Electrophilic Aromatic Substitution of Heteroaromatics
 

• Aromatic compounds which contain heteroatoms (e.g. O, N, S) are called heteroaromatics.
• The presence of the heteroatom influences the reactivity compared to benzene.
• The 5-membered ring heterocycles (furan, pyrrole, thiophene) are p-electron rich aromatics (6p electrons over 5 atoms)
• This makes them more reactive than benzene (since the aromatics the nucleophilic component in these electrophilic substitution reactions)
• For example furan is similar to an activated benzene like methoxybenzene
• Pyridine is less reactive than benzene (more like nitrobenzene) due to the electronegativity of N, it is described as a p-electron defficient aromatic
• The basic nature of the N atom of pyridine often interferes and interacts with the electrophile E+ which further deactivates the system

 

Compound Name	Line Drawing	3D Model
Furan
Thiophene
Pyrrole
Pyridine

The 5 membered heterocycles react in a similar way. As an example here is the general pattern for the electrophilic aromatic substitution reactivity of furan, complete with the 3 important resonance structures that justify the reactivity of the 2-position.
How many important resonance structures can you draw for reaction at the 3-position ?