Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Fluorination of an aromatic ring is easily accomplished by treating a diazonium salt with \(\mathrm{HBF}_{4}\). Which of the following conditions is correct about this reaction?(a) \(\mathrm{NaF} / \mathrm{Cu}\) (b) \(\mathrm{Cu}_{2} \mathrm{O} / \mathrm{H}_{2} \mathrm{O}\) (c) Only heat (d) \(\mathrm{NaNO}_{2} / \mathrm{Cu}\)

Step-by-Step Solution

Answer

(c) Only heat.

Step 1: Understand the Reaction

The diazonium salt is treated with another reagent to substitute the diazonium group with a fluorine atom. This is known as the Balz–Schiemann reaction. The reagent \(\mathrm{HBF}_4\) is used to create the tetrafluoroborate salt which, upon heating, decomposes to introduce the fluorine atom into the aromatic ring.

Step 2: Identify Suitable Conditions

The reaction involves forming an intermediate (tetrafluoroborate salt) and then applying heat. The role of heat is crucial to decompose the intermediate, but additional conditions or materials like metals are not necessarily used after forming the salt.

Step 3: Analyze Each Option

Let's examine each given condition: (a) \(\mathrm{NaF}/\mathrm{Cu}\): This is used in Sandmeyer reactions for chloro/bromo substitution, not suitable for fluorination via Balz-Schiemann. (b) \(\mathrm{Cu}_{2}\mathrm{O}/\mathrm{H}_{2}\mathrm{O}\): Often used for hydrolysis in Sandmeyer reactions, irrelevant here. (c) Only heat: Correct, as heating is necessary to decompose the tetrafluoroborate salt formed by \(\mathrm{HBF}_4 \). (d) \(\mathrm{NaNO}_{2}/\mathrm{Cu}\): Similar to option (a), not relevant for fluorination.

Step 4: Select the Correct Answer

Among the options, only utilizing heat accurately describes the condition required to complete the Balz-Schiemann reaction, turning a diazonium salt with \(\mathrm{HBF}_4\) into a fluorinated aromatic ring.

Key Concepts

aromatic fluorination

Aromatic fluorination is a unique process that allows a fluorine atom to be introduced into an aromatic ring. This process enhances the chemical properties of compounds, making them valuable in various applications, particularly in pharmaceuticals.
The process is significantly different from other halogenation reactions due to fluorine's high reactivity and the challenging nature of the C–F bond formation. This is where the Balz–Schiemann reaction comes in handy. By using diazonium salts combined with special reagents, fluorination becomes more manageable and safer.

• The reaction begins with the formation of a diazonium salt from an aniline derivative. This is typically achieved by reacting the aniline with nitrous acid.
• The diazonium group in the salt is then replaced by fluorine using a unique method that involves a tetrafluoroborate salt intermediate.
• This method of aromatic fluorination through the Balz–Schiemann reaction offers a practical route for incorporating fluorine atoms into aromatic compounds without the need for extreme conditions.

diazonium salts

Diazonium salts play a crucial role in the Balz–Schiemann reaction and other substitution reactions. These salts are usually derived from aromatic amines and have the general formula \( ext{R-N}_2^+ \).Diazonium ions offer a convenient leaving group, making them ideal for various substitution mechanisms.
A key feature of diazonium salts is their ability to undergo reactions that replace the diazonium group with different substituents. In the scope of aromatic fluorination, the diazonium salt undergoes a transformation involving \ etrafluoroborate ions, eventually resulting in the substitution of the diazonium group with a fluorine atom.

• The reactivity of diazonium salts towards this substitution makes them valuable intermediates in the synthesis of aryl fluorides.
• The formation of azo compounds is another important aspect where diazonium salts serve as intermediates.
• It's worth noting that the handling of diazonium salts should be done cautiously, as many can be quite unstable, especially in their dried forms.

tetrafluoroborate salt decomposition

Tetrafluoroborate salt decomposition is a critical step in the Balz–Schiemann reaction, responsible for introducing the fluorine atom into the aromatic compound. This process starts after the formation of a stable diazonium tetrafluoroborate salt.
The stability of the intermediate tetrafluoroborate salt ( R-N2BF4) allows for a safer handling of the reaction before the final decomposition.
Upon heating, the tetrafluoroborate salt decomposes effectively, resulting in the release of the diazonium group and the formation of an aryl fluoride.

• This decomposition step eliminates the need for harsh conditions or additional reagents, making it a relatively mild reaction process.
• The application of heat ensures that the intermediate converts to the desired fluorinated aromatic compound efficiently.
• Understanding the thermal decomposition of tetrafluoroborate salts is key to mastering this chemical transformation process and applying it effectively in synthetic chemistry.