Quinoline, also known as benzopyridine or azazonyl, is a compound formed by the parallel connection of pyridine and benzene, and it is a heterocyclic aromatic organic compound. At room temperature, it is a colorless hygroscopic liquid with a strong odor. When exposed to light, it gradually turns pale yellow and further brown. Its molecular formula is C9H7N. It is slightly soluble in water and readily soluble in many organic solvents such as ethanol and ether. There are two ways of combination, which are respectively called quinoline and isoquinoline. They exist in coal tar and bone tar. The crude quinoline obtained from coal tar contains approximately 4% isoquinoline. The melting point of isoquinoline is 26.5℃, the boiling point is 242.2℃(743 MMHG), and the density is 1.0986g/cm ³ (at 20℃). Its odor is completely different from that of quinoline. Both are basic. Isoquinoline is more basic than quinoline. Both can form salts with strong acids, such as picate and dichromate. Quinoline forms quaternary ammonium salts with haloalkanes, etc. It has a strong aromatic property, and the benzene ring part is prone to electrophilic substitution reactions at the 5,8 positions. For example, during nitration or sulfonation, 5- and 8-nitro and sulfoquinoline are produced. The pyridine ring is partially stable. During oxidation, the benzene ring is destroyed, while the pyridine ring remains unchanged. The infrared spectrum of quinoline has an absorption peak similar to that of 1-methylnaphthalene at a distance of 3.27 micrometers. The infrared absorption of isoquinoline is very similar to that of naphthalene. The properties of isoquinoline are similar to those of quinoline. Nitration and sulfonation occur at the 5th position of the benzene ring, while the nucleophilic reaction takes place at the 1st position. For instance, when reacting with sodium amide, 1-amino-isoquinoline is formed, and quinoline is aminated at the 2nd position. In industry, the property that the acidic sulfate of quinoline is soluble in ethanol while that of isoquinoline is insoluble is commonly used for separation.