Pharmacognosy - medicinal plants (herbs)
Pharmacognosy - medicinal plants (herbs)

Theme XVIII: Heterocycles

Pyrrole

Pyrrole study. Biological interest of the pyrrole ring. Structure or chemical formula as acid and base. Pyrrole importance.

The study of the pyrrole ring has great biological interest because this molecule has a close relationship with chlorophyll of plants, the hemoglobin in the blood and some alkaloids as nicotine and cocaine.


The heteroatom is nitrogen and is responsible for the basicity of many compounds. Nitrogen free electrons are involved in the electron cloud of the ring.

The conjugation of the pair of free electron of nitrogen found in p-orbitals, with p-orbitals of ring carbon atoms forms an electronic cloud which makes pyrrole less basic than other nitrogen compounds where the electron pair of the nitrogen is part of π-cloud. This phenomenon also allows the polarization of the NH bond and may give in the proton to the bases.


Pyrrole has the possibility to act as an acid and a base. To manifest each of these properties is necessary to develop the practical conditions for pyrrole to react against an acid, accepting protons by the nitrogen electron pair (base properties) and versus a base, where replaces the hydrogen from the NH bond (acidic).


This phenomenon is shown in the following equations:

As an acid

As a base

Pyrrole is therefore an amphoteric compound, ie, can behave as an acid and a base.


Pyrrole has high importance in the formation of numerous natural compounds forming the basis of porphyrins, which in turn, are the main skeleton of chlorophyll and hemoglobin.

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