Carboxylic acid

Chemical organic compounds, whose molecules have at least one carboxyl group (carbonyl-functional group of aldehydes and ketones combined , as well as hydroxyl-functional group of alcohols) in the structure have received a common name - carboxylic acids. Their formula can be represented as R-COOH, where R is a hydrocarbon monovalent functional group. Any carboxylic acid, unlike most inorganic acids, is weak and incompletely dissociates into ions.

The simplest examples are formic acid (methane) H-COOH. The name is explained by the history of its first receipt in 1670 of red ants by the English naturalist John Ray. Carboxylic acid with two or more carboxyl groups will be called dibasic (or dicarboxylic), tribasic (or tricarboxylic), and so on. The simplest example is oxalic acid with its formula C2H2O4, in the molecule of which there are two carboxyl groups. As a hexafluoric acid, it is possible to give mellitic (hexacarboxylic) acid, its formula C12H6O12. The molecule contains six carboxyl groups, which replaced hydrogen atoms in the benzene ring.

Organic acids, as a rule, are found in nature. For example, hexacarboxylic acid is contained in a honey stone found in brown coals).

There are many important natural compounds of this class. These include citric acid C6H8O7 (a few food additives E330-E333), which was first obtained from the juice of unripe lemons in 1784 by the Swedish pharmacist K. Scheele. Tartaric acid C4H6O6 is a food supplement E334). This carboxylic acid is widely distributed in nature. It is found in the fresh juice of many fruits.

If we consider any homologous series of these organic compounds, then there are regular changes in properties with increasing molecular weight. The properties of each compound depend on the structure of their molecules, that is, they are largely determined by the isomerism of carboxylic acids. The first representatives of the homologous series formed from formic acid, including acetic and propionic, refer to liquids. They have a sharp odor and are readily soluble in water. The higher representatives are solids, which do not dissolve in water.

The chemical properties of carboxylic acids are mainly determined by the effect of the carbonyl group on the hydroxyl group. Therefore, these compounds, in contrast to alcohols, have a pronounced acidic character.

For example, in aqueous solutions they can dissociate into ions, which proves the staining of the liquid after adding litmus to red. This indicates the presence of hydrogen cations. That is, the medium of their aqueous solutions is acidic (pH less than 7).

When interacting with metals or bases, carboxylic acids are able to form salts: 2CH3-COOH + Mg → (CH3-COO) 2Mg + H2 ↑.

Organic acids also enter into chemical reactions with carbonates, displacing carbonic acid: 2CH3-COOH + MgCO3 → (CH3-COO) 2Mg + H2O + CO2 ↑.

They react easily with ammonia, forming the salts: CH3-COOH + NH3 → CH3-COONH4.

Acidic properties of organic acids are enhanced by the presence of radical substituents in them with a negative induction effect. For example, under the action of chlorine on acid acetic, gradually replacing one atom of hydrogen by chlorine atoms and obtaining chloroacetic acid, then dichloroacetic acid and trichloroacetic acid, there is a sharp increase in acidic properties in them.

Any carboxylic acid can be obtained in several ways. The most common method is based on the oxidation reaction. As starting reagents, alcohols or aldehydes are taken. Another way to produce organic acids is the hydrolysis of nitriles, which occurs when they are heated with dilute mineral acids.