Heterocyclic compounds are widely found in nature, and most of the important biological compounds are heterocyclic compounds such as nucleic acids, certain vitamins, antibiotics, hormones, pigments and alkaloids. In addition, a variety of heterocyclic compounds with various properties have been synthesized, some of which can be used as drugs, insecticides, herbicides, dyes, plastics and the like. Of course there are other.
Five-membered heterocyclic compounds
For the electrophilic substitution reaction, the heteroatom increases the electron cloud density of the carbon atoms on the ring and activates the ring, respectively. They are more active than benzene, and their activity is similar to that of phenol and aniline. They can undergo the usual electrophilic substitution reactions such as nitration, sulfonation, halogenation and Friedel-Crafts reactions. Due to their high degree of activity and the sensitivity of furan and pyrrole to inorganic strong acids, their electrophilic substitution reactions require relatively mild conditions. For example, when furan and pyrrole are sulfonated, use pyridine-sulfur trioxide adducts; if sulfuric acid is used, the ring will be opened.
Six - membered heterocyclic compounds
Unlike pyrrole, the unused electron pair on the pyridine nitrogen atom does not participate in the large π system and binds to the proton. So the basicity of pyridine is stronger than pyrrole and aniline. Since the electronegativity of nitrogen is larger than that of carbon, the electron cloud density on the pyridine ring is lower, and the electron cloud density at the α position is lower than β. Therefore, pyridine and nitrobenzene similar to the general under strong conditions in order to occur electrophilic substitution reaction, and mainly in the β position.
The electrophilic substitution reaction of pyrimidine is more difficult than pyridine, and its nucleophilic substitution reaction is easier than pyridine.
Fused heterocyclic compounds
Quot; refers to a compound in which a benzene ring is complexed with a heterocyclic ring or a heterocyclic ring. Common are quinoline, indole and purine.