Problem 23 - Chemistry in 30 days

Question

Aniline is reacted with bromine water and the resulting produet is treated with an aqueous solution of sodium nitrite in presence of dilute HC1. The compound so formed is converted into tetrafluoroborate which is subsequently heated dry. The final product is (a) \(p\) - Ifromefluorobenzene (b) \(p\)-Bromoaniline (e) 2, 4, 6-Tribromofluorobenzene (d) \(1,3,5\)-Tribromobenzene

Step-by-Step Solution

Answer

The final product is 1,3,5-tribromobenzene (d).

Step 1: Reaction of Aniline with Bromine Water

Aniline reacts with bromine water in a substitution reaction that typically leads to the formation of 2,4,6-tribromoaniline. The amine group activates the ring, making it highly susceptible to multiple substitutions at the ortho and para positions.

Step 2: Diazotization

The resulting 2,4,6-tribromoaniline is treated with an aqueous solution of sodium nitrite and hydrochloric acid at low temperatures, converting the amine group into a diazonium salt, specifically 2,4,6-tribromobenzenediazonium chloride.

Step 3: Formation of Tetrafluoroborate

The diazonium salt is then treated with sodium tetrafluoroborate to form 2,4,6-tribromobenzenediazonium tetrafluoroborate. This process involves replacing the chloride ion with the tetrafluoroborate ion.

Step 4: Decomposition of Diazonium Tetrafluoroborate

Upon heating, the diazonium tetrafluoroborate decomposes, releasing nitrogen gas, leaving behind 1,3,5-tribromobenzene as the final product after the loss of the diazonium group.

Key Concepts

Aniline Reactions

Aniline is an aromatic compound that contains an amino group attached to a benzene ring. This unique structure makes aniline highly reactive, especially in reactions that involve nucleophilic substitution. When aniline reacts with bromine water, it undergoes an electrophilic substitution reaction. In this process, the amino group activates the benzene ring, making it more susceptible to attack by bromine. This reaction typically results in multiple substitutions at the ortho and para positions, leading to the formation of 2,4,6-tribromoaniline.
The outcome is impressive as it demonstrates the strong activating effect of the amino group that facilitates bromination to occur at three different positions on the benzene ring.

Diazotization Process

The diazotization process is a crucial step when modifying aromatic amines like aniline. This step involves converting the amine group present in 2,4,6-tribromoaniline into a diazonium group. The diazotization process is carried out by treating the amine with sodium nitrite in the presence of hydrochloric acid under low temperature conditions.

• The hydrochloric acid helps in forming the nitrous acid needed for the diazonium group formation.
• The reaction results in 2,4,6-tribromobenzenediazonium chloride, which is more reactive and serves as an important intermediate for further transformations.

Ensuring an efficient diazotization is key as the resulting diazonium compound is highly versatile for different synthetic applications.

Substitution Reaction

Substitution reactions are central to the transformation of aniline to its derivatives. In the case of aniline, the substitution occurs twice. First, the amine group undergoes direct substitution with the ring being substituted with bromine atoms, leading to tribromoaniline formation. Then, in the diazotization step, the amine group itself is replaced by a diazonium group.

• The initial substitution involves the aromatic ring being activated, allowing for the addition of bromine atoms.
• Later, the amine is substituted by the diazonium group, which facilitates further transformation to tetrafluoroborate in subsequent steps.

This two-step substitution process showcases the varied pathways available in organic chemistry when functional groups undergo transformation.

Formation of Tetrafluoroborate

The diazonium salt formed from 2,4,6-tribromoaniline is further reacted to form a more stable compound. This involves treating the diazonium chloride with sodium tetrafluoroborate. During this process, the chloride ion in the diazonium salt is replaced by a tetrafluoroborate ion, resulting in the formation of a new salt: 2,4,6-tribromobenzenediazonium tetrafluoroborate.
This substitution is crucial as the tetrafluoroborate ion adds stability to the compound, preventing it from premature decomposition. It is particularly useful as diazonium tetrafluoroborates are less sensitive to temperature changes compared to their chloride counterparts, making them easier to handle in laboratory settings.

Decomposition of Diazonium Compounds

The ultimate step in the outlined reaction sequence is the decomposition of diazonium compounds, specifically diazonium tetrafluoroborate. Upon heating, 2,4,6-tribromobenzenediazonium tetrafluoroborate undergoes thermal decomposition. This reaction leads to the release of nitrogen gas as a by-product, which is one of the distinctive features of diazonium decomposition.
The breakdown of the diazonium compound results in the removal of the diazonium group, ultimately yielding 1,3,5-tribromobenzene as the final product. This step highlights the usefulness of diazonium compounds in synthetic organic chemistry, offering pathways to form various aromatic compounds efficiently and effectively.