Carboxylic acids . They constitute a group of chemical compounds that are characterized by having a functional group called the carboxy group or carboxy group (-COOH); This occurs when a hydroxyl (-OH) and carbonyl (C = O) group coincide on the same carbon . It can be represented as COOH or CO 2 H.
[ hide ]
- 1 General
- 2 Nomenclature and physical properties
- 3 Synthesis of carboxylic acids
- 1 Industrial
- 2 In the Laboratory
- 4 Reactions of carboxylic acids
- 5 Sources
The carboxyl group, -C00H, is the characteristic functional unit of carboxylic acids, which can belong to the aliphatic, RCOOH, aromatic, ArCOOH, or heterocyclic series. This class of compounds also includes dicarboxylic and polycarboxylic acids (two or more -C00H groups in the molecule), as well as carboxylic acids that also have other functional groups. A wide variety of carboxylic acids are natural products, and these types of compounds serve countless functions both in nature and in industry.
The acetic acid , the acid component of the vinegar , is a carboxylic acid of great industrial importance, the citric acid , which gives the sour taste to citrus fruit , is an important intermediate for the metabolism of carbohydrates, the cholic acid is found in animal bile, and gallic acid , present in gills and other plant sources. Many derivatives of carboxylic acids, particularly esters and amides , are widely spread in nature and are of great importance and interest.
Nomenclature and physical properties
The IUPAC name of aliphatic carboxylic acids is obtained by putting the word acid before the corresponding alkane name and changing the ending -o of it for -oic.
CH 3 -CH 3 Ethane CH 3 -CH 2 -COOH Ethanoic acid (ethan + oico)
The carboxyl group defines the longest chain in the molecule and its carbon corresponds to number 1, the substituents attached to the chain being numbered in the usual way. The aromatic acids are named as substitution products of the benzoic acid , PhCOOH (mp 122 °), or the aromatic hydrocarbon from which they derive. Carboxylic acids are also often referred to as “alkanocarboxylic acids”, especially in the cycloalkanic series
|Structure||IUPAC name (Acid__)||Vulgar Name (Acid __)||eg ° C||mp, ° C|
|CH 3 COOH||Ethanic||Acetic||118||16|
|CH 3 CH 2 COOH||Propanoic||Propionic||142||– twenty-one|
|CH 3 CH 2 CH 2 COOH||Butanoic||Butyric||164||– 5|
|(CH 3 ) 2 CHCOOH||2-Methylpropanoic||Isobutyric||154||– 47|
|CH 3 (CH 2 ) 3 COOH||Pentanoic||Valerian||186||– 35|
|CH 3 (CH 2 ) 6 COOH||Octane||Capril||239||16|
|CH 3 (CH 2 ) 14 COOH||Hexadecanoic||Palmitic||139 / 1mm||63|
|CH 3 (CH 2 ) 16 COOH||Octadecanoic||Stearic||160 / 1mm||69|
Table 1: Some Properties of aliphatic carboxylic acids
Low molar mass acids (less than ten carbon atoms) are colorless liquids with a very unpleasant odor. The vinegar smell is due to acetic acid ; that of rancid butter with butyric acid . The caproic acid is found in hair and secretions of the cattle goats . Acids C5 to C10 have “goat” odors. The rest waxy and odorless solids at room temperature. Its melting and boiling points grow with increasing molar mass .
Carboxylic acids are generally weak acids, with only 1% of their molecules dissociated to give the corresponding ions, at room temperature and in aqueous solution. The lower acids are quite soluble in water. Its solubility in water decreases from butyric acid , with the increase of the hydrocarbon character of the molecule . All carboxylic acids are completely soluble in organic solvents .
Synthesis of carboxylic acids
Industrial production of carboxylic acids differs from those used on a small scale because they require specialized equipment.
- Aldehyde oxidation with air, using cobaltand manganese catalysts . The necessary aldehydes are easily obtained from alkenes by hydroformylation .
- Hydrocarbon oxidation using air. For the simplest alkanes, the method is not selective. Allylic and benzyl compounds undergo more selective oxidations. Alkyl groups on a benzene ring are oxidized to carboxylic acid, regardless of chain length. The formation of benzoic acidfrom toluene , terephthalic acid from p- xylene , and phthalic acid from o- xylene are some illustrative large-scale conversions. The acrylic acid is generated from propene.
- Dehydrogenation of alcohols, catalyzed by bases.
- Carbonylation is the most versatile method when accompanied by the addition of water. This method is effective for alkenes that generate secondary and tertiary carbocations , for example, from isobutylene to pivalic acid . In the Koch reaction , the addition of water and carbon monoxide to alkenes is catalyzed by strong acids. Acetic acid and formic acid are produced by the carbonylation of methanol, carried out with iodine and alkoxide , which act as promoters, and frequently with high pressures of carbon monoxide., generally involving several additional hydrolytic steps , in the Monsanto Process and Cativa Process . Hydrocarboxylations involve the simultaneous addition of water and CO. Such reactions are sometimes called ” Reppe Chemistry “:
HCCH + CO + H 2 O → CH 2 = CHCO 2 H
- Some long chain carboxylic acids are obtained by the hydrolysis of triglyceridesobtained from oils and fats from plants and animals. These methods are related to soap making .
At the laboratory
Preparation methods for small-scale reactions for the purposes of research, instruction, or the production of small amounts of chemicals often use expensive reagents.
- The oxidation of primary alcoholswith strong oxidizing agents such as potassium dichromate , Jones’s reagent , potassium permanganate , or sodium chlorite . The method is suitable for laboratory conditions compared to industrial air use , but the latter is more environmentally friendly since it leads to fewer inorganic by-products, such as chromium or manganese oxides .
- Oxidative breakdown of olefins, by ozonolysis , potassium permanganate , or potassium dichromate .
- Carboxylic acids can also be obtained by hydrolysis of nitriles, esters , or amides , generally with acid or basic catalysis .
- Carbonation of an organolithiumor Grignard reagent :
RLi + CO 2 RCO 2 Li
RCO 2 Li + HCl RCO 2 H + LiCl
- Halogenation of methyl ketones, followed by hydrolysis in the haloform reaction
- The Kolbe-Schmitt reaction, which provides a synthesis route to the salicylic acid , the precursor of aspirin
Carboxylic Acid Reactions
Carboxylic acids have a variety of reactions in which the intervening atom of hydrogen of the carboxyl group, -C00H, hydroxyl group, -CO-OH, or other parts of the molecule . Many of these reactions transform carboxylic acids into important derivatives .
a) Salt formation
The carboxylic acids in reaction with the conversion of into stable metal salts. Many of these metal salts are of technical utility. Thus, the basic copper acetate , Cu (OH) 2. 2Cu (000CH3) z, is used as an insecticide ; the aluminum acetate , A the (OCOCH3) ,, is used to impregnate the fibers of cotton in the previous etching staining; the calcium propionate , Ca (OCOC2H5) 2 and sodium benzoate , NaOCOPh, are used for storing food; and the sodium salts of long chain fatty acids are the common soaps
- b) Reduction to primary alcohols
They can be quantitatively reduced to primary alcohols by the action of Lithium Aluminum Hydride, which are agents of great reducing power.
- c) Halogenation α
When it is an aliphatic carboxylic acid with chlorineor brominein the presence of red phosphorus , the α hydrogen atoms of the carbon adjacent to the carbonyl group are replaced by Cl or Br. These Hell-Volhard-Zelinsky reactions can be carry out in steps by adequate control of the reaction conditions.