Question

Pyrrole undergoes electrophilic aromatic substitution preferentially at the 2 position as illustrated by the synthesis of 2 -nitropyrrole. Write resonance contributing structures for the intermediate formed by attack of \(\mathrm{NO}_{2}^{+}\) at the 2 and 3 positions of pyrrole. From examination of these intermediates, offer an explanation for preferential nitration at the 2 position.

Short answer

Answer: The preference for nitration at the 2 position of pyrrole can be explained by the increased stability of the intermediate at the 2 position due to resonance stabilization involving the nitrogen atom's lone pair of electrons. This makes the intermediate at the 2 position more stable compared to that at the 3 position, which is not stabilized by resonance the same way.

Step-by-step solution

Draw the structure of pyrrole

Pyrrole is an aromatic heterocycle that has a five-membered ring with one nitrogen atom. Here is the structure of pyrrole: H \ N - C = C - H | \ H C = C - H / H

Show the attack of \(\mathrm{NO}_{2}^{+}\) at the 2 and 3 positions

We will now illustrate the attack of \(\mathrm{NO}_{2}^{+}\) at the 2 and 3 positions of pyrrole: Attack at 2 position: O || N-O^+ \ C = H // C - N - C = C - H | H Attack at 3 position: O || N-O^+ \ C = H / \ C = C - H N - H \ H

Draw resonance structures for the intermediates at each position

We will draw resonance structures for the intermediates formed by the attack of \(\mathrm{NO}_{2}^{+}\) at the 2 and 3 positions of pyrrole: Intermediate at 2 position: Resonance Structure 1: O || N-O^+ \ C = H // C - N - C = C - H | H Resonance Structure 2: O || N-O^+ \ C - H // C = N - C - C = H \ H Intermediate at 3 position: Resonance Structure 1: O || N-O^+ \ C = H / \ C = C - H N - H \ H Resonance Structure 2: O || N-O^+ \ C - H / \ C - C = H N - H \ H

Analyze the resonance structures to explain the preferential nitration at the 2 position

By examining the resonance structures drawn in step 3, we can see that at the 2 position, there are delocalized electrons from the lone pair on the nitrogen atom that contribute to the resonance stabilization of the intermediate. This makes the intermediate at the 2 position more stable compared to that at the 3 position, which is not stabilized by resonance the same way. As a result, the preference for nitration at the 2 position can be explained by the increased stability of the intermediate at the 2 position due to resonance stabilization involving the nitrogen atom's lone pair of electrons.