Question

Nitration by nitric acid alone is believed to proceed by essentially the same mechanism as nitration in the presence of sulfuric acid. Write an equation for the generation of \(\mathrm{NO}_{2} \oplus\) from nitric acid alone.

Short answer

The generation of the $\mathrm{NO_2^+}$ cation from nitric acid alone proceeds through the following equation: \[ \mathrm{2HNO_3} \rightarrow \mathrm{H_2O} + \mathrm{NO_2^+} + \mathrm{NO_3^-} \].

Step-by-step solution

Identify the reaction species

Nitric acid has the chemical formula HNO3. We are looking for a reaction that produces NO2+.

Write down the first-step reaction

In the nitration process, the first step is the generation of a reactive ion. When nitric acid dissociates, it produces a hydronium ion and a nitrate ion. Write this dissociation as follows: \[ \mathrm{HNO_3} \rightarrow \mathrm{H_3O^+} + \mathrm{NO_3^-} \]

Generate the NO2+ ion

Nitric acid further dissociates into water and nitrogen dioxide cations (NO2+). Write this dissociation as: \[ \mathrm{H_3O^+} + \mathrm{NO_3^-} \rightarrow \mathrm{H_2O} + \mathrm{NO_2^+} \]

Combine the reactions to generate the NO2+ cation

Now, we have the two reactions responsible for creating the NO2+ cation. Combine these reactions to get the final equation for the generation of the NO2+ cation from nitric acid alone: \[ \mathrm{2HNO_3} \rightarrow \mathrm{H_2O} + \mathrm{NO_2^+} + \mathrm{NO_3^-} \]

Key concepts

Nitric Acid

Nitric acid, represented chemically as HNO₃, is a strong acid commonly used in various chemical processes, including nitration. Nitration is an important reaction where a nitro group, -NO₂, is introduced into an organic compound. This process can occur in the presence of nitric acid, either alone or in combination with other acids such as sulfuric acid, which enhances its effectiveness.
The distinctive property of nitric acid is its ability to dissociate to form various ions. When it dissociates, it produces hydronium ions (H₃O⁺) and nitrate ions (NO₃⁻). This ability to form ions is crucial because these ions participate in subsequent reactions that lead to the generation of other reactive species needed for nitration. Understanding how nitric acid dissociates and reacts is key to comprehending how nitration proceeds, especially how the nitrogen dioxide cation (NO₂⁺) is formed.

Nitrogen Dioxide Cation

The nitrogen dioxide cation, denoted as NO₂⁺, is a critical species in the nitration process. This cation is sometimes referred to as the nitronium ion. It's a positively charged ion that serves as an electrophile, meaning it seeks out electrons. Its formation is essential for introducing the nitro group into organic compounds during nitration.
The generation of NO₂⁺ involves several steps starting from its precursor, nitric acid. Initially, nitric acid dissociates into hydronium and nitrate ions. In the next step, the hydronium ion reacts with the nitrate ion to yield water and the nitronium ion (NO₂⁺).
Here is how you can visualize the critical reactions:

• Dissociation of nitric acid: \[ \mathrm{HNO_3} \rightarrow \mathrm{H_3O^+} + \mathrm{NO_3^-} \]
• Formation of NO₂⁺:\[ \mathrm{H_3O^+} + \mathrm{NO_3^-} \rightarrow \mathrm{H_2O} + \mathrm{NO_2^+} \]

These steps highlight the transformation of stable ions into a highly reactive cation capable of carrying out nitration.

Reactive Ion Generation

Reactive ion generation is an important phase in the nitration process as it ensures the creation of species like NO₂⁺ that facilitate effective nitration. In the case of nitration using nitric acid alone, understanding ion generation from this acid is fundamental.
When nitric acid dissociates, it initially produces a hydronium ion and a nitrate ion. This initial reaction sets the stage for further reactions following this dissociation. The subsequent reaction between the hydronium and nitrate ions culminates in the formation of water and the reactive NO₂⁺ ion. This two-step evolution from stable molecular components to reactive ions illustrates a general principle where acids like nitric acid activate themselves through dissociation, thereby generating ions that can engage in further chemical activity.
For students and chemists, recognizing these reactions as sequential steps allows a deeper insight into how reactive ions are generated and how these ions partake in complex series of reactions such as nitration.