What Is Hydroxybenzoic Acid?
Hydroxybenzoic acid is a compound in which a hydrogen in the phenyl group of benzoic acid (Ph-COOH) is replaced by a hydroxy group.
It is represented by the chemical formula C7H6O3 and has a molecular weight of 138.12. There are three isomers according to the position of the hydroxy group: 2-hydroxybenzoic acid (o-hydroxybenzoic acid), 3-hydroxybenzoic acid (m-hydroxybenzoic acid), and 4-hydroxybenzoic acid (p-hydroxybenzoic acid).
In particular, 2-hydroxybenzoic acid has anti-inflammatory and analgesic effects and is known by the common name “salicylic acid“; CAS registration numbers are 69-72-7 for salicylic acid (2-hydroxybenzoic acid), 99-06-9 for 3-hydroxybenzoic acid, 99-96-7 for 4-hydroxybenzoic acid, 96-7.
Uses of Hydroxybenzoic Acids
Among hydroxybenzoic acids, salicylic acid (2-hydroxybenzoic Acid) is a drug approved for use in skin diseases because of its anti-inflammatory, analgesic, and skin softening properties. Dosage forms include adhesive plasters, ointments, and liquids. The main indication as an adhesive plaster is exfoliation of verrucae, chicken eyes, and callosities. Indications for ointments and liquids include psoriasis, tinea, keratosis, eczema with keratosis, atopic dermatitis, and contusions.
Otherwise, the substance is sometimes used as a raw material for organic synthesis. 4-hydroxybenzoic acid is also an intermediate in ubiquinone synthesis and other processes in vivo, and is found in a wide range of organisms, including animals, plants, and microorganisms. Salicylic acid is also a substance widely found in plants. Against this background, it is sometimes used in research and development as a reagent for culture engineering, plant tissue culture, plant growth control reagents, and growth inhibitors.
Characteristics of Hydroxybenzoic Acids
1. Salicylic Acid (2-Hydroxybenzoic Acid)
Salicylic acid has a melting point of 159°C and a boiling point of 211°C. It is a colorless needle-like crystal at room temperature. It has a density of 1.443 g/cm3 and an acid dissociation constant pKa of 2.97. It is soluble in ethanol, ether, and acetone, and insoluble in water.
The method for synthesizing salicylic acid is known as the Kolbe-Schmidt reaction. In this reaction, carbon dioxide is applied to an alkali metal phenoxide at high temperature and pressure to carboxylate the ortho-position, and salicylic acid is obtained after neutralization with acid.
2. 3-Hydroxybenzoic Acid
3-hydroxybenzoic acid has a melting point of 201-205°C and is a white to pale yellow crystalline powder or powder at room temperature. It is soluble in ethanol, acetone, ether, and hot water and insoluble in cold water. 3-hydroxybenzoic acid is synthesized from 3-chlorobenzoic acid in pseudomonas sp.
3. 4-Hydroxybenzoic Acid
4-hydroxybenzoic acid has a melting point of 214-217°C and is a white to almost white crystalline powder or powder at room temperature. Its density is 1.443 g/cm3. It is readily soluble in ethanol and acetone, but insoluble in water and chloroform.
Industrially, 4-hydroxybenzoic acid is produced from potassium phenoxide and carbon dioxide. In the laboratory process, potassium salicylate and potassium carbonate are heated to 230°C and treated with acid after the reaction.
The ester of 4-hydroxybenzoic acid is also called paraben and is a substance used as a preservative.
Types of Hydroxybenzoic Acid
Hydroxybenzoic acid has three positional isomers, as mentioned above. All are commercialized and sold as reagents for research and development, and are available in easy-to-handle laboratory volumes such as 25 g, 100 g, and 500 g .
Of the three isomers, salicylic acid is a pharmaceutical product approved and marketed as a skin disease agent. Salicylic acid, other products include 10% salicylic acid ointment and liquid, and salicylic acid-containing adhesive bandages. Various manufacturers offer products for this drug.