Chapter 16: Q16P (page 820)

(a) Explain how pyrrole is isoelectronic with the cyclopentadienyl anion.
(b) Specifically, what is the difference between the cyclopentadienyl anion and pyrrole?
(c) Draw resonance forms to show the charge distribution on the pyrrole structure

Short Answer

Expert verified

Cyclopentadienyl anion Pyrrole ( 6π electrons

and aromatic)

6π electrons and aromatic

b) Cyclopentadienyl is acidic whereas pyrrole is basic in nature.

(c)

Resonance structure of pyrrole

Step by step solution

Step 1:Isoelectronic species

The molecule that has the same number of electrons as of the electrons present in the counter atom, those species are referred to as isoelectronic species. In these species, the electrons would be the same but different elements will be present.

Characteristics of cyclopentadienyl anion and pyrrole

a) The pyrrole is an aromatic five-membered heterocyclic with one nitrogen atom and two double bonds. although it may seem that pyrrole has only four pi electrons, the nitrogen atom has a lone pair of electrons.

The cyclopentadienyl anion is an aromatic five-membered ring, with two double bonds and a negative charge of lone pair of electrons. So, it also contains six pi electrons. Hence it is isoelectronic with cyclopentadienyl anion

Cyclopentadienyl anion Pyrrole ( 6π electrons and aromatic)

6π electrons and aromatic

b) These two compounds are aromatic compounds. The cyclopentadienyl is usually acidic because the loss of proton converts the non-aromatic diene to the aromatic cyclopentadienyl anion. The pyrrole has amiono group. So, it has a basic nature. The pyrrole has 4 pi electrons and the nitrogen atom has a lone pair of electrons. So, it has 6 pi electrons. The cyclopentadienyl anion has 4 pi electrons and the negatively charged of lone pairs of a methylene group. So, it also has 6 pi electrons.

The cyclopentadienyl anion does not have any hetero atom but pyrrole has a nitrogen atom (heteroatom).

c) The lone pair of electrons on the nitrogen atom undergoes resonance to form four resonance contributors. They are represented as: