Problem 10 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Aniline is reacted with bromine water and the resulting product is treated with an aqueous solution of sodium nitrite in presence of dilute hydrochloric acid. The compound so formed is converted to a tetrafluoroborate which is subsequently heated. The final product is: 1 CBSE 1998] (a) \(1,3,5\)-tribromobenzene (b) \(p\)-bromofluorobenzene (c) \(p\)-bromoaniline (d) \(2,4,6\)-tribromofluorobengene

Step-by-Step Solution

Answer

The final product is \(2,4,6\)-tribromofluorobenzene (d).

Step 1: Bromination of Aniline

Aniline reacts with bromine water to form a tribrominated compound. The amino group is a strong activating group that directs the bromination to the ortho and para positions, leading to the formation of \(2,4,6\)-tribromoaniline.

Step 2: Formation of Diazonium Salt

The tribromoaniline is treated with sodium nitrite \((NaNO_2)\) and hydrochloric acid \((HCl)\) to form \(2,4,6\)-tribromobenzenediazonium chloride. This process involves the conversion of the amino group into a diazonium group \(-N_2^+Cl^-\).

Step 3: Conversion to Tetrafluoroborate

The diazonium salt is treated with tetrafluoroborate ions \((BF_4^- )\), which leads to the formation of \(2,4,6\)-tribromobenzenediazonium tetrafluoroborate. This step replaces the chloride ion with the tetrafluoroborate ion.

Step 4: Heating the Tetrafluoroborate

Upon heating, the diazonium tetrafluoroborate undergoes a decomposition reaction, losing \(N_2\) gas and \(BF_3\), and replacing the diazonium group with a fluorine atom, forming \(2,4,6\)-tribromofluorobenzene.

Key Concepts

Bromination Reaction

Bromination is a fascinating chemical reaction where bromine atoms are added to an organic compound. When it comes to aromatic compounds like benzene, this process is known as electrophilic aromatic substitution. Aniline is a base structure that has an amine group attached to the benzene ring. The amine group is highly activating and directs any substituting group, such as bromine, to the ortho and para positions.

In this exercise, aniline reacts with bromine water, which results in bromine substituting at each ortho and para position. This occurs due to the high electron density from the amine group. As a result, we get a tribrominated product known as 2,4,6-tribromoaniline. This transformation is quite straightforward once the directing effects of the amine group are recognized. However, it's important to carefully control the conditions, as over-bromination can easily occur.

Diazonium Salt Formation

Diazonium salt formation is a key transformation in organic chemistry that involves transforming an amine into a diazonium group. When we treat the 2,4,6-tribromoaniline with sodium nitrite ( NaNO_2 ) and hydrochloric acid ( HCl ), a reaction takes place that converts the amino group ( -NH_2 ) into a diazonium group ( -N_2^+Cl^- ). This step is crucial because it provides a versatile intermediate for further chemical transformations.

The reaction is typically performed in cold conditions since diazonium salts are often unstable and may decompose at higher temperatures. This formation is a prime example of the transformation potential of aromatic amines, as it opens up pathways to introduce various functional groups to the benzene ring.

Aromatic Substitution

Aromatic substitution is a broad and critical concept in organic chemistry that encompasses the replacement of an atom or group in an aromatic compound with another atom or group. In the overall sequence shown here, we see two instances of aromatic substitution. First, during the bromination of aniline, bromine replaces hydrogen atoms at specific ortho and para positions. Second, during the diazonium salt reaction, a diazonium group is introduced, and eventually substituted by a fluorine atom.

This type of substitution typically follows a mechanism where the aromatic ring's stability is temporarily disturbed to allow the incoming substituent to replace the current one. Finally, when the diazonium tetrafluoroborate is heated, it undergoes decomposition to introduce a fluorine atom into the aromatic ring, leading to the formation of 2,4,6-tribromofluorobenzene. Understanding these substitution reactions is essential as they illustrate how aromatic compounds can be strategically modified for various chemical syntheses.