4-Hydroxybenzoic acid

Description

4-Hydroxybenzoic acid, also known as p-hydroxybenzoic acid (PHBA), is a monohydroxybenzoic acid, a phenolic derivative of benzoic acid. It is a white crystalline solid that is slightly soluble in water and chloroform but more soluble in polar organic solvents such as alcohols and acetone. 4-Hydroxybenzoic acid is primarily known as the basis for the preparation of its esters, known as parabens, which are used as preservatives in cosmetics and some ophthalmic solutions. It is isomeric with 2-hydroxybenzoic acid, known as salicylic acid, a precursor to aspirin, and with 3-hydroxybenzoic acid.
4-Hydroxybenzoic acid belongs to the class of aromatic acids, which may be used as a preservative in cosmetics, pharmaceuticals and as a synthetic flavoring agent in food products.

Description

4-Hydroxybenzoic acid (4-HBA) is a white crystalline solid that is widely used in organic synthesis. In particular, its esters, called parabens, are used as preservatives in cosmetics and pharmaceuticals.
In 1947, H. Gilman and C. E. Arntzen at Iowa State College (Ames) reported?the synthesis of 4-HBA?from?4-hydroxybenzaldehyde. Several other preparation methods were developed since then. Today it is produced commercially via the Kolbe–Schmitt reaction, which originated in the mid-1800s. Potassium phenoxide and CO2?are heated under pressure; the reaction mixture is then acidified to obtain the product. (Oddly, if the sodium salt is used, the product is 2-hydroxybenzoic acid, or salicylic acid.)
Recently, a new role for 4-HBA was discovered. Douglas Rasher*, Mark Hay, and colleagues at Georgia Tech (Atlanta) found that the sea slug?Elysia tuca?locates its food source, the seaweed?Halimeda incrassate,?by sensing 4-HBA?and halimedatetraacetate, which the plant produces. The slug then pierces the seaweed to obtain chloroplasts, photoreceptors that help the invertebrate convert sunlight into energy.
*Now at the University of Maine (Walpole).

The Uses of 4-Hydroxybenzoic acid

4-Hydroxybenzoic acid is used as an intermediate for dyes, antiseptics and active pharmaceutical ingredients. It is also used as a food preservative, corrosion inhibitor, anti-oxidant and an emulsifier. It reacts with 6-hydroxynaphthalene-2-carboxylic acid to prepare aromatic polyester called Vectran fiber. It acts as a precursor to prepare p-hydroxyphenylbenzoate, p-acetoxybenzoyl chloride and 4,4'-dihydroxybenzophenone. Further, it is used in paint additives, coating additives, processing aids, electrical and electronic products. In addition to this, it acts as a preservative in cosmetics and some ophthalmic solutions.