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Carboxylic Acids - Organic Chemistry Tutor

Compounds containing in their molecule carboxyl functional group are named organic or carboxylic acids. A carboxylic acid may be considered as a replacing product of the three hydrogen atoms from the CH3 radical in a hydrocarbonate with three OH groups; considering that the resulted compound is a very unstable one, it loses one water molecule becoming a compound that contains COOH functional group that is a carboxylic acid.

The general formula of an acid is R-COOH, in which R represents a hydrocarbonate radical. Carboxylic acids are very stable substances and due to this fact, they are easily obtained and widely extended in nature.

Nomenclature

Acids are considered as substitution derivatives of hydrocarbonates with COOH (carboxylic) group and this is why their name is being formed by adding the carboxylic termination to the initial hydrocarbonate. If in the substance's molecule there are more COOH groups, the name is formed by adding di, tri, etc. prefixes.

Examples:
C5H6-COOH - benzencarboxylic acid
C6H11-COOH - cyclohexancarboxylic acid
COOH-COOH - etandicarboxylic acid
C6H4(COOH)2 - benzendicarboxylic acid

Classification

Considering the number of carboxyl groups in the molecule, acids can be classified in: monocarboxylic acids, dicarboxylic acids, policarboxylic acids. Considering the hydrocarbonate radical's nature in the molecule, organic acids are being classified in saturated and unsaturated acids.

Saturated Monocarboxylic Acids

Saturated monocarboxylic acids contain in their molecule one single COOH functional group that is bounded with a radical that can be obtained from a saturated hydrocarbonate (cyclic or acyclic) or from an aromatic hydrocarbonate.

The most important saturated monocarboxylic acids are:
H-COOH - formic acid
CH3-COOH - acetic acid
CH3-CH2-COOH - propionic acid
CH3-(CH2)2-COOH - n-butyric acid
CH3-(CH2)3-COOH - n-valeric acid
CH3-(CH2)4-COOH - n-caproic acid

Obtaining methods

1) Hydrocarbonate's oxidation leads sometimes to obtaining carboxylic acids. Alkanes, especially the ones with a superior number of carbon atoms, that are oxidized in air atmosphere at approximately 100C in the presence of a catalyst (KMNO4), form acids with an inferior number of carbon atoms in their molecule.
2) Primary alcohols' and aldehides' oxidation.
3) Esters' saponification is a technique industrially used in order to obtain saturated monocarboxylic acids.
4) Nitriles' hydrolyze in acid or basic medium leads to acids.
5) Alkenes' carbonization due to the action of carbon oxide and water on the alkenes, at 250C-300C and 200 atmospheres, in the presence of metallic carbonyls.
6) Carbonation of the organo-metallic combinations.

Physical properties

Saturated monocarboxylic acids that have less than nine carbon atoms in their molecule are liquid at a regular temperature; the others are solid. Boiling points of saturated monocarboxylic acids have an alternant behavior meaning that acids with an even number of carbon atoms have boiling points higher than their neighboring acids that possess an uneven number of carbon atoms. The boiling points are also higher once the number of carbon atoms in the molecule rises; acids that have a forked catena have lower boiling points in comparison with their normal isomers.

Water solubility of monocarboxilic acids is hardened proportionally to the rising of their molecular masses; this is why formic and acetic acids are water soluble independent of their proportion while the acids containing more than 12 carbon atoms an water insoluble.

Chemical properties

1) In combination with hydroxides, oxides and metal carbonates, saturated monocarboxylic acids form salts that are soluble in water excepting some heavy metals salts.
2) With alcohol, they form esters.
3) When they are boiled, monocarboxylic acids are resistant unlike their salts which decompose.
4) Carboxylic acids are not strong acids and their strength is lost with the carbon atoms in the molecule. Formic and benzoic acids only are strong acids.
5) During oxidation and reduction, saturated monocarboxylic acids are resistant, excepting formic acid. With energic oxidation agents the molecule is destroyed.
6) Halogenation agents don't have an effect on saturated monocarboxylic acids if the temperature is low. On high temperatures a substitution process takes place, which can be progressive, but only in the hydrocarbonate radical of the acid, not in the functional group.

Saturated Dicarboxylic Acids

Dicarboxylic acids contain in their molecule two COOH functional groups. The most simple dicarboxylic acid is etandicarboxylic acid, HOOC-COOH, also known under de name of oxalic acid. Besides the rational names of the aliphatic saturated dicarboxylic acids, they also have common names:
HOOC-COOH - oxalic acid
HOOC-CH2-COOH - malonic acid
HOOC-(CH2)2-COOH - succinic acid
HOOC-(CH2)3-COOH - glutaric acid
HOOC-(CH2)4-COOH - adipic acid
In the aromatic series, the most important dicarboxlic acid is phtalic acid and its three isomers orto, meta and para.

Obtaining methods

Saturated dicarboxylic acids preparation can be possible in several cases using the same techniques as the ones for saturated monocarboxylic acids. This is the reason why oxidation is a method that can be applied for both the aliphatic and aromatic series. For example, in glycol's oxidation is obtained the oxalic acid. Adipic acid is obtained through the oxidation process of cyclohexanol. Nitriles' saponification can be also used in order to obtain dicarboxylic acids, for example, through cianacetic acid's saponification it results malonic acid.

Physical properties

Saturated dicarboxylic acids are crystalline substances, with alternant boiling points such as those of saturated monocarboxylic acids, meaning that acids with an even number of carbon atoms in their molecule boil at higher temperatures than their neighbors that contain an uneven number. Water solubility decreases proportionally with the substance's molecular mass. Acids with an uneven number of carbon atoms are more soluble in comparison with those with an even number. Due to the existence of two carboxyl groups in the molecule, those acids are stronger than saturated monocarboxylic acids.

Chemical properties

Oxalic acid when boiled at over 200C, decomposes in carbon dioxide and formic acid that can be divided in carbon monoxide and water( in the presence of sulphuric acid). Malonic acid, when boiled at 120-150C loses carbon dioxide and passes in acetic acid. Adipic acid loses when boiled, besides one water molecule, one carbon dioxide molecule becoming this way the corresponding cyclic ketone, cyclopentanone.


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