Understanding the Aniline Bromine Water Reaction

The reaction of aniline with bromine water is a classic example of electrophilic aromatic substitution. Aniline is an aromatic amine, meaning it has an amino ($\text{-NH}_2$) group attached directly to a benzene ring.

The amino group is a highly activating group for electrophilic substitution. It donates electron density into the benzene ring, particularly at the ortho (positions 2 and 6) and para (position 4) positions. This makes these positions much more reactive towards electrophiles compared to the meta positions.

Aniline Reaction with Bromine Water Mechanism

Bromine water ($\text{Br}_2$ in water) acts as a source of the electrophile, the bromine cation ($\text{Br}^+$). Due to the strong activation by the $-\text{NH}_2$ group, the benzene ring in aniline becomes highly susceptible to electrophilic attack. With excess bromine water, substitution occurs rapidly at all activated positions:

• One bromine atom substitutes at position 2 (ortho).
• One bromine atom substitutes at position 4 (para).
• One bromine atom substitutes at position 6 (ortho).

This multiple substitution happens because the activation is very strong, even after one or two bromine atoms are added. The product formed is 2,4,6-tribromoaniline.

Why 2,4,6-Tribromoaniline is Formed

Unlike many electrophilic substitution reactions where only one substitution might occur or conditions need to be controlled to get mono-, di-, or tri-substitution, the reaction of aniline with bromine water specifically leads to polysubstitution, yielding 2,4,6-tribromoaniline, especially when excess bromine water is used. This highlights the powerful activating effect of the $-\text{NH}_2$ group on the benzene ring.

Here is the structure of 2,4,6-tribromoaniline:

\( \text{C}_6\text{H}_2\text{Br}_3\text{NH}_2 \)

It is aniline with bromine atoms at positions 2, 4, and 6.

Comparing Products in Aniline Bromine Water Reaction

Let's look at the given options:

• 2-Bromoaniline: Only one bromine at position 2. (Requires controlled conditions, not excess bromine water).
• 3-Bromoaniline: Bromine at position 3 (meta). The amino group is ortho, para directing, so meta substitution is not favored.
• 2, 4-Dibromoaniline: Two bromines at positions 2 and 4. (Possible with limited bromine, but excess leads to trisubstitution).
• 2, 4, 6-Tribromoaniline: Three bromines at positions 2, 4, and 6. This is the product formed with excess bromine water due to the strong activation.

Therefore, the reaction of aniline with excess of bromine water gives 2, 4, 6-tribromoaniline. This is a key concept in the organic chemistry of aromatic amines and electrophilic substitution reactions.