Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

Under which of the following conditions would toluene \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{3}\), be converted into bromomethyl benzene, \(\mathrm{C}_{6} \mathrm{H}_{3}-\mathrm{CH}_{2} \mathrm{Br}\) ? (a) reaction with \(\mathrm{Br}_{2}\) in dark (b) reaction with \(\mathrm{Br}_{2} / \mathrm{FeBr}_{3}\) (c) reaction with \(\mathrm{Br}_{2}\) in sunlight (d) reaction with \(\mathrm{HBr}\)

Step-by-Step Solution

Answer

Reaction with Br2 in sunlight (option c) converts toluene to bromomethyl benzene.

Step 1: Understanding the Reaction Pathways

To determine under which conditions toluene is converted to bromomethyl benzene, recognize the type of reaction needed. Toluene must undergo a substitution at the benzylic position, which typically requires a free radical halogenation.

Step 2: Evaluating Option (a): Reaction with Br2 in Dark

In the dark, bromine molecules remain stable and do not generate free radicals. Therefore, there is no reaction with toluene as no benzylic hydrogen is abstracted. This condition is not suitable for forming bromomethyl benzene.

Step 3: Evaluating Option (b): Reaction with Br2 / FeBr3

This combination is typical for electrophilic aromatic substitution, which affects the aromatic ring rather than the benzylic position. Hence, it would result in bromination on the benzene ring, not across the methyl group.

Step 4: Evaluating Option (c): Reaction with Br2 in Sunlight

Sunlight provides energy to form bromine radicals from Br2, initiating a radical substitution reaction at the benzylic carbon, i.e., the formation of bromomethyl benzene. This fits the requirement for replacing a hydrogen at the benzylic position with a bromine.

Step 5: Evaluating Option (d): Reaction with HBr

HBr provides bromine as a nucleophile rather than generating radicals, and thus does not lead to the required substitution at the benzylic position. It is not suitable for converting toluene to bromomethyl benzene.

Key Concepts

Free Radical Halogenation

In free radical halogenation, a molecule like bromine (Br extsubscript{2}) breaks down into free radicals, which are highly reactive atoms or molecules with unpaired electrons. This process is usually initiated by heat or light, which provides the energy needed to overcome the bond energy of Br extsubscript{2}.
When toluene (C extsubscript{6}H extsubscript{5}-CH extsubscript{3}) is exposed to bromine in the presence of sunlight, the bromine molecules dissociate into bromine radicals. These radicals attack the benzylic hydrogen in toluene, forming a carbon radical at the benzylic position.

• This benzylic radical is then stabilized by a reaction with another bromine molecule, replacing one of the hydrogens with a bromine atom to form bromomethyl benzene (C extsubscript{6}H extsubscript{5}-CH extsubscript{2}Br).
• The free radical chain reaction continues until the bromine or toluene runs out.

Thus, sunlight enables this process, making option (c) the correct choice for forming bromomethyl benzene.

Benzylic Substitution

Benzylic substitution involves the substitution of a hydrogen atom on the carbon atom directly attached to the benzene ring, known as the benzylic position. This position has unique reactivity due to resonance stabilization, enabling it to undergo various reactions more readily than aliphatic carbon positions.
In the case of toluene, the benzylic site is where the methyl group (CH extsubscript{3}) connects to the benzene ring.

• For bromination at this site, it’s crucial to use conditions that promote the formation of radicals.
• Energy sources like light (option c) trigger the transformation efficiently, leading to the desired product.

Other methods that directly involve electrophilic or ionic reactions, such as options (b) and (d), do not favor benzylic substitution. This is because they either focus on the aromatic nucleus or lack the radical formation necessary for this transformation.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a common reaction where an electrophile replaces a hydrogen atom in an aromatic system. This is typical of aromatic compounds due to their electron-rich nature.
However, when considering tolulene, which consists of a benzene ring attached to a methyl group, if one looks to substitute a benzylic hydrogen, EAS is not the appropriate pathway; this would instead lead to the aromatic ring being substituted.

• Reaction option (b), using Br extsubscript{2}/FeBr extsubscript{3}, is the classic condition for EAS, directing the bromination to the electron-rich aromatic ring due to the electrophilic character of the bromine in the presence of the iron catalyst.
• Thus, while bromination occurs, it does not happen at the benzylic position as needed for producing bromomethyl benzene.

This highlights the importance of selecting the right reaction type for achieving specific site substitutions.