Heterocyclic Chemistry PDF

5-atom, six-π-electron aromatic heterocycles six π electrons Five-membered heterocyclic organic compound consists of two π bonds and one lone pair of electrons contributing to a π system. Thus it has six π electrons ( 4n +2 ). Structure of pyrrole, furan, and thiophene : The simplest of the five...

5-atom, six-π-electron aromatic heterocycles six π electrons Five-membered heterocyclic organic compound consists of two π bonds and one lone pair of electrons contributing to a π system. Thus it has six π electrons ( 4n +2 ). Structure of pyrrole, furan, and thiophene : The simplest of the five-membered heterocyclic compounds are pyrrole, furan, and thiophene, each of which contains a single heteroatom structures I, II, and III, we might expect each of these compounds to have the properties of a conjugated diene. Thiophene does not undergo the oxidation of a sulfide, and pyrrole does not possess the basic properties of amines. These heterocycles and their derivatives most commonly undergo electrophilic substitution: ( nitration, sulfonation, halogenation, Friedel–Crafts acylation, even the Reimer–Tiemann reaction and coupling with diazonium salts) , Except for a certain tendency to undergo addition reactions. The atom uses three sp2 orbitals, which lie in a plane and are 120° apart. A Friedel-Crafts reaction is an organic coupling reaction involving an electrophilic aromatic substitution of alkylation ( -R) and acylation( -COR) reactions. Reimer–Tiemann reaction is a chemical reaction used for the ortho-formylation of phenols. with the simplest example being the conversion of phenol to salicylaldehyde. A common example of the Reimer Tiemann reaction is the conversion of phenol to salicylaldehyde (2-hydroxy benzaldehyde), as shown below. Delocalization of the p electrons stabilizes the ring of pyrrol. As a result, it has an abnormally low heat of combustion; it tends to undergo reactions in which the stabilized ring to undergo substitution. Pyrrole, furan, and thiophene must be considered aromatic. Nitrogen’s pair of electrons, which is responsible for the usual basicity of nitrogen compounds, most amines, therefore, pyrrole is an extremely weak base (Kb = 2.5 x 10-14 ). By the same, there is a high electron density in the ring, It thus appears that pyrrole is better represented by IV, Pyrrole can be considered a hybrid of structures V-IX. Nitrogen is indicated by the ionic structures in which nitrogen bears a positive charge and the carbon atoms of the ring bear a negative charge. Furan and thiophene have structures that are analogous to the structure of pyrrole. Where nitrogen in pyrrole carries a hydrogen atom, the oxygen or sulfur carries an unshared pair of electrons in an sp2 orbital.