Question

Fluorobenzene \(\left(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{~F}\right)\) can be synthesized in the laboratory [2006] (a) by heating petrol with \(\mathrm{HF}\) and \(\mathrm{KF}\) (b) from aniline by diazotization followed by heating the diazonium salt with \(\mathrm{HBF}_{4}\) (c) by direct fluorination of benzene with \(\mathrm{F}_{2}\) gas (d) by reacting bromobenzene with NaF solution

Short answer

Option (b) is the best method: diazotization of aniline followed by HBF_4 treatment (Balz-Schiemann reaction).

Step-by-step solution

Analyze Option A

In option (a), it suggests heating petrol with hydrofluoric acid (HF) and potassium fluoride (KF). However, benzene or derivatives are not typically synthesized this way as the reaction conditions are extremely harsh, and the presence of other hydrocarbons in petrol makes targeted synthesis difficult. It is not a suitable or common method for synthesizing fluorobenzene.

Analyze Option B

Option (b) suggests starting from aniline and converting it to a diazonium salt, followed by heating with tetrafluoroborate (HBF_4). This is the Balz-Schiemann reaction. It is a classic and effective method for synthesizing aryl fluorides, including fluorobenzene. The diazonium salt is typically formed by treating aniline with nitrous acid in situ, followed by reaction with HBF_4 to yield the desired product.

Analyze Option C

Option (c) suggests direct fluorination of benzene with fluorine gas (F_2). Direct fluorination of benzene with F_2 is very challenging due to the high reactivity and difficulty in controlling the reaction conditions, often leading to over-fluorination and undesirable products. Therefore, it's not a practical method for synthesizing fluorobenzene.

Analyze Option D

Option (d) proposes reacting bromobenzene with sodium fluoride (NaF) solution. However, the reactivity of aryl halides such as bromobenzene does not readily allow for substitution of bromine with fluorine under mild conditions without a more specialized catalyst or elevated conditions. It's not a conventional synthetic route for fluorobenzene.

Conclusion: Select the Best Option

Upon analyzing all options, the most effective and standard method for synthesizing fluorobenzene is option (b), using diazotization of aniline followed by treatment with HBF_4, known as the Balz-Schiemann reaction.

Key concepts

Balz-Schiemann Reaction

The Balz-Schiemann reaction is a vital tool in the synthesis of aryl fluorides, like fluorobenzene. This chemical reaction involves transforming an aryl amine, such as aniline, into an aryl fluoride in a multi-step process. The process kicks off with the formation of a diazonium salt from the aromatic amine. Typically, aniline is treated with nitrous acid in situ to yield the diazonium salt.
Once formed, these diazonium salts are then reacted with tetrafluoroborate (HBF₄). Upon heating, this mixture decomposes to give the aryl fluoride and nitrogen gas, expelling boric acid as a side product. The Balz-Schiemann reaction is prevalent because it achieves single fluorination, which is often difficult due to fluorine's high reactivity. It provides a reliable way to introduce the fluorine atom into the aromatic ring.
Students learning about this reaction should focus on understanding the significance of diazonium salts and their stability, as well as the ability of HBF₄ to facilitate the transformation. This reaction finds its utility in the synthetic repertoire of both laboratory-scale chemistry and industrial applications of fluorinated aromatic compounds.

Diazotization

Diazotization is a cornerstone chemical process used to convert primary aromatic amines into diazonium salts. It serves as a critical initiation step in the Balz-Schiemann reaction when synthesizing aryl fluorides. The reaction generally involves the amine, such as aniline, reacting with nitrous acid, which is usually generated in situ from sodium nitrite and a strong acid like hydrochloric acid.
The resulting diazonium salt, characterized by its nitrogen-nitrogen triple bond (N₂⁺), is markedly more reactive than the parent amine. This added reactivity paves the way for subsequent targeted reactions like halogenation. However, diazonium salts can be unstable, especially when heated, which makes careful handling and control of the reaction conditions essential.
By understanding diazotization, students can appreciate how it acts as a gateway transformation, opening up a wide range of chemical modifications and enabling the synthesis of diverse aromatic derivatives, such as in the creation of aryl fluorides.

Aryl Fluorides

Aryl fluorides, such as fluorobenzene, are important organic compounds characterized by a fluorine atom bonded to an aromatic ring. These compounds are highly valued in pharmaceuticals, agricultural chemicals, and materials science because of the unique properties introduced by the fluorine atom. Fluorine imparts compounds with enhanced metabolic stability and lipophilicity, which is beneficial in drug design.
Creating aryl fluorides can be challenging due to the high reactivity of fluorine. Consequently, reactions like the Balz-Schiemann have been developed to reliably introduce fluorine into the aromatic ring without causing unwanted side reactions or decomposition. Understanding the synthesis and characteristics of aryl fluorides allows chemists to harness the beneficial properties of fluorine in a controlled, predictable manner, making it a topic of significant interest in modern synthetic chemistry.

Reaction Conditions

The success of reactions like the Balz-Schiemann relies heavily on precise reaction conditions. These conditions include the concentration of reactants, temperature management, and timing, especially during the formation and decomposition of diazonium salts. Due to the potential instability of diazonium salts, maintaining a cold environment and suitable acidity is crucial during their formation.
In the Balz-Schiemann reaction, careful addition of HBF₄ and controlled heating are essential to ensure the correct progression to aryl fluoride without side reactions. Reaction conditions must be finely tuned to prevent the decomposition or destruction of the diazonium salt before it is transformed into the desired product.
Understanding these reaction conditions provides students with the ability to troubleshoot and optimize syntheses based on diazotization and subsequent transformations. It underscores the importance of precision and control in chemical synthesis, particularly when dealing with sensitive intermediates like diazonium salts.