8-Hydroxyquinoline is an organic compound with the formula C9H7NO. It is a derivative of the heterocyclequinoline by placement of an OH group on carbon number 8. This light yellow compound is widely used commercially, although under a variety of names.
8-Hydroxyquinoline is a monoprotic bidentate chelating agent. Related ligands are the Schiff bases derived from salicylaldehyde, such as salicylaldoxime, salen and salicylaldehyde isonicotinoylhydrazone (SIH). In neutral solution, the hydroxyl is in the protonated form (pKa=9.89) and the nitrogen is not protonated (pKa=5.13). However, an excited-state zwitterionic isomer exists in which H+ is transferred from the oxygen (giving an oxygen anion) to the nitrogen (giving a protonated nitrogen cation).
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