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Carboxylic acids – structural features, classification and examples of representatives

Written by Hasem

Carboxylic acids belong to the class of organic compounds containing one or more carboxyl groups COOH. It is customary to distinguish between two groups of acids – monobasic (their general formula is R-COOH) and dibasic or dicarboxylic (oxalic acid with the formula HOOC – COOH can be attributed to them). There are also polybasic carboxyl groups; examples are citric acid, malonic acid and fumaric acid.

Description of the substance and parameters

The general formula of carboxylic acids (not to be confused with carbonic or carbonic acid) is CH2 + COOH; Under normal conditions, they have an oily liquid form with a characteristic unpleasant odor, however, starting from C10-carbons, the acid is a solid, insoluble in water. However, when the number of carbon atoms is more than 3, the substance is infinitely miscible with water. Since the acidic properties of this substance are relatively weak (with the exception of formic and acetic acid), their salts undergo reversible hydrolysis.

The electronic structure of the carboxyl group combines the presence of two functional groups – hydroxyl (-OH) and carbonyl (> C = 0). The length of the hydrogen bond in the dimer is 0.26 nm, the length of the C = O bond in various compounds is about 0.122 nm. Upon dissociation, a negatively charged anion stabilized by conjugation is formed.

Systematic nomenclature

In the name of these chemical compounds, first of all, the carbon atom of the COOH group is numbered. The salts formed during the reaction are named by adding the suffix “at” or “oat” to the name of the hydrocarbon (for example, potassium ethanedioate). The homologous series of limiting and unsaturated carbonic solutions is as follows:

The trivial name is formic acid, the systematic name is methane. Chemical formula – H-COOH. The number of double bonds is 0. The melting point is 8.4 degrees Celsius, the boiling point is 100.8.
Acetic (ethane). Formula CH3-COOH. The reagent has no double bonds, melts at 17 degrees, boils at 118.
Propionic (propane). CH3-CH2-COOH, melting point is -22 degrees, boiling point 141 is equal to degrees. The number of double bonds is zero.
Acrylic (propene). CH2 = CH-COOH. It boils at 141 degrees, melts at 12. There is 1 double bond (chemicals of this type especially easily add hydrogen when exposed to sodium amalgam, and dioxyacids are formed during oxidation).
Nylon (hexane). CH3 (CH2) 4-COOH. Melting temperature – minus 2 degrees, boiling point – 205.
Palmitic (hexadecane). C15-H31-COOH. It melts at 64 degrees, boils at 215. There are no double bonds.
Stearic (octadecane). C17-H35-COOH, boiling point is 232 degrees, melting point is 70.
Oleic (cis-octadecene-9). C17-H33-COOH. It boils at a temperature of 228 degrees, melts at 14. With a double bond, isomerism occurs – a trans isomer (elaidic acid) is formed, proceeding under the action of various catalysts such as aliphatic nitriles or nitrogen dioxide.

In addition, butanoic, linoleic, benzoic and oxalic acid bases are also included in the classification table of carboxylic acids of this type. Structural interclass and spatial isomerism (starting from C2), the position of the multiple bond, as well as the isomerism of the skeleton in the hydrocarbon radical are inherent in carbonic compounds.

Physical properties

The physical characteristics and properties of substances of the carboxyl group depend on the number of carbon atoms in their compounds. Those compounds containing no more than 3 C atoms belong to the lower class. They are readily soluble in water, have a pungent odor and are colorless. Acidic solutions containing more than 9 atoms in their composition are the highest; they are water-insoluble solids.

The density of acetic and formic acid is greater than unity; all other species have a lower density. Boiling of carboxylic compounds occurs at higher temperatures than that of alcohols. The bonds of oxygen and hydrogen molecules (O – H) are strongly polarized. Also, chemical compounds of this type can react through the formation of hydrogen bonds with the oxygen of the carbonyl dipole, which has significant electronegativity.

Acids in solid form exist as cyclic dimers; in liquid form, linear association can occur.

Chemical reactions and features

When reacting with metals, oxides, or with some of their hydroxides, carboxylic solutions exhibit the typical properties of all weak acids – the formation of salts of the corresponding metals, the displacement of a weaker acid solution from its salt, as well as the displacement of the carboxylic compound itself. In water, salts of carboxylic compounds are alkaline and hydrolyzed. The solution turns the litmus paper red. The following chemical formulas can be cited as an example:

2CH3-COOH + Mg => (CH3COO) 2-Mg + H2.
2CH3-COOH + CaO => (CH3COO) 2-Ca + H2O.
CH3-COOH + NaOH => CH3COONa + H2O.
2CH3-COOH + Na2SiO3 => 2CH3COONa + H2SiO3.
The high polarity of the O – H bond leads to significant dissociation of the carboxylic acid in aqueous solution. All compounds are weak electrolytes. The combustion reaction looks like this: CH3COOH + 2O2 => 2CO2 + 2H2O.

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