Question

Compare basic strength of furan , pyrole, thiophene, pyrdine

Answer 1

Thiophene, pyrrole and Furan are all five membered heterocyclic aromatic compounds, with the hetero atom being sulfur (S), nitrogen (N) and oxygen (O) respectively. They are aromatic as they are planar ring systems, and resonance is possible due to delocalization of the two pi bonds and the lone pair of electrons of the heteroatom. However, if we compare the electronegativity of the heteroatoms, we will see that O is more electronegative as compared to N, S being the least electronegative. Hence, O will have the least tendency, among the three, to donate its lone pair of electrons for resonance, and will rather draw the electronic density of the ring towards itself. The concentration of electron density on the heteroatom makes it a good electron donor (Lewis base). Thus the order of basicity will be

Furan > Pyrrole > Thiophene

In Pyrrole -N atom are present as heteroatom
In Furan -O atom are present as heteroatom