Question

\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3} \frac{\mathrm{Cl}_{2}}{\mathrm{Fe}}{ }^{\mathrm{C}}(\mathrm{A}) \stackrel{\mathrm{NBS} / \mathrm{CCl}_{4}}{\longrightarrow}(\mathrm{B})\)

Short answer

Answer: The final product formed is benzyl bromide (C6H5CH2Br).

Step-by-step solution

Reaction of Toluene with Cl2

For the first step, toluene (C6H5CH3) reacts with Cl2 in the presence of a Fe catalyst. The Fe catalyst helps to polarize Cl2, making it easier for one chlorine atom to react with the toluene's CH3 group. The H atom in the methyl group of toluene is replaced by Cl.

Formation of Intermediate Compound A

At the end of the first step, we obtain the compound A. The structure of compound A is chloromethylbenzene (C6H5CH2Cl).

Reaction of Chloromethylbenzene with NBS

In this step, N-bromosuccinimide (NBS) acts as a selective brominating agent, which is used to replace the H atom on the benzylic position of the chloromethylbenzene. CCl4 acts as a solvent in this reaction.

Formation of Final Product B

At the end of step 3, we obtain the final product B. The structure of compound B is benzyl bromide (C6H5CH2Br). So, the overall reaction can be described as: Toluene (C6H5CH3) + Cl2/Fe → Chloromethylbenzene (C6H5CH2Cl) + NBS/CCl4 → Benzyl Bromide (C6H5CH2Br)

Key concepts

Toluene Chlorination

Toluene chlorination is a significant reaction in organic chemistry that involves the substitution of a hydrogen atom in toluene with a chlorine atom. This reaction is a prime example of electrophilic aromatic substitution, where the aromatic ring remains intact, and the substitution occurs on the side-chain methyl group. The process requires the presence of a catalyst, typically iron (Fe), which helps to polarize the chlorine molecules, making it easier for the chlorine to attach to the methyl group.

During this reaction:

• Chlorine (\(\text{Cl}_2\)) is activated by the catalyst (Fe).
• One chlorine atom replaces a hydrogen atom in the methyl group of toluene, forming chloromethylbenzene.
• The overall reaction affects the methyl group, not the benzene ring.

This reaction sets the stage for further transformations in organic synthesis.

Halogenation Reactions

Halogenation reactions are essential in organic chemistry because they allow for the introduction of halogen atoms into organic compounds. This process can significantly change the physical and chemical properties of the original compound. Halogenation can be conducted on either the aromatic ring or the side chain of an aromatic compound, such as toluene.

With toluene chlorination, the focus is on the side chain, specifically the methyl group. A helpful aspect of halogenation reactions is their ability to be selective. The presence of different catalysts and halogenating agents can direct the reaction to the desired position on the molecule. This means:

• Reactions can be fine-tuned for specific outcomes, like side-chain modification.
• Selection of catalysts and conditions allows for control over the type and site of halogenation.
• Halogenation provides a way to create compounds that can undergo further chemical reactions.

This selectivity is particularly advantageous in synthesizing complex molecules, such as pharmaceuticals.

NBS Bromination

N-bromosuccinimide (NBS) is a convenient brominating agent, especially popular for its selectivity in brominating allylic and benzylic positions. The use of NBS ensures that the bromination occurs at the most reactive site, which in the case of chloromethylbenzene, is the benzylic position.

Here’s how NBS functions in this context:

• NBS provides a controlled release of bromine radicals, which facilitates selective bromination.
• Carbon tetrachloride (\(\text{CCl}_4\)) is often used as the solvent, allowing the reaction to proceed smoothly.
• The environment created by these reagents promotes substitution on the benzylic carbon, preserving the stability of the compound.

This selectivity is particularly beneficial for producing benzyl bromide from chloromethylbenzene, an important transformation in synthetic organic chemistry.

Chloromethylbenzene Formation

The formation of chloromethylbenzene is a crucial intermediate step in organic reactions involving toluene. After the initial chlorination of toluene, chloromethylbenzene is formed when a chlorine atom replaces one hydrogen of the methyl group.

This compound is particularly reactive at the benzylic position, making it an ideal candidate for further transformations, like bromination.

During its formation:

• Chloromethylbenzene retains the aromatic integrity of the benzene ring, while modifying the methyl side chain.
• This transformation is pivotal in producing derivatives like benzyl bromide.
• The intermediate's specific positioning allows for subsequent reactions that do not disrupt the aromatic ring.

Overall, chloromethylbenzene's reactivity makes it a valuable compound in synthetic pathways, highlighting the importance of stepwise transformations in organic synthesis.