Chemical organic compounds, the molecules of which in their structure have at least one carboxyl group (the carbonyl - functional group of aldehydes and ketones are combined in it , as well as the hydroxyl - functional group of alcohols) have received the common name - carboxylic acids. Their formula can be represented as R-COOH, where R is a monovalent hydrocarbon functional group. Any carboxylic acid, unlike most inorganic acids, is weak and does not completely dissociate into ions.
The simplest examples are formic (methane) acid H-COOH. The name is explained by the story of its first receipt in 1670 from red ants by the English naturalist John Ray. A 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, the molecule of which contains two carboxyl groups. Mellitic (hexacarboxylic) acid, its formula is C12H6O12, can be mentioned as the six-basic one. The molecule contains six carboxyl groups that have replaced hydrogen atoms in the benzene ring.
Organic acids are typically found in nature. So, for example, hexacarboxylic acid is found in honey stone found in brown coals).
There are many important natural compounds of this class. These include citric acid C6H8O7 (which consists of several food additives E330 – E333), which was first obtained from juice of unripe lemons in 1784 by Swedish pharmacist K. Scheele. Tartaric acid C4H6O6 is a food supplement E334). This carboxylic acid is widespread in nature. It is found in the fresh juice of many fruits.
If we consider any homologous series of these organic compounds, then regular changes in properties are observed in it with an increase in molecular weight. The properties of each compound depend on the structure of their molecules, that is, the isomerism of carboxylic acids determines them in many ways. The first representatives of the homologous series formed from formic acid, including acetic and propionic, refer to liquids. They have a pungent odor and dissolve well in water. Higher representatives are solids that 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, unlike 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 environment of their aqueous solutions is acidic (pH less than 7).
When interacting with metals or bases, carboxylic acids are capable of forming 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 easily react with ammonia to form salts: CH3 — COOH + NH3 → CH3 — COONH4.
The acidic properties of organic acids are enhanced by the presence of radical substituents with a negative induction effect. For example, under the action of chlorine on acetic acid, gradually replacing one hydrogen atom with chlorine atoms and producing chloroacetic acid, then dichloroacetic acid and trichloroacetic acid, a sharp increase in their acidic properties is observed.
Any carboxylic acid can be obtained in several ways. The most common is the method, which is based on the oxidation reaction. Alcohols or aldehydes are taken as starting reagents. Another method for producing organic acids is the hydrolysis of nitriles, which occurs when they are heated with dilute mineral acids.