Question

Explain the order of acidity in indene fluorene and cyclopentadiene derivative

Answer 1

This is a better question as you might expect. And the answer is not so obvious.

These are the pKa values of your structures (in DMSO)

Fluorene is the least acidic, followed by indene, and cyclopenta-1,3-diene is the most acidic.

The acidity of cyclopentadiene is a textbook example. When we would deprotonate it, we obtain the aromatic cyclopentadienyl anion explaining its unusual stability and therefore also the acidity of the cyclopentadiene.

But if we follow the same logic we would expect that indene and fluorene would be even more acidic. Deprotonating would equally form a ‘cyclopentadienylish’ (is that a word?) anion.