Question

The diazotization of aniline first involves the formation of \(\mathrm{NO}^{+}\) (nitrosonium ion) by the dehydration of nitrous acid with sulfuric acid. The aniline nitrogen then acts as a nucleophile and eventually loses water. Propose a mechanism for the formation of the dizaonium salt of aniline. Use curved arrows to show all electron movement.

Short answer

The mechanism involves nitrosonium ion formation, nucleophilic attack by aniline, and formation of diazonium salt.

Step-by-step solution

Formation of Nitrosonium Ion

The mechanism starts with the dehydration of nitrous acid (HNO2) by sulfuric acid (H2SO4). The sulfuric acid acts as a strong acid and protonates the nitrous acid, which then undergoes dehydration to form the nitrosonium ion (NO+). This is represented by the following steps and electron flow: 1. Protonation of HNO2 to form H2NO2+. 2. Loss of a water molecule from H2NO2+ to form the nitrosonium ion NO+.

Nucleophilic Attack by Aniline

The lone pair of electrons on the nitrogen atom of aniline acts as a nucleophile and attacks the electrophilic nitrosonium ion (NO+). This forms an intermediate cation. The curved arrow originates from the lone pair on the nitrogen of aniline and points to the nitrosonium ion, illustrating the nucleophilic attack.

Formation of Diazonium Salt

The resulting intermediate cation from the nucleophilic attack is initially unstable and rearranges. The nitrogen atom in the intermediate loses a hydrogen ion, leading to the formation of the diazonium salt. This step involves the removal of a proton from the intermediate to stabilize the diazonium ion. The final product is the diazonium cation (ArN2+), where Ar represents the aryl group from aniline.

Key concepts

Nitrosonium Ion

In the diazotization mechanism, the formation of nitrosonium ion is a crucial initial step. The process starts with nitrous acid ( HNO_2 ), which acts as a source of the nitrosonium ion ( NO^+ ). When nitrous acid reacts with sulfuric acid ( H_2SO_4 ), it undergoes protonation and then dehydration.
This transformation can be broken down into two main sub-steps:

• Protonation: The sulfuric acid, a strong acid, donates a proton ( H^+ ) to nitrous acid, forming an intermediate known as protonated nitrous acid ( H_2NO_2^+ ).
• Dehydration: Water ( H_2O ) is expelled from H_2NO_2^+ , resulting in the formation of the nitrosonium ion. The nitrosonium ion is electrophilic, making it ready for the next phase in the reaction where it can accept electrons.

The creation of NO^+ is significant as it acts as an electrophile, integral for the following nucleophilic attack.

Nucleophilic Attack

Once the nitrosonium ion ( NO^+ ) is formed, the process continues with a nucleophilic attack by aniline. Aniline, a known aromatic amine, possesses a lone pair of electrons on its nitrogen atom. This lone pair is crucial for the nucleophilic action. It involves the donation of these electron pairs to the electrophile, which in this case, is the nitrosonium ion.
Here are the steps for the nucleophilic attack:

• The nitrogen in aniline, being electron-rich, uses its lone pair of electrons to form a bond with the electron-poor NO^+ .
• This interaction results in the formation of an unstable intermediate cation. This intermediate holds both the original structure of aniline and an attached nitrosonium component.

This step is critical as it sets the stage for eventual diazonium salt formation. Understanding the nucleophile and electrophile interactions through curved arrow notation is key to mastering the reaction mechanism.

Diazonium Salt Formation

Following the nucleophilic attack, the intermediate cation evolves into a stable diazonium salt. The transformation demands the rearrangement and stabilization of the intermediate, specifically by losing a proton. This is how the process unfolds:

• The intermediate cation, initially unstable, undergoes rearrangement which facilitates the expulsion of a hydrogen ion ( H^+ ).
• The removal of this proton helps stabilize the positively charged nitrogen, culminating in the formation of a diazonium ion ( ArN_2^+ ). Here, Ar stands for the aryl group derived from aniline.
• The final product, diazonium salt, is characterized by its diazonium group ( N_2^+ ), making it a versatile intermediate for various organic synthesis reactions.

The formation of the diazonium salt is paramount as these compounds are precursors in the synthesis of azo dyes among other organic molecules. Understanding this part of the mechanism showcases the chemistry behind key industrial processes.