Carboxylic Acids


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Carboxylic Acids

 

a class of organic compounds that contain a carboxyl group:

Carboxylic acids can belong to the aliphatic (fatty), alicyclic, aromatic or heterocyclic series, depending on the nature of the radical bonded to the —COOH group. A distinction is made among monobasic, dibasic, and polybasic acids (monocarboxy-lic, dicarboxylic, and polycarboxylic, respectively) according to the number of carboxyl groups in the molecule. Carboxylic acids may be saturated or unsaturated and may contain double or triple molecular bonds.

Most carboxylic acids have trivial names, many of which are related to their origin in nature—for example, formic, malic, valeric, and citric acids. According to the Geneva nomenclature, the names of carboxylic acids are derived from the designations of hydrocarbons with the same number of carbon atoms, with addition of the ending “-ic” and the word “acid”—for example, methanoic (formic) acid and ethanoic (acetic) acid. Carboxylic acids are frequently regarded as hydrocarbon derivatives. For example, the acid used in the construction of HC≡C—COOH is called acetylene carboxylic acid.

Acidic properties are determined by the ability of carboxylic acids to dissociate in an aqueous solution:

RCOOH⇄RCOO + H+

As a rule, carboxylic acids are weaker than mineral acids. The dissociation constants of monobasic saturated fatty acids range from 1.7 X 10−4 (formic acid) to 1.3 X 10−5 (higher homo-logues) at 25°C. The strength of carboxylic acids is also largely dependent on the electrophilic activity of the radical bonded to the carboxyl. The addition of an electronegative substituent such as NO2, CN, or CI in a position adjacent to the carboxyl group markedly increases the acidity— for example, cyanoacetic acid, CNCH2CCOOH, is approximately 200 times as strong as acetic acid, CH3COOH. The effect of substituents decreases in proportion to the distance from the carboxyl. Dicarbonic acids are more powerful than monocarbonic acids, since the effect of one carboxyl on the other increases as they become closer together. For example, in the acid series, oxalic acid, HOOC—COOH, is stronger than malonic acid, HOOCCH2COOH, which in turn is stronger than succinic acid, HOOC(CH2)2COOH. The acidity of unsaturated acids is higher than that of saturated acids; the nearer the double bond to the carbonyl, the greater its effect. For example, acrylic acid, CH2=CH—COOH, is four times as strong as propionic acid, CH3—CH2—COOH. Aromatic acids are stronger than saturated aliphatic acids (the dissociation constant of benzoic acid is 6.5 X 10−5).

Carboxylic acids are liquids (the lower fatty acids) or solids (the higher fatty acids and aromatic acids). (See Table 1.) The lower members of the unsaturated carboxylic acids in the fatty series are highly soluble in water, the middle members (C4-C10) and the aromatic acids are soluble to a limited extent in organic solvents, and the higher fatty acids are insoluble in water; like the aromatic acids, they dissolve readily in alcohol, ether, and benzene.

The most important chemical property of carboxylic acids is the capacity to become derivatives. Upon reaction with bases, carboxylic acids yield salts: RCOOH + NaOH → RCOONa + H2O. Esters are readily formed upon treatment of carboxylic acids with alcohols in the presence of mineral acids: RCOOH + R’OH → RCOOR’+ H2 O. Acid halides of carboxylic acids RCOX (X is a halogen atom) are formed upon the action of acid halides of mineral acids on carboxylic acids (for example, PCI3, POCl3, and SOCI2). Carboxylic acid anhydrides, (RCO) 2O, are produced by heating the acids with dehydrating agents. Acid halides and anhydrides of carboxylic acids are used as acylating agents. The dehydration of ammonia salts of carboxylic

