The carbon compounds containing a carboxyl group -COOH are known as carboxylic acid. It consists of a carbonyl group attached to a hydroxyl group, hence its name carboxyl. Carboxylic acids are weak acids, they give H+ ions on ionisation. Carboxylate ions have more acidic strength than phenoxide ion due to delocalisation of lone pair of electrons over more electronegative two oxygen atoms.
Carboxylic acid andcarboxyl group and its nomenclature, uses, physical and chemical properties are given below:-
Nomenclature and Structure of Carboxyl Group
The common names end with the suffix, i.e., acid and have been derived from Latin or Greek names of their natural sources.
(i) IUPAC name of carboxylic acid is alkanoic acid.
(ii) Their names are derived by replacing the terminal ‘e’ from the name of corresponding straight chain alkane by suffix ‘oic acid’
(iii) Suitable numbers are assigned to the substituents.
(iv) Prefix di, tri, tetra are added before the ‘oic acid’ form acids containing 2,3, 4 carboxylic groups, respectively
Structure of Carboxyl Group:
In carboxylic acids, bonds to the carboxyl carbon are coplanar and are separated by about 120°. The carboxylic carbon is less electrophilic than carbonyl carbon because of the following. possible resonance structure shown below
In structure (I) and (III), carboxyl carbon is electrically neutral. As a result, the carboxyl carbon of resonance hybrid is less positive. However, like carbonyl group, carboxyl group is also polar due to the resonance structures (II) and (III).
Methods of Preparation of Carboxylic Acids:
Carboxylic acids are prepared by the following methods
(i) From Primary Alcohols and Aldehydes
Primary alcohols and aldehydes are readily oxidised to corresponding carbóxylic acid with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline medium or potassium dichromate (K2 Cr2O7,) and chromium trioxide (CrO3) in acidic medium. Aldehydes are easily oxidised to the corresponding carboxylic acids even with mild oxidising agents such as Tollen’s reagent.
(ii) From Nitriles and Amides
Nitriles are first hydrolysed to amides and then to acids in the presence of H+or OH as catalyst. Mild reaction conditions are used to stop the reaction at the amide stage.
(iii) From Alkyl Benzene
Aromatic acids are obtained by vigorous oxidation of alkyl benzene with chromic acids or acidic or alkaline KMnO4. During these oxidations, the aromatic nucleus remains intact but the entire chain is oxidised to-COOH group irrespective of the length of carbon chain.
(iv) From Grignard Reagents
Grignard reagents react with dry ice (CO2) in ethereal solution to produce salts of carboxylic acids, which on acidification with mineral acids give corresponding carbóxylic acid.
(v) From Acyl Halides and Anhydrides
Acid chlorides when hydrolysed with water give carboxylic acids or more readily hydrolysed with aqueous base to give carboxylate ions which on acidification provide corresponding carboxylic acid. Anhydrides on the other hand are hydrolysed to corresponding acid(s) with water.
(vi) From Esters
Acidic hydrolysis of esters gives directly carbóxylic acid while basic hydrolysis gives carboxylates, which on acidification give corresponding carboxylic acids.
Physical Properties of carboxylic acids:
The physical properties of carbóxylic acids are described below
(i) Aliphatic carboxylic acid up to nine carbon atoms are colourless liquids at room temperature with unpleasant odour while higher carbóxylic acids are wax like solids an odourless due to their low volatility.
(ii) Solubility Simple aliphatic carbóxylic acid having up to four carbon atom are miscible in water because of hydrogen bonds formation with water. Solubility decreases with increase in the number of carbon atoms. Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol, chloroform, etc
(iii) Boiling points As compared to hydrocarbons, aldehydes and ketones, carboxylic acids have higher boiling points because they have high extent of hydrogen bonding with water, due to which they exists as associated molecules.
(iv) Melting points The melting points of aliphatic carbóxylic acids do not show a regular pattern rather they show oscillation or alteration effect ie, melting point of an acid containing even number of carbon atoms is higher than the next member containing odd number of carbon atoms
Chemical reactions of carboxylic acids:
Flowchart for the chemical reactions of monocarboxylic acids
Uses of Carboxylic Acid:
(i) Methanoic acid is used as in rubber, textile, dyeing, leather and electroplating industries.
(ii) Ethanoic acid is widely used as solvent and it is also used as vinegar in food industry.
(iiï) In making of nylon-66, hexanedioic acid (Adipic acid) is used.
(iv) In perfume industry, esters of benzoic acid is used.
(v) Sodium benzoate is a very good food preservative.
(vi) Higher fatty acids are extremely useful in soaps and detergent production.