Question

Why is imidazole a stronger acid (pK_a = 14.4) than pyrrole (pK_a~ 17)?

Short answer

Imidazole is a stronger acid (pK_a = 14.4) than pyrrole (pK_a~ 17), because the second ring present in Imidazole, upon which the electrons left behind when the proton is removed can be delocalized making imidazole to be a stronger acid than pyrrole.

Step-by-step solution

Step-by-Step Solution Step 1: Acidic strength

The exact strength of a given acid HA in water solution is described using the acidity constant (${\mathrm{K}}_{\mathrm{a}}$)for the acid-dissociation equilibrium.

Acid strengths are normally determined using role="math" localid="1652454146167" $\text{p}{\mathrm{K}}_{\mathrm{a}}$values rather than ${\mathrm{K}}_{\mathrm{a}}$values, where the $\text{p}{K}_a$is the negative common logarithm of $K_a$:

$\text{p}K\text{a=-log}{K}_a$

The stronger acid has a smaller$\mathbf{p}{\mathit{K}}_a$, and a weaker acid has a larger$\mathbf{p}{K}_a$value.

Identification of  Imidazole as a  stronger acid than pyrrole

Imidazole is a stronger acid (pK_a = 14.4) than pyrrole (pK_a= 17), because the second ring present in Imidazole, upon which the electrons left behind when the proton is removed can be delocalized making imidazole to be a stronger acid than pyrrole, because electronegativity of nitrogen is more than that of carbon, so nitrogen easily delocalized electrons as compared to carbon.

The $\text{p}{\mathrm{K}}_{\mathrm{a}}$values also indicate that imidazole is stronger acid than pyrrole, because as $\text{p}{\mathrm{K}}_{\mathrm{a}}$value increases, acidic strength decreases.