a main component of human gall. It should be noted as extraordinary that only one cholic acid stereoisomer is to be found in human gall, though, due to the eleven stereocenters of cholic acid, 2048 different stereoisomers are conceivable.

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Fig.2
Cholic acid.

Global large-scale production of acetic acid amounts to about 10 million tons per year. Acetic acid is further processed in many industrial syntheses, such as the syntheses of vinyl acetate, dyes and adhesives. Acetic acid may be synthesized, for instance, by a cobalt acetate-catalyzed oxidation of acetaldehyde (ethanal) with atmospheric oxygen.

Fig.3
Acetaldehyde as starting product in acetic acid synthesis.

The company Monsanto has developed a more efficient process for the synthesis of acetic acid, in which methanol is treated with carbon monoxide in the presence of a rhodium catalyst.

Fig.4
Methanol as starting product in acetic acid synthesis - Monsanto process.

An additional vital synthetic pathway for the large-scale production of acetic acid is the liquid-phase oxidation of butane or naphtha.

The leaving ability of a leaving group depends on the hardness and basicity of the leaving base (group). The softer the base, the higher its leaving ability.

Thus, the reactivity of carboxylic acid derivatives in many reactions is determined by the hardness and basicity of the leaving group.

Tab.1
pKa values of acyl compounds.
Acyl compoundModelLeaving groupConjugated acidpKa
Fig.5
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Fig.6
Br- HBr-9
Fig.7
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Fig.8
Cl- HCl-7
Fig.9
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Fig.10
-OCOR RCOOH~3 to 5
Fig.11
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Fig.12
-OR ROH~15 to 16
Fig.13
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Fig.14
-OH H2O15.7
Fig.15
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Fig.16
-NH2 NH336
Fig.17
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Fig.18
R- RH~50
Fig.19
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Fig.20
H- H2extremely high

Exercise: Carboxylic Acids and Their Derivatives

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