Assertion (A): Cyclopenta-1,3-diene ( $p K_{a} \approx 16$ ) is far more acidic than cyclohepta-1,3,5-triene ( $p K_{a} \approx 36$ ). Reason (R): The cyclopentadienyl anion is a planar cyclic species with six pi electrons, whereas the cycloheptatrienyl anion would have eight pi electrons.

Answer: A

Both statements are true. Deprotonating cyclopentadiene gives a planar cyclic anion with

4 n + 2 = 6

pi electrons (

n = 1

), which is aromatic and strongly stabilised. The cycloheptatrienyl anion would have 8 pi electrons (

4 n

), which is antiaromatic and unstable. So R explains A.