What is Quinoline?

Quinoline is a heterocyclic aromatic organic compound with the chemical formula C9H7N.A colorless hygroscopic liquid with a strong odour.Aged samples, especially if exposed to light, turn yellow and then brown.Quinoline is only slightly soluble in cold water, but readily soluble in hot water and most organic solvents.Quinoline itself has few applications, but many of its derivatives are useful in different applications.A prominent example is quinine, an alkaloid found in plants.More than 200 biologically active quinoline and quinazoline alkaloids have been identified.4-Hydroxy-2-alkylquinolines (HAQ) are associated with antibiotic resistance.

Occurrence and isolation

German chemist Friedlieb Ferdinand Runge first extracted quinoline from coal tar;he called quinoline leukol (Greek for “white oil”).Coal tar remains the main source of commercial quinolines.In 1842, French chemist Charles Gerhardt obtained a compound by dry distillation of quinine, strychnine or cinchonine with potassium hydroxide;he named the compound Chinoilin or Chinolein.The compounds of Runge and Gephardt appear to be different isomers because they react differently.However, German chemist August Hoffmann eventually realized that the difference in behavior was due to the presence of pollutants, and that the two compounds were in fact identical.The only report of quinolines as natural products comes from the Peruvian stick insect Oreohoetes peruana.They have a pair of thymus glands which, when disturbed, excrete a foul-smelling fluid containing quinoline.

Like other nitrogen heterocycles such as pyridine derivatives, quinolines are frequently reported as environmental pollutants associated with oil shale or coal processing facilities and have also been found at legacy wood processing sites.Due to the relatively high solubility of quinoline in water, it has significant mobility in the environment, which may facilitate water pollution.Quinoline is readily degraded by certain microorganisms such as Rhodococcus strain Q1 isolated from soil and paper mill sludge.

Quinolines are present in small amounts in crude oil in the raw diesel fraction. It can be removed through a process called hydrodenitrogenation.

Applications

Quinoline is used in the manufacture of dyes and in the preparation of hydroxyquinoline sulfate and nicotinic acid.It is also used as a solvent for resins and terpenes.Quinoline is mainly used in the production of other specialty chemicals. According to a report published in 2005, approximately 4 tons are produced annually.Its main use is as a precursor to 8-hydroxyquinoline, a multifunctional chelating agent and insecticide precursor.Its 2- and 4-methyl derivatives are precursors of cyanine dyes. Oxidation of quinoline yields quinolinic acid (pyridine-2,3-dicarboxylic acid), a precursor to a herbicide sold under the name “Assert.” Reduction of quinoline with sodium borohydride in the presence of acetic acid is known to yield Kairoline A.(C.f. Kairine).There are several antimalarial derivatives of quinoline, including quinine, chloroquine, amodiaquine, and primaquine.The enantioselective reduction of quinolines to tetrahydroquinolines was performed using several catalyst systems.