Pyridine (molecular formula C6H5N) is a six-membered heterocyclic compound containing a nitrogen atom, that is, a compound generated by replacing a -CH= in a benzene molecule with nitrogen. It is similar to benzene, has the same electronic structure, and is still aromatic, so it is also called nitrogen benzene and nitrogen-benzene. It is a colorless liquid with a special odor at room temperature, with a melting point of -41.6℃ and a boiling point of 115.2℃. It forms an azeotropic mixture with water, and the boiling point is 92-93℃. (This property is used in industry to purify pyridine), and the density is 0.9819g/cm3. It is easily soluble in most organic solvents such as water, ethanol, ether, and can also be used as a solvent. Pyridine was originally separated from bone tar, and then it was found that coal tar, coal gas, shale oil, and petroleum also contain pyridine and its homologues, such as 2-methylpyridine and 2,6-dimethylpyridine. Pyridine and its derivatives are more stable than benzene, and their reactivity is similar to that of nitrobenzene. Due to the electron-withdrawing effect of the nitrogen atom in the ring, the electron density at positions 2, 4, and 6 is lower than that at positions 3 and 5. Typical aromatic electrophilic substitution reactions occur at positions 3 and 5, but the reactivity is lower than that of benzene, and nitration, halogenation, sulfonation and other reactions are generally not easy to occur. In addition, these substitution reactions are all carried out in an acidic medium, and pyridine forms positively charged ions, making it difficult for electrophilic reagents to approach. The halogens of 2- or 4-halopyridine are all active. Due to the low electron density at positions 2 and 6, nucleophilic substitution reactions can occur at these positions, such as reaction with sodium amide or potassium hydroxide to obtain the corresponding 2-aminopyridine or 2-hydroxypyridine. Structure