Question

Explain why cyclopentadiene $(\mathbf{p}\mathrm{K}\mathbf{a}=\mathbf{15})$is more acidic than pyrrole$(\mathbf{p}K\mathbf{a}~\mathbf{1}7)$, even though nitrogen is more electronegative than carbon.

Short answer

The ability of cyclopentadiene to change from non-aromatic compound to aromatic compound is the reason for its higher acidity.

Step-by-step solution

Step-by-Step SolutionStep 1:Aromaticity of cyclopentadiene and pyrrole

For a compound to be aromatic, it should be cyclic, planar, fully conjugated and has $4\mathrm{n}+2\mathrm{\pi }$ electrons.

Cyclopentadiene is a non-aromatic compound while pyrrole is an aromatic compound. This is because pyrrole has a lone pair of electrons on the nitrogen atom. So, it satisfy the localid="1652449296981" $4\mathrm{n}+2$rule of aromaticity since the lone pair of electrons give a total of six localid="1652449301599" $\mathrm{\pi }$electrons. But cyclopentadiene has only four localid="1652449308503" $\mathrm{\pi }$electrons. So, it does not satisfy the $4\mathrm{n}+2$rule of aromaticity.

Step 2: Aromaticity of conjugate base of cyclopentadiene and pyrrole

Cyclopentadiene loses a proton to form its conjugate base. Now, this conjugate base has six $\mathrm{\pi }$electrons, so that it satisfies the rule of aromaticity. Now, it forms a stable compound from an unstable compound.

While in pyrrole, it loses a proton to form its conjugate base, it is also an aromatic compound.

Given below is the representation of aromaticity of cyclopentadiene and pyrrole and their conjugate bases:

Aromaticity of cyclopentadiene and pyrrole and their conjugate bases

Acidity of cyclopentadiene and pyrrole

Since cyclopentadiene has the ability to form a stable compound from an unstable compound, it is more acidic than pyrrole. From the$\mathrm{pKa}$values also, it is clear thatcyclopentadiene is stronger acid than pyrrole.