Thiophene is a 5-membered aromatic ring system ring. As with the oxygen atom in furan, the sulfur atom contributes two electrons to the pi-system and also contains on sigma lone pair. Thiophene is an "electron-rich" aromatic system since there are 6 pi-electrons distributed over 5 atoms. Thiophene contains particularly electron-rich C-atoms since the S-atom is less electronegative and more prone to release electron density. Thiophene very readily undergoes electrophilic aromatic substitution.