Pyrrole undergoes electrophilic aromatic substitution preferentially at the 2 position as illustrated by the synthesis of 2-nitropyrrole. Pyrrole 2-Nitropyrrole

Pyrrole is an aromatic compound containing a nitrogen atom in a 5-membered ring. The nitrogen donates its lone pair of electrons to the ring's pi-system, making it aromatic and highly reactive. The structure of pyrrole is: ``` NH // \ H C \\ // C=C H H ```

In this reaction, we will be nitrating pyrrole to form 2-nitropyrrole. The electrophile in this reaction is the nitronium ion (\(\text{NO}_2^+\)), which will be generated from the reaction of nitric acid (\(\text{HNO}_3\)) with a strong acid catalyst, usually sulfuric acid (\(\text{H}_2\text{SO}_4\)): \(\text{HNO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{NO}_2^+ + \text{HSO}_4^- + \text{H}_2\text{O}\)

The nucleophile in electrophilic aromatic substitution reactions is the aromatic compound itself. In this case, pyrrole is the nucleophile, and it will use the pi electrons from its aromatic ring to attack the electrophile, \(\text{NO}_2^+\).

The nucleophilic attack occurs when the pi electrons in the aromatic ring of pyrrole attack the electrophilic nitronium ion, forming a cationic intermediate and breaking nitrogen's double bond with a carbon atom: ``` NH + [O=N=O]^+ // \ | H C ---> C \\ // // \ C=C H C H H + \ N // \ H C \\ C=C H H ```

To restore the aromaticity of the ring, the positive charge on the nitrogen atom must be neutralized, which requires the transfer of a proton. The nitrogen atom in the 1-position will donate its proton to the conjugate base of the strong acid catalyst (in this case, \(\text{HSO}_4^-\)) and regain the double bond with the carbon atom: ``` NH + \text{HSO}_4^- ^+ --\ | \\ --/ \ \|/ H C ---> C C v // \ \ \\ // H C C=C O H H // N / \\ O O ```

The final product of this electrophilic aromatic substitution reaction is 2-nitropyrrole, with the nitro group (\(\text{NO}_2\)) being added to the 2-position of pyrrole: ``` NH // \ H C \\ // C=N H O \ O^-