Question

Reaction of \(\mathrm{Br}_{2}\) and \(\mathrm{FeBr}_{3}\) with phenol can lead to three possible substitution products. Show the structure of each and name them.

Short answer

Products are 2-bromophenol, 4-bromophenol, and 2,4,6-tribromophenol.

Step-by-step solution

Introduction to Electrophilic Aromatic Substitution

The reaction of bromine (\(\mathrm{Br}_2\)) in the presence of iron(III) bromide (\(\mathrm{FeBr}_3\)) with phenol involves an electrophilic aromatic substitution mechanism. The phenolic group is activating and ortho, para-directing, leading to substitution at the ortho and para positions.

Identify the Ortho Positions

The phenol has a hydroxyl group attached to the benzene ring. The ortho positions are adjacent to the hydroxyl group. In the case of phenol, these are the positions 2 and 6 on the benzene ring.

Identify the Para Position

The para position is opposite the hydroxyl group on the benzene ring. For phenol, this is position 4 on the benzene ring.

Formation of 2-Bromophenol

A bromine atom can be substituted at one of the ortho positions (position 2). The structure is a bromine atom at position 2 with the hydroxyl group remaining at position 1. This product is named 2-bromophenol.

Formation of 4-Bromophenol

Alternatively, substitution could occur at the para position (position 4). Here, a bromine atom is placed at position 4. This product is named 4-bromophenol.

Formation of 2,4,6-Tribromophenol

In some cases, all three positions (2, 4, and 6) adjacent to the hydroxyl group can be brominated. The result is a structure with bromine atoms at positions 2, 4, and 6, and is called 2,4,6-tribromophenol.

Key concepts

Phenol

Phenol is a simple aromatic compound, characterized by a hydroxyl group (\(-\mathrm{OH}\)) attached directly to a benzene ring. This structure influences its reactivity and the nature of its chemical reactions, particularly electrophilic aromatic substitution (EAS). The hydroxyl group is highly reactive in electrophilic substitution reactions because it is an activating group.
The presence of the \(-\mathrm{OH}\) group increases the electron density on the benzene ring, making it more reactive towards electrophiles like \(\mathrm{Br}_2\). This electron-donating effect is vital in understanding phenol's behavior in chemical reactions.
As the substituent exerts a strong activating effect, it significantly enhances the reaction rate with electrophiles compared to benzene. Furthermore, it plays a crucial role in directing where an incoming substituent might attach on the benzene ring. Understanding phenol's reactivity helps students predict and explain outcomes in bromination reactions.

Ortho Para-Directing Groups

Ortho and para-directing groups are substituents that influence the position on the benzene ring where electrophilic substitution reactions commonly occur. These groups have the ability to activate the aromatic ring by donating electrons through resonance or inductive effects. Consequently, positions directly next to (ortho) and opposite (para) to these substituents are more reactive towards incoming electrophiles.
In the case of phenol, the hydroxyl group is ortho and para-directing. Due to resonance, it effectively increases electron density at the ortho (positions 2 and 6) and para (position 4) locations. These positions become favorable spots for further substitution.

• Ortho positions: These are the two adjacent positions to the \(-\mathrm{OH}\) group.
• Para position: This is directly opposite to the \(-\mathrm{OH}\) group.

When phenol undergoes bromination, it tends to yield brominated products at these strategic positions due to the electron-donating nature of the hydroxyl group.

Bromination

Bromination is a type of electrophilic aromatic substitution where a bromine atom is introduced to the benzene ring. For phenol, this reaction is facilitated by using \(\mathrm{Br}_2\) in the presence of \(\mathrm{FeBr}_3\), which generates the electrophilic species \(\mathrm{Br}^+\) needed for the substitution.
During bromination, \(\mathrm{Br}_2\) interacts with \(\mathrm{FeBr}_3\) to form a highly reactive complex that facilitates the attack of bromine on the electron-rich positions of phenol. The reaction can yield several possible products:

• 2-Bromophenol: Bromine atom substitutes at one ortho position.
• 4-Bromophenol: Bromine atom substitutes at the para position.
• 2,4,6-Tribromophenol: An intensive reaction where bromine atoms substitute at all three ortho and para positions.

This selectivity in bromination highlights the role of the hydroxyl group as an ortho and para director and shows how different conditions can manipulate product distribution in chemical syntheses.