carboxylic acid

carboxylic acid, any of a class of organic compounds in which a carbon (C) atom is bonded to an oxygen (O) atom by a double bond and to a hydroxyl group (―OH) by a single bond. A fourth bond links the carbon atom to a hydrogen (H) atom or to some other univalent combining group. The carboxyl (COOH) group is so-named because of the carbonyl group (C=O) and hydroxyl group.

The chief chemical characteristic of the carboxylic acids is their acidity. They are generally more acidic than other organic compounds containing hydroxyl groups but are generally weaker than the familiar mineral acids (e.g., hydrochloric acid, HCl, sulfuric acid, H₂SO₄, etc.).

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Carboxylic acids occur widely in nature. The fatty acids are components of glycerides, which in turn are components of fat. Hydroxyl acids, such as lactic acid (found in sour-milk products) and citric acid (found in citrus fruits), and many keto acids are important metabolic products that exist in most living cells. Proteins are made up of amino acids, which also contain carboxyl groups.

Compounds in which the ―OH of the carboxyl group is replaced by certain other groups are called carboxylic acid derivatives, the most important of which are acyl halides, acid anhydrides, esters, and amides.

Click Here to see full-size tableCarboxylic acid derivatives have varied applications. For example, in addition to its use as a disinfectant, formic acid, the simplest carboxylic acid, is employed in textile treatment and as an acid reducing agent. Acetic acid is extensively used in the production of cellulose plastics and esters. Aspirin, the ester of salicylic acid, is prepared from acetic acid. Palmitic acid and stearic acid are important in the manufacture of soaps, cosmetics, pharmaceuticals, candles, and protective coatings. Stearic acid also is used in rubber manufacture. Acrylic acid is employed as an ester in the production of polymers (long-chain molecules) known as acrylates. Methacrylic acid serves as an ester and is polymerized to form Lucite. Oleic acid is used in the manufacture of soaps and detergents and of textiles.

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Nomenclature of carboxylic acids and their salts

The IUPAC name of a carboxylic acid is derived from that of the longest carbon chain that contains the carboxyl group by dropping the final -e from the name of the parent alkane and adding the suffix -oic followed by the word “acid.” The chain is numbered beginning with the carbon of the carboxyl group. Because the carboxyl carbon is understood to be carbon 1, there is no need to give it a number. For example, the compound CH₃CH₂COOH has three carbon atoms and is called propanoic acid, from propane, the name for a three-carbon chain, with -oic acid, the suffix for this class of compounds, appended. If the carboxylic acid contains a carbon-carbon double bond, the ending is changed from -anoic acid to -enoic acid to indicate the presence of the double bond, and a number is used to show the location of the double bond.

Most simple carboxylic acids, rather than being called by their IUPAC names, are more often referred to by common names that are older than their systematic names. Most simple carboxylic acids were originally isolated from biological sources; because their structural formulas were often unknown at the time of isolation they were given names that were generally derived from the names of the sources. For example, CH₃CH₂CH₂COOH, butyric acid, first obtained from butter, was named after the Latin butyrum, meaning “butter.” The acids containing an odd number of carbon atoms greater than nine generally do not have common names. The reason is that long-chain carboxylic acids were originally isolated from fats (which are carboxylic esters), and generally these fats contain carboxylic acids with only an even number of carbon atoms (because the process by which living organisms synthesize such fatty acids puts the molecules together in two-carbon pieces).

When common names are used, substituents on the hydrocarbon chain are designated by Greek letters rather than by numbers, and counting begins not with the carboxyl carbon but with the adjacent carbon. For example, the common name of the following compound γ-aminobutyric acid, abbreviated GABA. Its IUPAC name is 4-aminobutanoic acid. GABA is an inhibitory neurotransmitter in the central nervous system of humans.

Salts of carboxylic acids are named in the same manner as are the salts of inorganic compounds; the cation is named first and then the anion, as in sodium chloride. For carboxylic acids, the name of the anion is derived by changing the ending -oic acid of the IUPAC name or -ic acid of the common name to -ate. Some examples are sodium acetate, CH₃COONa; ammonium formate, HCOONH₄; and potassium butanoate (potassium butyrate), CH₃CH₂CH₂COOK.