Problem 159 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Phenol reacts with bromine water to give: \(\quad\) |KCET 1996] (a) 2-bromophenol (b) 4-bromophenol (c) 2,4 dibromophenol (d) \(2,4,6\)-tribromophenol

Step-by-Step Solution

Answer

(d) 2,4,6-tribromophenol

Step 1: Identify the Reaction

Phenol, represented as \(C_6H_5OH\), is known for undergoing electrophilic substitution reactions. When bromine \(Br_2\) is in water, it typically results in the substitution of hydrogen atoms on the aromatic ring.

Step 2: Understand Phenol's Reactivity

Phenol has an \(-OH\) group that activates the benzene ring toward electrophilic substitution, especially at the ortho and para positions. This means that phenol is very reactive and usually results in multiple bromine substitutions.

Step 3: Consider Bromine Substitution

When phenol reacts with bromine water, the process is not limited to a single bromination. The highly activated nature of the benzene ring due to the \(-OH\) group leads to the tendency to undergo tri-bromination at the ortho and para positions.

Step 4: Identify the Product

The typical outcome of phenol reacting with bromine water is \(2,4,6\)-tribromophenol, which results from bromination at all of the positions ortho and para to the \(-OH\) group. Therefore, the correct answer is \(2,4,6\)-tribromophenol.

Key Concepts

Phenol Reactivity

Phenol is an intriguing and highly reactive aromatic compound, mainly due to the presence of the hydroxyl group often symbolized as (-OH) . This group plays a crucial role, elevating the reactivity of phenol through electron donation. It does this by donating electron density into the benzene ring via a resonance effect.

• The hydroxyl group (-OH) is what we refer to as an activating group.
• This activation enhances the likelihood of electrophilic substitution reactions on the aromatic ring.

Phenol's electron-rich nature makes it far more reactive compared to benzene, especially in reactions with electrophiles like bromine (Br_2) . Understanding how the hydroxyl group influences reactivity provides insight into why phenol reacts so readily with bromine to form products like 2,4,6-tribromophenol.

Benzene Ring Activation

The benzene ring in phenol is highly active, mainly because of the electron-donating effect of the attached (-OH) group. This (-OH) group causes an increase in electron density throughout the ring, making it more appealing to electrophiles looking for electrons.
This is described as activating the ring for electrophilic substitution reactions. Two terms frequently used in this context are "ortho" and "para" positions, which refer to specific locations on the benzene ring relative to the (-OH) group. These sites are particularly electron-rich.

• The ortho position is adjacent to the (-OH) group.
• The para position is directly opposite the (-OH) group in the benzene ring.

Activation typically results in substitution occurring predominantly at these positions, leading to products like 2,4,6-tribromophenol when reacting with bromine in water.

Bromine Substitution

Bromine substitution in phenol is fascinating because phenol doesn't simply react with bromine (Br_2) in a single-step or one-to-one manner.
When phenol is introduced to bromine water, it doesn't just stop at mono-brominated products. Instead, given the activation of the benzene ring by the (-OH) group, it progresses to tri-bromination. This results in the formation of a 2,4,6-tribromophenol.

• The benzene ring's ortho and para positions are preferentially substituted by bromine.
• The (-OH) group enhances the reactivity so significantly that it results in full tri-bromination.

In general, understanding the role of these positions and the influence of the hydroxyl group is crucial to predicting the product outcome in reactions involving phenol and bromine.