Introduction, nomenclature, examples of aliphatic carboxylic acids

Organic compounds that contain one or more carboxyl groups, (-COOH) have acidic properties and are known as carboxylic acids. The word carboxyl rightly represents this functional group in the sense that it is a combination of the carbonyl group and the hydroxyl group as shown below:

They may be considered as carboxyl derivatives of hydrocarbons where one or more hydrogen atoms are replaced by carboxyl groups.

For example:

Acids containing one carboxyl group are called monocarboxylic acid and those with two or three carboxyl groups are called di- and tri- carboxylic acids respectively.

Introduction of carboxylic acids

Replacement of hydroxyl group in carboxylic acids with a halogen, carboxylate, alkoxy or amino group gives functional derivatives of carboxylic acid known as acyl halides, acid anhydrides, esters or amides respectively.

Nomenclature of carboxylic acids

Since carboxylic acids are amongst the earliest organic compounds to be isolated from nature, a large number of them are known by their common names. The common names end with the suffix-ic acid and have been derived from Latin or Greek names of their natural sources. For e.g., formic acid (HCOOH) was first obtained from red ants (formica means ant), acetic and (CH₃COOH) from vinegar (acetum means vinegar) butyric acid (CH₃CH₂CH₂COOH) from rancid butter (butyrum means butter) caproic acid (CH₃CH₂CH₂CH₂COOH) from goats (caper means goat). The positions of the substitutes are indicated by Greek letters a, b, g, d etc the a-carbon being the one directly attached to the carboxyl group, b - the next and so on.

In the IUPAC system, the monocarboxylic acids are named as alkanoic acids. The name of the acid is derived by replacing the terminal 'e-' of the corresponding alkane with '-oic acid'. Carboxyl carbon is always given number one while numbering the carbon atoms of the parent chain.

Examples:

Structure of carboxylic group

Electron and neutron diffraction studies show that carboxyl group has a planar structure. Thus the carboxylic carbon and the two oxygen atoms in carboxylic group are sp² hybridized. The carboxylic carbon forms are (sp² - sp²) s - bond with two oxygen atoms. One the oxygen atoms form (sp² - s) s - bond with a hydrogen atom. One half filled unhybridized p-orbital of the carbon and the unhybridized p-orbital of the oxygen atom undergo sideways overlap. This results in the formation of the delocalized p-electron cloud. This is confirmed by the C-O single bond length in formic acid being shorter than the C-O bond length in ethanol.

The carboxyl group has the following resonating structures.

The third resonance structure (C) has all atoms with their full quota of electrons (closed shell structure) and thus is more stable and more important contribution to the resonance hybrid than the second structure (B) in which the positively charged carbon atom has only six electrons in its valence shell (open shell structure). This in two important resonance structures [A] and [C] the carboxyl carbon is electrically neutral.

The carboxyl group is also polar due to resonance structures [B] and [C].

In aldehydes and ketones, only one structure is electrically neutral. As a result, the carboxyl carbon of the resonance hybrid, in acids, is less positive and less electrophilic than the carbonyl carbon of aldehydes and ketones.

Examples of carboxylic acids

(i) Methanoic acid [Formic acid HCOOH]

It occurs in a variety of biting and stingins plants and insects. It is manufactured by the reaction of carbomonoxide with sodium hydroxide under pressure at 473 K. Sodium methanoate is obtained which is acidified and distilled to give the acid.

Methanoic acid is a colorless, pungent smelling liquid with a boiling point 373.5 K. Due to the presence of aldehyde - like hydrogen, it is a powerful reducing agent. It reduces Tollen's reagent and Fehling's solution.

It is used in rubber, textile, dyeing, leather and electroplating industries.

(ii) Ethanoic acid (Acetic acid CH

₃COOH)

It is the main constituent of vinegar and is obtained by fermentation of molasses in the presence of air.

Industrially, it is obtained by oxidation of ethanol with air in the presence of cobalt acetate catalyst.

Also the carbonylation of methanol in the presence of rhodium catalyst gives acetic acid.

Ethanoic acid a colorless liquid with a pungent odor boils at 391 K. It freezes at an unusually high temperature (Melting point 298 K) forming ice like crystals. The water free ethanoic acid obtained by melting of the crystals is called glacial acetic acid. It is used in the manufacture of rayon and in plastic, rubber and silk industries. It is also used as a solvent. Vinegar is used in the food industry.

Benzoic acid is prepared by oxidation of toluene. In the form of esters, benzoic acid is used in perfume industry. The salts of benzoic acid like sodium benzoate are used as food preservatives.

1,2-Benzene dicarboxylic acid (Phthalic acid) is used in the manufacture of plasticizers and resins.

1,4-Benzene dicarboxylic acid (terephthalic acid) is a basic raw material for polyester.