Fundamentals of Organic ChemistryChemistryGeneral Organic Chemistry
Pyrrole and pyridine are both basic and can form salts with acids as shown below: Which of the following statements is correct regarding the aromatic character of the four species (i), (ii), (iii), and (iv)?
Options
- AOnly (i) and (iii) are aromatic.
- BOnly (i), (ii), and (iii) are aromatic.
- COnly (i), (iii), and (iv) are aromatic.
- DAll four species are aromatic.
Correct answer
C. Only (i), (iii), and (iv) are aromatic.
Step-by-step solution
Species (i) is pyrrole. The lone pair of electrons on the nitrogen atom participates in delocalization with the two bonds, providing a total of 6 electrons ( 4n+2 , where n=1 ). The ring is planar and fully conjugated, making it aromatic. Species (ii) is N-protonated pyrrole. The lone pair on the nitrogen atom is used to form a bond with the H^+ ion and is no longer available for delocalization. The nitrogen atom becomes sp^3 hybridized, which breaks the continuous conjugation of the ring. Thus, species (ii) is non