Question

What is the order of reactivity for furan, pyrrole, thiophene and benzene (towards electrophilic aromatic substitution)?

• A. benzene > thiophene > furan > pyrrole
• B. pyrrole > furan > thiophene > benzene
• C. furan > thiophene > pyrrole > benzene
• D. thiophene > pyrrole > furan > benzene

Hint:

Compare how easily each heteroatom lone pair is pushed into the ring based on electronegativity and orbital overlap.

Answer 1

The Correct Option is B

Step 1: In furan, pyrrole and thiophene, the heteroatom donates one lone pair into the ring pi system to complete the aromatic sextet. Benzene has no such extra donor, so all three heterocycles react faster with electrophiles.

Step 2: Among the three heterocycles, donation depends on electronegativity and orbital overlap.

Step 3: Nitrogen in pyrrole is least electronegative, so it donates most effectively, giving pyrrole the fastest reaction.

Step 4: Oxygen in furan is more electronegative than nitrogen but still uses a 2p orbital with good overlap, so furan reacts faster than thiophene.

Step 5: Sulfur in thiophene uses a larger, more diffuse 3p orbital with poor overlap, so thiophene donates least efficiently of the three, though still faster than benzene.

\[\boxed{\text{pyrrole} > \text{furan} > \text{thiophene} > \text{benzene}}\]