8-Hydroxyquinoline

8-Hydroxyquinoline is an organic compound derived from the heterocycle quinoline. A colorless solid, its conjugate base is a chelating agent, which is used for the quantitative determination of metal ions.
In aqueous solution 8-hydroxyquinoline has a pK_a value of ca. 9.9 It reacts with metal ions, losing the proton and forming 8-hydroxyquinolinato-chelate complexes.
The aluminium complex, is a common component of organic light-emitting diodes. Substituents on the quinoline ring affect the luminescence properties.
In its photo-induced excited-state, 8-hydroxyquinoline converts to zwitterionic isomers, in which the hydrogen atom is transferred from oxygen to nitrogen.

History

8-hydroxyquinoline was first obtained by Hugo Weidel and his student Albert Cobenzl in 1880. They decarboxylated so-called oxycinchoninic acid and characterized the resulting compound as melting at about 70°C. They identified that the hydroxy group is on the benzene ring and called the compound oxyquinoline and α-quinophenol.
In the following year more chemists found other ways to make the compound. Zdenko Hans Skraup discovered a way to synthesize substituted quinolines from substituted phenols and described three isomers of oxyquinoline, identifying the structure of 8-hydroxyquinoline. and his student Karl Bedall made the compound from a sulphonic acid independently at about the same time, but misidentified its structure.
By 1888 azo dyes were made from the compound.
In the 1920s insoluble chelates of 8-hydroxyquinoline were discovered.

Bioactivity

The complexes as well as the heterocycle itself exhibit antiseptic, disinfectant, and pesticide properties, functioning as a transcription inhibitor. Its solution in alcohol is used in liquid bandages. It once was of interest as an anti-cancer drug.
A thiol analogue, 8-mercaptoquinoline is also known.
The roots of the invasive plant Centaurea diffusa release 8-hydroxyquinoline, which has a negative effect on plants that have not co-evolved with it.