Page 33 - Org 3 theoritical book 2024-25

Clinical Pharmacy PharmD - 2024/2025 Level 2 Pharmaceutical Organic Chemistry-3 (PC 305)

Thus, the order of aromaticity of such rings is actually the reverse. In other words,

furan is the least aromatic of all the three compounds followed by pyrrole and then

thiophene which is the most aromatic in the series.

Aromaticity : benzene > thiophene > pyrrole > furan

Acidic properties:

Pyrrole is weakly acidic,

When heated with solid KOH, it forms potassium salt, which is hydrolyzed back to

pyrrole when treated with water.

This acidic property of pyrrole can be explained as follows:

The N-H bond of pyrrole is weak as the lone pair of electrons on N-atom is

involved in resonance, making the N positively charged, so making it easier to

remove the proton.
Basic properties:

• Generally, each of the five-membered heterocycles can act as a base by accepting a

proton or utilizing the lone pair on the heteroatom with Lewis acids.

• In dilute acids, pyrrole readily forms a trimer and in concentrated solutions it

polymerizes to a dark colored polymeric product known as pyrrole red.