Question

Under which of the following conditions would toluene \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{3}\), be converted into bromomethyl ben8 zene, \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2} \mathrm{Br} ?\) (a) reaction with \(\mathrm{Br}_{2}\) in dark (b) reaction with \(\mathrm{Br}_{2} / \mathrm{FeBr}_{3}\) (c) reaction with \(\mathrm{Br}_{2}\) in sunlight (d) reaction with \(\mathrm{HBr}\)

Short answer

Toluene converts to bromomethylbenzene with \( \mathrm{Br}_{2} \) in sunlight (option c).

Step-by-step solution

Identify Reaction Types

Review each option to determine the type of reaction it indicates. Reaction (a) suggests non-catalyzed bromination, (b) indicates a bromination with an iron catalyst, (c) involves photochemical conditions, and (d) involves hydrogen bromide, which may not lead to substitution at the methyl position.

Understand Reaction Conditions

Understand that bromination at the methyl group in toluene requires radical conditions, typically initiated by light (photochemical reaction). The methyl group needs light-induced bromination to form bromomethylbenzene, which involves substitution at the side chain.

Evaluate Electrophilic Aromatic Substitution

Recognize that options (a) and (b)—dark and FeBr₃-catalyzed—primarily lead to electrophilic aromatic substitution rather than side chain bromination. They likely result in substitution on the aromatic ring, forming different products.

Recognize Radical Bromination

Option (c) provides photochemical conditions, leading to the formation of bromine radicals that target the methyl group, transforming it into bromomethylbenzene (\(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CH}_{2}\mathrm{Br}\)).

Eliminate Incorrect Options

Reject option (d), as \(\mathrm{HBr}\) doesn’t provide conditions for bromination at the \(\mathrm{CH}_{3}\) group, and option (a), as the reaction occurs in dark conditions leading to aromatic ring substitution.

Confirm the Correct Condition

Confirm that the correct condition for forming bromomethylbenzene is provided by option (c), which utilizes \(\mathrm{Br}_{2}\) in sunlight to produce the desired side chain bromination.

Key concepts

Radical Bromination

In the world of organic chemistry, radical bromination is a fascinating process where bromine is used to replace a hydrogen atom on a molecule through a radical mechanism. This type of reaction is especially useful for converting methyl groups into bromomethyl groups, like in the conversion of toluene into bromomethylbenzene.

Radical bromination requires energy, often provided by light or heat, to initiate the reaction. The energy helps produce bromine radicals. These are highly reactive species that have an unpaired electron. Once formed, these radicals target the weakest bond in the molecule, typically a hydrogen atom bonded to a carbon.

The process involves the following stages:

• Initiation: The energy splits the bromine molecule ( Br_2 ) into two bromine radicals.
• Propagation: A bromine radical abstracts a hydrogen from the methyl group on toluene, creating a new radical that can react with another bromine molecule.
• Termination: Two radicals combine to form a stable product, including the desired bromomethylbenzene.

Understanding this process is crucial because it shows how to harness radical chemistry to target specific parts of a molecule, leading to valuable chemical transformations.

Photochemical Reaction

Photochemical reactions are chemical reactions initiated by the absorption of light. These reactions involve energy transfer, often leading to the breaking of chemical bonds. In the case of radical bromination of toluene, sunlight provides the energy needed for bromine molecules to split into radicals.

This splitting or "homolytic cleavage" is essential for producing reactive bromine radicals. These radicals play a key role in targeting the methyl group of toluene for substitution, leading to the formation of bromomethylbenzene.

Some important points about photochemical reactions include:

• Light as a catalyst: Although light does not alter its own state, it provides the necessary activation energy.
• Selective excitation: Photons are absorbed by specific bonds, selectively exciting and activating them for reaction.
• Quick processes: Once initiated, these reactions can proceed very rapidly, owing to the highly reactive nature of the radicals involved.

Photochemical reactions open up unique pathways to chemical transformations that might be less feasible under thermal conditions.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a common reaction type where an electrophile replaces a hydrogen atom on an aromatic ring, like benzene. This reaction is characterized by preserving the aromaticity of the product, which is a driving force for the reaction to occur.

In the context of bromination, when toluene is reacted with a brominating agent and an appropriate catalyst, such as FeBr₃, bromine can add to the benzene ring rather than the methyl group, resulting in an electrophilic aromatic substitution:

• Catalyst role: Lewis acid catalysts like FeBr₃ help form a more electrophilic bromine species.
• Stability of the cation: The aromatic ring stabilizes charged intermediates through resonance.
• Substitution pattern: Directed generally by substituent groups already present on the ring, in this case, methyl directing ortho/para positions.

While electrophilic aromatic substitution is prominent, it is crucial to know that it contrasts with radical bromination. Instead of targeting the side chain, these reactions focus on the aromatic system, leading to different products.