Heterocyclic chemistry sulfur, electronic structure

The other two chapters deal with the application of molecular orbital theory to heterocyclic chemistry. The groups surveyed are (a) sulfur heterocycles and (6) azines. The chapters, which are authored by R. Zahradnik and J. Koutecky, discuss the relevance of theoretical calculation to reactivity, electronic structure, and other physicochemical properties of the compounds. The new techniques of theoretical chemistry have been applied with great success to carbocyclic compounds their significance in heterocyclic chemistry will surely increase. 

Thiophene is an aromatic compound. Its structure can be assumed to be derived from benzene by replacement of two annular CH groups with sulfur. The sulfur atom in this five-membered ring acts as an electron donating heteroatom by contributing two electrons to the aromatic sextet and thiophene is thus considered to be an electron-rich heterocycle. The chemistry of thiophene and other five-membered heterocycles, for example pyrrole and furan, are consequently dominated by this effect. 

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