Salicylic aldehyde (2-hydroxybenzaldehyde) is an

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

with the formula . Along with 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde, it is one of the three

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

s of hydroxybenzaldehyde. This colorless oily liquid has a bitter almond odor at higher concentration. Salicylaldehyde is a precursor to coumarin and a variety of chelating agents.

Production

Salicylaldehyde is produced by condensation of phenol with formaldehyde to give hydroxybenzyl alcohol, which is oxidized to the aldehyde. Salicylaldehydes in general are prepared by ortho-selective formylation reactions from the corresponding phenol, for instance by the Duff reaction, Reimer–Tiemann reaction, or by treatment with paraformaldehyde in the presence of magnesium chloride and a base. : Reimer-Tiemann_Reaction_Scheme

Natural occurrences

Salicylaldehyde is a characteristic aroma component of buckwheat. Salicylaldehyde also occurs in the larval defensive secretions of several leaf beetle species that belong the subtribe Chrysomelina.Pauls, G., Becker, T., et al. (2016)
Two Defensive Lines in Juvenile Leaf Beetles; Esters of 3-nitropropionic Acid in the Hemolymph and Aposematic Warning.
''Journal of Chemical Ecology'' 42 (3) 240-248. An example for a leaf beetle species that produces salicylaldehyde is the red poplar leaf beetle '' Chrysomela populi''.

Reactions and applications

Salicylaldehyde is mainly used commercially as a precursor to coumarin. The conversion entails condensation with acetic anhydride (" Perkin synthesis"). Salicylaldehyde Derivatives

#Oxidation with

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

gives catechol (1,2-dihydroxybenzene) ( Dakin reaction). # Etherification with chloroacetic acid followed by cyclisation gives the heterocycle

benzofuran Benzofuran is the heterocyclic compound consisting of fused benzene and furan rings. This colourless liquid is a component of coal tar. Benzofuran is the structural nucleus (parent compound) of many related compounds with more complex stru ...

(coumarone). The first step in this reaction to the substituted benzofuran is called the Rap–Stoermer condensation after E. Rap (1895) and R. Stoermer (1900). #Salicylaldehyde is converted to chelating

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s by condensation with amines. With

ethylenediamine Ethylenediamine (abbreviated as en when a ligand) is the organic compound with the formula C2H4(NH2)2. This colorless liquid with an ammonia-like odor is a basic amine. It is a widely used building block in chemical synthesis, with approximately ...

, it condenses to give the ligand salen. Hydroxylamine gives salicylaldoxime. #Condensation with diethyl malonate gives 3-carbethoxycoumarin (a derivative of coumarin) by an aldol condensation. Elbs persulfate oxidation gives gentisaldehyde (2,5-dihydroxybenzaldehyde).

Internal hydrogen bonding

Due to the ''ortho'' positioning of the hydroxy- and aldehyde groups, an internal

hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

is formed between the groups. The hydroxy group serves here as the hydrogen bond donor, and the aldehyde as hydrogen bond acceptor. This internal hydrogen is not found in the other hydroxybenzaldehyde isomers. When the aldehyde is reacted with an amine to form an imine, the internal hydrogen bond is even stronger. In addition, tautomerisation further increases the stability of the compound. The internal hydrogen bond also ensures that the aldehyde (or corresponding imine) is held into the same plane, making the whole molecule essentially flat.

References

{{Authority control Flavors Hydroxybenzaldehydes 2-Hydroxyphenyl compounds