Salicylaldehyde

Salicylaldehyde (2-hydroxybenzaldehyde) is the chemical compound with the formula C₆H₄CHO-2-OH.^[1] Along with 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde, it is one of the three isomers of hydroxybenzaldehyde. This colorless oily liquid has a bitter almond odor at higher concentration and a characteristic buckwheat aroma at lower concentration. Salicylaldehyde was identified as a characteristic aroma component of buckwheat.^[2] 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 or potassium hydroxide in a Reimer-Tiemann reaction:

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.

Reactions

Salicylaldehyde undergoes a variety of chemical reactions including:

• Salicylaldehyde is converted to chelating ligands via condensation with amines. With ethylenediamine, it condenses to give the diprotic ligand H₂salen. Hydroxylamine gives salicylaldoxime.
• Oxidation with hydrogen peroxide gives catechol (1,2-dihydroxybenzene) (Dakin reaction).^[3]
• Condensation with diethyl malonate gives a derivative of the heterocycle coumarin^[4] via an aldol condensation.
• Etherification with chloroacetic acid followed by cyclisation gives the heterocycle benzofuran.^[5] The first step in this reaction to the substituted benzofuran is called the Rap-Stoermer condensation after E. Rap (1895) and R. Stoermer (1900).^[6][7]

References