4Hydroxybenzoesäure

4-Hydroxybenzoic acid, also known as p-hydroxybenzoic acid (PHBA), is an organic compound in the class of hydroxybenzoic acids. Its structure features a benzene ring bearing a carboxyl group and a hydroxyl group in the para position relative to each other. The molecular formula is C7H6O3. PHBA is a white to off-white crystalline solid.

Properties include its status as a weak monoprotic acid with a pKa around 4.0–4.3. It is sparingly soluble in cold water and more soluble in hot water, and it dissolves in many organic solvents. In chemical reactions, it readily forms salts with bases, which can improve solubility.

Occurrence and production: PHBA occurs in nature in trace amounts in some plants. Industrially, it is produced and sold as a chemical intermediate and building block for further transformations. A common laboratory and industrial route involves oxidation of p-hydroxybenzaldehyde to the corresponding acid.

Uses: It serves as a precursor for the synthesis of esters such as methyl paraben and other parabens, which are used as preservatives in cosmetics and some foods. It is also employed as an intermediate in pharmaceutical synthesis and in polymer-related applications.

Safety: 4-hydroxybenzoic acid is of low acute toxicity but can be irritating to skin and eyes. It should be handled with standard laboratory precautions, avoiding inhalation of dust and contact with skin or eyes. Proper storage and disposal according to applicable regulations are recommended.

See also: parabens; hydroxybenzoic acid isomers; benzoic acids.