Conjugation and ultraviolet bands of five-membered heterocycles

Abstract

The problem of interpreting and correlating the properties of five-membered ‘aromatic’ heterocycles can be solved by re-examining the experimental data and applying a semi-empirical quantum-chemical calculation. The important objections (based mainly on the u.v. absorption spectra) to the idea that the compounds are essentially perturbed cis-dienes are shown not to be valid; the amount of homocyclic conjugation appears to increase in the order furan < thiophen < pyrrole, the case of thiophen requiring special consideration; in no case does the conjugation correspond to that of a typically aromatic heterocycle like pyridine.