hydroxybenzoïques

Hydroxybenzoïques (hydroxybenzoic acids) are a class of aromatic carboxylic acids derived from benzoic acid by substitution of one or more hydrogen atoms on the benzene ring with hydroxyl groups. They include monohydroxy isomers—ortho (2‑), meta (3‑) and para (4‑)—and polyhydroxylated derivatives such as di‑ and tri‑hydroxybenzoic acids. Representative compounds are salicylic acid (2‑hydroxybenzoic acid), p‑hydroxybenzoic acid, gentisic acid (2,5‑dihydroxybenzoic acid) and gallic acid (3,4,5‑trihydroxybenzoic acid).

These compounds exhibit varied physical and chemical properties depending on hydroxyl position and number. Hydroxyl substituents influence acidity, solubility and hydrogen‑bonding behavior; for example, intramolecular hydrogen bonding in the ortho isomer alters both acidity and solubility compared with para and meta isomers. Hydroxybenzoic acids are moderately polar and occur naturally as plant metabolites, components of lignin and tannins, and intermediates in microbial degradation pathways.

Industrial and laboratory uses include precursors for pharmaceuticals (salicylic acid derivatives), preservatives (esters of p‑hydroxybenzoic acid, known as parabens), and building blocks in organic synthesis. Major synthesis routes include oxidation of alkylbenzenes and carboxylation reactions such as the Kolbe‑Schmitt process for salicylic acid. Analysis and identification are commonly performed by chromatographic techniques (HPLC, GC‑MS).

Toxicological and environmental profiles vary by compound; some derivatives are widely accepted in consumer products while others are restricted or studied for endocrine and ecological effects.