Problem 178 - A Complete Resource Book in Chemistry for JEE Main

Question

Fluorobenzene \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~F}\right)\) can be synthesized in the laboratory \([2006]\) (a) by heating petrol with HF and 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

Step-by-Step Solution

Answer

Fluorobenzene is synthesized using diazotization of aniline and treating with \( \mathrm{HBF}_4 \).

Step 1: Eliminate based on safety and conditions

Option (a) suggests heating petrol with HF and KF. This is not a standard or safe method to introduce fluorine into a benzene ring, primarily due to the reactivity and danger of using HF. Option (c) involves direct fluorination of benzene with \( \mathrm{F}_2 \) gas which is highly reactive and not typically used due to safety concerns.

Step 2: Consider diazotization method

Option (b) involves synthesizing fluorobenzene from aniline by diazotization, followed by heating the diazonium salt with \( \mathrm{HBF}_4 \). This is a recognized method known as the Balz-Schiemann reaction, which is commonly used for introducing fluorine into aromatic compounds safely.

Step 3: Analyze halogen exchange possible

Option (d) discusses reacting bromobenzene with a NaF solution. Halogen exchange (Finkelstein reaction) in aromatic compounds is not efficient for synthesis of fluorobenzene, as fluorobenzene is not typically formed this way. Thus, this method is less feasible compared to diazotization.

Key Concepts

Diazotization

Diazotization is a fascinating chemical process largely used in organic chemistry. It involves converting a primary aromatic amine into a diazonium salt. This reaction is crucial because it sets the stage for a series of subsequent reactions that are useful in synthesizing various aromatic compounds.

• The general process begins with a primary aromatic amine, such as aniline, which is treated with nitrous acid (often generated in situ using sodium nitrite and an acid like HCl).
• This creates an aromatic diazonium salt, a versatile intermediate that can participate in multiple reactions.

An important aspect of diazotization is its ability to be used for further chemical modifications. These diazonium salts are known for their stability at low temperatures, making them ideal candidates for various substitution reactions, like replacing the amine group with different functional groups.
The Balz-Schiemann reaction specifically leverages this diazonium salt to introduce a fluorine atom into the aromatic ring, which is a classic example of utilizing diazotization in organic synthesis.

Fluorobenzene Synthesis

Fluorobenzene synthesis via diazotization is a classic method known for its efficiency and practicality. This process, typically the Balz-Schiemann reaction, introduces a fluorine atom into the aromatic ring, providing a pathway to generate fluorinated aromatic compounds.

• The process begins with aniline, an aromatic amine, which undergoes diazotization to form a diazonium salt.
• Subsequently, this diazonium salt is treated with \( \text{HBF}_4 \), facilitating the replacement of the amine group by a fluorine atom.
• Upon heating, the diazonium group is expelled, and fluorobenzene is formed.

This reaction is remarkably efficient and allows chemists to safely introduce fluorine into aromatic compounds. The choice of \( \text{HBF}_4 \) is significant, as it stabilizes the diazonium salt precursor and enables the smooth transition to the fluorinated product.
Fluorine's unique properties, such as high electronegativity and the ability to form strong carbon-fluorine bonds, contribute to the significance of this compound in various industrial and pharmaceutical applications.

Aromatic Compounds

Aromatic compounds, sometimes referred to as arenes, are a fascinating class of organic molecules known for their stability and unique properties. These compounds have one or more planar rings with delocalized pi-electron clouds, which confer distinct chemical behaviors.
Common examples include benzene, toluene, and naphthalene. These molecules exhibit resonance, which makes them exceptionally stable compared to other hydrocarbons.
Fluorobenzene, the target product in the discussed synthesis, is an aromatic compound characterized by a benzene ring bonded to a fluorine atom. This substitution impacts the compound's chemical properties, enhancing its stability and altering its reactivity.

• The aromatic ring facilitates various substitution reactions, which are fundamental in organic chemistry.
• Additions of different substituents can significantly change the electronic properties of the compound, affecting its reactivity and potential applications.

Understanding the chemistry of aromatic compounds is crucial for any organic chemist. They serve as the backbone for numerous synthetic processes and are the subject of ongoing research due to their potential uses in medicine, materials science, and more.