Table 1. Some representative carboxylic acids and their properties
 FormulaMelting point (°C)Boiling point °C)Density * (g/cm3)
*Figures in parentheses are temperature(°C) **1 mm Hg = 133.322 N/m2
Aliphatic (fatty) acids    
Formic acid ……HCOOH8.4100.51.220(20)
Acetic acid ……CH3COOH16.6118.21.049(20)
Pelargonic ……CH3(CH2)7COOH12.3255.60.906(20)
Palmitic acid ……CH3(CH2)14COOH62.83900.841(80)
Stearic acid ……CH3(ch2)16COOH69.63600.839(80)
   (with decomposition) 
Adipic acid ……HOOC(CH2)4COOH153.52651.366(20)
   (100 mm Hg)** 
Acrylic acid    CH2=CHCOOH12.3140.01.062(16)
Methacrylic acid ……CH2=C(CH3)COOH161631.105(20)
Oleic acid ……CH3(CH2)7CH=CH(CH2)7COOH16223 (10 mm Hg)0.895(18)
Aromatic acids    
Benzoic acid ……C6H5CooH121.7249.21.322(20)
Cinnamic acid ……C6H5CH=CHCOOH1363001.245(20)
Terephthalic acid ……p=HOOCC6H4COOH300 (sublimates)

acids (1) and the reaction of acid halides with ammonia (2) yield acid amides:

RCOONH4 → RCONH2 + H20

RCOC1 + 2NH3 → RCONH2 + NH4C1

Various methods are used to produce carboxylic acids. The oxidation of primary alcohols and aldehydes yields carboxylic acids containing the same number of carbon atoms. Ketone oxidation is accompanied by rupture of the C—C bond. Dicar-bonic acids are formed from cyclic ketones—for example, adipic acid from cyclohexanone:

Saturated hydrocarbons can undergo destructive oxidation to form a mixture of products, among which are carboxylic acids. Approximately 350 kg of carboxylic acid are obtained from 1 ton of paraffin using this method. Oxidation of the side chain of fatty aromatic hydrocarbons or polynuclear aromatic hydrocarbons yields aromatic carboxylic acids—for example, phthalic acid is formed by the oxidation of o-xylol or naphthalene:

Unsaturated hydrocarbons are oxidized at the double bond:

An important method of synthesizing carboxylic acids is the hydrolysis of their nitriles, which are readily produced by the reaction of halide derivatives of hydrocarbons with sodium cyanide:

RC1 + NaCN → RCN → RCOOH

At the present time, the synthesis of carboxylic acids by carbonization (that is, addition of a CO group to organic compounds) is widely used in industry:

Certain carboxylic acids are obtained from natural products. For example, salts of higher fatty acids (soaps) and glycerol are prepared by alkaline hydrolysis (saponification) of fats and oils. Citric acid is produced from the haulm of the cotton plant and from the stems of common tobacco plants (after the extraction of nicotine from them). Many carboxylic acids are prepared by the fermentation of carbohydrates in the presence of a certain species of bacteria (butyric-acid, lactic, citric-acid, and other types of fermentation).

Carboxylic acids are widespread in nature; they exist in the free state and as derivatives (primarily esters). For example, pelargonic acid is contained in volatile geranium oil, and citric acid is found in lemons. Glycerides of higher normal carboxylic acids of the fatty series are constituents of animal and vegetable fats and oils, among which palmitic, stearic, and oleic acids predominate.

Carboxylic acids and their derivatives, as well as numerous compounds that contain other functional groups in addition to the carboxyl (such as amino acids and hydroxy acids), are of considerable biological importance and have found various practical applications. Formic and acetic acids, for example, are used in the dyeing and printing of textiles; acetic acid and acetic anhydride are used in the manufacture of cellulose acetate. Amino acids are a component of proteins. Salicylic acid, p-aminosalicylic acid, and others are used in medicine.

Higher fatty carboxylic acids are widely used as raw material for the manufacture of soap, lacquers and paints, and surface-active agents, as emulsifiers in the production of rubber, and as plasticizers in the manufacture of vulcanized rubber. Adipic acid is one of the base products in the manufacture of polyamide fiber (nylon), terephthalic acid is used in the production of polyester fiber (lavsan and terylene), and the polymer nitrile of acrylic acid (Orion) is used as a synthetic fiber whose properties are similar to those of natural wool. The polymers and copolymers of me-thacrylic acid esters are used as organic glass.

REFERENCES

Nenitescu, C. D. Organicheskaia khimiia, vols. 1–2. Moscow, 1962–63. (Translated from Rumanian.)
Nesmeianov, A. N., and N. A. Nesmeianov. Nachala organicheskoi khi-mii, books 1–2. Moscow, 1969–70.
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