Problem 104 - A Complete Resource Book in Chemistry for JEE Main

Question

Reaction of ethyl benzene with bromine in presence of \(\mathrm{FeBr}_{3}\) gives (a) 2-bromoethyl benzene (b) 2 -bromoethyl benzene (c) 3-bromoethyl benzene (d) 4 -bromoethyl benzene

Step-by-Step Solution

Answer

The major product is (d) 4-bromoethyl benzene.

Step 1: Identify the Type of Reaction

The given reaction is an electrophilic aromatic substitution (EAS) reaction. Bromine ({Br}) in the presence of a Lewis acid catalyst like \({FeBr}_3\) acts as an electrophile. The goal of this reaction is to introduce a \({Br}\) atom onto the benzene ring, which is typical of halogenation reactions in aromatic systems.

Step 2: Recognize Substitution Specifics

Ethyl benzene has an ethyl ({C}_2{H}_5) group attached to the benzene ring. The ethyl group is an ortho/para-directing and activating group, meaning a bromine atom is more likely to substitute at the positions ortho or para to the ethyl group on the benzene ring due to increased electron density in those positions.

Step 3: Determine the Possible Products

The ortho positions relative to the ethyl group are positions 2 and 6, while the para position relative to the ethyl group is position 4. However, due to steric hindrance, substitution at the ortho positions is less favorable than at the para position. Thus, the major product is expected to be a bromine substituting at the para position.

Step 4: Conclude the Major Product

Considering the directing effects of the ethyl group and sterics, bromination will primarily occur at the para position, resulting in 4-bromoethyl benzene.

Key Concepts

Halogenation Reaction

Halogenation reactions are a fundamental part of organic chemistry, especially when it comes to the chemistry of aromatic compounds. In a halogenation reaction, a halogen such as chlorine or bromine is introduced into another compound. For aromatic compounds like benzene, this process often involves an electrophilic aromatic substitution (EAS), where a halogen replaces a hydrogen atom on the ring.
This type of reaction is widely used because it allows chemists to modify benzene rings by introducing halogen atoms, which can then be used for further chemical transformations. The presence of a Lewis acid catalyst is frequently necessary, as it helps to generate a more reactive electrophilic species from the halogen molecule.

Ortho/Para Directing Groups

Ortho/para directing groups play a critical role in determining the position where new substituents will attach in aromatic substitution reactions. These groups are typically electron-donating and increase electron density at the ortho and para positions on the benzene ring, making them more attractive to electrophiles.
The ethyl group in ethylbenzene is a classic example of an ortho/para directing group. In the presence of electrophiles, such as the bromine in our reaction example, it directs incoming substituents primarily to the para position. Why? Because it provides more stability and less steric hindrance compared to the ortho position, where there could be crowding due to nearby groups.

Lewis Acid Catalyst

In many aromatic halogenation reactions, a Lewis acid catalyst is essential for improving the reaction's efficacy. A Lewis acid may not always be an acid in the traditional sense, but it acts by accepting electron pairs. For example, \[ \mathrm{FeBr}_3 \] is a commonly used Lewis acid catalyst in bromination reactions.
By bonding with bromine, \[ \mathrm{FeBr}_3 \] effectively forms a complex that increases the electrophilicity of the bromine. This process makes it easier for the bromine to attack the electron-rich benzene ring, thus facilitating the substitution. The use of Lewis acids is crucial because it often increases the reaction speed and yields of the desired product.

Bromination of Benzene

Bromination of benzene is a key electrophilic aromatic substitution reaction, which introduces a bromine atom into the benzene ring. This procedure is well-established and often requires a catalyst because bromine by itself is not reactive enough to overcome the stability of the benzene ring's delocalized pi electrons.
When \[ \mathrm{FeBr}_3 \] is present, it coordinates with bromine to form a more reactive species. As a result, the bromine can effectively substitute a hydrogen atom on the benzene ring, typically in positions that are influenced by existing substituents. In the case of ethylbenzene, the ethyl group directs the bromine to the para position, leading to the formation of 4-bromoethyl benzene as the major product.