Hydroxybenzaldehydes

Abstract

Among the hydroxybenzaldehydes, only mono‐, di‐ and trihydroxybenzaldehydes are known. Salicylaldehyde (2‐hydroxybenzaldehyde) and 4‐hydroxybenzaldehyde represent more than 99% of the hydroxybenzaldehydes' market. The physical and chemical properties of hydroxybenzaldehydes are described. 2‐Hydroxy and 4‐hydroxybenzaldehydes react in a similar manner as phenol toward electrophiles. The aldehyde function can be reduced or oxidized to form a variety of compounds depending on the reagents and conditions used. Hydroxybenzaldehydes undergo the normal reactions of aromatic aldehydes. Most industrial methods for the manufacture of hydroxybenzaldehydes are based on phenol, but other routes exist, starting from hydroxybenzoic acids, by reduction, or from cresols, by oxidation. Both p ‐hydroxybenzaldehyde and salicylaldehyde have a low to moderate acute oral toxicity, but salicylaldehyde is, in addition, corrosive toward skin and appreciably irritating to the eyes. The hydroxybenzaldehydes are used primarily as chemical intermediates to a variety of products. The largest single use of salicylaldehyde is in the manufacture of coumarin. Both are used as intermediates in the synthesis of agrochemicals, pharmaceuticals, fragrances, and in electroplating. Condensation products of salicylaldehyde and diamines have important chelation properties, and have wide use in the stabilization of petroleum products, and in chemical processing, especially as oxidation catalysts. These and other applications to polymer chemistry are described.