4-Hydroxybenzoic acid

Description

4-Hydroxybenzoic acid, also known as p-hydroxybenzoic acid (PHBA, 4-HBA), 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.

Chemical properties

4-Hydroxybenzoic acid is an odorless white crystalline astringent powder. In pure form, it imparts a sweetish taste at first, which quickly becomes acrid and disagreeable

Occurrence

4-Hydroxybenzoic acid is a phenolic acid that has been found in a variety of fruit peels and leaves. It can be found naturally in coconut. It is one of the main catechins metabolites found in humans after consumption of green tea infusions. It is also found in wine, in vanilla, in Macrotyloma uniflorum (horse gram), carob and in Phyllanthus acidus (Otaheite gooseberry).

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.

Definition

ChEBI: 4-hydroxybenzoic acid is a monohydroxybenzoic acid that is benzoic acid carrying a hydroxy substituent at C-4 of the benzene ring. It has a role as a plant metabolite and an algal metabolite. It is a conjugate acid of a 4-hydroxybenzoate.

Preparation

Prepared from p-bromophenol or p-hydroxybenzaldehyde, or from saponification of methyl salicylate, a major component of oil of wintergreen.
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 4-Hydroxybenzoic acid. (Oddly, if the sodium salt is used, the product is 2-hydroxybenzoic acid, or salicylic acid.)
Org. Synth. 1934, 14, 48
DOI: 10.15227/orgsyn.014.0048

Purification Methods

Crystallise the hydroxyacid from water. [Beilstein 10 IV 345.]