Salicylaldehyde

Salicylaldehyde, or 2-hydroxybenzaldehyde, is the chemical compound with the formula C₆H₄CHO-2-OH ^[1]. This colourless oily liquid has a bitter almond odor. Salicylaldehyde is a key precursor to a variety chelating agents, some of which are commercially important. It can be prepared from phenol and chloroform by heating with sodium hydroxide in a Reimer-Tiemann reaction.

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Salicylaldehyde is a common highly-functionalized arene that has often been exploited as a precursor to still other chemicals, which are shown in the figure, from the left: catechol, benzofuran, a salicylaldehydimine (R = alkyl or aryl), 3-carbethoxycoumarin.

• Salicylaldehyde is converted to chelating ligands via condensation with amines. With ethylenediamine, it condenses to give the popular diprotic ligand H₂salen. Hydroxylamine gives salicylaldoxime.
• Oxidation with hydrogen peroxide gives catechol (1,2-dihydroxybenzene).^[2]
• Condensation with diethyl malonate gives a derivative of the heterocycle coumarin.^[3]
• Etherification with chloroacetic acid followed by cyclisation gives the heterocycle benzofuran.^[4]

Other hydroxybenzaldehydes

Three isomers exist of hydroxybenzaldehyde with the hydroxyl group in the ortho, meta, or para position.

3-Hydroxybenzaldehyde has been prepared from m-nitrobenzaldehyde in a sequence of nitro group [[organic reduction, diazotisation of the amine and its hydrolysis ^{[7] [8]}

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