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car·box·yl·ic acid (kär'bŏk-sĭl'ĭk) ![]() |
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One of a large family of organic substances widely distributed in nature, and characterized by the presence of one or more carboxyl groups (—COOH). These groups typically yield protons in aqueous solution. In the type formula, R(CXY)nCOOH, symbols R, X, and Y can be hydrogen, saturated or unsaturated groups, carboxyl, alicyclic, or aromatic groups, halogens, or other substituents, and n may vary from zero (formic acid, HCOOH) to more than 100, provided that the normal carbon covalence of four is maintained.
Physical and chemical properties of carboxylic acids are represented, grossly, by the resultant of the various chemical groupings present in the molecule. A short-chain aliphatic acid, wherein the carboxyl is dominant, is a pungent, corrosive, water-soluble liquid of abnormally high boiling point (because of molecular association), with specific gravity close to 1 (higher for formic and acetic acids). With increasing molecular weight, the hydrocarbon grouping overbalances the carboxyl; sharpness of odor diminishes, boiling and melting points rise, the specific gravity falls toward that of the parent hydrocarbon, and the water solubility decreases. Thus the typical high-molecular-weight saturated acid is a bland, waxlike solid.
Acids are used in large quantities in the production of esters, acid halides, acid amides, and acid anhydrides. They find wide use in the manufacture of soaps and detergents, in thickening lubricating greases (stearate soaps), in modifying rigidity in plastics, in compounding buffing bricks and abrasives, and in the manufacture of crayons, dictaphone cylinders, and phonograph records. The solvent action of acids finds use in manufacture of carbon paper, inks, and in the compounding of synthetic and natural rubber. Because of the stability of saturated fatty acids toward oxidation, these are often used as solvents for carrying out oxidation reactions upon sensitive compounds.
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| Veterinary Dictionary: carboxylic acid |
An organic compound containing the carboxy group (−COOH), which is weakly ionized in solution forming a carboxylate ion (−COO−).
| Wikipedia: Carboxylic acid |
Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -C(=O)OH, usually written -COOH or -CO2H. [1] Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids are called carboxylates.
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Carboxylic acids are polar. Because they are both hydrogen-bond acceptors (the carbonyl) and hydrogen-bond donors (the hydroxyl), they also participate in hydrogen bonding. Together the hydroxyl and carbonyl group forms the functional group carboxyl. Carboxylic acids usually exist as dimeric pairs in nonpolar media due to their tendency to “self-associate.” Smaller carboxylic acids (1 to 5 carbons) are soluble with water, whereas higher carboxylic acids are less soluble due to the increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be rather soluble in less-polar solvents such as ethers and alcohols.[2]
Carboxylic acids tend to have higher boiling points than water, not only because of their increased surface area, but because of their tendency to form stabilised dimers. Carboxylic acids tend to evaporate or boil as these dimers. For boiling to occur, either the dimer bonds must be broken, or the entire dimer arrangement must be vaporised, both of which increase enthalpy of vaporisation requirements significantly.
Carboxylic acids are typically weak acids, meaning that they only partially dissociate into H+ cations and RCOO- anions in neutral aqueous solution. For example, at room temperature, only 0.02 % of all acetic acid molecules are dissociated. Electronegative substituents give stronger acids.
| Carboxylic Acids | pKa |
|---|---|
| Formic acid (HCO2H) | 3.77 |
| Acetic acid (CH3CO2H) | 4.76 |
| Chloroacetic acid (CH2ClCO2H) | 2.86 |
| Dichloroacetic acid (CHCl2CO2H) | 1.29 |
| Trichloroacetic acid (CCl3CO2H) | 0.65 |
| Trifluoroacetic acid (CF3CO2H) | 0.5 |
| Oxalic acid (HO2CCO2H) | 1.27 |
| Benzoic acid (C6H5CO2H) | 4.2 |
Ionization of a carboxylic acid gives a carboxylate anion, which is stabilized because the negative charge is shared (delocalized) between the two oxygen atoms. Each of the carbon-oxygen bonds in a carboxylate anion has partial double-bond character.
Carboxylic acids often have strong odors, especially the volatile derivatives. Most famous are acetic acid (vinegar) and butyric acid (rancid butter). On the other hand, esters of carboxylic acids tend to have pleasant odors and many are used in perfumes.
The simplest series of carboxylic acids are the alkanoic acids, RCOOH, where R is a hydrogen or an alkyl group. Compounds may also have two or more carboxylic acid groups per molecule. The mono- and dicarboxylic acids have trivial names.
