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 3-Hydroxybenzaldehyde is an organic compound with the formula . It is a colorless solid although most samples appear tan. Two other isomers of hydroxybenzaldehyde exist. Preparation It has been prepared from 3-nitrobenzaldehyde in a sequence o ...

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 Hydroxybenzaldehydes are phenolic aldehydes. The term may refer to: * Salicylaldehyde Salicylic aldehyde (2-hydroxybenzaldehyde) is an organic compound with the formula . Along with 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde, it is one of ...

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

chelating agent Chelation () is a type of bonding of ions and their molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These l ...

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 reaction Formylation refers to any chemical processes in which a compound is functionalized with a formyl group (-CH=O). In organic chemistry, the term is most commonly used with regards to aromatic compounds (for example the conversion of benzene to benz ...

s from the corresponding phenol, for instance by the

Duff reaction The Duff reaction or hexamine aromatic formylation is a formylation reaction used in organic chemistry for the synthesis of benzaldehydes with hexamine as the formyl carbon source. The method is generally inefficient. The reaction is named after J ...

, Reimer–Tiemann reaction, or by treatment with

paraformaldehyde Paraformaldehyde (PFA) is the smallest polyoxymethylene, the polymerization product of formaldehyde with a typical degree of polymerization of 8–100 units. Paraformaldehyde commonly has a slight odor of formaldehyde due to decomposition. Par ...

in the presence of

magnesium chloride Magnesium chloride is an inorganic compound with the formula . It forms hydrates , where ''n'' can range from 1 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which ...

and a base. : Reimer-Tiemann_Reaction_Scheme

Natural occurrences

Salicylaldehyde is a characteristic aroma component of

buckwheat Buckwheat (''Fagopyrum esculentum'') or common buckwheat is a flowering plant in the knotweed family Polygonaceae cultivated for its grain-like seeds and as a cover crop. Buckwheat originated around the 6th millennium BCE in the region of what ...

. Salicylaldehyde also occurs in the larval defensive secretions of several

leaf beetle The beetle family Chrysomelidae, commonly known as leaf beetles, includes over 37,000 (and probably at least 50,000) species in more than 2,500 genera, making it one of the largest and most commonly encountered of all beetle families. Numerous s ...

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 Coumarin () or 2''H''-chromen-2-one is an aromatic organic chemical compound with formula . Its molecule can be described as a benzene molecule with two adjacent hydrogen atoms replaced by an unsaturated lactone ring , forming a second six-me ...

. The conversion entails condensation with

acetic anhydride Acetic anhydride, or ethanoic anhydride, is the chemical compound with the chemical formula, formula . Commonly abbreviated , it is one the simplest organic acid anhydride, anhydrides of a carboxylic acid and is widely used in the production of c ...

Perkin synthesis The Perkin reaction is an organic reaction developed by English chemist William Henry Perkin in 1868 that is used to make cinnamic acids. It gives an α,β-unsaturated aromatic acid or α-substituted β-aryl acrylic acid by the aldol condensati ...

").

#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 Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is an organic compound with the molecular formula . It is the ''ortho'' isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amounts. It ...

(1,2-dihydroxybenzene) (

Dakin reaction The Dakin oxidation (or Dakin reaction) is an organic redox reaction in which an '' ortho''- or ''para''-hydroxylated phenyl aldehyde ( 2-hydroxybenzaldehyde or 4-hydroxybenzaldehyde) or ketone reacts with hydrogen peroxide (H2O2) in base to ...

). #

Etherification In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R′ re ...

with

chloroacetic acid Chloroacetic acid, industrially known as monochloroacetic acid (MCA), is the organochlorine compound with the formula . This carboxylic acid is a useful building block in organic synthesis. It is a colorless solid. Related compounds are dichlo ...

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 Salicylic aldehyde (2-hydroxybenzaldehyde) is an organic compound with the formula . Along with 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde, it is one of the three isomers of hydroxybenzaldehyde. This colorless oily liquid has a bitter almond ...

after E. Rap (1895) and R. Stoermer (1900). #Salicylaldehyde is converted to

chelating Chelation () is a type of bonding of ions and their molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These l ...

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 Salicylaldoxime is an organic compound described by the formula C6H4CH=NOH-2-OH. It is the oxime of salicylaldehyde. This crystalline, colorless solid is a chelator and sometimes used in the analysis of samples containing transition metal ions, ...

. #Condensation with

diethyl malonate Diethyl malonate, also known as DEM, is the diethyl ester of malonic acid. It occurs naturally in grapes and strawberries as a colourless liquid with an apple-like odour, and is used in perfumes. It is also used to synthesize other compounds ...

gives 3-carbethoxycoumarin (a derivative of

) by an

aldol condensation An aldol condensation is a condensation reaction in organic chemistry in which two carbonyl moieties (of aldehydes or ketones) react to form a β-hydroxyaldehyde or β-hydroxyketone (an aldol reaction), and this is then followed by dehydration t ...

Elbs persulfate oxidation The Elbs persulfate oxidation is the organic reaction of phenols with alkaline potassium persulfate to form '' para''-diphenols. The reaction is generally performed in water at room temperatures or below, using equimolar quantities of reagents. ...

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

isomers. When the aldehyde is reacted with an amine to form an imine, the internal hydrogen bond is even stronger. In addition,

tautomerisation In chemistry, tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relo ...

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