Fundamentals of Organic ChemistryChemistryGeneral Organic Chemistry
Cyclopentadiene has a p K_ a = 15 , whereas cyclopentane has a p K_ a > 50 . This is because:
Options
- ACyclopentadiene is a 4 anti-aromatic compound.
- BCyclopentadiene is particularly unstable.
- CCyclopentadiene is a 4 non-aromatic compound and after deprotonation it is aromatic.
- DCyclopentane contains no lone pairs.
Correct answer
C. Cyclopentadiene is a 4 non-aromatic compound and after deprotonation it is aromatic.
Step-by-step solution
Cyclopentadiene contains an sp^3 hybridized carbon atom that interrupts the conjugation of its two double bonds ( 4 electrons). Therefore, it is a non-aromatic compound. Upon deprotonation at the sp^3 carbon, it forms the cyclopentadienyl anion. The lone pair on the resulting carbanion participates in resonance with the two double bonds, creating a continuous, planar cyclic system with 6 electrons. According to Hückel's rule ( 4n+2 electrons, where n=1 ), the cyclopentadienyl anion is aromatic. The strong aromatic