Carboxylic acids are most readily identified as such by infrared spectroscopy. They exhibit a sharp band associated with vibration of the C-O vibration bond (νC=O) between 1680 and 1725 cm−1. A characteristic νO-H band appears as a broad peak in the 2500 to 3000 cm−1 region.[2] By 1H NMR spectrometry, the hydroxyl hydrogen appears in the 10-13 ppm region, although it is often either broadened or not observed owing to exchange with traces of water.
Straight chain mono- and dicarboxylic acids occur in many natural settings and many are produced industrially on a large scale. They are used in the production of polymers, pharmaceuticals, solvent, and food additives. Vinegar, a dilute solution of acetic acid, is biologically produced from the fermentation of ethanol. It is used in food and beverages, but is not used in industry. Industrially important carboxylic acids include acrylic and methacrylic acids (polymers, adhesives), adipic acid (polymers), citric acid (beverages), ethylenediaminetetraacetic acid (chelating agent), fatty acids (coatings), maleic acid (polymers), propionic acid (food preservative), terephthalic acid (polymers).
Industrial routes to carboxylic acids generally differ from those used on smaller scale because they require specialized equipment.
Preparative methods for small scale reactions for research, instruction, or for production of small amounts of fine chemicals often employ expensive consumable reagents.
Many reactions afford carboxylic acids but are used only in specific cases or are mainly of academic interest:
The most widely practiced reactions convert carboxylic acids into esters, amides, carboxylate salts, acid chlorides, and alcohols. Carboxylic acids react with bases to form carboxylate salts, in which the hydrogen of the hydroxyl (-OH) group is replaced with a metal cation. Thus, acetic acid found in vinegar reacts with sodium bicarbonate (baking soda) to form sodium acetate, carbon dioxide, and water:
Carboxylic acids also react with alcohols to give esters. This process is heavily used in the production of polyesters. Similarly carboxylic acids are converted into amides, but this conversion typically does not occur by direct reaction of the carboxylic acid and the amine. Instead esters are typical precursors to amides. The conversion of amino acids into peptides is a major biochemical process that requires ATP.
The hydroxyl group on carboxylic acids may be replaced with a chlorine atom using thionyl chloride to give acyl chlorides. In nature, carboxylic acids are converted to thioesters.
The carboxylic acid can be reduced to the alcohol by hydrogenation or using stoichiometric hydride reducing agents such as lithium aluminum hydride.
The carboxylate anion R-COO– is usually named with the suffix -ate, so acetic acid, for example, becomes acetate ion. In IUPAC nomenclature, carboxylic acids have an -oic acid suffix (e.g., octadecanoic acid). In common nomenclature, the suffix is usually -ic acid (e.g., stearic acid).
| Carbon atoms | Common name | IUPAC name | Chemical formula | Common location or use |
|---|---|---|---|---|
| 1 | Formic acid | Methanoic acid | HCOOH | Insect stings |
| 2 | Acetic acid | Ethanoic acid | CH3COOH | Vinegar |
| 3 | Propionic acid | Propanoic acid | CH3CH2COOH | |
| 4 | Butyric acid | Butanoic acid | CH3(CH2)2COOH | Rancid butter |
| 5 | Valeric acid | Pentanoic acid | CH3(CH2)3COOH | Valerian |
| 6 | Caproic acid | Hexanoic acid | CH3(CH2)4COOH | Goat fat |
| 7 | Enanthic acid | Heptanoic acid | CH3(CH2)5COOH | |
| 8 | Caprylic acid | Octanoic acid | CH3(CH2)6COOH | Coconuts and breast milk |
| 9 | Pelargonic acid | Nonanoic acid | CH3(CH2)7COOH | Pelargonium |
| 10 | Capric acid | Decanoic acid | CH3(CH2)8COOH | |
| 12 | Lauric acid | Dodecanoic acid | CH3(CH2)10COOH | Coconut oil and hand wash soaps. |
| 14 | Myristic acid | Tetradecanoic acid | CH3(CH2)12COOH | Nutmeg |
| 16 | Palmitic acid | Hexadecanoic acid | CH3(CH2)14COOH | Palm oil |
| 18 | Stearic acid | Octadecanoic acid | CH3(CH2)16COOH | Chocolate, waxes, soaps, and oils |
| 20 | Arachidic acid | Eicosanoic acid | CH3(CH2)18COOH | Peanut oil |
Other carboxylic acids include:
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