Quinoline

Quinoline is a heterocyclic aromatic organic compound. It has the formula C9H7N and is a colourless hygroscopic liquid with a strong odour. Aged samples, if exposed to light, become yellow and later brown. Quinoline is only slightly soluble in cold water but dissolves readily in hot water and most organic solvents.

Quinoline is mainly used as a building block to other specialty chemicals. Approximately 4 tonnes are produced annually according to a report published in 2005. Its principal use is as a precursor to 8-hydroxyquinoline, which is a versatile chelating agent and precursor to pesticides. Its 2- and 4-methyl derivatives are precursors to cyanine dyes. Oxidation of quinoline affords quinolinic acid (pyridine-2,3-dicarboxylic acid), a precursor to the herbicide sold under the name "Assert".

Like other nitrogen heterocylic compounds, such as pyridine derivatives, quinoline is often reported as an environmental contaminant associated with facilities processing oil shale or coal, and has also been found at legacy wood treatment sites. Owing to high water solubility quinoline has significant potential for mobility in the environment, which may promote water contamination. Fortunately, quinoline is readily degradable by certain microorganisms, such as Rhodococcus species Strain Q1, which was isolated from soil and paper mill sludge.

Isolation and synthesis
Quinoline was first extracted from coal tar in 1834 by Friedlieb Ferdinand Runge. Coal tar remains the principal source of commercial quinoline. It can be synthesized using various methods:
 * Combes quinoline synthesis using anilines and β-diketones.
 * Conrad-Limpach synthesis using anilines and β-ketoesters.
 * Doebner-Miller reaction using anilines and α,β-unsaturated carbonyl compounds.
 * Friedländer synthesis using 2-aminobenzaldehyde and acetaldehyde.
 * Skraup synthesis using ferrous sulfate, glycerol, aniline, nitrobenzene, and sulfuric acid.
 * Povarov reaction using an aniline, a benzaldehyde and an activated alkene.
 * Camps quinoline synthesis utilizing an o-acylaminoacetophenone and hydroxide
 * Knorr quinoline synthesis, using a β-ketoanilide and sulfuric acid.
 * Gould-Jacobs reaction starting from an aniline and ethyl ethoxymethylenemalonate
 * Niementowski quinoline synthesis, using anthranilic acid and ketones.

Applications
In manufacturing of dyes, preparation of hydroxyquinoline sulfate and niacin. Solvent for resins and terpenes.