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

Understanding Free Radical Halogenation is crucial when studying organic chemistry reactions involving the introduction of a halogen atom into organic molecules. It's a type of chemical reaction that proceeds through the radical mechanism, which involves species with unpaired electrons called free radicals. This type of reaction is initiated by external energy sources like heat or light, which in our case is sunlight.

Free Radical Halogenation can occur on aliphatic chains, like the methyl group (-CH₃) in toluene. When toluene is exposed to bromine (Br₂) under sunlight, the light's energy causes the homolytic cleavage of the Br₂ molecule into two bromine radicals. These highly reactive species then abstract a hydrogen from the methyl group of toluene, forming a new radical that rapidly combines with another bromine radical to form bromomethyl benzene. The process can be summarized in three stages: initiation, propagation, and termination. This reaction is regioselective, often favoring the formation of the more stable radical intermediate, which in this case is the primary radical generated from the methyl group of toluene.

Such reactions are highly useful in the synthesis of halogenated compounds, which serve as building blocks for more complex molecules in the pharmaceutical and material science industries.

Electrophilic Aromatic Substitution (EAS) is another fundamental reaction in organic chemistry, which specifically alters the compounds containing an aromatic system, such as benzene rings. Unlike Free Radical Halogenation, EAS involves the attack of an electrophile on the pi system of the aromatic ring, temporarily breaking its delocalized structure before it is restored in the final product.

The role of a catalyst, like FeBr₃ in the reaction with Br₂, is to enhance the electrophilic nature of the bromine, making it more reactive towards the electron-rich aromatic ring. This reaction mechanism includes the formation of a carbocation intermediate, typically formed at positions that are ortho or para to groups that can donate electrons, such as the methyl group in toluene.

EAS is not the preferred method when bromomethyl benzene formation is desired, as it would target the benzene ring and not the methyl group. Knowing this distinction is essential when selecting conditions for specific chemical transformations, such as distance learning from an online educational platform where precise outcomes are required for successful experimentation and understanding of chemical processes.

The formation of bromomethyl benzene from toluene is an illustrative example of Free Radical Halogenation. To specifically obtain bromomethyl benzene, we need to target the methyl group attached to the benzene ring, rather than the ring itself. This goal can be achieved under conditions that favor Free Radical Halogenation, which proceeds via the formation and reaction of free radicals.

The conversion of the methyl group to a bromomethyl group (-CH₂Br) involves the abstraction of a hydrogen atom from the methyl group. This reaction is most effective under conditions that generate free radicals, such as exposure to sunlight (condition c in the exercise). Sunlight provides the energy for bromine molecules to dissociate into radicals, which are necessary intermediates for this transformation.

For students learning about organic transformations, it is important to understand that although bromination can take place on both aliphatic and aromatic systems, the choice of reaction conditions, like the presence of light or specific catalysts, will determine the site of halogenation in a molecule like toluene.