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

Option (c) is correct, as sunlight provides the energy needed for free radical bromination.

Step 1: Identify the target compound

We want to convert toluene ( C_{6}H_{5}-CH_{3}) into bromomethyl benzene ( C_{6}H_{5}-CH_{2}Br). This requires replacing one of the hydrogen atoms in the methyl group (-CH_{3}) with a bromine atom.

Step 2: Understand the type of reaction needed

The conversion of toluene to bromomethyl benzene involves a free radical substitution reaction. In this type of reaction, a hydrogen atom in the methyl group is replaced by a bromine atom.

Step 3: Assess conditions needed for free radical substitution

Free radical substitutions generally require the presence of heat or light to initiate the formation of radicals. This often involves using bromine ( Br_{2}) under specific conditions.

Step 4: Evaluate given options

Option (a) involves  Br_{2} in dark, which does not provide the energy needed for radical formation. Option (b)  Br_{2} /  FeBr_{3} is an electrophilic aromatic substitution, not a radical substitution. Option (d) involves HBr, not conducive for radical conditions. Option (c) involves sunlight, a source of energy required to form radicals.

Step 5: Conclusion

Since option (c) suggests  Br_{2}^{-} in sunlight, it provides the necessary energy to form radicals which then substitute a hydrogen atom in the methyl group, making it the correct choice.

Key Concepts

Free Radical Substitution

Free radical substitution is a fundamental concept of organic chemistry where atoms or groups of atoms within molecules are replaced by radicals. A radical is an atom or group with an unpaired electron, making it highly reactive.

In the context of converting toluene (\(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{3}\)) to bromomethyl benzene (\(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2} \mathrm{Br}\)), free radical substitution involves replacing a hydrogen atom in the methyl group with a bromine atom. This process requires the formation of bromine radicals, which then engage with the toluene molecule to substitute one of the hydrogen atoms for a bromine atom.

This type of reaction typically requires energy in the form of heat or light, as it helps in breaking the bond between the two bromine atoms to generate bromine radicals.

Bromination

Bromination is a specific type of halogenation where a bromine atom is introduced into a molecule. In the case of toluene, bromination specifically replaces a hydrogen atom in the methyl group to form bromomethyl benzene.

The bromination process follows a radical mechanism. It starts with the homolytic cleavage of bromine (\(\mathrm{Br}_2\)) molecules into two bromine radicals. These radicals interact with the toluene molecules, effectively substituting a hydrogen atom in the methyl group with a bromine atom.

Bromination is selective and often favored due to the moderate reactivity of bromine, which allows controlled substitution reactions as opposed to more reactive halogens like fluorine or chlorine.

Reaction Conditions

Reaction conditions are crucial for any chemical process and significantly impact the outcome of the reaction. In free radical substitution reactions like bromination of toluene, the presence of energy is necessary to facilitate the homolytic cleavage of bromine molecules.

Energy sources such as heat or ultraviolet light are typically used to initiate radical formation, which is why option (c) in the exercise, involving bromine in sunlight, is accurate. Sunlight provides the high-energy photons needed to break the \(\mathrm{Br}_2\) bond, creating radicals that will react with the toluene molecules.

Understanding these reaction conditions ensures that the radical mechanism is favored over other potential reaction pathways, resulting in successful methyl group substitution.

Methyl Group Substitution

Methyl group substitution refers to the replacement of a hydrogen atom within the methyl (\(-\mathrm{CH}_3\)) group of toluene by another atom or group, typically through a chemical reaction.

In the context of bromination, this substitution process involves free radicals that selectively target and replace one of the three hydrogen atoms attached to the carbon in the methyl group. The result is the production of bromomethyl benzene, where one hydrogen is substituted by a bromine atom, forming \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_2 \mathrm{Br}\).

Methyl group substitution changes the chemical properties of the original compound, potentially altering its reactivity, polarity, and other physical characteristics.